ANSI/ISA-TR75.25.02 Annex A - Valve Response and Control Loop Performance - Sources, Consequences, Fixes, and Specifications

The absence of valve response requirements, the need to fill in a leakage class on valve specification forms, an emphasis on minimizing cost and in some cases, pressure drop, and a perception that excess capacity is good for future capability may lead one to think that valves typically designed for on-off service are a good option for throttling control because of lower cost, tighter shutoff, and lower pressure drop. Often these valves designed for on-off service employ actuators and assemblies including linkages and shaft connections with severe inherent limitations that greatly reduce control loop performance. 

This annex provides the sources, consequences, fixes, and examples of valve response nonlinearities to understand the ramifications of such a decision and concludes with examples of specifications and tests to help a good throttling valve meet application performance objectives. A broad view of nonlinearities is taken to include anything that changes the valve’s response. 

The knowledge presented is intended to give guidance (including examples of specifications and tests in Table A.1) to improve loop performance and should not be taken as requirements. The goal is to make suppliers and users aware of the impact of valve response on loop performance so that better decisions are made as to the offering and selection of throttling control valves. The need for tight shutoff can be met by a separate on-off valve coordinated with the throttling valve.