Control valves are designed to throttle and not necessary to close 100%.
A control valve’s ability to shut off has to do with many factors as the type of valves for instance. A double seated control valve have very poor shut off capability. The guiding, seat material, actuator thrust, pressure drop, and the type of fluid can all play a part in how well a particular control valve shuts off.
The ANSI standard FCI 70-2: Control Valve Seat Leakage, establishes a series of six seat leakage classes for control valves and defines the test procedures.
It Is also know as dust tight and can refer to metal or resilient seated valves.
This class establishes the maximum permissible leakage generally associated with commercial double-seat control valves or balanced single-seat control valves with a piston ring seal and metal-to-metal seats.
This class establishes the maximum permissible leakage generally associated with Class II (4.2.2), but with a higher degree of seat and seal tightness.
This class establishes the maximum permissible leakage generally associated with commercial unbalanced single-seat control valves and balanced single-seat control valves with extra tight piston rings or other sealing means and metal-to-metal seats.
This class is usually specified for critical applications where the control valve may be required to be closed, without a blocking valve, for long periods of time with high differential pressure across the seating surfaces.
It requires special manufacturing, assembly and testing techniques. This class is generally associated with metal seat, unbalanced single-seat control valves or balanced single-seat designs with exceptional seat and seal tightness.
This class establishes the maximum permissible seat leakage generally associated with resilient seating control valves either unbalanced or balanced single-seat with “O” rings or similar gap less seals.
There are two types of leakage from a valve, namely; fugitive emissions from the valve to atmosphere, and leakage through the valve but contained within the piping system.
Fugitive emissions can both be detrimental to the environment and a potential safety hazard. Valves are considered to be the major contributors to fugitive emission losses.
Leakage through the valve can also be a safety hazard, and can be detrimental to the process.
Common causes of leakage through the valve include:
Common causes of leakage to atmosphere:
Both of them can have several causes
There are actually six different seat leakage classifications as defined by ANSI/FCI 70-2 1976(R1982).
The most common used are
CLASS IV is also known as metal to metal. It is the kind of leakage rate you can expect from a valve with a metal plug and metal seat.
CLASS VI is known as a soft seat classification. Soft Seat Valves are those where either the plug or seat or both are made from some kind of composition material such as Teflon or similar.
The different valve leakage classifications are as follows :
Identical to Class II, III, and IV in construction and design intent, but no actual shop test is made. Class I is also known as dust tight and can refer to metal or resilient seated valves.
Intended for double port or balanced singe port valves with a metal piston ring seal and metal to metal seats.
Intended for the same types of valves as in Class II.
Intended for single port and balanced single port valves with extra tight piston seals and metal to-metal seats.
Intended for the same types of valves as Class IV.
Class VI is known as a soft seat classification. Soft Seat Valves are those where the seat or shut-off disc or both are made from some kind of resilient material such as Teflon. Intended for resilient seating valves.
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