The most modern treatment provides the best drinking water

Transcription

1 Drinking water The most modern treatment provides the best drinking water Process measuring and control technology technology for Germany s largest nanofiltration plant Last year, a nanofiltration plant unparalleled in Germany was put into operation in a municipal water works. The plant filters unwanted materials from the drinking water and ensures a water supply of the best possible quality. Pressure measurement technology, conductivity measurements and modern process engineering help the Dinslaken GmbH municipal works to implement optimum monitoring and control of the plant. In the spring of 2010, the largest nanofiltration plant in Germany was officially put into operation in a municipal water works. In the future, it will supply the City of Dinslaken with high quality drinking water (Figure 1). The plant was necessary because due to coal mining in the water catchment area, changes were to be expected in the flow of the groundwater in the coming years. Fig. 1: Overall view of the water works with the new nanofiltration plant in the background The result: Rhine bank filtrate and a number of trace materials could get into the raw water of the extraction wells. To remove these unwanted materials from the drinking water, the Dinslaken municipal works installed a state-of-the-art nanofiltration plant in the Voerde-Löhnen water works. The plant can treat m³ of water per hour. With a pressure drop across the membrane, ions and particles in the nanometer range are filtered out (Fig. 2, fig. 3). siemens.com/sensors/environmental

2 Fig. 2: The nanofiltration plant with eleven blocks Sophisticated procedure At the present time, the nanofiltration plant is operating at partial load levels, in other words, somewhat more than half of the water passes through the filter plant. As things stand, the contaminants in the Rhine have not yet reached the water catchment area. Full treatment will start as soon as the Rhine bank filtrates contaminate the extraction wells. Nobody knows when this will happen because the advance of the Rhine water cannot be predicted with any accuracy. To avoid any risk, the municipal works have set up measuring stations close to the Rhine that will warn the operators when the Rhine water penetrates the groundwater aquifers (underground layers that carry groundwater). The permeate is so pure, that it no longer contains many natural mineral components. With full treatment, a hardness increase process with dolomitic filter material ensures the drinking water quality. Currently, the permeate and raw water passes through a neutralizing plant before reaching the two water storage tanks. The result is top quality drinking water. From here, the water is pumped into the conveyor pipeline and fed into the supply network of the Dinslaken municipal works via a pressure intensifier. The company of Wetzel + Partner Ingenieurgesellschaft of Moers in Germany was responsible for planning the process, mechanical and electrical engineering involved in the modernization of the water works. The process engineering implementation of the nanofiltration plant was handled by A+H Anlagentechnik of Isselburg, Germany. Siemens Industry Solutions was awarded the contract to implement the automation and electrical engineering by the Dinslaken Voerde-Löhnen water works including the nanofiltration. Siemens also renewed the entire power supply and distribution and set up a backup network to provide maximum protection against failure. Medium voltage switchgear and a low voltage main and secondary distribution center were also installed. Fig. 3: Functional schematic of the nanofiltration plant ( operating at full load). Courtesy of Wetzel + Partner Ingenieurgesellschaft mbh Better safe than sorry The municipal works decided to modernize and expand the existing process control technology for the entire water works based on the process control system SIMATIC PCS 7 (version 7). The redundantly designed process control system ensures totally integrated operator control and a uniform visualization of the entire plant including the external plants (Figure 4). Using a modified, active mosaic diagram, all the existing parts of the main and external plants can also be monitored. The installed control technology was designed so that the control center of the Dinslaken water works can control and monitor individual, important parts of the main and external plants such as the pressure intensifier system, water storage tanks and clean water pumps using Ethernet communication. The special challenge: All the work had to be performed while the water works remained fully operational because the water supply had to be kept available. Fig. 4: SIMATIC PCS 7 view of the nanofiltration plant in the Voerde Löhnen water works 2

3 Fig. 5: Mosaic panel in the control room of the water works Fig. 7: Hach Lange type 3411SC contacting two-pole conductivity measurement Diagnostics made easy To press the raw water through the pressure pipe into the filter blocks of the nanofiltration plant, an average operating pressure of 8.2 bar is set. To achieve this, eleven conveyor pumps each with a power of 150 kw were installed. Solids can block or damage the membranes of the filter blocks. To remove sand and particles from the raw water, two pre-filters are installed in series before each nanofiltration block. The first filter contains filter elements with a 5 µm mesh aperture, the second pre-filter has a mesh aperture of 1 µm. Since these filters act as a safeguard, they are also known as police filters. The performance of the filters is monitored by SITRANS P300 relative pressure transmitters before and after the first pre-filter and after the second pre-filter. If either of the filters is contaminated, the differential pressure increases considerably. This allows simple detection of any blockage of the filters (Figure 6). Trust is good, control is better The nanofiltration plant consists of eleven identically designed filter blocks. Each block has two filter stages. The first filter stage contains ten and the second stage five pressure pipes. 60 % of the raw water leaves the filter block as cleaned permeate, 40 % as concentrate containing the filtered out materials. This concentrate is cleaned again in the second filter stage. The total yield is 80% (see Table 1). Each filter block is equipped with five conductivity measurements from Hach Lange (Figure 7). The electrodes are installed directly in the pipe using weld-on supports. To achieve the best possible accuracy, two-pole contacting electrodes were selected. For the lower conductivities (permeate), a sensor made of stainless steel with a cell constant K=0.10 of the type 3411SC is used. The sensor for the higher measured values (raw water and concentrate) is made of graphite and has a cell constant of K=1.0 (type 3412SC). Both electrode types pre-calibrated in the factory and require practically no calibration or maintenance in this application. Temperature compensation is performed automatically by the integrated temperature sensor Pt100. Cleaning stage Feed Permeate Concentrate 1st stage 2nd stage Total 110 m³ natural water 45 m³ concentrate of the first cleaning stage 65 m³ 45 m³ 23 m³ 22 m³ 88 m³, recovery 80 % 22 m³ Fig. 6: Pressure measuring transmitter SITZRANS P300 in the prefiltration Table 1 3

4 Fig. 8: The SC1000 controller shows the five conductivity measurements of a filter block directly on site Fig. 9: Filter block with conductivity and pressure measurement Measuring the conductivity of the raw water allows the state of the raw water to be monitored. A typical measured value would be 840 µs/cm. The measurement in the permeate outflow following the first filter stage is used to monitor the filtering performance of the first membrane. Normally, the conductivity is approximately 12 µs/cm. The conductivity measurement is used to detect a break in the membrane following which the measured value increases tenfold or more. A sampling point allows the defective pressure pipe to be identified. The concentrate from the first filter is cleaned again in the second filter stage. This is monitored by a conductivity measurement in the permeate outflow. Finally, the two permeates are mixed and also measured (typical conductivity value: 15 µs/cm). The collected concentrate from the second filter stage (conductivity for example 3.4 ms/cm) flows back into the Rhine. The display of an SC1000 controller allows all five measured conductivity values of a filter block to be displayed simultaneously on site (Figure 8). Measuring technology with intelligence An ideal conveyor pressure for the filter membrane has a significant effect on the control of the process. If the pressure is too low, the filter performance is reduced, if it is too high, energy consumption is increased unnecessarily. To achieve high measurement accuracy, relative pressure transmitters of the type SITRANS P300 are used to measure the pressure. These are outstanding for their extremely low characteristic curve deviation ( %) and low long-term drift of 0.25 % in five years. Two SITRANS P300 transmitters monitor the conveyor pressure in the inflow of stage 1 and 2 of each filter block. If there is a membrane break, there is a jump not only in the conductivity but also in the pressure after the filters. For this reason, additional pressure measurements are installed in the permeate outflow and the collected concentrate outflow. The measuring transmitter has comprehensive diagnostics functions that inform the control system of any problems. An additional aim of the monitoring is to detect blockage of the membranes by measuring the difference in pressure between the individual stages. 4

5 Successful overall composition In the nanofiltration plant in the Voerde-Löhnen water works of the Dinslaken GmbH municipal works, tried and tested, intelligently deployed measurement techniques such as pressure measurements from Siemens or conductivity measurements from Hach Lange ensure reliable operation. Fig. 10: Hydrostatic fill level measurement SITRANS P MPS in raw water container 1 (left) Up to now, the customer is completely satisfied with the running of the plant. The use of modern control technology opens up numerous new possibilities: When the water works are unmanned (for example during the night), disruptions are signaled centrally and standby personnel are informed. Employees can then connect to the plant and operate and monitor it from home using a standby client. If it becomes necessary to clean the membranes, the cleaning pump injects cleaning chemicals (lye and acid) to circulate in the filter blocks. The deposits flushed out of the membrane filters are removed from the cleaning solution via an intermediate filter element to protect the membranes. The monitoring of this police filter (construction 5 µm and 1 µm) is also handled by the SITRANS P300. The pressure measuring transmitter also monitors the pneumatic pressure required for controlling the pneumatic armatures. The fill levels in the raw water, permeate and concentrate containers and in the pump shafts are measured by the hydrostatic pressure transmitters of the type SITRANS P MPS. This simple-to-use pressure transmitter consists of a piezo resistance sensor flush mounted on the front with a housing and measuring membranes of stainless steel. The pressure and conductivity measurements are very precise and practically maintenance free. The trouble-free operation up to now is the best proof that we made the right choice in selecting measurement technology from Siemens. With the higher-level process control system SIMATIC PCS 7, the plant is always visible and can be controlled very conveniently from a central location. Dirk Bittner, water works foreman in the Voerde Löhnen water works 5

6 Siemens AG Industry Sector Sensors and Communication Process Instrumentation KARLSRUHE GERMANY Subject to change without notification 04/2012, Siemens AG The information provided in this Case Study contains descriptions or characteristics of performance which in case of actual use do not always apply as described or which may change as a result of further development of the products. An obligation to provide the respective features shall only exist if expressly agreed in the terms of contract. Availability and technical specifications are subject to change without prior notice. All product designations may be trademarks or product names of Siemens AG or supplier companies whose use by third parties for their own purposes could violate the rights of the owners.