why must globe & gate vale be isolated from system in test(in the

[qeqe]

when we test the one test package ,sometimes in the end line we have gate&globe valve , we isolated the valve from line and cap the line.why cannot we close the valve and dont use cap.

 

[Hush]

Presumably the test pressure is higher than the working pressure of the valve. To test across the valve sealing mechanism above operating pressure risks damaging the valve and voiding your warranty.

 

[zdas04]

Few people allow testing against a closed valve (especially a throttling valve) because inherent leakage past valves is irrelevent to the strength test and the resulting pressure drop in the test is unacceptable in the final result. Basically, valves may leak (some don't many do) and throttle valves (like globe valves) leak worse than block valves. You don't want to have to pay for a re-test when the only problem is a valve that isn't guarenteed to be bubble tight.

 

[abeltio]

Another consideration is... what are you testing?
1. Package Test
If you are testing the integrity of the package itself, a warranty issue with the package vendor is one thing.
Is the valve OUTSIDE THE BATTERY LIMIT of the package? i.e. the valve was not supplied by the package vendor... then they do not want to have their package test affected by a leaky valve.
2. System Test
If you are testing the whole system and you cannot afford to have leaks then you have to RETEST after the valve was re-installed (two flanged joints that were not tested yet).

May be the system test is outside the scope of your vendor so plant personnel may have to do the last test...

HTH
Saludos.
a.