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why must gate vale must be open &ball valve half-open duratin in test
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For more information, please visit LCB chlorine globe valve.
(Mechanical)
(OP)
10 Jan 03 02:38in testin the line in test package why gate&glob must be open and ball valve half-open?
(Petroleum)
10 Jan 03 08:46I 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
(Mining)
10 Jan 03 09:10David has made a very clear explanasion of what is happening but your question is not exactly detailed enough.
More details please.
Mark
(Mechanical)
12 Jan 03 12:41Pressure 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).
(Mechanical)
13 Jan 03 16:56Line 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.
(Chemical)
15 Jan 03 07:05I 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.
(Petroleum)
24 Jan 03 09:24The 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.
(Petroleum)
3 Mar 03 15:52Gate 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.
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Any valve that refuses to fully shut off won't be improved by tightening the handle further.
The usual problem with a Globe Valve is that the rubber valve seal has aged and cracked, needing replacement. This means you have to shut the water off upstream of the valve and remove the body cartridge so the rubber seal can be changed out.
Some valves have a globe shape, but get misidentified by that shape because they actually are Gate Valves which have a vertical wedge shaped metal gate that gets pushed down into a tapered slot. These fail because sediment accumulates in the slot on the downwards side and the gate no longer can be pushed fully home so the taper seals. This requires the water to be turned off upstream so you can take the top off the valve, extract the gate and grub the sediment out using a piece of welding rod so it will flush out when you reassemble the valve.
I've gone over to using quarter turn ball valves when I have to go to this much trouble. They are better than Globe Valves because the opening is the same size as the inner diameter of the pipe when the valve is full open, the water doesn't have to turn 90 degrees to flow through a restriction as it does in a Globe valve and while the gate valve has the same lack of restriction as the ball, the ball valve has no slot for rust, sand and sediment to accumulate in while it spends most its life wide open.
If you are looking for more details, kindly visit Y Type Bellows Seal Globe Valve.