Product category:
Valves and manifolds
News Release from: Drallim Industries | Subject: Valve Partial Stroke Testing
Edited by the Processingtalk Editorial
Team on 21 February 2003
The benefits of valve partial stroke
testing
Valve signatures and partial stroking can be achieved with the simple addition of a pressure transmitter and purpose designed microcomputer can diagnose many types of valve problem
Many papers have been written over the last five years on the benefits of partial stroke valve testing There are many techniques for performing such testing, most of which are mentioned in the draft ISA technical report TR84.0.03
This article was originally published on Processingtalk on 24 Feb 2003 at 8.00am (UK)
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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.
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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 SIL ratings? 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.
There are solutions: there are a number of methods for partially stroking valves.
One of the simplest, which can be applied to valves from any manufacturer, 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 your own chest in order to measure pulse or blood pressure.
During the period it takes the valve to close, one can observe the solenoid venting, the actuator spring expanding, the valve body coming off its seat, and the valve body sealing.
In order to perform a partial stroke of the valve, the microcomputer simply re-energises the solenoid valve after it detects the specific signature of the stroke.
If the operation of the solenoid has slowed due to wear or partial breakdown of its coil, the solenoid operation will be delayed.
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.
Many other valve problems can be diagnosed.
The above examples are merely intended to show examples.
The benefits resulting from this method of partial stroking and valve signature can be used with any valve (e.g, linear/rotary, pneumatic/hydraulic), regardless of manufacturer.
The addition of the pressure transmitter and microcomputer are a simple and inexpensive retrofit to any existing installation.
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.
Referenceswhich might be consulted are as follows.
"Non Disruptive Testing of Emergency Shutdown Final Element Devices", Essam, Proceedings of Texas A and M Instrumentation Symposium, January 1997.
"Increase plant safety with online valve testing", Gruhn et al., InTech, February 1998.
"Smart ZV's - The Smart Choice for Emergency Isolation Valve Functional Testing", Flanders and Van Slooten, Saudi Aramco Journal of Technology, Fall 1999.
"Partial-Stroke Testing of Block Valves", Summers, Control Engineering, November 2000.
"Use HART for On-Line ESD Valve Testing", Adler, Moore Industries White Paper, 2000.
"Ins and Outs of Partial Testing", Mostia, Control, August 2001.
ISA dTR84.0.03, Testing of Process Sector Safety Instrumented Functions (SIF) Implemented as or Within Safety Instrumented Systems (SIS), 2002.
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