Leakage Classification of Control Valves

Seat Leakage Classifications

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

The most commonly used are

  • CLASS IV
  • 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 VI 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 Class IV in construction and design intent, but no actual shop test is made. Class I is also known as dust-tight and can refer to metal or resilient seated valves.

Class II – Valve Leakage Classifications

This type of valve is intended for double port or balanced single port valves with a metal piston ring seal and metal to metal seals.

  • 0.5 % leakage of full open valve capacity
  • Service dP or 50 psid ( 3.4 bar differential ), whichever is lower at 50 to 125 Deg F.
  • The test medium is at 45 to 60 psig is the test fluid.

Typical Constructions:

  • Balanced, single port, single graphite piston ring, metal seat, the low seat load
  • Balanced, double port, metal seats, high seat load.
  • Class II Valve Leakage Classification:
  • Class II Valve Leakage Classification:

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 Deg F.
  • The test medium at 45 to 60 psig is the test fluid.

Typical constructions:

  • Balanced, double post, 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 Deg F.
  • The test medium at 45 to 60 psig is the test fluid.

Typical constructions

  • Balanced, single port, Polytetrafluoroethylene (PTFE) piston ring, lapped metal seats, the 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/inch of orifice diameter /psi differential
  • Service dP at 50 to 125 Deg F.

Typical Constructions:

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

 Class VI – Valve Leakage Classifications

Class VI is known as a soft classification. Soft Seat Valve is 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
  • The pressure is the lesser of 50 psig or operating pressure.
  • The leakage limit depends on valve size ranges from 0.15 to 6.75 ml/inch for valve sizes 1 through 8 inches.
Leakage from 1 Inch to 8 Inch port diameter of valve