High Pressure Drain Into A Low Pressure Closed Drain 2

[Mikey M]

Greetings,

I do have a question for my experienced and knowledgeable engineers. I am working on designing a gas processing plant. The gas comes in at a very high pressure (1532 psig) into a choke valve, where the pressure is reduced to 675 psig and the processing continues.

My question has to do with a drain line from the high pressure side going into a low pressure closed drain. There is a closed spectacle blind between 2 ball valves that are supposed to be closing off the low pressure side. There is a spec break after a ball valve from 600# to 150#. I have attached a schematic diagram for illustration.

In my limited experience, I've always installed an RO on the drain line to protect the low pressure drain. In this case, the RO is not installed. I have read that if it is a manual draining scenario that this arrangement is permissible.

Can anyone tell me if the arrangement without the RO is correct and if so what is the logic.

 

[georgeverghese]

You're right, a thick plate RO would be better to make this draining operation safe. Without this RO, even if this draining operation were to be carried out under supervision / work permit, there is no guarantee the LP closed drain system wouldnt be overpressured. Unless you can show by calculation, that the wide open flowing capacity with the combination Cv of both ball valves in series cannot overpressure the LP CD system.
Obviously, if both ball valves are close to each other, both of these should be good for the lowest temp seen during letdown from 1532psig.
My process department HoD once told me more high flying process engineers have had their careers ruined by goof ups on drain systems design than with any other system.

 

[LittleInch]

Can anyone tell me if the arrangement without the RO is correct and if so what is the logic.

Well not without more data such as sizes, pressure, flow capacity of the drain and its vent, when is it used etc. Is there an open path from the connection to atmosphere?, i.e. no valves which could be closed? Any risk of hydrates forming?

It's a pretty classic HAZOP type question / comment / action "Consider flow capacity of the closed drain system at full flow through the drain to avoid overpressure...", basically as George puts it.

But running a class 600 system at 1532 psig is interesting. The latest ASME B16.5 doesn't go above 1500 psig for any material below 100F and can be lower. ??

Relying on procedures to not open the drain until the pressure in the main header is below the class 150 rating is something which has moved away from normal and acceptable to unacceptable due to various cock-ups and errors in operation and people not following the said procedure.

The drain system should be exactly that - a drain and not a pressure let down system. That's the purpose of a vent system. The trick is making sure process B (draining) follows process A (venting and de-pressurisation) or making sure if it happens the other way around you don't destroy your drain system...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MortenA]

Well i often see that the 2. valve is a globe or a needle valve. But since you show two ball vales i think that the RO would be appropriate (in addition the the spec. blind - although could the spec blind be the RO (not having a full ID compared to piping ID?)

 

[mk3223]

As one of the "positive" mechanical isolation configuration, the double blocks with a blind is acceptable by operation for depressurizing the system during the system shutdown process. And, a bleed valve is missed between two ball valves.

 

[georgeverghese]

There is no vent line shown anywhere on this sketch, so I got the impression that this drain could be used as a high pressure drain. Hence the previous response concurring with your suggestion for an RO on this HP drain.

If there is a vent line some where along this piping, typical draining procedure would be to vent / depressure first, and then only drain down. Where mandatory operating practices are in place for this 2 step operation to be be done under PtW, an RO is not required. In such cases, an RO may actually impede / block the drain line since there is little diff pressure between the system being drained and the LP drains system.