Electricity + Control July 2016

VALVES + ACTUATORS

Assessing control valves and their performance

Jim Shields, Fluke Corporation

What are the types, what can they be tested for?

W hen assessing control valves and their performance, you need to understand the different types of valves and what they can be tested for. For open/close shutoff valves with- out analog control the tests are pretty simple. Do the valves open and close? When open, do they open all the way? When they close, do they close completely? Testing is mostly observational – looking at the valve and watching cause and effect in the process during the cycle. Control valves are a different ‘beast’ altogether These valves open and close proportionally, and vary the degree of travel depending on the percent of span of the 4 to 20 mA signal ap- plied to them. Observing the valve’s position, as reported on the visual

travel indicator, gives the technician or operator a rough indication of percent of travel for a particular setting when in operation - but does not provide any assurance of how the valve will operate under dynamic and changing conditions. The most sophisticated valve performance tests require removing the valve and testing its performance on a ‘valve prover.’ This is an expensive test device, out of range for most instrument shops. The valve prover is often only used by valve manufacturers in testing the valve when shipped, or by highly qualified field service engineers. It offers a very complete test, but the tool is not feasible for most instrument shops. So, what is the technician to use for testing a control valve? What is a meaningful test that can be used as a baseline? Since most valves use a 4 to 20 mA input signal, any test tool with an mA output signal

Electricity+Control July ‘16

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