Relieving Rate from Infinite Compressible Flow Reservoir?
Relieving Rate from Infinite Compressible Flow Reservoir?
(OP)
Hi guys. Once again life is a cruel teacher; it makes us take the exam before we've had a chance to study. Such is life in the oil patch these days. I should know how to do this but I'm stumped.
I need to size a PSV. Therefore I need to specify the required relief rate. The problem is the source is essentially an infinite compressible flow reservoir.
The scenario is blocked discharge for the tubeside of a S/T HX. The system consists of the following: a very large 70% quality 800 psig steam header of essentially infinite capacity and a steam lateral coming off the header feeding the HX. In the lateral, in series, are an ESD valve, a restriction orifice, a letdown valve, a second ESD valve, and finally a temperature control valve. This PSV is located just downstream of the TV.
The PSV relieves to atmosphere therefore the dP across the PSV is 800 psid. The scenario is blocked discharge with all the upstream control valves stuck in the wide open position. OK that means my Cv's are all known.
Q is a function of dP and Cv. I have two unknowns so I'm stuck. Do I assume choked flow at each of the control valves using the critical pressure ratio of 0.545 for steam, and therefore that fixes the dP across each device in the lateral?
Thanks guys!! Pete
I need to size a PSV. Therefore I need to specify the required relief rate. The problem is the source is essentially an infinite compressible flow reservoir.
The scenario is blocked discharge for the tubeside of a S/T HX. The system consists of the following: a very large 70% quality 800 psig steam header of essentially infinite capacity and a steam lateral coming off the header feeding the HX. In the lateral, in series, are an ESD valve, a restriction orifice, a letdown valve, a second ESD valve, and finally a temperature control valve. This PSV is located just downstream of the TV.
The PSV relieves to atmosphere therefore the dP across the PSV is 800 psid. The scenario is blocked discharge with all the upstream control valves stuck in the wide open position. OK that means my Cv's are all known.
Q is a function of dP and Cv. I have two unknowns so I'm stuck. Do I assume choked flow at each of the control valves using the critical pressure ratio of 0.545 for steam, and therefore that fixes the dP across each device in the lateral?
Thanks guys!! Pete





RE: Relieving Rate from Infinite Compressible Flow Reservoir?
Now if the PSV set point was less than source pressure it gets a lot more difficult to calculate. For that I would start with the setpoint of the PSV and assume that it has an infinite flow rate (i.e., no build up in the upstream piping above the setpoint). Assuming that the difference between the PSV setpoint and the source cannot give me critical flow in any of the intermediate restrictions I'd set up a series of equations where P(down) of the first restriction equals P(up) of the second restriction, etc. (basically disregarding friction in the pipe at this time). That gives you a maximum mass flow rate. You can either use that (conservative) or use that mass flow rate to calculate friction in the pipe to determine if it is material (it will be if the PSV setting is well below the source pressure). Use either the mass flow rate from the valves or the mass flow rate represented by both friction and valve losses to pick a PSV size and orifice.
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.
The plural of anecdote is not "data"
RE: Relieving Rate from Infinite Compressible Flow Reservoir?
You have an 800psig supply, but a relief valve set at 800 psig downstream of three restriction devices, just doesn't make sense. If you said 400 psig then you have something to work with. Given that most spring reliefs are no more accurate than 10% on set pressure it might not go off.
Before I go much further, can you sort this issue out for us please.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Relieving Rate from Infinite Compressible Flow Reservoir?
David Simpson, PE
MuleShoe Engineering
Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat
RE: Relieving Rate from Infinite Compressible Flow Reservoir?
The required PSV set pressure is 200 psig. Thanks guys! Pete
RE: Relieving Rate from Infinite Compressible Flow Reservoir?
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Relieving Rate from Infinite Compressible Flow Reservoir?
RE: Relieving Rate from Infinite Compressible Flow Reservoir?
David Simpson, PE
MuleShoe Engineering
Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat
RE: Relieving Rate from Infinite Compressible Flow Reservoir?
If its more than 600 then you won't have choked flow and then iterate backwards to find the same mass flow which has the sum of the pressure drops equaling 600, based of course on which item follows which so you use the right pressure and density. ZDAS04 sets it out better than me.
Difficult to do manually so I think you need an analysis program.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Relieving Rate from Infinite Compressible Flow Reservoir?
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Relieving Rate from Infinite Compressible Flow Reservoir?
David Simpson, PE
MuleShoe Engineering
Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat
RE: Relieving Rate from Infinite Compressible Flow Reservoir?
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Relieving Rate from Infinite Compressible Flow Reservoir?
RE: Relieving Rate from Infinite Compressible Flow Reservoir?
I've used SiNet (EPCON) for more complex than average flow problems. But like zdas04 my past experiences with it lead me to believe that on a compressible flow case with three successive chokes, first, I'd be surprised if it converged, and second, I'd be very skeptical if it did!
My company has an in-house program that would make short work of it. It assumes ideal gas behavior, but the flow equations are rigorous for high speed gases, and I've never seen it go crazy on choked flow conditions or multiple chokes. You have to know what you are doing so the right "fudge factors" (flow coefficients) are used when needed, but I believe it would do it. This doesn't really help sshep, but the right program's do exist, they just may not be in the public/mainstream domain.
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.