While many control valves throttle in mid-travel positions throughout their operating lives, some applications include a requirement for the control valve to stop, or nearly stop, fluid flow through the valve. The term that describes the ability of a control valve to stop flow is shutoff.
To provide varying degrees of shutoff, sliding stem valves are available with several different types of plug and seat constructions. In rotary-shaft valves, seals of various configurations and materials provide a range of shutoff options.
There are several conditions under which it may be important for a control valve to either partially or fully shut off flow.
Process – A requirement for full or partial shutoff may be related to the proper operation of a process or system; for example, if a control valve in a blending
operation leaks excessively in the closed position, the leakage may make it impossible to maintain the desired ratio of blended ingredients.
Seat Life – Potential damage to the valve is another reason why shutoff may be desirable; for example, if a valve in a boiler feedwater application leaks, the high velocity streams that leak across the seat can result in rapid erosion of critical seating surfaces.
ANSI Test Criteria B16.104 And Section 4.1.7 Of SAES-J-700
Shutoff is ordinarily stated in terms of classes of seat leakage as defined in the American National Standard for Control Valve Seat Leakage (ANSI B16.104). Section 4.1.7 of SAES-J-700 states that all seat leakage classifications shall be specified in accordance with ANSI B16.104.
The ANSI standard defines leakage test setups and test procedures, and it lists the allowable leakage rates for several classes of seat leakage.