Double whammy: the benefits of valve signatures and partial stroke testing

Transcription

1 Double whammy: the benefits of valve signatures and partial stroke testing By Paul Gruhn, P.E., C.F.S.E., L&M Engineering and Derek Essam, Drallim Abstract Many papers have been written over the past five years on the benefits of partial stroke valve testing (see refs1-6). There are many techniques for performing such testing, most of which are mentioned in the draft ISA technical report TR Partial stroke valve testing is one of the most effective techniques for enabling a single valve to achieve high SIL performance. One aspect of testing, however, has not been covered that of valve signatures. The problem Valve assemblies, especially large ones, can be damaged during shipment and installation. One very effective way of determining if and what type of any damage has been done is to take a signature of the valve stroke when new at the factory and compare it with the signature after installation at the site. During normal operation the valve may be partially stroked and the results recorded. Valve signatures and partial stroking can be achieved with the simple addition of a pressure transmitter and purpose designed microcomputer. Quarter turn valves, especially ball valves, have a known tendency to stick when not used for long periods. Unfortunately, this is the normal operation of most safety instrumented systems dormant. The traditional method of testing such valves would be to close them fully. Since this results in stopping production, this can generally only be done when the process is down. The interval between scheduled downtimes for many processes is measured in years. Such infrequent testing of valves means the overall system can typically only achieve a SIL 1 rating. What is to be done for systems needing to meet higher SILs? The only available options are to either use redundant valves an economically unattractive solution or to test the valves more frequently. Taking a process down merely for the purpose of valve testing is also economically unattractive. However, if the primary failure mode of certain valves is failing stuck, it is not necessary to fully close them in order to detect this failure mode. A partial stroke test will reveal the problem without affecting production. A solution There are a number of methods for partially stroking valves. One of the simplest, which can be applied to any manufacturer s valve, may in fact be the most effective. By merely adding a pressure transmitter in the exhaust line of the actuator and capturing the signature profile of the valve during closure with a microcomputer, a surprising amount of information can be obtained. Comparing later strokes of the valve against the original signature can reveal almost any problem. It is not even necessary to measure actual valve movement, just as it is not necessary to open one s chest in order to measure pulse or blood pressure.

2 In Figure 1, the blue line shows a normal valve signature of a valve closing fully. During the five seconds it takes the valve to close, it is possible to observe the solenoid venting, the actuator spring expanding, the valve body coming off its seat, and the valve body sealing. Fig. 1: Full valve stroke (blue line) and partial close signature (red line) In order to perform a partial stroke of the valve, the microcomputer simply re-energizes the solenoid valve after it detects the specific signature of the stroke. (After the blip shown in Figure 1 at two seconds.) The resulting partial stroke test curve is shown as the red line. Detecting various failures If the operation of the solenoid has slowed due to wear or partial breakdown of its coil, the solenoid operation will be delayed. An example of this impending failure is shown in Figure 2.

3 Fig. 2: Degraded solenoid performance (red line) Restrictions to the flow of air through either the solenoid valve, the exhaust/vent ports of the quick exhaust valve, or the actuator cause varying degrees of blockage and a number of different conditions that will ultimately affect system performance. Depending upon the degree of restriction/blockage, the effects could range between a slowing of operation to prevention of closure. Figure 3 shows an example of a partially blocked quick exhaust valve vent port.

4 Fig. 3: Partially blocked solenoid vent port (red line) Many other valve problems can be diagnosed. The above examples are merely intended to show examples. Benefits This method of partial stroking and valve signature can be used with any valve (e.g., linear / rotary, pneumatic / hydraulic), regardless of manufacturer. Retrofitting the pressure transmitter and microcomputer to an existing installation is a simple and inexpensive procedure. The testing can be performed automatically, faults can be alarmed immediately and results recorded. In order to achieve the full benefit of signatures, however, installation with the new valve assembly at the factory is recommended. References Non Disruptive Testing of Emergency Shutdown Final Element Devices, Essam, Proceedings of Texas A&M Instrumentation Symposium, January Increase plant safety with online valve testing, Gruhn et al., InTech, February Smart ZV s The Smart Choice for Emergency Isolation Valve Functional Testing, Flanders & Van Slooten, Saudi Aramco Journal of Technology, Fall Partial-Stroke Testing of Block Valves, Summers, Control Engineering, November 2000.

5 Use HART for On-Line ESD Valve Testing, Adler, Moore Industries White Paper, Ins and Outs of Partial Testing, Mostia, Control, August ISA dtr , Testing of Process Sector Safety Instrumented Functions (SIF) Implemented as or Within Safety Instrumented Systems (SIS), About the authors Paul Gruhn, P.E., C.F.S.E., is a safety system specialist with Siemens in Houston, Texas. Paul is an ISA Fellow, a member of the ISA SP84 committee, is the developer and instructor for ISA s 1 and 3-day courses on Safety Systems, is the primary author of the companion book available through ISA, has published numerous articles, and is the developer of commercial software for modelling the performance of control and safety systems. He is also a member of the System Safety Society the National Society of Professional Engineers. Paul has a B.S. degree in mechanical engineering from Illinois Institute of Technology, in Chicago, Illinois, is a licensed Professional Engineer in Texas, and a Certified Functional Safety Expert (a TÜV certification). Derek Essam has been a director of Drallim Industries for the past ten years. Founded in 1958, Drallim s business segments span from large scale country wide monitoring systems used in the power and telecommunications sectors, to factory automation monitoring systems. Additional segments cover military load carrying and helicopter shipboard restraining systems and pneumatic Interlok valves for the hydrocarbons industry. A core business sector over the past ten years has been in the field of on-line testing of emergency valves (ESDV). Drallim Industries are the original developers of ESDV partial stroking technologies and have more installations of ESDV testing systems than any other company. Drallim have now combined its monitoring technologies into the area of ESDV monitoring and now offer the worlds most advanced product in this field.

6 Derek Essam is a recognised expert in the field of ESDV testing systems, holding more patents on this subject than any other individual. Originally studying as an electrical engineer, Derek then went on to specialise in pneumatic and hydraulics at Coventry Technical College in the UK. Derek has combined his skills in these fields with computer technologies, to promote the issues of the fatal consequences should an ESDV fail to operate. Essam will be promoting the subject of whole life cycle issues from monitoring the valve, actuator and controlling components at the automation phase, through construction and into service. In addition to the new construction phase, Essam will cover the problems that existing plants face, when considering retrofitting a testing/monitoring system to existing valve and actuators.