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Actuated Ball Valves (Shutdown Valves) Seat Tightness

Seat tightness of the valves has a very close relation with a leakage class per ANSI/FCI 70.2. Some oil &

gas project specified the requirement of leakage class in their shutdown valve (actuated ball valves) and

some of them specified the requirement of seat tightness per BS 6755 PART I (superseded by BS EN 12266-1).

The leakage class per ANSI/FCI is specified in term of Class I to Class VI, while seat tightness per BS 6755

is specified in term of Leakage Rate from Rate A to Rate G. So which one that we should use for shutdown valves,
per ANSI/FCI or BS 6755?
First of all, there is no debate regarding the leakage class in the control valves. Almost all control valve datasheet

specified leakage class per ANSI/FCI. But for shutdown valves, there is a debate whether we specify per ANSI/FCI

or per BS 6755. The main idea to select the most suitable leakage class or seat tightness is the main purpose of the valves.


A control valve is mainly use for throttling or modulating services, while the shutdown valve is use for isolation services.

The control valve isn’t designed to isolate the plant which will be exposed to high differential pressure.

But shutdown valve is designed to expose in the high differential pressure. For example, when we need to shutdown

a high pressure suction scrubber, then we need to close all shutdown valves in the upstream and downstream of the vessel.

While the vessel is isolated by this valves, the downstream or upstream pressure of valves might be loss or goes to

atmospheric pressure. Thus the shutdown valves will hold this high differential pressure.


The ANSI/FCI 70.2 leakage class is obtained by applying a test procedure with the upstream pressure of the valves is

around 45 to 60 psig and the downstream pressure is let to atmospheric pressure. On the other hand, the BS 6755 seat

tightness rate is obtained by applying a test procedure with the upstream pressure of the valves is minimum 1.1 of the

design differential pressure. From this different test procedure, we can select and conclude that the most suitable standard &

codes to specify the seat tightness or leakage class for shutdown valve is BS 6755 or BS EN 12266. Why? Because

ANSI/FCI leakage class isn’t intended for leakage rate at high differential pressure applied to the valves,

while the BS 6755 (or BS EN 12266) does that.