LOW-COST PROCESS CONTROL IN FOOD AND DRINK PROCESSING

Transcription

1 GG220 GUIDE ENVIRONMENTAL TECHNOLOGY BEST PRACTICE PROGRAMME LOW-COST PROCESS CONTROL IN FOOD AND DRINK PROCESSING GOOD PRACTICE: Proven technology and techniques for profitable environmental improvement

2 LOW-COST PROCESS CONTROL IN FOOD AND DRINK PROCESSING This Good Practice Guide was produced by the Environmental Technology Best Practice Programme Prepared with assistance from: Entec UK Limited Crown copyright. First printed November This material may be freely reproduced in its original form except for sale or advertising purposes. Printed on paper containing 75% post-consumer waste.

3 SUMMARY In today s competitive markets, no company in the UK food and drink industry can afford to pour money down the drain. Effective process control can reduce costs, conserve raw materials, use resources more efficiently and minimise waste disposal. Implementing low-cost measures to improve process control offers companies the opportunity to gain competitive advantage. Some companies have saved up to /year by improving their process control techniques. Improving process control by focusing on waste minimisation can reduce production costs by up to 5%. This Good Practice Guide is intended to help food and drink processing companies use lowcost process control techniques as part of an integrated approach to improve their performance by: reducing raw material consumption and product loss; reducing water use and effluent generation. The Guide, which is applicable to all sectors of the food and drink industry, describes: the benefits of process control; how to select the best control option; measurement of temperature, pressure, level, flow, ph, turbidity and conductivity; best practice for hygienic installation of measurement and control devices; the latest developments in process control technology; examples of companies that have reduced costs and wastes by installing or improving process control. Summary tables are provided to help in the selection of a suitable control technology for a particular application. The Guide concentrates on proven, low-cost control techniques available from a wide range of suppliers and suitable for a variety of applications. Industry Examples demonstrate that even simple options can produce significant cost savings and that all sizes of company can benefit from implementing or improving process control.

4 CONTENTS Section Page 1 How to use this Guide 1 2 The benefits of process control Reducing waste through better process control 3 3 Identifying the best options Will process control help to reduce waste? Selecting the best control option 6 Measurement and control techniques 8.1 Elements of a control system 8.2 Temperature measurement 9.3 Pressure measurement 13. Level measurement 15.5 Flow measurement 19.6 Analytical measurement 23.7 Process controllers/transmitters 27.8 Control devices 27 5 Ensuring a hygienic environment 31 6 Process control technology developments 32 7 Industry Examples Product recovery using a turbidity switch CIP optimisation using a conductivity probe 35 8 Action plan 37

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6 1 HOW TO USE THIS GUIDE All companies control their processes sufficiently to achieve an acceptable yield of saleable product. However, many companies could increase their profits by improving their process control and reducing waste. This Good Practice Guide is intended to help food and drink processing companies adopt low-cost process control techniques as part of an integrated approach to improve their business and environmental performance by: 1 reducing raw material consumption and product loss; reducing water use and effluent generation. Some companies have saved up to /year by improving their process control techniques. Five measurements are important for process control in the food and drink industry, ie temperature, pressure, level, flow and analysis (ph, turbidity and conductivity). The Guide, which is applicable to all sectors of the food and drink industry, presents a step-by-step approach to implementing or improving process control. The Guide describes: The benefits of process control (see Section 2). How to select the best control option (see Section 3). The different control techniques available and their relative merits (see Section ). Best practice for equipment installation in hygienic applications (see Section 5). The latest developments in process control technology (see Section 6). Examples of companies that have reduced waste and costs by installing or improving process control. Industry Examples are given throughout the Guide and more detailed information about two applications at Taw Valley Creamery are presented in Section 7. Section contains summary tables that indicate key criteria for each technique, including: Relative capital, installation and maintenance costs. Operational reliability and constraints, eg accuracy, repeatability or expected drift, temperature and maintenance. Potential applications in the food and drink industry. The tables are intended to help companies select a suitable control technology for a particular application and provide them with sufficient information to discuss their requirements with suppliers. 1

7 The Guide concentrates on proven, low-cost control techniques that are available from a wide range of suppliers and suitable for a variety of applications. As shown by the Industry Examples, even simple measures can produce significant cost savings and all sizes of company can benefit from implementing or improving process control. 1 Simple, low-cost measure reduces trade effluent charges A fish processing company employing 20 people installed a flow regulator on its skinning machine. Installing the regulator reduced the water flowrate from 26 litres/minute to 10 litres/minute. The process still operates efficiently and the company is currently saving 915/year. When trade effluent charges increase next year, the saving is expected to be 1 700/year. The regulator cost less than 50 and was easy to install. 2

8 2 THE BENEFITS OF PROCESS CONTROL UK food and drink processing companies are under pressure to cut costs and remain competitive. For example, as water companies seek to recoup the cost of complying with the EC Urban Waste Water Treatment Directive many companies face increases of 200% or more in trade effluent charges. Food and drink processing companies will have to comply with the provisions of the Integrated Pollution Prevention and Control (IPPC) Directive 1 and water, raw material, energy and waste disposal costs continue to rise. Improving process control by focusing on waste minimisation can reduce production costs by up to 5%. The use of low-cost process control techniques can help companies achieve a significant proportion of these potential savings. 2 The benefits of improved process control include: increased yield of saleable product; improved product quality; less need for rework; reduced loss of raw materials and product; less waste and effluent; optimum use of process water; optimised water and detergent demand for cleaning; more efficient operation of the site s effluent treatment plant (ETP). Controlling product loss to effluent saves over /year Product from a food manufacturing process was lost to drain during the separation stages. As well as losing valuable product, this caused the site s effluent to breach the company s discharge consent. A hygienic turbidity meter and flow meter allowed greater control of the process, resulting in savings of /year from increased product yield alone. 2.1 REDUCING WASTE THROUGH BETTER PROCESS CONTROL Almost all processes generate some waste. Improved control can provide more saleable product, generate less waste and provide increased profits. When considering the need for better process control, important questions for companies are: Can we improve the control of our process? How can we do it? How difficult will it be? What will it cost? Will the benefits justify the effort and cost? To find the answers to these questions, companies should start by identifying their sources of major waste streams and calculating the true cost of waste to the business. Much of the cost of waste is hidden, but includes raw material/ingredient costs, energy consumption, water consumption, effluent generation, packaging, consumables and wasted time and effort. 1 For advice on current and forthcoming legislation affecting the food and drink industry, contact the Environment and Energy Helpline on freephone

9 The true cost of waste can be 5-10 times the cost of disposal. Most companies can achieve savings of at least 1% of turnover through waste minimisation. Improved process control techniques will help to minimise waste and effluent by, eg: 2 preventing off-specification product by improving the control of raw material additions; preventing product spoilage during storage by improving temperature control; reducing loss of product to drain by closing discharge valves automatically when a low level or phase interface is detected; eliminating overfilling of vessels or tanks by stopping the feed flow at a preset level; minimising the water used for cleaning by controlling the flow rate, pressure or duration; minimising product giveaway by controlling the feed rate or fill level. For free advice and publications on how to implement a systematic waste minimisation programme, contact the Environment and Energy Helpline on freephone Efficient use of materials, water and energy is important for all manufacturing processes. Better management will produce higher efficiencies.

10 3 IDENTIFYING THE BEST OPTIONS Process control allows better management of: process inputs, eg control of material flows into vessels; process conditions, eg temperature control within production vessels; product handling and storage, eg controlling the level in storage tanks; effluent generation, eg analysis of liquid streams for recovery/re-use. Effective control requires understanding and measurement of the process conditions. The use of process control techniques in the food and drink industry helps to save money by: minimising the amount of valuable raw materials and products that go to waste; minimising the use of water; minimising the generation of effluent for disposal WILL PROCESS CONTROL HELP TO REDUCE WASTE? Once the sources of waste from the process have been identified, the next step is to decide whether improved process control will help to reduce the waste. To do this, find the answers to the following questions. The method is illustrated with a worked example (see the shaded text). Example Where is the waste generated? Identify from which stage of the process, and from which items of equipment, the waste arises. What is the cause of the waste? Find out why the waste is being generated at this point in the process. Could better process control help to reduce the waste? Consider whether better control of flow, level, pressure, temperature or another parameter would reduce the amount of waste generated. What are the process control options? Use the selection tables and supporting information given in Section to identify possible options. Water is lost to drain due to a tank overflowing. The tank is topped up periodically, but supplies water to the process continuously. Tank is being overfilled. The operator responsible for topping up the tank has to leave the tank filling while he attends to other duties. The water supply is frequently left on for too long and the tank overflows to drain. Yes. This source of waste would be eliminated if the supply of water to the tank was stopped before the tank overflow level was reached. Three possible controls could be fitted, ie: a float valve that shuts off the water supply when the water reaches a pre-determined level in the tank; a level switch or continuous indicator that sends a signal to close an actuated valve when the water reaches a pre-determined level in the tank; a flow meter that measures the flow of water out of the tank and sends a signal that adjusts the position of a control valve in the water supply line. 5

11 3.2 SELECTING THE BEST CONTROL OPTION The initial assessment to decide if process control will help to reduce waste usually produces a list of several options. To select the most suitable process control option for an application, it is necessary to appreciate the requirements for process control, the potential cost and other issues. Once these factors are understood, the summary tables in Section and other information in the Guide can be used to assess which option is most appropriate and cost-effective. The checklist below is intended to help companies identify and consider important issues. The worked example from Section 3.1 continues as shaded text. Example 3 Are measurement data required? If data are needed (eg on flow, level and/or temperature), it is also necessary to consider: Location - are readings required locally on the plant or in a control room? Point of measurement - eg is it in a pipe, an open tank or a closed vessel? Accuracy and repeatability - the tighter the control required, the more important these criteria become. Is control only required? Flow and level control are often the simplest and cheapest process control options to install and operate. Are both measurement and control required? If the measured data are to be used for automatic process control, then it is necessary to consider: Response time - how quickly does the control loop need to respond to correct a change in the process conditions? In general, the more critical it is for the process to operate at, or very close to, its set point, then the quicker the control loop has to respond. Accuracy and repeatability. What will it cost? The costs of installing and maintaining the instruments and control devices should be considered as well as the capital cost. Are there any other operational issues? A number of other issues should be considered, eg: Consistency with overall control philosophy - for example, manual flow control valves may not be appropriate for a process controlled primarily from a remote control room. Compatibility with control system standards - many processes are operated to standards that specify the instruments and equipment to be used in particular situations. Data on water flows or tank level would only be used for controlling water overflow from the tank. Data are not needed for process monitoring. The water overflow could be prevented by a controlonly device, eg a float valve. A control loop to prevent water overflow from the tank will be based on measurements of tank level or water flow rate. It does not require a rapid response time or high levels of accuracy or repeatability. The float valve will be the cheapest of the three options to purchase, install and maintain. No. 6

12 This decision process can be applied to situations where the process could be controlled using more than one process parameter. This is the case in the worked example shown opposite, where either level control or flow control could be used to prevent water overflow from the tank. To select the best process control option: identify possible process control options; identify important criteria, eg cost, operational reliability and constraints, and applications; use the information in this Guide to evaluate the options and choose the best solution. 3 7

13 MEASUREMENT AND CONTROL TECHNIQUES This Section describes the different types of measurement and control devices suitable for use in food and drink processing. Some measurement devices provide a signal for automated control and others give a reading that provides information for manual action. Information on equipment suppliers can be obtained from the Environment and Energy Helpline on freephone and also from the Instrument Engineer s Yearbook, published by the Institute of Measurement and Control (InstMC) 2. The Yearbook can be searched by company name, products and services, trade names, and agents and distributors. The latest edition, published in February 1999, costs ELEMENTS OF A CONTROL SYSTEM Most process control systems are made up of three distinct elements..1.1 Measurement sensors (see Sections.2 -.6) The measurement sensor monitors the condition or properties of the process. This provides data on how the process is actually performing..1.2 Controllers/transmitters (see Section.7) Controllers compare the actual process data with a set point - the desired value of the controlled variable. From this comparison, the controller generates a signal, which is used to regulate the operation of the final control device. It is quite common for one process condition to be controlled by measuring another condition. For example, the flow of steam heating a reaction vessel is controlled by the temperature inside the vessel. Alternatively, the level within a tank is controlled by measuring the flow out of the tank..1.3 Control devices (see Section.8) These are usually some sort of control valve, operated either manually or by an actuator on receipt of a signal. The actuator translates the control signal into a mechanical action, which is used to manipulate the particular process variable, eg flow. The simplest type of automatic control is on-off, or two-position action, where the valve is either open or closed. More complex control devices use variable-position action, where the valve can be moved and held in any position between fully open and fully closed Gower Street, London WC1E 6AA. Tel: Fax: World wide web:

14 .2 TEMPERATURE MEASUREMENT Raw material waste and effluent generation can be reduced by measurement and control using temperature sensors. Table 1 gives some example uses of temperature sensors and Table 2 outlines some typical applications in food and drink processing. Plant Condition/activity Reason for control Storage vessels Refrigeration or heating systems Minimise deterioration and waste of materials Reaction vessels and Heat inputs, eg steam jackets Minimise production of out-of-specification transfer lines products Heat exchangers Temperature of solutions for CIP 3 Ensure quality of cleaning and minimise and SIP - different duties require potential for contamination different temperatures, eg C for milk vessels Table 1 Examples of use of temperature control in food and drink processing If temperature sensors are needed to control the temperature of materials in reaction vessels or during transfer, consider their potential for an additional or dual function, eg monitoring the temperature of cleaning solutions. Sector/product Dairy Pasta Chocolate Meat Application To maintain milk temperature during pasteurisation by controlling the flow of steam or hot water Integrated measurement and control of flow and temperature for water addition to pasta dough kneaders Temperature sensors to minimise temperature drop during product transfer, thus minimising product deterioration To maintain the temperature of thawing baths for frozen meat by controlling the water flow Table 2 Typical applications of temperature measurement in food and drink processing Table 3 summarises the main types of temperature measurement devices and their principal selection criteria. Use Table 3 and the guidelines given in Section 3.2 to choose the best option for an identified opportunity to save money through better temperature control. 3 Cleaning-in-place (CIP) - automatic cleansing of production equipment achieved by chemical cleaning (eg using acid and alkaline agents), mechanical cleaning using the shear forces of pumped fluids, the temperature of the cleansing solution and/or the length of the cleansing cycle. Sterilisation-in-place (SIP) - automatic process following cleaning-in-place using high-temperature, saturated steam to ensure sterilisation of production equipment. 9

15 Notes: 1 An indication of the percentage of the span (H = ±0.05%; M = ±0.2%). Improved by using self-diagnostic equipment. 2 An indication of the acceptable replication of a measurement for a consecutive number of times before maintenance is required. 3 L: < 100; M: ; H: > 500. Cost will vary according to supplier and extent of control system required. L: < 500; M: ; H: > Includes the costs of cleaning and frequency of calibration. Will depend on local labour charges. Key: H High M Moderate L Low E Electrical N None Y Yearly Applications in the food and drink industry Type of device Selection criteria Maintenance cost Installation cost Capital cost Maintenance interval Accuracy Repeatability Power source Operating range ( C) Note Widely used, especially for accurate measurement over a limited range. Devices available for both hygienic and non-hygienic applications. L M M Y E H H 20 to +650 Resistance thermometers Widely used. Devices available for both hygienic and non-hygienic applications. L M M Y E M H 270 to Thermocouples Limited use only for local measurement or indication. Modern transmitters offer more advantages. Devices available for non-hygienic applications only. L L L Y N M M 270 to +810 Filled-system thermometers Limited use for surface temperature measurement only. Do not provide a signal for process control purposes. L L H Y E M M 25 to Optical thermometers Table 3 Temperature measurement selection 10

16 .2.1 Resistance thermometers Resistance thermometers - or resistance thermometer detectors (RTDs) - are used extensively in the food and drink industry. They are particularly popular for applications requiring accurate measurement of temperatures over a relatively narrow range, eg the pasteurisation of milk. It is important to check with the supplier that the instrument will provide sufficiently accurate measurements. The accuracy of RTDs can depend on the metal used in the thermometer and the design of the thermowell (see Section.2.5). Improved temperature control increases yields by 15% A company that ferments molasses to produce alcohol identified poor temperature control as the reason for reduced yield and increased waste. Process operators used local temperature readings from thermocouples to manually adjust the flow of cooling water. Temperatures in the fermentation vessel should have been between 38 C and 0 C. In practice, they frequently exceeded 5 C. Installation of an improved temperature-monitoring system using resistance thermometers and automatic control of the cooling water flow, together increased production yields and reduced waste by 15%..2.2 Thermocouples Thermocouples are one of the most reliable types of temperature probe and are used widely in food and drink applications. They are particularly suitable when accurate measurement is required over a wide range of temperatures. The repeatability of the temperature measurements is satisfactory for most applications, but if this is vital, a resistance thermometer may be better. Fitting thermocouples saves /year Installing thermocouples to provide temperature control allowed a meat processing company to reduce its water supply costs by up to 10%. Thermocouples on the water inlet and outlet to a chilling and washing system, feed into an automated control valve which optimises the flow rate. The control system has reduced water use, energy use and effluent generation significantly, while maintaining sufficient flow rate to meet the process s hygiene requirements. Savings of /year were achieved for an initial investment of 3 000, giving a payback period of 12 weeks..2.3 Filled-system thermometers Filled-system thermometers are designed to indicate the temperature at the point of measurement. They contain a gas or liquid that changes in volume, pressure or vapour pressure with temperature and therefore do not need an external power supply. Although their design is simple and rugged, their use in the food industry is limited because: there is a risk of contamination of the process by the sensor s working fluid in the event of damage to the device; the control signal provided by the device is limited compared with that from electrical measurement devices; they are uneconomic when remote readings or control are required..2. Optical thermometers (pyrometers) These portable, hand-held devices, which are used to monitor surface temperature, are particularly suitable for taking multiple measurements around a site, eg searching out hot spots. However, pyrometers cannot be used to provide a signal for process controllers. 11

17 Optical pyrometers rely on the human eye to balance the colour of a radiant filament with the colour of the hot surface, and use a standard colour/temperature key. Infrared pyrometers measure heat emitted from the surface of an object, eg a vessel wall, automatically..2.5 Installation requirements for temperature sensors Temperature can be measured with either an intrusive or a non-intrusive device. With both types of device, there is a delay in the response of the control system. This lag is greater with a non-intrusive installation, where the vessel or pipe wall has to heat up to the process temperature before an accurate reading is detected. With intrusive devices, the protective sheath has to heat up to the process temperature before it is detected by the sensor. Both thermocouples and resistance thermometers are intrusive devices. The fastest response will be achieved by equipment that can be immersed directly in the process liquid, although hygiene remains an important concern. Thermowells These allow intrusive measurements to be taken in hygienic applications by preventing direct contact between the temperature device and the process material (see Fig 1). The external surface of the thermowell should have a smooth finish and be free of areas that could encourage bacterial growth. Purchase and installation of a thermowell costs about 150 (1999 prices). Dip systems In terms of temperature measurement, dip systems achieve the fastest response because the sensor is in direct contact with the process fluid (see Fig 1). However, if an intrusive sheath is needed to meet hygiene requirements, this will lengthen the response time. Surface-mounted This is a non-intrusive type of system, where the temperature sensor is attached to the outside surface of a vessel or pipework (see Fig 1). Such systems are cheaper than thermowells, but are not as responsive to changes in the process due to the time lag introduced by the pipe or vessel wall. Controller, indicator or recorder Signal transmission Temperature sensor Connecting head Thermowell Dip system Surface-mounted Fig 1 Components of a temperature control system 12

18 .3 PRESSURE MEASUREMENT Pressure sensors are typically used for the indirect control of other parameters, eg flow or level. Table gives some examples of how such control can reduce waste and effluent generation in food and drink processing. Typical applications are shown in Table 5. Device Condition/activity Reason for control Pressure sensors in transfer lines Controlling pump speed, pressure Minimise waste from material and flow velocity damaged by shear or friction forces Differential pressure system Monitoring levels in storage or Minimise material loss from reaction tanks overflow or production downtime due to lack of stock Differential pressure system Monitoring pressure drop across Minimise waste filters to control cleaning cycles and optimise operation Table Examples of use of pressure measurement in food and drink processing Sector/product Brewery Fruit juice Dairy products Application Monitoring and controlling the pressure of sparge water for mash filters Monitoring the differential pressure across filters and initiating a cleaning cycle as required Monitoring and controlling flow velocities in pipelines (via pump control) to avoid friction damage to product Table 5 Typical applications of pressure measurement in food and drink processing Table 6 summarises the main types of pressure measurement devices and their principal selection criteria. Use Table 6 and the guidelines given in Section 3.2 to choose the best option for an identified opportunity to save money through better pressure control..3.1 Diaphragm pressure sensors Single-point pressure sensors convert the force on a membrane or diaphragm into an electrical output. For example, a diaphragm pressure sensor in an open storage tank can be used to measure the head above the sensor and thus the liquid level in the tank. Both dry and oil-filled sensors are available; dry sensors can reduce inaccuracies resulting from frequent temperature changes..3.2 Differential pressure sensors Differential pressure cells detect the pressure difference between two points and convert this into an electronic signal that can be used in control devices. One application is to use a sensor to measure the pressure drop across an orifice plate within a transfer line and thus determine liquid flow in the line. Differential pressure sensors reduce water use for filter cleaning by 30% A fruit juice manufacturer filtered the product to remove fruit pulp solids before bottling. Filter cleaning with a water sparge was initiated at regular intervals according to a timer. However, the company recognised that this resulted in the filters being cleaned more frequently than necessary. Differential pressure sensors were installed on three filter units and the cleaning cycle is now initiated by a signal from these sensors. This signal is given when a preset pressure across the filters is reached. Water consumption for filter cleaning has fallen by 30%, saving 8 000/year in water and effluent costs. The cost of the modifications was 6 000, giving a payback period of nine months. 13

19 Notes: 1 An indication of the percentage of the span (H = ±0.1%; M = ±0.5%). Improved by using self-diagnostic equipment. 2 An indication of the acceptable replication of a measurement for a consecutive number of times before maintenance is required. 3 L: < 100; M: ; H: > 500. Cost will vary according to supplier and extent of control system required. L: < 500; M: ; H: > Includes the costs of cleaning and frequency of calibration. Will depend on local labour charges. Key: H High M Moderate L Low E Electrical Q Quarterly Applications in the food and drink industry Type of device Selection criteria Maintenance cost Installation cost Capital cost Maintenance interval Accuracy Repeatability Power source Operating temperature ( C) Operating range (bar) Note Widely used. Devices available for both hygienic and non-hygienic applications. L L M Q E M H 20 to +85 Vacuum to 0 Diaphragm pressure sensors Used for pressure control where process and device are separated. If necessary, maintenance requirement can be yearly. M L H Q E H H 0 to Differential pressure sensors Table 6 Pressure measurement selection 1

20 .3.3 Installation requirements for pressure sensors Pressure sensors used in food and drink applications generally require seals and surfaces designed specifically for hygienic applications. General guidelines for the installation of sensors within pipework are shown in Fig 2. Installation of sensors in storage vessels, eg for level measurement, depends on the specific application. Advice should be sought from the supplier. For hygienic applications, units can be mounted flush with the vessel wall. Minimum of two diameters before bend Fig 2 Correct and incorrect locations of pressure devices. LEVEL MEASUREMENT The two main categories of level sensor are: Level-detecting sensors - these basic sensors indicate whether or not a fluid is present at a specific point in a vessel (usually a high or low point). Most applications are connected to a visual indicator, a visual or audible alarm, or on-off control of flows in or out of the vessel. Level-measurement sensors - these allow continuous monitoring of actual fluid levels, with associated variable controls, eg increasing or slowing pumping rates. Table 7 shows some examples of how level sensors can be used to reduce material waste and effluent generation. Plant Storage or reaction tanks Storage vessels Vessels with automatic transfer controls Reason for control Prevent overflow and waste of material or water Provide information for stock control Minimise waste from out-of-date stock or production losses due to material not being available Minimise waste from transfer losses or inaccurate batch recipes Table 7 Examples of use of level sensors in food and drink processing 15

21 A number of different types of level sensor are available. Issues to be considered when choosing which type of level sensor include: The potential for foaming. This can cause false signals for echo devices such as ultrasonics. In such cases, a vibrating level switch, a conductivity switch or a hydrostatic device may be best. The presence of viscous liquids and fats. These can build up on probes and switches leading to inaccurate measurements. For viscous liquids, vibrating level switches are a popular choice. The possibility of uneven liquid surfaces being created during mixing. These can cause false echoes from sonic devices. Table 8 shows some typical applications for level sensors in food and drink processing. Product/activity Liquid food materials CIP/SIP Application To monitor the level in storage tanks to avoid overfilling and loss to drain As a level switch in vessel cleaning to optimise the amount of water/detergent used and to protect against overspill Table 8 Typical applications of level measurement in food and drink processing Table 9 summarises the main types of level-measurement sensors available and their principal selection criteria. Use Table 9 and the guidelines given below and in Section 3.2 to choose the best option for an identified opportunity to save money through better level control...1 Float valves These simple mechanical devices are based on a buoyant unit. No reading is given, but the device will shut off the flow to a tank or vessel automatically when a predetermined level is reached. Float valves are simple and low-cost, but are easily damaged leading to an overflow and material losses. If the device is in a remote location, such losses can go undetected for a long time. Simple float valve produces savings of over /year A large vegetable-processing company saved over /year in reduced water costs, effluent charges and operator time by installing level controls on the water supply tanks to the flume system that transported the vegetables. Previously, an operator adjusted the water supply controls manually. This led to excessive water overflow from the tanks when the operator was occupied elsewhere. A simple float valve was identified as offering a low-cost solution with a payback period of only a few months. The valve now controls the water flow to the tanks, freeing the operator to work elsewhere...2 Mechanical indicators and float switches These devices use a float connected to a counterweight or rod that gives a direct level reading on a gauge board or indicator. They can also be connected to high-level and low-level switches. The simple equipment has low installation and maintenance costs, but provides only limited accuracy...3 Capacitance level switches These devices consist of a metallic probe inserted into a vessel to be in contact with the contents. The signal from the probe indicates whether the probe is immersed in the fluid, but does not indicate the depth of immersion. 16

22 Notes: 1 The range over which the device is calibrated: the height between the equipment mounting and maximum liquid level. Applicable only to sensors mounted on the top of the vessel, eg ultrasonic or microwave devices. 2 An indication of the percentage of the span (H = ±0.1%). Improved by using self-diagnostic equipment. 3 An indication of the acceptable replication of a measurement for a consecutive number of times before maintenance is required. L: < 100; M: ; H: > 500. Cost will vary according to supplier and extent of control system required. 5 L: < 500; M: ; H: > Includes the costs of cleaning and frequency of calibration. Will depend on local labour charges. 7 Limited by the practical static head, ie the pressure due to the head or height of the fluid above the point of measurement. Key: H High M Moderate L Low E Electrical P Pneumatic N None Mn Monthly Y Yearly N/A Not applicable Applications in the food and drink industry Type of device Selection criteria Maintenance cost Installation cost Capital cost Maintenance interval Accuracy Repeatability Power source Operating temperature ( C) Operating range (metres) Note Control at a single point. No measurement. Float valves N/A N/A N/A N Mn M L L Control at a single point. Local measurement only. < L L N Mn M L L Mechanical indicators and float switches Widely used. Devices available for hygienic and non-hygienic applications. < H H E Y M/H L/M L Capacitance level switches Widely used. <20 <100 H H E Y M/H L/M L Vibrating level switches Widely used. < H H E Y L/M L/M M Conductivity switches Control and measurement at a single point. Devices available for hygienic and non-hygienic applications. Note H H E/P Mn H L/M M Hydrostatic devices Control and measurement at a single point. Devices available for hygienic and non-hygienic applications. < H H E Mn H M L Ultrasonic sensors Control and measurement at a single point. Devices available for hygienic and non-hygienic applications. < H H E Mn H H L Microwave devices Table 9 Level measurement selection 17

23 Capacitance probes are typically used to initiate on-off control, eg when a vessel has filled to a certain level, the signal from the probe stops the supply flow. Capacitance probes can also detect the interface between two different liquids. This feature is used extensively in CIP systems, eg detection of the interface between product and wash water initiates the next stage of the sequence. Interface detection using a capacitance level switch leads to savings of /year A brewery recognised that it was losing beer worth more than 1 million/year in its effluent. A waste minimisation audit showed that 80% of all beer losses were from a vessel that separated the beer from the dead yeast cells. Clear beer was run off from the vessel using a fixed standpipe, before the bottom phase, containing the yeast cells, was discharged to drain. However, the position of the interface between the two phases depended on the type of beer, and any beer below the level of the standpipe was lost to drain. The process was modified so that the yeast phase was drained off first, until a capacitance level switch at low level in the vessel detected the interface. The beer was then run off to storage, and finally the residue in the vessel was discharged to drain. Process modifications cost 9 500, but with a reduction in beer losses and effluent charges worth /year, the payback period was only five days... Vibrating level switches These switches are based on a tuning fork. The tuning fork is set vibrating by a piezoelectric crystal matched to its resonant frequency in air. When the fork is covered by liquid, the frequency of vibration changes. This change in vibration is converted into an electrical signal, thus indicating the presence of fluid at this level. Vibrating level switches are being used increasingly instead of float switches as they have no separate moving parts, are easy to install, require little maintenance and are fail-safe. Typical applications include viscous sauces or mixtures, eg wet yeast in the brewing industry...5 Conductivity switches These switches sense a change in the electrical resistance between two conductors. They are simple and moderately inexpensive, and ideal for on-off control. However, use is limited to applications with conductive liquids. Typical applications include processes with foaming products, eg milk, beer and carbonated drinks...6 Hydrostatic devices In a hydrostatic device, a transducer converts the static pressure of a liquid head acting on a diaphragm into an electrical signal. The device is one of the simplest types of level control that can be used for both monitoring and control. Therefore, it is one of the most popular in the food industry. The sensors are reliable and accurate, provide stable measurement in agitated vessels, and are not affected by either foaming or materials with variable electrical properties. However, hydrostatic devices are not suitable for applications where solids could build up on the diaphragm because this affects the accuracy of the reading. They are also unsuitable for materials held continuously at temperatures above 100 C Ultrasonic sensors Such devices consist of an ultrasonic sensor mounted above the maximum liquid level, which sends an ultrasonic pulse to the liquid surface and then receives the echo. The time delay between emitting and receiving the pulse is converted to an electronic signal. This signal serves as a measure of the distance between the sensor and liquid surface. Ultrasonics is a non-contact technique, making it ideal for hygienic applications. The technique is therefore used extensively throughout the food and drink industry. The main advantages are insensitivity to changing material characteristics and no problems with build-up of materials or corrosion of the sensor head by the liquid. A potential disadvantage is that surface foaming will affect accuracy.

24 ..8 Microwave devices Microwave devices are similar to ultrasonic devices in that they use a non-contact, reflection-based technique. The main difference is that microwave devices can be used in more extreme process conditions than ultrasonic devices because the speed of transmission of microwaves is not affected by process temperatures, pressure or vacuum. However, surface foaming still affects the reflection of the signal and thus the accuracy of the reading...9 Installation requirements for level sensors Care should be taken to ensure that accurate readings are obtained. Actual installation requirements for specific instruments should be discussed with the supplier. Switch points A vibrating fork system should be installed as low in the vessel as possible to ensure that a low switch point is achieved. With high-level float switches, the float must be placed so that process fluids do not overflow continuously. Hydrostatic sensors An isolating valve is essential for hydrostatic level sensors installed directly in a vessel. This enables the unit to be removed for cleaning and maintenance, without having to close down the whole process. Reflection-based sensors With ultrasonic sensors, the sensor mounting arrangement can affect the quality of the reflected echo. When installing a reflection-based sensor, it is important that: the edges and walls of vessels should be as smooth as possible to avoid false scattered echoes; the sensor is placed away from existing equipment to avoid false echoes; the sensor is not near filling points; the sensor is placed approximately half the vessel radius from the wall and angled towards the exit cone to ensure measurement to the bottom of the vessel..5 FLOW MEASUREMENT Table 10 gives some examples of how flow measurement and control technologies can reduce material waste and effluent generation in food and drink processing. Some typical applications are given in Table 11. Equipment Condition/activity Reason for control Transfer lines Accurate addition of materials Minimise excessive use of materials and formation to reaction vessels of out-of-specification products Steam supplies Maintaining correct operating Minimise waste from underheated or overheated temperatures materials and products Cleaning systems Water use Optimise use and minimise effluent generation Table 10 Examples of use of flow control in food and drink processing Product/activity Soft drinks Bulk solids Milk powder CIP Application Flow measurement and feedback control for raw material addition For example, determining the flow of potato crisps to a flavour drum to ensure correct ratio of flavouring Flow measurement for accurate batching of ingredients into the process as specified in recipes Flow measurement to ensure fixed volume of water issued for each cleaning stage Table 11 Typical applications of flow measurement in food and drink processing 19

25 Flow control saves vegetable-processing company /year Manual flow control valves were installed on the water supplies to the conveying system at a vegetable-processing company. This allowed the operators to make sensitive adjustments to the water flow. In addition, a particular valve setting and water flow rate could easily be repeated. With the valves set for the optimum flow rate, the company saved about /year in water and wastewater costs. The payback period was three months. Table 12 summarises the main types of flow measurement devices and their principal selection criteria. Use Table 12 and the guidelines given below and in Section 3.2 to choose the best option for an identified opportunity to save money through better flow control. Flow meters with no internal measuring element, eg electromagnetic meters, are particularly suited to hygienic applications..5.1 Variable area (rotameter) A rotameter provides simple, local indication of the flow rate. It consists of a tapered metering tube and a float that is free to move up and down within the tube. Fluid enters at the bottom of the tube, passes upward around the float and out at the top. A rotameter is a low-cost device providing a direct reading, but it is only suitable for relatively low flows and does not provide an associated control action..5.2 Positive displacement meters These devices provide high accuracy and excellent repeatability, with no need for a separate power supply. A unit consists of a number of fixed volume chambers that rotate about an axis within the meter housing. The meter records the number of discrete volumes of liquid flowing through the unit. Positive displacement meters can be used to provide both local indication and a signal for control. Positive displacement meter reveals opportunities to reduce water use A company manufacturing ready-made meals installed a positive displacement meter to increase its understanding of water use in the equipment cleaning bay. After several weeks of monitoring, it became clear that water consumption varied significantly and that it was not related to production levels. Following discussions with the operators in the area, improved cleaning procedures were introduced. The result was an immediate reduction in water use of 80 m 3 /week. This resulted in savings of 3 000/year in water and effluent costs, giving a payback of ten weeks on the 600 paid to install the meter..5.3 Turbine meters Turbine meters consist of a bladed rotor that converts fluid flow into rotary motion. A pick-up coil provides an electrical signal for flow rate indication and control. Turbine meters are simple, reliable devices when used with low viscosity fluids. Unlike positive displacement meters, the unit can be cleaned in situ. Care is needed not to make the rotor run too fast as this can reduce both accuracy and meter life significantly. Typical applications include measuring flows of milk, beer and water..5. Electromagnetic meters In electromagnetic meters, a voltage is generated that is proportional to the average velocity of the liquid flowing through the pipe. These meters are ideal for use in hygienic applications because there is no direct contact between meter and material. Electromagnetic meters can also be used to provide a control signal. 20

26 Notes: 1 Turndown is equivalent to measuring span and is the ratio of upper and lower range values. 2 An indication of the percentage of the span (H = ±0.75%; M = ±2%). Improved by using self-diagnostic equipment. 3 An indication of the acceptable replication of a measurement for a consecutive number of times before maintenance is required. L: < 100; M: ; H: > 500. Cost will vary according to supplier and extent of control system required. 5 L: < 500; M: ; H: > Includes the costs of cleaning and frequency of calibration. Will depend on local labour charges. 7 High accuracy providing that fluid conductivity exceeds 1 µs/cm and flow is turbulent. Key: H High M Moderate L Low E Electrical N None Mn Monthly Q Quarterly Y Yearly N/A Not applicable Applications in the food and drink industry Type of device Selection criteria Maintenance cost Installation cost Capital cost Maintenance interval Accuracy Repeatability Power source Operating temperature ( C) Operating range (turndown) Note Local measurement only. Mainly low-flow and non-hygienc applications. 10: M M N Y L L L Variable area (rotameter) Hygienic and non-hygience applications. Not suitable for high viscosity materials or CIP. 10: H H N Y H M M Positive displacement meters Widely used for water. Turbine meters 10: H H N Y M/H M M Non-intrusive sensor, thus minimising hygiene issues. H E Y H H L 100: H (note 7) Electromagnetic meters Non-intrusive sensor, thus minimising hygiene issues. 20: M M E Q H H M Ultrasonic transit time meters Mainly used for steam and gas, but can be used for low viscosity fluids. N/A 0-00 H H E Y H H L Vortex shedding meters Widely used. Devices available for hygienic and non-hygienic applications. : M M E Mn H M M Differential pressure meters Table 12 Flow measurement selection 21

27 Electromagnetic meters can be cleaned in place, as they are able to withstand the high temperatures (up to 180 C) of CIP or SIP. Typical monitoring applications range from flows of milk and water to more complex products, eg yoghurt, molasses and rice pudding. Electromagnetic flow meter helps to reduce raw material use A confectionery manufacturer found that there was considerable variation in the weight of chocolate coating on its products. A number of measures were implemented to improve the control of chocolate addition. These included installation of an electromagnetic flow meter and a control valve to control the feed rate accurately. The modifications cost to implement, but reduced chocolate consumption by 10 tonnes/year. This reduction was worth /year, giving a payback period of six months..5.5 Ultrasonic transit time meters These devices work by measuring the time taken for an ultrasonic beam to traverse a length of pipe, both with and against the flow of liquid in the pipe. The electrical signal produced by the meter can be used to provide both an indication of flow and a signal for control. Maintenance requirements are low as there are no moving parts and the pressure drop is negligible. However, the pipe must be full and the fluid relatively clean and free of air bubbles to obtain accurate readings..5.6 Vortex shedding meters Vortex shedding meters can monitor flow over a wide range, providing a linear output signal. In the food and drink industry, such meters are used mainly to measure steam used for heating and cleaning duties. These meters are easy to install, have no moving parts and therefore maintenance requirements are low. However, applications are limited to steam and low-viscosity liquids. Another disadvantage is that, at the minimum predicted flow rate, the device output drops to zero, even though there is still some flow occurring..5.7 Differential pressure meters Differential pressure meters are robust, simple, well-established devices. A differential pressure, which is proportional to flow, is generated across an element such as a restricting orifice. This pressure difference is converted to an electronic signal for both flow indication and for use as a control signal. These meters are best suited to the measurement and control of reasonably constant flow rates. Differential pressure meters are most commonly used for flow of services, eg boiler water, where high operating pressures and temperatures have to be endured. They are not suitable when accurate measurement is required, as wear at the orifice or deposits on the orifice reduce absolute accuracy rapidly..5.8 Installation requirements for flow measurement Hygiene is vital when using flow controls. The wetted parts should be made from materials that are smooth and easy to clean, while design and installation should aim to minimise dead spaces. The equipment should also be able to withstand CIP at high temperatures. Even when this is possible, good practice requires easy dismantling of the equipment to facilitate cleaning. In processes where process fluids may solidify at low temperature, heat tracing may be necessary to ensure that this does not occur around the equipment. As with all instruments, the position of the device is important for accuracy, eg: Rotameters should be mounted in vertical pipelines, but do not require straight lengths of pipe before or after the point of installation. 22

28 Positive displacement meters do not need straight lengths, but they do require a fluid backpressure, otherwise cavitation can occur which may damage the meter. To maximise accuracy, electromagnetic flow meters require 3-5 straight pipe diameters upstream of the meter and 2 straight pipe diameters downstream, along with turbulent flow. Ultrasonic transit time meters require straight pipe diameters upstream and 5 diameters downstream. In addition, the meter coefficients for laminar and turbulent flow have to be considered. Vortex shedding meters insert directly into the pipeline, but require up to 50 straight pipe diameters upstream and 5 pipe diameters downstream. Accuracy is reduced if the flow is turbulent (ie the Reynolds number is greater than )..6 ANALYTICAL MEASUREMENT The most widely used analytical measures in the food and drink industry are ph, conductivity and turbidity. Table 13 shows some example uses of analytical measurement and control techniques to reduce material waste and effluent generation in food and drink processing. Device Activity Reason for control ph sensor Control of additions of acid or alkali Minimise waste from overdosing and to reaction vessels production of out-of-specification product ph sensor Monitor effluent streams for use in Minimise use of fresh caustic or acid for mixing and neutralising prior to effluent treatment discharge Conductivity sensor Monitor levels of dissolved salts Minimise fresh water use and effluent prior to water re-use generation Conductivity sensor Monitor well water Minimise production of poor-quality product (that becomes waste) due to use of unsuitable process water Turbidity sensor Monitor quality of process water Minimise effluent from out-of-specification process water or products Turbidity sensor Monitor CIP systems Optimise re-use of cleaning water, thus minimising effluent generation Table 13 Examples of use of analytical measurement in food and drink processing A range of fluid properties can be measured on-line to check the quality of the materials being used and the products being produced. In many processes, accurate control of specific parameters is essential to prevent the generation of significant quantities of waste. Some typical applications are shown in Table 1. Sector/activity Dairy Jam production Bottling Product recovery CIP Application ph analysis of milk deliveries to minimise losses through mixing of unsuitable milk with other raw materials ph measurement to identify correct gelling point Conductivity monitoring of caustic used in bottle-washing solutions Turbidity monitoring of process waste streams to determine viability for recovery (see Industry Example in Section 7.1) Conductivity monitoring for plant valve control, based on differences between product, eg between beer and water or caustic and acid (see Industry Example in Section 7.2) Table 1 Typical applications of analytical measurement in food and drink processing 23

29 Table 15 summarises the main types of analytical measurement devices and their principal selection criteria. Use Table 15 and the guidelines given below and in Section 3.2 to choose the best option for an identified opportunity to save money through better analytical control. With analytical measurements, maintenance, accuracy and repeatability depend substantially on the process media. It is therefore impossible to be specific about these criteria in Table ph probes ph probes measure the acidity or alkalinity of a liquid. ph is important in many applications, eg: controlling milk quality; monitoring cream and cheese ripening; fermentation processes; the production of infant foods; water and wastewater treatment. The probes can be fixed permanently into a process line or dipped manually into tanks or storage vessels. Various devices are available, ranging from simple probes and transmitters to self-diagnostic probes that alert operators to equipment faults and can be maintained and calibrated without removal. ph control helps company meet its discharge consent A confectionery company uses a ph probe to control caustic dosing of an effluent stream. As well as ensuring compliance with its discharge consent, ph control helps to minimise the potential for expensive corrosion in the drainage system due to acid or alkaline attack..6.2 Conductivity - electrode cells Conductivity measurements are used to determine the purity of water or the concentration of an acid or alkali stream. Electrode cells are very accurate, contact-type sensors in which the process fluid passes between two electrode plates. Applications include monitoring process water for re-use (thus minimising effluent generation) and monitoring boiler water to minimise build-up of deposits on hot surfaces. Conductivity meter reduces detergent use for CIP Optimisation of detergent use during CIP at Taw Valley Creamery involved the installation of conductivity meters on all CIP sets. Approximately 15% less detergent is now used per set and cleaning times are shorter. For more details, see the Industry Example in Section Conductivity - inductive sensors Conductivity can also be measured using inductive sensors. These non-contact sensors use two electromagnetic coils around the process fluid and are therefore suitable for hygienic applications. The units, which have a greater range than the electrode cell, are used in monitoring CIP or bottlewashing systems, and for product monitoring in breweries, beverage industries and dairies. Conductivity meters reduce detergent use in dairy CIP system Conductivity meters were installed in a dairy CIP system to help reduce detergent use. The conductivity probe detects whether water or detergent is in the line and, if detergent is present, diverts flow to a detergent recovery tank. Rinse water recycling and reduced use of detergent produced savings of /year, giving a payback period of four months on the investment. Additional benefits from the reduced use of detergents included a reduction in effluent chemical oxygen demand. 2

30 Notes: 1 Operating range depends on the type of eqiupment: ph scale 1-1; conductivity ms/cm; turbidity g/litre solids concentration. 2 An indication of the percentage of full-scale. Improved by using self-diagnostic equipment. 3 An indication of the acceptable replication of a measurement for a consecutive number of times before maintenance is required. L: < 100; M: ; H: > 500. Costs are approximate and will vary according to the supplier and the technology. 5 L: < 500; M: ; H: > Includes the costs of cleaning and frequency of calibration. Will depend on local labour charges. Key: H High M Moderate L Low E Electrical Mn Monthly Y Yearly Applications in the food and drink industry Type of device Selection criteria Maintenance cost Installation cost Capital cost Maintenance interval Accuracy Repeatability Power source Operating temperature ( C) Operating range Note Simple systems that are low to medium cost to purchase, install and maintain. ph probes M/H L/H E Mn/Y L/M L/M L/M Widely used for services control, eg monitoring boiler water, process water and steam condensate. 160 M/H L/H E Mn/Y H M M 0.0 µs/cm ms/cm Conductivity electrode cells Ideal where there is contact with the product, thus minimising hygiene concerns. M/H L/H E Mn/Y H M L/M 5 to ms/cm Conductivity inductive sensor A sampling device may be needed M/H L/H E Mn/Y H M M mg/litre Turbidity scattered light devices Particularly useful for effluent monitoring and treatment g/litre 0-50 M/H L/H E Mn/Y H M M Turbidity suspended solids meters Table 15 Analytical measurement selection 25

31 .6. Turbidity - scattered light devices These devices use a light diffusion method to measure the light scattered through a right angle by the particles or bubbles in the liquid. This method is used for low to average turbidity measurements, including the measurement of turbidity in distilled water. Sampling devices should be used when it is difficult to install the turbidity meter into a process line. This will help improve hygiene. Turbidity meter reduces product loss and avoids need to build an effluent treatment plant Product lost to drain during the separation stages of a food manufacturing process caused the site to breach its effluent discharge consent and to lose valuable product. Greater control of the process was achieved by installing a hygienic turbidity meter and flow meter. The increase in product yield produced savings of over /year. The lower suspended solids content of the effluent avoided the need for substantial capital expenditure on an on-site effluent treatment plant..6.5 Turbidity - suspended solids meters These devices are based on a light absorption method that measures the amount of light transmitted through matter in the process fluid. They are used for measuring medium to high levels of turbidity. Turbidity switch increases product yield A turbidity switch was installed at Taw Valley Creamery to allow recovery of whey previously lost when equipment was taken off-line for CIP. This resulted in less whey being lost to drain and savings of /year. For more details, see the Industry Example in Section Installation requirements for analytical measurement Correct installation of analytical sensors is crucial to ensure accurate measurement. ph probes To avoid spurious readings, the fluid velocity should not exceed 2 metres/second. The electrode should always be wetted first to avoid loss of function. Conductivity meters Although fluid flow is not essential, it provides a self-cleaning effect. Air pockets should be avoided. The equipment should be able to compensate for the change in the conductivity of a fluid with temperature. Turbidity meters Turbidity meters should preferably be installed in vertical pipes with an upward flow. To avoid incorrect measurements from floating or settled debris, horizontal pipes should have meters installed at the side and not at the bottom or top. Like ph probes, the fluid velocity should not exceed 2 metres/second to avoid spurious readings. Inline mounting should preferably be in vertical piping with upward flow and the optical unit facing the direction of flow for maximum self-cleaning. To minimise deflection of the beam, gas bubbles and degassing should be avoided. Installation in tanks should not be at the bottom or at the top, to avoid fouling by deposition and floating debris respectively. 26

32 .7 PROCESS CONTROLLERS/TRANSMITTERS There are many different types of process transmitter, but they generally perform the following functions: receive data as a signal from the measurement device(s); compare the measured data to a set point; transmit a signal to make the control device, eg an actuated valve, perform the necessary action. Transmitters can either be incorporated into the measurement sensor or be a separate unit. Process transmitters used in the food and drink industry tend to fall into two categories: limit detectors and continuous transmitters..7.1 Limit detectors This type of instrument provides a two-level output signal to the control device performing the action. Typical examples of their use include: to change the position of an on-off valve; to switch a pump or agitator on or off..7.2 Continuous transmitters Continuous transmitters are designed to provide and receive electrical signals between 0-20 ma. This standard setting ensures that sensors, actuators and other control modules produced by different manufacturers are compatible. The simplest systems use a single sensor to provide process data to a transmitter linked to a single control device. The most complex control systems have multiple sensors, transmitters and control devices. The sensors may be linked to a programmable logic controller (PLC) or a dedicated computer control system. These complex systems are typically designed by equipment suppliers to control whole process units, eg a CIP system..8 CONTROL DEVICES Valves are the most common control device, with both manual and automatic control systems making extensive use of valves. Valves are often used to modify a flow rate to control a different process parameter, eg the temperature of chocolate in the shaping process is measured and, if necessary, adjusted by controlling the flow rates of heating and cooling water. Table 16 summarises the main types of control devices and their main selection criteria. Use Table 16 and the guidelines given below and in Section 3.2 to choose the best device for an identified opportunity to save money through better control..8.1 Manual control devices Globe and angle valves These valves are used for manual control of liquid flow rates and are best suited to applications with low viscosity liquids containing no solids. Globe and angle valves have a relatively high pressure drop across the valve; angle valves have a lower pressure drop than conventional globe valves. Needle control valves These valves are used for accurate manual control of liquid flow rates. The graduated scale on the valve hand wheel allows sensitive flow adjustments to be made and a particular valve setting to be repeated. Needle control valves are best suited to applications with low viscosity liquids and minimal solids content. 27

33 Notes: 1 Turndown is equivalent to measuring span and is the ratio of upper and lower range values. 2 Accuracy is largely dictated by that of the monitoring device; the valve can be selected to match this. 3 L: < 100; M: ; H: > 500. Cost will vary according to supplier and extent of control system required. L: < 500; M: ; H: > Includes the costs of cleaning and frequency of calibration. Will depend on local labour charges. 6 Depends on size and type of device. 7 Depends on valve design. 8 Check setting of manual valve regularly to ensure it has not changed. Key: H High M Moderate L Low E Electrical P Pneumatic Q Quarterly W Weekly N None Applications in the food and drink industry Type of device Selection criteria Maintenance cost Installation cost Capital cost Maintenance interval Accuracy Repeatability Power source Operating temperature ( C) Operating range (turndown) Note Variety of devices available to set flow rate at specified rate, eg a restricting orifice, or to control flows manually, eg needle valves. L/M L L N W/Q and Note 8 Note 6 Maximum of Manual or fixed control devices Used for two-point, eg on-off, control only. Note 6 Note 7 E Q L/M L L Solenoid actuated valves More expensive option, but essential when flow rates have to be controlled accurately and need to respond quickly to changes in process conditions. Note 6 Note 7 E or P Q M/H M/H L/M Automatic control valves Table 16 Control device selection 28

34 Constant flow valves reduce mains water use by 7.5% A food manufacturing company identified that excessive water consumption by its vacuum pumps was due to a higher flow than necessary for the seal water. Although the maximum flow for the service liquid should have been 2.7 m 3 /hour, the actual flow was found to be almost 11.5 m 3 /hour, ie over four times the design requirement. Installing constant flow valves to ensure the correct flow rate to each of the water ring vacuum pumps reduced water use by approximately m 3 /year (7.5% of the site s mains water consumption). Water and effluent costs fell by /year, giving a payback on the investment of less than one month. Additional benefits include reduced power consumption and wear of the vacuum pumps. Butterfly valves Butterfly valves are used for the manual control of liquid flows, but they generally provide less accurate control than globe, angle or needle valves. However, butterfly valves have a lower pressure drop and can be used with more viscous fluids or those containing solids. Weir diaphragm valves These simple, low-cost devices offer moderate control. They are particularly suitable for applications with fluids that are either viscous or contain solids..8.2 Fixed control devices Restricting orifice Restricting orifice plates fitted into a pipeline provide a constant flow at a predetermined rate. An insert with a particular orifice diameter - equivalent to a specific flow rate - is selected according to the flow rate required. Orifice plates have two main problems: erosion can enlarge the orifice, thus increasing the flow rate; solid material can build up on the orifice, reducing its size and hence reducing the flowrate. Flow regulators Flow regulators are used to provide a constant flow at a predetermined rate. The flow through the regulator can be adjusted within a limited range, but these devices are designed with the intention that adjustment will be infrequent. Flow regulators save chicken-processing company over /year A programme to monitor water use at a chicken-processing company identified several areas around the site where water consumption was excessive. Flow regulators were installed to fix the water supply to particular processes at the rate required by the process. The modifications cost less than and resulted in water savings worth over /year..8.3 Solenoid actuated valves These valves are two-position devices where a solenoid is used to open or close a valve on receipt of a control signal. This simple, low-cost control technique is applicable throughout the food and drink industry and is frequently used to control water supply. Solenoid control reduces water use by 0% A fish processing company installed a solenoid system on the water supply to a pre-wash system. Water had previously flowed continuously, causing overflowing and washing of debris to drain (thus increasing trade effluent charges). The solenoid enabled the water supply to be shut off when the conveyor belt was not in use. Water use by the process fell by 0%, saving 2 500/year and giving a payback period of five weeks. 29

35 .8. Automatic control valves A number of different types of control valve, eg globe, butterfly, ball and three-way valves, can be used for automatic control of flow rates. The flow rate is determined by the valve position, which is controlled by an actuator. Most actuators contain a diaphragm, which is moved and held in any position between fully open and fully closed by an air pressure controller. Accurate and variable control of the flow rate through a valve is provided. The flow rate is adjusted automatically by the control loop, without intervention from an operator. To ensure correct positioning of the valve stem, a positioner can be fitted that adds or exhausts air from the diaphragm according to the signal from the controller. This modification can minimise the effect of process fluid friction, lag or changes in fluid pressure..8.5 Other devices Monitoring and control devices other than valves are also operated by control systems. Some examples are outlined below. Pumps Signals from monitoring devices can be used to operate pumps in both normal and emergency situations. Examples of normal use include stopping and starting the material transfer or the addition of ingredients to a vessel. An example of an emergency situation is using a level-switch signal to stop a pump and thus prevent overfilling of a storage tank. Alarms Visual and audible alarms can be set off by signals from monitoring devices to alert operators to abnormal conditions in the production process, eg overpressure 5 or high temperature. Depending on the importance of the controlled parameter, alarms can be either placed locally or activated through a programmable logic controller (PLC) to give a warning in the plant control room, or a combination of both. Recording instruments A permanent record of process operation can be provided by signals from monitoring devices. This is achieved by circular or strip charts using pen and fluid inks, or data loggers. Visual indication Visual indication of the monitored parameter can be provided by signals from monitoring devices. This can be either local indication, eg a level gauge attached to a storage vessel, or remote indication, eg the level measurement shown on the main PLC control system in the plant control room, or a combination of both. 5 An increase above normal operating pressure, often as a result of valves closing suddenly or when pumps are operating. 30

36 5 ENSURING A HYGIENIC ENVIRONMENT Monitoring and control equipment must be installed to ensure that a hygienic or sterile environment is maintained in the production process. Contamination of food and drink products by microorganisms or other materials, eg small particles or fibres from foreign matter, is a major concern in the industry. Such contamination can harm companies, in terms of both financial losses and their reputation with retailers and consumers. Fig 3 provides a checklist for the installation of process control equipment in a hygienic environment. Comply with all regulations concerned with the composition of materials used in food production, eg Materials and Articles in Contact with Food Regulations No cracks, spaces, notches, dead cavities, fissures or splits. Corners and edges must have a minimum radius ( 3 mm). No threads to be in contact with the product. Highly polished surface finish, ie minimal surface roughness. Seals with defined initial compression. Installation designed to be free from leaks. Fast and simple assembly and dismantling. Non-welded metallic joints to be avoided. Robust and shockproof assembly. Suitable for cleaning by CIP and SIP systems. Passive and non-reactive surfaces, ie not absorbent, adsorbent or chemically reactive. 5 Fig 3 Installation checklist for hygienic process control 31

37 6 PROCESS CONTROL TECHNOLOGY DEVELOPMENTS Food and drink processing imposes particular demands on the instrumentation and control industry, eg: the effects of temperature shock; tolerance to sudden pressure changes; moisture protection of the electronics; corrosion resistance of sensors during CIP and SIP; elimination of nooks and crevices in hygienic applications. Current developments that take account of these demands and have potential applications in food and drink processing include: 6 improved communication systems - equivalent to local area networks (LAN) - linking networks of sensors and controls; laser technologies - improved accuracy that is not limited by fluid type; improved reliability - further development of self-diagnostic equipment to reduce inspection requirements; improved calibration techniques - making maintenance easier and quicker, and reducing health, safety and environmental risks; new materials, eg use of ceramics in sampling equipment, to improve accuracy and reduce maintenance due to use of hard-wearing materials; improved equipment manufacturing techniques - higher specification machining and techniques, eg plasma welding, resulting in more reliable and accurate equipment; use of audible sonics, eg for level measurement, at higher accuracy (no loss of echo) and lower cost than ultrasonics. For information about the latest developments in process control, contact the Environment and Energy Helpline on freephone or equipment suppliers. 32

38 7 INDUSTRY EXAMPLES Both these Industry Examples are from Glanbia Foods Limited s Taw Valley Creamery in North Tawton, Devon. Taw Valley Creamery employs 120 people in the manufacture of cheese, whey products and butter. 7.1 PRODUCT RECOVERY USING A TURBIDITY SWITCH Whey, a by-product of cheesemaking, is concentrated using an evaporation stage prior to spray drying to make whey powder. The evaporators are regularly cleaned in place, which involves flushing out residual concentrated whey prior to detergent cleaning. An investigation revealed that a significant quantity of whey was being lost to drain when the evaporator was taken off-line for CIP. 7 Evaporators used to concentrate whey Bench-scale tests were carried out with different strengths of whey solution to investigate ways of detecting recoverable water/whey mixtures. Discussions with suppliers and a review of existing applications suggested a turbidity probe was the best option. The choice of a turbidity probe was heavily influenced by the generic reliability and simplicity of the device. It also had to be compatible with the existing process control system. 33

39 A turbidity probe was installed at the end of the fill line to the concentrate tank as part of an automatic recovery system. The probe detects the presence of water/whey mixtures and sends this information back to the control system. Recovery of concentrated whey to the whey storage tank is controlled via a densiometer, while the turbidity probe controls the recovery of the water/whey mixture to a separate tank (see Fig ). The mixture is subsequently mixed with raw whey to be reprocessed. When the turbidity is between certain set points indicating the presence of whey, the flow is automatically diverted to the recovery tank via actuated valves. When the detergent cycle starts, the probe signal is overruled to ensure no acid or alkaline detergent is diverted into the whey tank. Contamination of the recovered whey is avoided and only clear or detergent-containing water is discharged to drain. Programmable logic controller Evaporation plant Turbidity probe Concentrated whey storage Whey recovery tank Raw whey storage Drain Fig Flow diagram of whey recovery process 7 No problems have occurred since the whey recovery system began operating in When the system was first installed, training was provided for operators to make them aware of how the new system operated and its benefits. Operating costs are negligible and the turbidity probe requires little maintenance. The probe is cleaned in place when the main evaporator is cleaned. Planned maintenance is performed annually. The system is designed to be fail-safe. If a valve or automation failure occurred, concentrated whey would not be lost to drain, but would be saved in the whey recovery tank. The benefits of installing a turbidity probe at Taw Valley Creamery include: cost savings of around /year; increased product yield; less whey lost to drain; lower effluent treatment plant costs; a payback of eight months on combined capital and installation costs. 3

40 7.2 CIP OPTIMISATION USING A CONDUCTIVITY PROBE At Taw Valley Creamery, the stages of the cleaning-in-place (CIP) cycle are: water flush to rinse out residual product; detergent (acid or caustic) clean for a set period; water rinse. These stages were previously controlled by individual timers to limit the amount of detergent lost at the end of cleaning. Because a number of lines and vessels are cleaned by each CIP set, the cleaning cycles vary. The timing of the opening and closing of the drain valve was therefore a compromise. This resulted in significant quantities of detergent being discharged to drain. To minimise the amounts of water and detergent discharged to drain, the Company investigated methods of enhancing the CIP control system. Conductivity measurement was identified as suitable for distinguishing between acid detergent, caustic detergent and water. Conductivity measurement would indicate when detergent and/or water could be recovered, and whether the right amount of detergent was being used. A conductivity probe was installed initially on one CIP set. However, it was so successful in allowing optimisation of the cleaning cycle for all circuits that all existing CIP sets were retrofitted with conductivity probes and included in the specification for all new sets. 7 Cleaning-in-place equipment 35