PSVs Venting to Tank 1

[engg9009]

There is a PSV set at 1440psi piped to a 400bbl tank through a 2" line. The maximum gas rate through the PSV would be about 10 mmscf/d. The tank is venting through two 8" vents.

Does anyone know how I could calculate the pressure in the tank the instant that the PSV vents gas to the tank?

 

[zdas04]

I'd double check the PSV calc before I put much work into figuring out if the lid will blow off the tank. It looks to me like you are between an "E" and "F" orifice if you've talking about 10 MMSCF/day. An "E" works out to 8.9 MMSCF/d and an "F" to 12.7 MMSCF/d for a 0.7 SG gas at 80[°]F.

Then you need to decide how much pressure you feel safe putting on the tank (it should be a pretty small number).

Then you need to look at the length of the 8" vents. If it they are longer than about 3D, then you need to do a pipe flow calc using the dP you decided was safe above. If the vents are shorter than 3D then you need to do a nozzle calc with the dP to get the flow rate up the vents.

If the two vents can move significantly more than what you calculate then you are fine (i.e., the maximum pressure won't approach the safe value). When I get pushed into a corner and have to do this kind of calculation I typically use a safety factor of 2 (i.e., can each pipe move the entire volume by itself at the maximum dP?).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[engg9009]

One of the 8" vents should be able to move all the gas by itself without exceeding the maximum tank pressure (1 psig).

It just seems like with that amount of gas there might be a pressure surge greater than 1 psi the instant that the PSV pops. Does that make sense?

 

[zdas04]

Not really. A 400 bbl tank is 15' in diameter. A pretty big space to instantly fill up. I'd be careful about the direction the end of the PSV pipe was pointing to make sure that you couldn't fluid cut the tank with a high velocity stream really close to the wall, but the stream will dissipate in air within a couple of yards.

Have you ever watched the end of a vent pipe during a blowdown? It starts out kind of raggedy and then within a few seconds it coalesces into a teardrop shape with the center of the teardrop being very dense. That teardrop is formed by the standing wave that is choking the flow. If you look a foot or so past the teardrop it looks like the stream has dissipated (it hasn't, but it has slowed below 0.6 Mach and is no longer exhibiting velocity enhanced density or compressible flow). At the point that the stream seems to disappear it is no longer dangerous to steel. That point is usually within 2-3 ft of the end of the pipe. Same thing would happen in your tank.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[engg9009]

Thanks for the help

 

[BigInch]

How much backpressure did you allow on the PSV when you sized it?

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek