Valve Leakage Classes

Some times the specifications are provided as such, But it is always advisable to go for API 598 for non-control valves.

Dear Nithya

First of all Leakage Stantard ANSI/FCI 70-2 do not state that class VI is exclusive for soft seated control valves. FCI 70-2 states that Class VI is "associated" with soft seated valves. There are control valve manufacturers advertising metal to metal plug/seat meeting class VI leakage rating.

FCI 70-2 states that the standard is for control valves only. It should not be used with manual or automatic block valves. Unfortunately manufacturers and project engineers ignore this reccomendation.

For block valves you should use API 598, API 6D or MSS-SP-61.

API 6D is used for pipeline valves and is complemented by ISO 5208.

The above standars do not have leakage classes like FCI 70-2.

For example, API 598 Standard publishes tables informing maximum allowable leakage related to body size and trim material within a specific test time limit.

Recommend purchase API598 standard for more information on other important test requirements.

Best regards

David Livingstone Villar Rodrigues

Control valves have actuators that provide the force to open/close and seat. The seating force is calculated and tested against standards to provide the closure to meet the leakage class, based on the trim configuration. A handwheel only valve usually can provide much more (probably damaging with a valve wrench) force than the engineered actuator forces required for stable plug movement and seating. The manual seating force is uncontrolled so repeatability to a standard is not possible. The same is true when a handwheel override is used with the control valve. Seating to standards is out the window.

NEVER use an automated control valve for positive isolation.