8 replies to this topic

[irprocess]

Hi Everyone,

As I looked over in P&ID drawing for gas refinery,I noticed that for Flow control valve with fail to close state and also it's size grater than 4" needs to consider vent/drain valves in upstream and downstream of FCV.I want to know is there any standard for this or it is related to project criteria!

Thanks and Regards

[Zauberberg]

I don't know about codes but common sense dictates to provide drain/vent connections on both sides in case of a fail-close valve. Imagine that you have to repair the valve, and prior to repairing you need to remove it from the system. If there were no drain/vent connections on both sides of the valve, how would you drain the liquid inventory and depressurize the section in between the two isolation valves? The control valve will be in fully closed position, thus completely separating the upstream and the downstream section.

For a fail-open valve, one drain/vent connection is sufficient - following the same logic.

[irprocess]

Dear Sir,

I think, the pressure of piping/pipeline should be considered as important item for considering vent/drain connection in both side of FCV and also this is applicable for rating upper than 150#.

[Evgeny]

Dear Zauberberg

In my practice we always put only 1 drain valve for the control valve.
Could you explain in which cases you applied 2 drains on each side?
Thanks

Evgeny

[Zauberberg]

Hello Evgeny (Russian?),

Imagine the following scenario: the control valve is in high-pressure, or highly toxic service. And the valve has left stuck in fully closed position. You have to isolate the valve, depressurize both sections (in between the control valve and isolation valves, on both sides), and remove it for maintenance. Now, if the valve is stuck closed, there is no chance to depressurize/deinventory both sections with a single drain/vent connection. I had a look into bunch of P&IDs here in the plant and I see the same concept. And it seems to be perfectly logical to me.

[Evgeny]

Hi,

yes, im russian

I agree that you would need bleeding valves on both sides to drain before you replace the valve.

On the other hand I find it (valve arrangement) totally depends on the client's approach to safety, opetating etc.

If they are willing to design all FC valves based on worst case scenario when they would need to replace valve during operation then yes - 2 bleeding valves. And I agree -its much easier than isolate whole section.
However what I have seen and I checked on many PIDs normally only 1 bleeding valve is applied. And it seems there is no logic (at least from process view) but only safety thoughts.

So, after I analyzed it my conclusion is there is no logic and it very varies from plant to plant.

But I would like to hear other observations and thoughts on the matter


Evgeny

[djack77494]

But I would like to hear other observations and thoughts on the matter

Evgeny,
I have always specified drain or vent valves on both sides of a fail closed control valve that could be isolated and accessed without a total shutdown. I could accept omitting the second valve for maybe low pressure air or water services where any risk would be minimal, but doing so could lead to confusion as others might see that arrangement and draw the wrong inferences from it. Might that be your situation? Anyway, when dealing with hazardous or flammable fluids or with high temperatures or pressures, the small cost of an additional 3/4" valve is well spent.

[Evgeny]

But I would like to hear other observations and thoughts on the matter

Evgeny,
I have always specified drain or vent valves on both sides of a fail closed control valve that could be isolated and accessed without a total shutdown. I could accept omitting the second valve for maybe low pressure air or water services where any risk would be minimal, but doing so could lead to confusion as others might see that arrangement and draw the wrong inferences from it. Might that be your situation? Anyway, when dealing with hazardous or flammable fluids or with high temperatures or pressures, the small cost of an additional 3/4" valve is well spent.

Hi djack
I doubt it is the case as I see it from as-built PID of the plant. And in 95% valves its 1 drain valve.
I will check it out later with operators of the plant and let you know.

I know 3/4 drain valve is really tiny and nothing costing thing to continue this discussion. I am just used to see a reason behind each (even tiny) detail and to work out most reasonable solution based on everyone's experience.

Thanks
Evgeny

[Himanshu Sharma]

Sir,

I would like to mention that Control Valve vent and drain arrangement varies from Project to Project.This is a usual point while discussing Design Basis of the refinery.

For what i have seen there are two usual arrangements.

1.Control valves with Drains/vents on both side.
2.a)Fail Close Control Valve having drain on upstream side.
b)Fail open Control Valve having drain on downstream side.

Option 1 is pretty much discussed and clear.
For option 2,Draining is done from Low Point Drains/High Point Vents provided by piping.

As i am writing this it just clicked me that control valves are normally at grade and since line has to progressively rise towards any equipment there is bound to be a "U" formation where in Piping must provide a Low point drain !!!

Now it boils down to having another drain in control valve assembly or low point drain (a few meters away)...choice is yours !

In usual engineering practices you show all drains of CV but low point drains are there on isometrics only,thus there is drain nearby you just cant see it on P&ID!!!