why must gate vale must be open &ball valve half-open duratin in test

I assume that you are saying that, in a specific test set up, when using three different valves of the same nominal size, the apparent opening of the valves is different depending on the type of the valve (gate/globe/ball)

If my interpretation is correct, your observation is probably a result of the different pressure drops across the seat which will be specific to the internal physical arrangement of the valve. A ball valve could be the least pressure drop just because it might have a full size port which aligns precisely with the pipe, whereas the other two valves have more complicated flow paths and protrusions to create internal turbulence (pressure drop). Actually, I would expect the ball to be the best and the globe valve to be the worst.

David

David has made a very clear explanasion of what is happening but your question is not exactly detailed enough.

More details please.

Mark

Pressure drop is almost never an issue during a test, if the fluid can pass, the pressures will equilze because the rate of pressurization is much too slow for friction drops to be a factor.

A possible explaination:
- Gate valve full open (and on the back seat) to keep test pressure off the packing gland.
- Globe valve full open (and on the back seat) to keep pressure off the packing gland.
- Ball valve part open to pressurize the body cavity (full open doesn't pressurize the area between the ball and the casing).

Line is pressure tested with Ball valves in half open position so that the seats are not subjected to 1.5 times design pressure which can and do damage them.

I don't know exactly why the globe and ball valve should be kept in the position (opening) you said, but gate valve should be, always, in fully open or closed position, as any partially open opsition will increase the speed throught the valve and damage the disc and the seat, not allowing it to fully close anymore. It will leak.

The reason is to test the body of the valve. If a ball valve is fully open, the seats could prevent the body of the valve from seeing the full test pressure. Gate & globe valves don't seal the body when fully open.

Gate and globe valve must be fully open during the pressure test in order to check the "back seat" of the valve. Ball valves must be semi open in order to test the body.