Control Valve Handbook Fourth Edition (Fisher, Emerson by LLC Fisher Controls International, Emerson Process

By LLC Fisher Controls International, Emerson Process Management

Keep an eye on Valve guide by way of Fisher/Emerson procedure administration. 4th variation, 2005.

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NACE is no longer an abbreviation. OSHA: Abbreviation for Occupational Safety and Health Act. ) Operating Medium: This is the fluid, generally air or gas, used to supply the power for operation of valve positioner or automatic controller. Operative Limits*: The range of operating conditions to which a device can be subjected without permanent impairment of operating characteristics. Range: The region between the limits within which a quantity is measured, received, or transmitted, expressed by stating the lower and upper range values (for example: 3 to 15 psi; -40 to +212_F; -40 to +100_C).

This, of course, will mean a penalty of increased process variability. The second way oversized valves hurt process variability is that an oversized valve is likely to operate more frequently at lower valve openings where seal friction can be greater, particularly in rotary valves. Because an oversized valve produces a disproportionately large flow change for a given increment of valve travel, this phenomenon can greatly exaggerate the process variability associated with dead band due to friction.

The input stage of these positioners is often a low static gain transducer module that changes the input signal (electric or pneumatic) into movement of the spool valve, but this type of device generally has low sensitivity to small signal changes. The result is increased dead time and overall response time of the control valve assembly. Some manufacturers attempt to compensate for the lower performance of these devices by using spool valves with enlarged ports and reduced overlap of the ports.

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