New applications for ultrasonic sensors in process industries

Transcription

1 Ultrasonics 44 (2006) e1379 e New applications for ultrasonic sensors in process industries Alf Püttmer * Siemens AG, Automation and Drives, PI13, Oestliche Rheinbrueckenstr. 50, Karlsruhe, Germany Available online 5 June 2006 Abstract Today, sophisticated instruments and algorithms used for process control allow the efficient production of substances with the quality guaranteed to the customer. However, production and management techniques are changing. This article aims to analyze trends resulting from those changes from the point of view of ultrasonic sensors. The focus will be on plant asset management. One part of plant asset management is maintenance management. Its goal is cost saving by avoiding unexpected shutdowns of single components or the complete plant. However, new maintenance concepts like condition-based or predictive maintenance require reliable methods of condition monitoring. This article will discuss these new applications for ultrasonic sensors. A new ultrasonic measurement based on acoustic emission analyses for condition monitoring of high pressure process pumps is introduced and economic benefits for the user are discussed. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Process monitoring; Condition monitoring; Ultrasonic sensor; Acoustic emission 1. Introduction Conversion of gases, steams, liquids and solids, conversion of different forms of energy, separation, mixing and storage of products are among the many operations found in process industries. Ultrasonic sensors for level, flow, concentration and particle distribution already play an important or increasingly important role in process automation [1]. New developments in machinery, aggregates and management of a process plant require new types of sensors. The driving forces and resulting technological trends will be discussed below. 2. Technological trends The economics-driven requirement of increased efficiency of process plant operation can not only be achieved by cuts in personnel expenses, but by optimization of the plant. Two goals of this optimization are * Tel.: ; fax: address: alf.puettmer@siemens.com (i) High and reproducible product quality and (ii) reduced plant downtime. The first goal forces the development of new production processes and improved process control; the second one requires a sophisticated asset management including performance management and maintenance management. Both approaches require sensors, which provide information for [2] process control, process safety, monitoring legal regulations (e.g., emissions), plant and machinery condition monitoring. These considerations imply that future sensor applications are closely linked to future techniques of production and asset management Process control A detailed analysis of state-of-the-art applications of ultrasonic sensors can be found in [1]; an analysis of future applications of sensors in process control was published in [2] as a technology roadmap. As an example of a demand X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi: /j.ultras

2 e1380 A. Püttmer / Ultrasonics 44 (2006) e1379 e1383 for ultrasonic-based sensors, the technology roadmap points out that ultrasonic tomography for the measurement of the spatial distribution of concentration, density or temperature in reactors may be among the techniques that solve future problems in process control Asset management Asset management involves plant performance monitoring with the help of calculated performance indicators and maintenance management. Typically, maintenance management systems are now integrated in process control systems. The aim is to shift from corrective or time-based maintenance to condition-based or predictive maintenance (Fig. 1). User benefits are reduced downtime, reduced number of unplanned shutdowns, extended maintenance intervals, reduced number of on-site visits, reduced time for repair. Fig. 1. Maintenance concepts in process plants: Corrective = replacing a defective part; time based = periodically replacing parts that are not yet damaged; condition based = replacing parts with incipient damages; predictive = predicting the remaining life time of a part and scheduling maintenance based on that knowledge. Both condition based and predictive maintenance require condition monitoring. However, this is only possible when the condition of machines and aggregates is actually measurable online during normal machine operation. The large economic potential of condition monitoring drives the development of condition monitoring systems or machinery health transmitters and their integration into process control systems. Applications of condition monitoring are all components in the plant, which lead to a shutdown in the case of a failure, such as pumps, compressors, fans, turbines, valves, pipes. Some failures in these devices may be detected by temperature measurement, e.g., bearing failures. However the response is slow. Others may be detected by pressure measurement, e.g., leaks in valves or pipelines. But monitoring the pressure loss in a closed section of a pipeline can not be done online. More advantageous is the application of advanced techniques like vibration measurement or acoustic emission (AE). Clearly, temperature or pressure measurement is much simpler and less expensive, but vibration or AE provide more diagnostic information and have a rapid response to changes in machinery condition. The structure of an AE measurement system is shown in Fig. 2. The difference between vibration and AE is the frequency range. Vibration measurements use the sonic frequency range, AE uses the ultrasonic range. Since attenuation of ultrasonic waves is higher, AE is more sensitive to the distance between source and sensor. This improves both the signal-to-noise ratio and cross-sensitivity to other sources of noise within one machine. Depending on the application, each technique has its own advantages and disadvantages (Table 1, [3,4]). A summary of AE sources and related applications is given in Table 2. Detailed descriptions of applications can be found in [3 8]. One application example is discussed in detail in the next section. 3. Application example: condition monitoring of high pressure process pumps 3.1. Technology Fluids and slurries are usually conveyed by reciprocating positive displacement pumps in a pressure range Fig. 2. Structure of an AE-based condition monitoring system. The application-related experience is included in signal processing and the alarm setpoint.

3 A. Püttmer / Ultrasonics 44 (2006) e1379 e1383 e1381 Table 1 Comparison of vibration and ultrasound/ae measurement for condition monitoring Frequency range [3] Vibration AE <25 khz >25 khz Low cross-sensitivity to typical + environmental noise Low cross-sensitivity to noise of + normal machinery operation High sensitivity to minor imbalance + or misalignment Low sensitivity to sensor alignment + Localization of the measurement to + the machine/component monitored Simple signal processing + Simple application when operational + speed of the machine varies + advantage, disadvantage Table 2 Sources of noise and examples of condition monitoring Noise source Application in condition monitoring Impact Bearing condition monitoring Friction Particle monitoring in pipes after filters Blocking monitoring of pneumatic particle conveying systems Cavitation Leakage monitoring (liquid media) of valves, pipes, etc. Cavitation monitoring of centrifugal pumps Valve condition monitoring of reciprocating positive displacement pumps Turbulence Leakage monitoring (liquid or gaseous media) of valves, pipelines, etc. >10 bar. Those pumps comprise a suction and a discharge valve. Wear of both valves depends on variable parameters like composition of the fluid/slurry, pressure and temperature, is hardly predictable and can be high. Damaged valves have leaks. Such leaks grow fast and cause reduced efficiency of the pump. Later they can lead to expensive consequent damage to the pump. These problems make it a highly attractive condition-monitoring application. Condition-monitoring approaches like correlating operational speed, pressure and flow fail, since they are too insensitive. Therefore a condition-monitoring system based on AE was developed [5]. It is based on the fact that the acoustic emission level L of a valve depends on the hydraulic power P hyd L ¼ 10 log g P hyd ð1þ P 0 with the efficiency coefficient for energy conversion from hydraulic to acoustic g and a reference power P 0. The hydraulic power of the valve depends on pressure difference Dp and flow rate Q: P hyd ¼ Dp Q ð2þ The valves are periodically opened and closed. The hydraulic power of both an open valve and a working valve are small (either no pressure loss or no flow). Leaks however cause some flow when the pressure is high. Hence, the acoustic emission is high. Additionally, g is increased when cavitation is present in the flow. This is usually the case in leaks under high pressure differences. Sensor signals and calculated levels measured on a pump valve are shown in Figs. 3 and 4. The difference between a working and defective valve is significant User benefits Benefits for the user become obvious when analyzing the data obtained in a field test during a period of two years. Fig. 5 shows a period of three months and documents the development of a leak. The decision to repair the valve was only based on the experience of the pump operator/ maintenance personnel. That means a person listens to Fig. 3. Signal of the AE sensor and calculated level of a working valve of a reciprocating positive displacement pump. Operating speed of the pump is 0.8 Hz, pressure 70 bar.

4 e1382 A. Püttmer / Ultrasonics 44 (2006) e1379 e1383 Fig. 4. Signal of the AE sensor and calculated level of a defective valve of a reciprocating positive displacement pump. Operating speed of the pump is 0.8 Hz, pressure 70 bar. Fig. 5. Trend data of the AE level of a valve of a reciprocating positive displacement pump logged during a field test. Two alarm thresholds allow distinguishing between incipient damages and full damages of the valve. the pump operating noise and decides about the activities. Due to the high level of operating noise in the audio range it is hardly possible to find the defective valve even when reduced pump output indicates the existence of one or more defective valves. Often several valves have to be opened and the defective valve has to be identified by visual inspection. This is time-consuming and expensive. It can clearly be seen from Fig. 5 that the AE measurement indicates an incipient leakage roughly one month before the operator replaced the valve. This is sufficient time to plan a regular shutdown. The advantages of condition-monitoring systems can be summarized: automatic inspection, automatic identification of the failure location, thus reduced downtime for repair, indication of beginning valve defects up to four weeks in advance, Fig. 6. Example calculation of the expected life cycle costs of a reciprocating positive displacement pump. Investment costs are higher for the pump with condition monitoring. Maintenance and repair costs are higher for the pump without condition monitoring (only costs for material and labor have been taken into account; costs due to lost production and energy costs due to reduced efficiency have not been considered). avoidance of expensive consequent damages, increased efficiency of the machine. These advantages lead to reduced life-cycle costs. Fig. 6 compares the life-cycle costs of a pump with and without condition monitoring. The potential savings after 30 years of operation are approximately 50% when condition-based maintenance is used. 4. Conclusions and prospects New production concepts like condition-based maintenance require new sensors. This article has shown that ultrasonic sensors could play an important role in machinery condition monitoring. The acoustic emission technique

5 A. Püttmer / Ultrasonics 44 (2006) e1379 e1383 e1383 has its distinct advantages compared to the vibration technique, making it particularly attractive in noisy environments. A condition-monitoring system for reciprocating positive displacement pumps has been demonstrated and its benefits for the user have been shown. However, the successful application of advanced maintenance management concepts requires condition-monitoring systems for all key components in a process plant. References [1] P. Hauptmann, N. Hoppe, A. Püttmer, Application of ultrasonic sensors in the process industry, Measurement Science and Technology 13 (2002) R73 R83. [2] NAMUR, Technologie-Roadmap Prozess-Sensoren , 2005 (in German). [3] T. Holroyd, Acoustic Emission and Ultrasonics, Coxmoor Publishing Company, [4] R.K. Miller, P. McIntire, Nondestructive Testing Handbook, Acoustic Emission Testing, American Society for Nondestructive Testing vol. 5 (1987). [5] A. Püttmer, H.M. Nägel, Optimization of pump maintenance, PRO- CESS worldwide (4) (2004) [6] J. Frohly et al., Ultrasonic cavitation monitoring by acoustic noise power measurement, J. Acoust. Soc. Am. 108 (2000) [7] S.V. Shepherd, Adapting acoustic monitoring technology to detect bulk solids flow, Sensors 18 (9) (1990). [8] R. Ohba, Leak detection under overwhelming ambient noise conditions, Proceedings of Sensors and their Applications (1995)