Control Valves Leakage Classifications

Control valves are designed to throttle and not necessary to close 100%.

Contents

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.

Control Valves Leakage Classifications

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.

Class I

It Is also know as dust tight and can refer to metal or resilient seated valves.

Class II

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.

Class III

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.

Class IV

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.

Class V

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.

CLASS VI

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.

Types of Leakage

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.

Reasons of Valves leak

Common causes of leakage through the valve include:

Valve is not fully closed. This can be due to various reasons, including;
Valve seat is prevented from closing fully due to dirt, rust, or line debris
Insufficient actuator travel
The seat is damaged, e.g. scored
The seal is damaged

Common causes of leakage to atmosphere:

Gasket between valve-body and valve-bonnet is damaged
Stem Pakking is worn, loose or damaged

Both of them can have several causes

Details of Seat Leakage Classifications

There are actually six different seat leakage classifications as defined by ANSI/FCI 70-2 1976(R1982).

The most common used are

CLASS IV
CLASS VI

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.

Valve Leakage Classifications

The different valve leakage classifications are as follows :

Class I – Valve Leakage Classifications

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.

Class II – Valve Leakage Classifications

Intended for double port or balanced singe port valves with a metal piston ring seal and metal to metal seats.

0.5% leakage of full open valve capacity.
Service dP or 50 psid (3.4 bar differential), whichever is lower at 50 to 125 deg F.
Test medium air at 45 to 60 psig is the test fluid.

Typical constructions:

Balanced, single port, single graphite piston ring, metal seat, low seat load
Balanced, double port, metal seats, high seat load

Class III – Valve Leakage Classifications

Intended for the same types of valves as in Class II.

0.1% leakage of full open valve capacity.
Service dP or 50 psid (3.4 bar differential), whichever is lower at 50 to 125oF.
Test medium air at 45 to 60 psig is the test fluid.

Typical constructions:

Balanced, double port, soft seats, low seat load
Balanced, single port, single graphite piston ring, lapped metal seats, medium seat load

Class IV – Valve Leakage Classifications

Intended for single port and balanced single port valves with extra tight piston seals and metal to-metal seats.

0.01% leakage of full open valve capacity.
Service dP or 50 psid (3.4 bar differential), whichever is lower at 50 to 125oF.
Test medium air at 45 to 60 psig is the test fluid.

Typical constructions:

Balanced, single port, Teflon piston ring, lapped metal seats, medium seat load
Balanced, single port, multiple graphite piston rings, lapped metal seats
Unbalanced, single port, lapped metal seats, medium seat load
Class IV is also known as metal to metal

Class V – Valve Leakage Classifications

Intended for the same types of valves as Class IV.

The test fluid is water at 100 psig or operating pressure.
Leakage allowed is limited to 5 x 10 ml per minute per inch of orifice diameter per psi differential.
Service dP at 50 to 125 deg F.

Typical constructions:

Unbalanced, single port, lapped metal seats, high seat load
Balanced, single port, Teflon piston rings, soft seats, low seat load
Unbalanced, single port, soft metal seats, high seat load

Class VI – Valve Leakage Classifications

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.

The test fluid is air or nitrogen.
Pressure is the lesser of 50 psig or operating pressure.
The leakage limit depends on valve size and ranges from 0.15 to 6.75 ml per minute for valve sizes 1 through 8 inches.