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    Leakage Classification of Control Valves

    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. Seat Leakage Classifications There are actually six different seat leakage classifications as defined by ANSI/FCI 70-2 2006 (European equivalent standard IEC 60534-4). The most common 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 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. 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, 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 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 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.


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