Integrating valves, systems, and processes can allow partial stroke testing (PST) of emergency shutdown valves, increasing safety and extend time between full stroke testing, thus saving money. But beware of the '7 misunderstood points’ of PST.
　　7 misunderstood points on safety and partial stroke valve testing
　　Valves are expected to operate even if they haven’t been operated for awhile. To ensure proper operation in emergency shutdown (ESD) service, valves must be actuated. To do so, most p

rocesses must be interrupted—inconvenient at bestCONTROL ENGINEERING China版权所有, costly at worst, but certainly not as costly as a valve actuation failure in a safety instrumented function (SIF).
　　Integrating partial stroke testing (PST), if processes allow, can stretch duration between tests by one to three years and reduce the probability of failure on demandCONTROL ENGINEERING China版权所有, said Sandro Esposito, global product marketing manager for Dresser Masoneilan. Esposito spoke on the subject at the Yokogawa Corp. of America 2008 User Conference & Technology Fair in April.
　　Depending on the processCONTROL ENGINEERING China版权所有, documenting and doing the system integration for that kind of change can save millions of dollars in hardware and maintenance costs while increasing process efficiency, Esposito said. Because of the change in system design and integration of varied practiceswww.cechina.cn, people may be unwilling to consider partial stroke testing. Attitudes may change with greater understanding of IEC 61508, IEC 61511, and ANSI/ISA-TR96.05.01, Partial Stroke Testing of Automated Block Valves, recently approved by ISA and registered with the American National Standards Institute.
　　When Esposito hears of resistance to new ideas, he quotes Albert Einstein, who said: “The world we have created is a product of our thinking; it cannot be changed without changing our thinking.”
　　Actuation of the process isolation elements (the on/off valve that brings the plant to a safe state) represents the weakest link in the ESD system控制工程网版权所有, Esposito said.
　　Sources of error are many,