Can I use multiple blowdown valves to depressure a sour gas plant? 2

[almostbroken]

If I size a blowdown valve to depressure a sour gas plant from 1000 psig to 100 psi in 15 minutes I end up with a large flare system that is not warranted for other failures.

My question is does anyone know if I can stage the depressuring with a smaller valve that restricts the max flow to 12mmscfd for the first minute or so until the peak pressure is lost, then open a larger valve to keep near the 12 mmscfd with the lower pressure?

Almost Broken

 

[zdas04]

I can't see that as anything but a reasonable idea. I would probably size the valves so that once a valve was open it would stay open until the target pressure (e.g., open a 1-inch, when pressure drops to the next step open a second 1-inch, at the third step open a 1.5 inch leaving the other two open, all three of them is a two inch so I'd open a 2-inch next, etc). That seems like a lot better idea than trying to throttle the flow with a valve to stay under a reasonable flare capacity.

David

 

[maddocks]

Here's a few things to think about:

1.) In general, it's better to have more blowdown valves to bleed off different sections of the plant. Do NOT blowdown the amine contactor backwards (or any trayed tower for that matter). You'll be planning a long turnaround if you do that.

2.) Timed valves can be difficult to manage - especially if one of your scenario's is loss of control system power. The valves should be fail-safe - that is, they fail open on loss of air or control power. If the 2nd valve needs to be timed, you should consider using volume bottles so that the air bleeds down at a preset rate and then the valve opens when you need it to. I wouldn't depend on a pressure controlled blowdown - things don't always work the way you expect. You need the blowdown to ALWAYS work. No matter what.

3.) You can use 1/4 turn ball valves if you want and then trim them with restriction orifices to keep the flow reasonable.

Hope this helps - oversizing is sometimes necessary to ensure blowdown times fit your requirements.