Integrating valves, systems, and processes can allow partial stroke testing (PST) of emergency shutdown valves控制工程网版权所有, increasing safety and extend time between full stroke testing控制工程网版权所有, 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 best, costly at worstwww.cechina.cn, 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 demand, said Sandro EspositoCONTROL ENGINEERING China版权所有, 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 process, 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 practices控制工程网版权所有, 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控制工程网版权所有, 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, Esposito said.
　　Sources of error are many,