9 replies to this topic

Dear all,
I have a pressure regulator set at 100 kPaG (Cv=0.06), the outlet pressure range specified in the catalog is 0-50psig. This is diaphragm type. I am trying to calculate the maximum flow rate of this regulator during its failure mode. So I need to know the inlet pressure and the outlet pressure of the fluid. The problem is the outlet pressure. Is it the maximum or minimum outlet pressure that is specified in the catalog or else ?? The regulator is connected with a 1/4" inch tube with length of 100m ?
Your prompt reply is highly appreciated.
Thank you

[Art Montemayor]

Your description doesn't make for good sense.

You say you are regulating a pressure. This, in conventional communication terms means that you are taking a high pressure gas and regulating it to a lower, constant pressure. So how can it be that you have a regulator set for 100 psig and with an outlet pressure specified as 0 - 50 psig? This doesn't make any sense and obviously your information is wrong or you are interpreting information in a wrong way. You must get your basic data correct and accurate.

If you are trying to calculate the maximum flow rate of this regulator during its failure mode, then all you have to know is its Cv value and the driving force through the regulator - the pressure drop. You probably will have sonic flow if the ratio of the inlet pressure to the outlet pressure is more than 2:1. Do you have any concept or understanding of what the Cv is or what it represents?

Await your response.

Dear Art Montemayor,
Thank you for your reply.
Please kindly re-read my post, I did write that my regulator is set at 100 kPaG (not 100 psig as you mentioned above).
What I know about Cv is that it measures the efficiency in allowing the flow of fluid in a device and is calculated by the formula: Cv = Q*sqrt(SG/dP). I know the inlet pressure but not sure about the outlet pressure. If the pipe is connected to an atmospheric vent, so is the outlet pressure the atmospheric pressure ?
Best regards

[fallah]

Atmospheric pressure+DP due to moving fluid friction

[latexman]

kdthinh212,

K_regulator = 891d⁴/Cv² = 967
using d = 0.25"

K_tubing = fL/D = 0.034(328 ft.)(12in./ft.)/(0.25 in.) = 535

The tubing is significant to the final flow that will flow through the system. The system includes everything from the source of the gas to the atmospheric vent.

Consult the technical literature of your regulator vendor to determine the maximum flow through the regulator. Fisher has good technical literature on this. Second choice would be to refer to Crane Technical Paper 410, available for sale on the internet (Google it). The inlet line to the regulator and outlet line from the regulator to atmosphere can be handled by methods in Crane Technical Paper 410. There are several ways this problem can be solved. One is to use the information from the inlet line, regulator, and outlet line to characterize a single, compressible flow system. Two, you can treat the inlet, regulator, and outlet line as three separate compressible flow systems and you act to converge the results where these three system meet. Method two will be more accurate, but will require more work.

[Qalander (Chem)]

Dear Latexman Hello/Good Evening,

I must appreciate your appropriate guiding info to kdthinh212; as this sets the way forward.
Best regards
Qalander

[shan]

Hi kdthinh212,

I have to presume you know the process conditions of the valve upstream. Otherwise, there is no solution for the problem.

There is no such thing “maximum” flow through a failed valve. If your upstream process conditions, valve Cv, and piping configuration are fixed, there is only one flow to satisfy all the conditions. Otherwise, please inform me what your “minimum” flow through a failed valve is.

OK. Now the question is how we calculate the flow.

1. The easy way is to run a Flarnet simulation. It will take you less than 5 minutes to get the results if you know how to run the software. You may apply a blowdown valve as your pressure regulator and use flare headers as your piping system.

2. The tough way (actual not that tough) to conduct a hydraulics calculation. A) Assume a flow rate; Back calculate pressure drop from the piping outlet (atmospheric pressure) for each elements involved (piping, valve etc.) C) If the calculated valve upstream pressure is equal to the known valve upstream pressure, you get the answer. If not, please start from A) again. I will not give you more than 30 minutes credit to get the correct answer if you build all the calculation formulas into an Excel to do the try and error.

Regards

Shan

Thanks latexman, Qalander and shan for your informative reply but I am still a bit confusing...

Ok so here is my system: A process pipe is partly withdrawn by a sample probe to an analyser system. Between the take-off point and analyser system, I have a pressure regulator close to the take off point. And then, it is connected by 1/4" inch stainless steel pipe of 100 m length to a relief valve for safeguarding purpose (The relief valve has Cv of 0.1 and is set at 300kPaA). After passing through the relief valve, the sample fluid then enters to the analyser. My boss suspects that if during regulator failure mode, the so-called "maximum flow through the regulator" would exceed the flow capacity of the pressure relief valve. (The diverted fluid will pass through the ATM vent). So he wants me to compare the "maximum flow rate through pressure regulator" and the flow capacity of the relief valve at that condition to check whether it is satisfactory or not.

Let's say if the inlet pressure to regulator is at 5795kPaA and 21oC, the regulator Cv is 0.06 and set at 100kPaG, the relief valve Cv is 0.1 and set at 200kPaG. The fluid is natural gas. If I want to calculate the flow rate during regulator failure mode, I need to consider the Cv of both regulator and relief valve and friction loss of the pipe. Is it right? What about the setting level of the relief valve, does it affect my calculation?
What do you think about flow capacity of the relief valve. I found out by assuming the inlet pressure of the relief valve is process pressure (upstream pressure of the regulator). However, it doesnt seem correct since pressure drop along the regulator and pipe is also significant

Best regards

kdthinh212

[latexman]

kdthinh212,

You have a compressible flow problem that may look something like this:

NG source -> inlet line to regulator -> regulator -> 100 m of 1/4" line -> safety valve -> outlet to atmosphere

You know the P and T of the NG source. You know the characteristics of the system and that it discharges to atmosphere. We have given you excellent literature and software references. Your college text on fluid flow would be a good reference too. Now it's up to you to solve the problem. If you have a mentor at work or know a more experienced engineer in your company that has solved similar problems, I advise you to seek them out. But first, on one sheet of paper draw out a PFD of the problem, label it clearly and correctly, and document all the data you know on it, because "a picture is worth a thousand words". Basically, you want to solve the compressible flow problem and see if the pressure and temperature of the NG in the line between the regulator and safety valve exceeds the regulator's or line's or PSV's design basis or the Code (Maximum Allowable Working Pressure) that governs them. If it does, your boss was right. If it doesn't, it's fine.

I don't think anyone on this forum is going to solve the problem for you. If you have some specific questions, we'll be glad to advise you, but the rest is up to you. Oh yeah, if you do have some specific questions, please post that one page PFD with it. Thanks and

[Qalander (Chem)]

kdthinh212,

You have a compressible flow problem that may look something like this:

NG source -> inlet line to regulator -> regulator -> 100 m of 1/4" line -> safety valve -> outlet to atmosphere

You know the P and T of the NG source. You know the characteristics of the system and that it discharges to atmosphere. We have given you excellent literature and software references. Your college text on fluid flow would be a good reference too. Now it's up to you to solve the problem. If you have a mentor at work or know a more experienced engineer in your company that has solved similar problems, I advise you to seek them out. But first, on one sheet of paper draw out a PFD of the problem, label it clearly and correctly, and document all the data you know on it, because "a picture is worth a thousand words". Basically, you want to solve the compressible flow problem and see if the pressure and temperature of the NG in the line between the regulator and safety valve exceeds the regulator's or line's or PSV's design basis or the Code (Maximum Allowable Working Pressure) that governs them. If it does, your boss was right. If it doesn't, it's fine.

I don't think anyone on this forum is going to solve the problem for you. If you have some specific questions, we'll be glad to advise you, but the rest is up to you. Oh yeah, if you do have some specific questions, please post that one page PFD with it. Thanks and

Dear kdthinh212 Hello,
I must say that at times one image/flowsheet can say/express it all better than many other narrations to invoke correct/accurate response; as identified by latexman indeed.
Best regards
Qalander