opening/closing of valve in HP service 1

[allegro5]

Could it be problemmatic to operate the valve (ballvalve)when the pressure is shut in the system either side of the valve of if shut in only one side. I mean would this affect the movement of the ball.

We have a liquid service operating at 1500Lb rating

Does it require a bleed drain in the system to depressurize the systm in some circumstances.???????????

Would be gratful for any advice

 

[jsummerfield]

How large is the pipe? Which problem concerns you?

If automated, the actuator needs to be large enough to open and close the valve with full differential pressue. The piping system needs to accommodate the hammer from opening and closing the valve.

Many large gas applications use a pressuring valve across a main block valve. Perhaps this would not apply to liquid. Drains are a normal thing in the piping system.

John

 

[StephenA]

The valve manufacturer should tell you, but there does not seem much point in a valve that cannot be used at the working pressure.

The bleed on gas main valves is to avoid the valve freezing as it is closed or opened and the gas expands on one side.

You might need to depressurise the system to change the valve.

 

[allegro5]

The sizes are 6 and 10 inch. I dont really understand the mechanics of this potential problem, but the valve in question is a pig receiving/loading valve that has a safety mechanism to stop it opening when loading the pig. There are isolating valves either side which obviously need to be closed when working on the valve. I have read somewhere that lock in can occur because of the high pressure and it may become difficult to operate the valve. I dont know if this related to the handwheel operation.

 

[odem]

A substantial pressure difference between the upstream and downstream side of a valve can make it difficult to open a valve. On most mainline block valves (MLBV) a bypass or equalization line (consisting of a smaller valve on either side of the MLBV and some associated piping) is constructed around the MLBV to allow for some equalization around the valve before it is opened, thereby making it easier to operate the valve.

Let me explain, if you have full line pressure (1500psig) on one side of the ball, slab gate, or expanding gate and 0psig on the other side, the pressure actually forces the ball, gate, etc. toward the seat on the low pressure side, this is what cause the valve to contain pressure or product. Vavles are actually designed to work off of differential pressure. If the pressure is high enough the friction can get so great between the seat and the ball that it can be hard to operate. Now, if you have some way to equalize the pressure around the valve the friction will decrease and the valve will be eastier to open. I hope that made sense.

In the situation you described, you shouldn't have a problem if you have some sort of equalization around the trap valve. In most cases you are going to want to equalize the pressure in the pig launcher/recever prior to launching the pig. If you don't, you can actually cause th pig to "burp" back up into the l/r rather then send it down the line.

Hope that helps.


ODEM