Location of drain valve at control valve

[PAN]

In general, our existing plant installs control valve with by-pass. There is also a drain valve upstream the control valve location.

However, our consultant indicate drain valve location at downstream of the control valve for our new project. Is this the general design practice? Any comment or advice for my reference is appreciated.

 

[SeanB]

If your control valve has a bypass you probably have block valves on either side of the control valve. Design practices I have read state that you only require one drain on the upstream side. However, in the case of high pressure steam or hazardous fluid you should also have one on the downstream side.

Normally I put a drain on both sides on the control valve between the control valve and the block valves. This is my preference and my clients. Drain valves tend to plug with dirt and debris so I like having a redundant valve.

 

[Montemayor]

Pan:

Your description is not clear. I'll have to assume that you mean that there is a "bleed" valve installed between the two block valves - which are downstream and upstream of the control valve (CV). When you say "There is also a drain valve upstream the control valve location", this could be anywhere upstream.

The reason for a drain (or bleed) valve is to depressure the fluid that remains between the two block valves around the control valve. This is a safety (& practical) necessity. Some engineers rely on the normal position of the CV to presume that fluid might be trapped if the CV is normally closed (NC). I never rely on this presumption. The bleeding off of the trapped, pressurized fluid can be done either downstream or upstream of the CV, between the block valves, by manually actuating the stem action to ensure that the valve opens and the two sides of the CV are equalized. This will allow you to bleed the fluid on either the downstream or upstream side with confidence that no residual pressure exists within the two block valves upon removing the CV.

Additionally, I would add the caveat that if the fluid is toxic or hazardous, then you should have two (2) positive bleed valves: one downstream and one upstream of the CV and both within the block valves. This ensures that all fluid is positively drained before disengaging the CV for removal.

I hope this helps.

 

[KenA]

I've seem general practice to provide a single drain (or bleed) if the control valve is fail open and two drains (one upstream and one downstream of the CV) if the control valve is fail closed.

 

[SmartEngineer]

Additional to above responses, I think that the need of a bleed valve also depends on the piping route as some bleeds are provided even with control valve stations as to drain accumulated or trapped liquid in the piping.
Also, one bleed valve either downstream or upstream is good enough for doing the intended purpose of it (i.e. depressuizing, puging)
Most of the applications I have seen only one bleed valve is usually provided for control valve station.

Hope this would help

Cheers
SmartEngineer

 

[djack77494]

Also, don't forget that the bleed valves can function as liquid drain valves. Depending on the system particulars, this function could make it desirable to locate the bleed on one side or the other or both sides of the control valve.
Doug

 

[sshep]

Hey PAN,

In general I like the convention of having the bleed valve downstream of the control valve, especially if the bleed goes to a hard piped collection system. This is based on alot of start-up experience where the process must be temporarily rerouted (i.e. via hose or hardpiped drain system), such as downstream problems, contaminated process, water run operations, chemical cleaning, steaming-outs, draining, etc. In these cases the control valve-bleed valve combination gives control ability at the panel that does not exist with a simple bleed valve in the field. This might seem a minor consideration, except that I have done it so very often. In-fact in plant start-ups where I find an upstream bleed convention is used, I have often used the trick of lining up the process to flow through the bypass and then backwards through the control valve to achieve the same effect (this requires another block valve downstream of control valve station).

For depressuring it seems basically unimportant which side the bleed is on. If the valve does not open from the panel, you can generally handjack the valve or manually put air pressure to the actuator. This is true for almost every situation except maybe for the rare case of stem breakage in a high pressure flow down valve.

Of course two bleeds is always great for pressure survey's, closed loop sampling, and such!!!

best wishes,
sshep

 

[PAN]

Montemayer,
Sorry for my unclear description. You are right. The control valve is installed between two block valves.

I'm not sure about the comment from HAZOP review. Should we install the drain valve in high pressure side between the two block valves for all cases?