Control valves are designed to throttle flows and they are not necessary able to close 100% with no leakage.

The shut off ability has to do with type of valve. Double seated control valves have very poor shut off capabilities. In addition - the guiding, the seat material, the actuator thrust, the pressure drop and the type of fluid all play part in how well particular control valves shuts off.

Seat Leakage Classifications

There are actually six different seat leakage classifications defined by ANSI/FCI 70-2 2006 (European equivalent standard IEC 60534-4).

The most commonly used are

• CLASS IV
• CLASS Vl

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 a metal seat.

CLASS Vl is known as 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 Polytetrafluoroethylene (PTFE) or similar.

Valve Leakage Classifications

Class I - Valve Leakage Classifications

Identical to Class II, III, and IV in construction and design intent, but no actual shop test is made. Cass 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 oF
• 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 125 oF
• 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 125 oF
• Test medium air at 45 to 60 psig is the test fluid

Typical constructions:

• Balanced, single port, Polytetrafluoroethylene (PTFE) 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 10-4 ml per minute per inch of orifice diameter per psi differential
• Service dP at 50 to 125 oF

Typical constructions:

• Unbalanced, single port, lapped metal seats, high seat load
• Balanced, single port, Polytetrafluoroethylene (PTFE) piston rings, soft seats, low seat load
• Unbalanced, single port, soft metal seats, high seat load

Class Vl - Valve Leakage Classifications

Class Vl 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 Polytetrafluoroethylene (PTFE). 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