Sizing PSV Header Piping

[polyroly]

Hello All,

I have just started with a new company that is in mid construction of their facility. I have been asked to size a header pipe (if it is possibly to connect 4 PSV's to a common header to discharge out of the building) or route separate discharge lines out of the building.

The PSV's have already been purchased and the set pressure is 250 psi which matches the MAWP of the vessels that they are connected to. All 4 vessels will be relieving hydrocarbon vapor in a fire scenario.

I have sized individual pipe sizes based on adiabatic expansion. I would like to know what effect the superimposed backpressure will have on each PSV if they are connected to a header, ie if PSV 'A' pops how will this impact the other 3, etc, etc.

I haven't looked at PSV's in quite some time so any help would be apperciated.

 

[pennpiper]

I have a few questions for you "polyroly"

1. Where (what country) is this project being built?

2. Was this project engineered and designed by a qualified Engineering and Design Company?

3. What are the Design Codes (OSHA, ASME B31.3, etc.) for this project?

4. Are you venting these PSVs to atmosphere? or to a closed collection system then to a Flare Stack?

5. What size are the PSVs (pipe size of the inlet and outlet)?

6. What is your background relative to piping engineering and design?

 

[polyroly]

The project is in Canada and yes it was designed by a qualified engineering firm. The design firm and my company aren't on good terms right now so I doubt I will get any help if I call them for any help. I don't have any data sheets on the PSV's so I don't know what scenario they are sized for. I just founf out that I need to verify that the PSV's that are installed are correctly sized.

I have been reading API 520 and 521 for more background info. The PSV's are venting to atmosphere and there is not flare system. The inlet and outlet sizes are 1". Yes I have experience in piping design....just rusty on PSV's

My new question is this. How do I calculate the vapor generation rate to determine that the existing relief valves are large enough.

 

[CMA010]

A 1" inlet and a 1" outlet sound like standard 1"x1" thermal expansion PSV's, not PSV's sized for an external fire scenario.

Is it in Canada allowed to discharge hydrocarbons to atmosphere?

What type of equipment (function) are we talking about here?

 

[meanderson3d]

polyroly,

You need to determine what your maximum relief rate will be for each valve and then calculate your back pressure backwards from your vent or flare. This will give you the pressure at each point in the header, which will tell you what the superimposed pressure is at each valve.

Note that in a fire scenario, if all the vessels are close together or in the same containment area, you could have a valid scenario where multiple relief valves are relieving at the same time. Make sure you examine that case too.

Andy
New Orleans, LA
Petronyx Consulting Engineers, LLC

http://www.petronyx.com

 

[SNORGY]

Fellow Canadians...pipe up if I am wrong here.

CMA010:

We don't normally like to discharge hydrocarbons to atmosphere, but I think we can be "permitted" (at least in my Province) to do so if a dispersion model proves that the ground level concentrations and total emissions in terms of tonnages per year are held within the established environmental guidelines enforced by the Regulator.

Included in the consideration of ground level concentrations are things like LEL, etc. Vapour clouds are considered a bad thing - in much the same way as they were considered a bad thing in Texas City even though sustained operation with relief valves routed to LP vent stacks instead of a proper flare system was permitted for years via some grandfathering arrangements or other.

Usually the issue we see a lot of is that landowners, aboriginals and trappers don't want continuous flaring. Some even go so far as to force - under the threat of violence - the plant to shut down if they see a flare. Coincidentally, operating companies (Facility Owners) don't really want to spend money on sweet fuel gas pipelines either (or, in the alternative, propane). Thus, with nothing to keep the pilot lit, in sweet service gas without carcinogenic aromatics in significant concentrations, PSVs sometimes get routed to atmosphere. On compressor skids, for example, sometimes (I think) PSVs and (for sure) packing eductor systems are routed to a vent pipe running out the back of the skid into the aerial cooler discharge plenum.

We also see a lot of "pop tanks" up here...typically a little API 12D/12F unit that collects high pressure relief from, say, an oil treater. The pop tank itself, in turn, is often vented to atmosphere in its vapour space but at least the oil and water are contained.

So...sorry about the long-winded answer regarding whether or not we are allowed to vent HC to atmosphere up here. Whether or not it's the right thing to do, we sure seem to be doing it a lot.

Regards,

SNORGY.

 

[polyroly]

Thank you all for your insite.

The PSV's that I am referring to are on four small pressure vessels that constitute a research and development operation. The flows are quite small and it is a batch process.

I agree with you Snorgy, we are allowed to vent HC to atmosphere, even if it doesn't seem like the right thing to do.