control Valve Leakage Classifications

          In many process applications, it is important that the control valve be able to completely stop fluid flow when placed in the “closed” position. Although this may seem to be a fundamental requirement of any valve, it is not necessarily so. Many control valves spend most of their operating lives in a partially-open state, rarely opening or closing fully. Additionally, some control valve designs are notorious for the inability to completely shut off (e.g. double-ported globe valves). Given the common installation of manual “block” valves upstream and downstream of a control valve, there is usually a way to secure zero flow through a pipe even if a control valve is incapable of tight shut-off. For some control valve applications, however, tight shut-off is a mandatory requirement. For this reason we have several classifications for control valves, rating them in their ability to fully shut off. Seat leakage tolerances are given roman numeral designations, as shown in this table

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^-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, 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.

Typical constructions:

• Unbalanced, single port, soft seats, low load

                   It is from this leakage test procedure that the term bubble-tight shut-off originates. Class VI shut-off is often achievable only through the use of “soft” seat materials such as Teflon rather than hard metal-to-metal contact between the valve plug and seat. Of course, this method of achieving bubble tight shut-off comes at the price of limited operating temperature range and the inability to withstand nuclear radiation exposure.