Piping inlet loss on PSVs on long header

[Ammonianeer]

I just recently ran into a problem I had not considered before:

I am working on specifying a PSV for a new ~3000ft 8" 900# steam line. The MAWP of the line is 1108psig. The line is supplied by a steam system operating at 1500 psig (via a let down FV), and feeds a system with a MAWP of 990 psig. The 990 psig system does not currently have any relief valves installed that are not upstream of non-return valves.

The difficulty is that the downstream side of the line operates at 900 psig and approx. 60 psi is required to overcome the frictional losses of the 3000ft long line. Therefore, if the PSV's are installed on the upstream side (at the source) and set at 990 psig they would have to operate at ~97% of set pressure (900+60, too high). If they are installed on the downstream (lower pressure) end of the line the inlet loss from the supply valve (FV) to the PSV during a blocked flow scenario would be much >3%. Note that the upstream side would not be over-pressured under this scenario (MAWP 1108 psig), but the downstream valve would likely chatter.

There is a line in API 520 dealing with this, requiring the pressure loss due to the incremental flow generated by the PSV, rather than the total flow in the pipe, to be <3%. This increment would vary with operating conditions (normal flow will vary greatly) and could be large (90% of rated capacity), so I do not think this helps.

Does anyone have any advice / suggestions on how to deal with this scenario? Are there any other codes / standards / practices to deal address this type scenario?

Thanks!

 

[JoeWong88]

Ammonianeer,
I have read your post few times. However, i have difficulties in imagining your system.

Engineer like drawing than wordind. Can you please prepare a sketch for your system and mark the necessary information in the sketch to ease understanding ?



JoeWong
Chemical & Process Technology

 

[MMII]

Ammonianeer,

what is clear is that the pipe that you want to protect is the 990psig one. So the PSV should be installed in this pipe.

The 3% indicated in the API is referred to the DP between the piece of equipment to be protected (in this case the 990 psig line) to the PSV itself. So in your case this 3% do not apply to the DP from the FV to the 990psig pipe (3000ft), forget about it.

A different problem is to calculate the flowrate to be discharged. To calculate it, yes, you have to consider the DP in the 3000ft line.

I hope this helps you.

 

[Latexman]

It’s not clear to me whether the 990 psig MAWP system is protected by relief valves, or not. JoeWong is right; a sketch would have been nice.

I would protect each system independent of the others. The pipe can be protected by a relief valve at the most upstream position to minimize inlet pressure drop. It can be set at or near 1108 psig to get some room between the operating pressure (960 psig) and the set pressure.

The 990 psig MAWP system should be protected by its own relief valve. Hopefully, this system has enough volume to it compared to the relief flow that it does not cycle too frequently. I assume its relief inlet is designed so it will not chatter (dP < 3%).


Good luck,
Latexman