Partial stroke testing

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). 

Oliveira, L.R Abramovitch, R. 2008, A Numerical Approach to Calculate PFD Values for koon Architectures with and without Partial Stroke Testing , DNV Internal Report, to be submitted for publication, 2008. 

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. 

Develc )ed based cai die idea from Partial stroke testing for 6nal elements T. Karte, IJ. Kiesl uen Samson AG ... 

Discussions on Partial Stroke Test and Full Stroke Test... 

Partial stroke testing for final elements T. Karte, I.J. Kiesbauer Samson AG Conference Proceedings of Petroleum and Chemical Industry Conference (PCIC) Europe, 2005. 

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

K. Bingham, Partial Stroke Testing with Positioners and/or Logic Solvers, April 2014- http // www.acm.ca/resource-centre/articles/partial-stroke-testing-positioners-andor-logicsolvers. 

J. Cahill, Credit for Partial Stroke Tests in Verification Process, Emerson Process Expert, 2011, in http //www.emersonprocessxperts.com/20H/01/credit for part/. 

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

ESD). Finally, a discussion on SIS would be incomplete unless certification methods, FMEDA, proof testing, and partial stroke testing, etc. are not covered. 

The p factor for the undetected common cause failures is fixed at 2% (0.02). The Pq factor for the detected common cause failures is fixed at 1% (0.01). The proof test is a full, perfect proof test as opposed to a partial stroke test. The mean time to repair (MTTR) is 8 hours for all elements. 

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. 

Revealed by partial stroke test PFD Revealed by proof test PFD Revealed by demand (ESD or real) PFD... 

Requirements for partial stroke testing (Interval, Coverage)... 

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

Partial stroke test Interval - Final element 9 months Coverage - Final element 90 %... 

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