PSV lower than the Flare Header

[Weko]

Hi,

I am designing a CNG plant. The PSV on the compressor package and CNG tube package will be supplied inside the skid. So the PSV will be lower than the flare header.

Is it ok if the PSV is lower than the flare header?

 

[Chance17]

For many reasons - this is a poor enginering design
which should not be openly endorsed.
I suspect it does not meet "CODE".
But it is not an absolute "killer bullet".
Just make sure the PSV design (process & mechanical)accounts
for a standing liquid level in the outlet pipe.
If you can influence elevation of the PSV - you should make the change.

 

[BigInch]

Not if the vented gas is supposed to go to the flare. Hopefully the gas that is supposed to be going to the flare won't come to the compressor psv.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[4Pipes]

I would suspect blasting a slug of liquid down the flare system might be a killer factor. Slug flow events have severely damaged more than than one flare system.

 

[racookpe1978]

I have done no relief valve calc's that go to flare systems, so forgive my ignorance.

What is the "proper" configuration: A relief valve that is higher than the flare inlet, with the valve outlet piping configured to drain its output down to the flare? Seems like that would allow burning fluids back down the pipe after the relief valve closes.

 

[Weko]

Thanks all for the input.

After internal discussion. We can have an option to put a "sub"flare header. With pocket drain for the liquid.



But the CNG compressor vendor and the CNG Tube Skid vendor said that they normally discharge the psv gas to open air...

that made me wonder, what condition does a gas allowed to be discharge to the air.

 

[BigInch]

Gas can be vented to a "Safe" location in "open" air. It does not always have to go to a flare. Compressor in a building is usually vented up and out a stack, perhaps above the roof line, as long as the outlet is away from ignition sources.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[Weko]

So how do I decide it is safe (far away from the ignition source)?

Is there any standard that tells the minimum safe distance?

If we can eliminate the flare system, it surely will be more cheaper.

 

[BigInch]

You need to see, or make, an electrical area classification drawing, in which a classification is determined for each point or area on the plot plan according to it's potential ignition hazard. A gas vent located above the roof for example, might be given a Class I, Div I (i don't know, just made that up) because it could be hazardous to operate an open electrical switch within 50 feet of it. A sphere of 50 ft diameter would be drawn around that vent point signifying that any electrical apparatus placed within would have to be rated for Class I, Div I use, further meaning it should have enough protective devices or an enclosure sufficiently tight to prevent sparks or heat sources from igniting any gas that happens to be expelled from that vent. In your case you would draw a sphere of diameter equal to whatever distance is specified and if you have no ignition sources within that distance, you could locate the vent there.

See this for OSHA guidelines on the subject.

http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_id=10708&p_table=STANDARDS

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[Weko]

thx a lot for the link