Seat Leakage Classifications

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.
The type of valves for instance.
A double seated control valve has 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.

There are actually six different seat leakage classifications as defined by:

The most common used are CLASS IV and 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 Vl 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

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 II - Valve Leakage Classifications

Intended for double port or balanced single 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, 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 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 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 Vl - Valve Leakage Classifications

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.