Pipe Spool With Flanges of Different Classes at both Ends

[Abdude]

Good day to all,

I just joined a plant where we have both of a pumps suction and discharge main header's equipped with drains to atmosphere that are long enough to be classified as dead legs.

A design was made to connect a piping spool with different flange classes on each end to the flanges of the drains valves. The pressure differential between the suction and discharge of said pumps is large (hundreds psi).

I'm an inexperienced engineer and am concerned that said design is risky and will not handle the pressure differential of introducing the discharge flow into the suction line of those pumps.
Please help in answering the following questions:

- Is it an acceptable design and safe design to have a piping spool with 2 different flange classes on each end?
- What would be a pressure regulating device of justifiable cost for a service that will be operated occasionally? (Once in 2-4 months)

Comment: Welding shorter drains is out of the question, as a shut down would incur heavy production costs.

 

[zdas04]

As long as your safety devices protect the lowest rated component, then you can use any flange class you want to. I have frequently installed spool pieces with ASBM B16.5 Class 600 flanges on one end and Class 150 flanges on the other end. As long as I have protections in place for the Class 150 flanges it is all good.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[mk3223]

Instead of guessing any reason, suggest a P&ID could be handy for you to find out what's the process Pressure and temperature the pipe spool is to be operated, which may give you a engineering basis of the different flange ratings on both ends of the spool.

 

[LittleInch]

Well drain valves are not intended to be opened when there is flowing fluid at pressure in the system.

Normally with drain systems you have to make sure that the drain line is essentially open ended into a vessel or similar before you can do what you suggest.

Normally the HAZOP would have considered this potential operation and either added noted to the P&ID or determined that there wasn't a Hazard. You sometimes get spectacle blinds, Locked closed valves or similar or there is no potential for overpressure ( on further valves which could be closed before you get to atmosphere.

Sometimes you will get a pressure break marked on the P&ID immediately downstream of the drain valve, but because the drain valve itself is usually the same flanges on both sides, then the mating flange needs to be a higher rating, but that's Ok.

We would need to see a P&ID to make more comments.

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

 

[racookpe1978]

Granted, the "design" is set up so the long pipe acts as a "throttling device" and thus the "end of the pipe" is not ever expected to be high pressure.

But.

What happens when the drain valve is NOT opened (due to error, failure to operate, stuck shut, or busted controller or no power (fails closed) or is frozen in place (cold, corrosion, solidified process fluid, maintenance error, etc, etc, etc. The entire pipe upstream of the drain valve (any possible plug or freeze point) must be rated for the full system pressure, or be relieved off with a safety valve.

 

[MickMc]

If my understanding of intent is correct is to add spectacle blind at least at higher pressure side so positive isolation an be effected in event of any failure, it can be turned during draining.