The term “Control Valve Seat Leakage Class” comes from ANSI / FCI 70-2. This is a control valve seat leakage specification Standard.
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Valve Leakage Classifications
Class I - Valve Leakage Classifications Identical to Class II, III, and IV in construction and design intent, but no actual shoptest 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 125oF.
• 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.
• Balanced, double port, soft seats, low seat load
• Balanced, single port, single graphite piston ring, lapped metal seats, medium 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 oF.
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, 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.