why must globe & gate vale must be open duratin in test

[qeqe]

in piping we full open the gate & and globe valve
after that we test the line
why?

 

[PAN]

This might be your test package for test whole system. You can test each line without pass through the valves. However, you have to use more times for testing.

 

[TheBlacksmith]

What do you mean by open fully; as opposed to shut or partially open? Some valves have to be backseated when open to prevent leakage around the valve stem.

 

[ahmedsabry1976]

under any condition can we tie flat face flange to raised face flange

 

[PAN]

ahmedsabry1976,

Using raised face flange with flat face
thread378-13410

Flat face and raised face
thread378-29732

 

[TerryOsser]

qeqe - because if the valves where closed the line pressure might not reach the rest of the line. Gate valves are isolation valves. Globe can have tight shut off. I guess you want to pressure test the whole line???

 

[22082002]

I assume that you are checking a line where globe and gate valve come in that section.
You cannot belive that these two valves even if they are adjacent, will hold the pressure. No matter how tight you close, they will pass and they do pass. Even if these two valves pass two drops of water, you can see noticable pressure drop. If you need very small leak through these valves, you will need mention, specially, tight-shut-off valves and class..., depending on the leak you expect.

So, it is best to keep these two valves and do the test, so you are sure whole segment is subjected to test pressure.