Partial valve stroke testing

Actuator shaft failure FaiFDanget Partial valve stroke test ... 

There are several different types of "partial valve stroke testing" that have been discussed under a common name. The simplest approach involves a system where the digital output of a PLC is momentarily de-energized in a remote actuated valve that is energized in normal operation. The PLC monitors a switch moimted to detect that the valve assembly has moved as a result of the momentary off pulse. 

The effectiveness of any diagnostic including a partial valve stroke test can be determined by a FMEDA. Table 11-3 shows an abbreviated FMEDA for a ball valve. 

The analysis can be done for valves, actuators or combination actuator/ valve devices. Table 11-4 shows an abbreviated FMEDA for a gate valve with piston actuator. This analysis shows that partial valve stroke testing... 

What diagnostic coverage would be expected using a high quality partial valve stroke test for a pneumatic rack and pinion actuator in severe service conditions ... 

In the SIL 3 Case 2 design, three conventional transmitters are connected to the safety PLC and the two valve assemblies use three way solenoids instead of digital valve positioners. No partial valve stroke testing is done. The results of the PFDavg calculations are listed in Table 12-10. 

Another alternative design might utilize one valve with partial valve stroke testing. As can be seen in Chapter 11, the effectiveness of a partial valve stroke test depends on the t rpe of valve and the service conditions. Figure 14-7 shows the particular data from an actuator/valve FMEDA for the reactor example. 

A design using a single valve with partial valve stroke testing would meet SIL 2 per the results shown in Figure 14-8. In addition, the MTTFS also meets the minimum criteria of five years. Of course there may be many other designs that meet both the SIL 2 integrity criteria and the minimum MTTFS of five years. 

Figure 14-8. Calculation Results for Reactor SIF Using loot Final Element with Partial Valve Stroke Testing...

The FMEDA method also provides diagnostic coverage factors for the mechanical components when external diagnostic techniques are applied such as partial valve stroke testing. 

Final Elements 2 valves (loo2 voting) with full assessment and certified as per lEC 61508 and partial valve stroke testing at a frequency at least ten times more often than the expected average demand interval. 

In the process industries the most common final element is a remote actuated valve. This assembly usually consists of a pneumatic/hydraulic control assembly, an actuator and a valve (Figure 11-1). The control assembly may be as simple as a three-way solenoid, a smart partial valve stroke box or a complex electro-pneumatic assembly with solenoids, test switches, pneumatic booster relays and quick exhaust valves. 

Self — diagnostics features of devices Real time identification of dangerous failures Valve partial/full stroke testing possible Reduced manual proof testing burden Operability ... 

Care must be taken in implementing PVST, particularly in high-energy conditions. In very high flow conditions the partial stroke test of the SIS valve could incur a response in the BPCS system and the resetting of the PVST could introduce an additional variable to the BPCS. This could impact otherwise unknown instabilities in the BPCS or SIS sensing system leading to a spurious trip. 

Figure 12-4 consists of a single Safety Transmitter, safety PLC, and single shutdown valve without tight shutoff requirements with a digital valve positioner. The digital positioner would be provided with the capability for Partial Stroke Testing (PST). 

As final element is the most vulnerable component in a safety loop so, redundancy is another important issue here. Use of redundant valve in series and parallel mode is a related to process concerned but in any case it is normally a costly affair. However, proof testing at regular intervals could be an economic alternative to ensure reliability and availability of final element in the safety loop. This proof testing for final elements is popularly known as partial stroke testing discussed later in this chapter. 

In a running facility or plant, if testing is to be performed on solenoid operated on-off shut down valves to assess the health of valve, and then only way is to carry out partial stroke testing of the valve. This may give rise to chances for spurious tripping of the plant. In the following sub-clauses, partial stroke and full stroke testing shall be covered. 

B.G. Liptak, Instrument Engineers Handbook Process Control, Chapter 6.10 Emergency partial Stroke testing of block valves A.S. Summers. 

What You Need To Know About SIS and Partial Stroke Testing of BSD Valves, Metso Automation write up www.metsoautomation.com. 

The literature reveals that partial stroke testing can reveal approximately 70% (arguably 40—80%) of faults. However, this is widely debated and there are many scenarios and valve types where this is by no means achieved. The whole subject is dealt in-depth (including a Uter-ature search) in Technis GuideUnes T658. 

Despite optimistic claims by some, obtaining SIL 3 from partial stroke testing of a single valve is not that easy. Technis Guidelines T658 show that SIL 2 is a more likely outcome. 

Partial Stroke Testing (PST) is sometimes used as a supplement to full proof testing of safety valves. 

Comparison of partial stroke curves from past tests can indicate the gradual degradation of valve components. Use of overlay graphics, identification of unhealthy shifts in servodrive, increases in valve friction, and changes in dynamic response provide information leading to a diagnosis of needed maintenance. 

FIG. 8-86 Online partial stroke diagnostic used to validate the operability of a pneumatically operated safety shutdown valve, (a) Input command generated by the diagnostic and resulting travel, (b) Actuator pressure measured over the tested range of travel. 

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