Understanding ANSI Leakage Classes
10.22.2025 |
The ANSI Leakage Classes document outlines definitions and allowable leakage rates for control valves from Class I through Class VI. These classifications guide valve selection, testing, and performance expectations. Use the links below to jump to specific class definitions and notes:
Leakage Class Overview
Control valves are designed primarily for throttling and not always to fully seal under all conditions. The ANSI standard defines six seat leakage classifications (Classes I–VI) that specify the maximum allowable leakage under test conditions. Class IV (metal-to-metal) and Class VI (soft-seat) are the most commonly used.
Classes I, II & III
Class I
Class I is equivalent in design intent to Class II, III, or IV, but no specific test is performed. It is sometimes referred to as “dust tight.” It applies to either metal or resilient seated valves.
Class II
For designs such as double-port or balanced single-port valves with metal piston ring seals and metal seats. The allowable leakage is 0.5% of full-open valve capacity. Test medium is air at 45–60 psig.
Class III
Same valve types as Class II. The allowable leakage is 0.1% of full-open capacity. Test conditions mirror Class II (air, same pressures).
Class IV
Class IV is intended for single-port or balanced single-port valves with extra tight piston sealing, metal-to-metal seating, and high seat loads. The allowable leakage is 0.01% of full-open capacity.
Test conditions: air medium at 45 to 60 psig, or operating differential pressure (whichever is lower), within a temperature range of 50 to 125 °F.
Class V
Class V is intended for valves similar to Class IV types. The test medium for Class V is water (or operating fluid), generally at the operating pressure.
The leakage limit is specified as 0.0005 ml per minute per inch of orifice diameter, per psi differential.
Class VI
Class VI is known as the soft-seat classification. In this class, the seat or shut-off member (or both) are made of a resilient material (e.g. PTFE). The test medium is air or nitrogen.
The test pressure is the lesser of 50 psig or the operating pressure differential. Leakage limits vary by valve port size, from 0.15 to 6.75 ml/min for valve sizes 1 to 8 inches. Larger sizes have scaled allowances.
For example:
| Port Size (inches) | Leakage (ml/min) |
|---|---|
| 1" | 0.15 |
| 1½" | 0.30 |
| 2" | 0.45 |
| 2½" | 0.60 |
| 3" | 0.90 |
| 4" | 1.70 |
| 6" | 4.00 |
| 8" | 6.75 |
To summarize
The ANSI leakage classes provide a structured way to quantify how much leakage is permissible for different classes of control valves under specified conditions. While Classes IV and VI are most commonly used, understanding all six helps in making informed decisions about valve design, testing, and specification.
For full details, you can download the original ANSI Leakage Classes PDF.