Hi Latexman, the elevation difference is 34.5 ft.
Pavan
Okay.
Are you designing for a certain sizing flow, or are you evaluating an existing 3J4 PSV to see what it will do? (I suspect the latter is the case.)
Hi Latexman,
I am validating the existing PSV by re-analyzing the overpressure scenarios.
The existing PSV is 3L4 but as per my calculations I find a 3J4 orifice is sufficient. We do not need an oversized PSV.
As per my study the closed liquid outlets and Failure of Liquid Level control valve + By pass valve are the two applicable scenarios. Both these scenarios lead to two phase relief and the Control valve failure scenario is the sizing case. The fluid is subcooled condensate at PSV inlet but two phase in the PSV outlet. I need to evaluate if the fluid is flashing across the orifice or in the downstream outlet line which is were I am stuck right now and need help. I will try and alternate method based on the paper “Discharge of Two Phase Flashing Flow in a Horizontal Duct” - JC Leung & MA Grolmes as attached at the earliest and check the result against the API 521 Section 5.5.10 method.Discharge of Two Phase Flashing Flow in a Horizontal Duct - JC Leung & MA Grolmes.pdf unless you have some better suggestions. (258.1 KB)
Thanks and Regards,
Pavan Kumar
What’s the sizing case flow rate? 124,525 lb/hr? Knowing this will help once I’m back in the office next week.
The basis for my HEM program is:
- Simpson, L. L., Chem. Eng., pp 98-102, Aug. (1991).
- Simpson, L. L., Navigating the Two-Phase Maze, “Proc. of International
Symposium on Runaway Reations and Pressure Relief Design,” G. A. Melhem and H. G. Fisher eds., AIChE/DIERS, New York (1995).
Hi Latexman,
The required relief rate for the sizing case ( Control Valve scenario ) is 119362 lb/hr. The mass flux assuming there is no back pressure is Gmax=5954.2 lbm/ft2-sec. With 3J4 orifice the rated capacity is 124525 lb/hr. But this will be different if the back pressure is more than the pressure of 29.45 psia which is the pressure at which the mass flux maximized. Once I determine the back pressure I will re-evaluate the mass flux and required orifice area which in-turn will lead me to the different rated capacity and different back pressure. After a couple of iterations I will be able to get the correct PSV size.
Thanks and Regards,
Pavan Kumar
Great. I think about the only thing missing is a characterization of the inlet pipe and fitting to evaluate the inlet pressure drop. What’s the diameter (3" Sch. 40?), length, and fittings in the inlet line. My program will evaluate that too.
Hi Latexman,
The inlet pipe is 3" Sch 40 with a length of just 2’10" with no fittings except for the sharp edged pipe entrance. I evaluated the inlet pressure loss using Darcy Equation as the condensate is subcooled water at the PSV inlet and got a DP = 0.4 psi which is 0.55% of PSV set pressure of 72.5 psig. It will be great if you can verify all the calculations with your software, especially the outlet DP.
Thanks and Regards,
Pavan Kumar
Okay, sounds good. My plan next week is to run my PSV program, which detects 2-phase transitions and gives warnings, but does no 2-phase dP calculations, and, then, run my HEM program on two separate components, the PSV nozzle and the outlet pipe/fittings. Depending on what happens between the PSV nozzle and the tailpipe, I’ll converge between them, and let you know what I find. This shouldn’t take too long; the programs crunch most of the numbers. I just have to characterize the water properties at 2 or 3 data states (probably 3 with subcooled liquid) so the HEM program can interpolate while it is integrating. So a little isethalpic steam table work on my part.
That will be great. But are you going to verify the PSV outlet line pressure drop for two phase flashing flow?.
Thanks and Regards,
Pavan Kumar
Yes, yes. That’s what HEM does.
Hi Latexman,
I can send my Excel workbook which I used to analyze my scenarios and the PSV sizing which has all the information that you may need for the calculation. Do you want me to send it?.
Thanks and Regards,
Pavan Kumar
I have the two spreadsheets you uploaded to the site in the thread above. If it’s different from those, please do. If not, no need.
Hi Pavan @pxyarala ,
First I calculated the backpressure at the PSV outlet flange with rated flow using my program HEM:
Estimate of backpressure with 3 data states (Model F).
Lengths, diameters and losses for pipes.
Mass flow rate = 124525 lb/hr
Surrounding pressure = 14.70 psia
Pipe length = 45.92 ft
Inside diameter = 4.026 in
Total turb. fitting VH losses = 1.20
Total laminar fitting losses = 0
Elevation difference (up is +) = -34.50 ft
Pipe sand roughness = 0.00180 in
========================== PROPERTIES ==========================
DATA STATE A B C
========== ======= ======= =======
Pressure, psia 94.45 28.80 14.70
Wt. fraction gas 0.00000 0.00001 0.03757
Gas density, lb/ft3 0.22690 0.07003 0.03731
Liquid density, lb/ft3 58.890 58.875 59.830
================================================================
PIPING RESULTS:
Upstream press. = 27.35 psia
PIPE EXIT:
Pressure = 28.81 psia
Thrust / area = 14.7 psi
Velocity = 6.6 ft/s
Wt. frac. gas = 0.00000
Friction f = 0.01702
Next, I evaluated the “installation” using my program SafSiz:
CASE:
Liquid expands and vents through a bellows Spec. K pressure relief valve.
Relief device flowrate was specified.
DESIGN CONDITIONS:
Set pressure = 72.5 psig; Back pressure = 12.6 psig
Overpressure = 10 %; Input sizing temp. = 248.0 F
INSTALLATION RESULTS:
Nozzle area = 1.287 in2; Rated coeff. = 0.650
Inlet pressure loss is 0.6 psi, based on the actual flowrate.
Flowrate (lb/hr) = 125987 (rated) 139986 (actual)
Installation-rated flowrate for bellows liquid PRV = 125562 lb/hr
Maximum replacement valve capacity = 272.3 gpm water at 70 F.
PHYSICAL PROPERTIES:
Properties based on WATER at 248.0 F and 94.4 psia.
Liquid density = 58.89 lb/ft3;
Use HEM to obtain a Stored Energy estimate, if required.
WARNING:
Flashing may occur in the discharge piping. If so, evaluate with HEM.
ADDITIONAL DATA:
=================== PIPE & FITTING SUMMARY ===================
Inlet
=========
Diameter, in 3.068
Length, ft 2.8
Reynolds number 2.585E+05
Fitting fric. factor 0.0189
# Contractions 1
# 45 degree elbows 0
# 90 degree elbows 0
# Gate Valves 0
# Rupture disks 0
Other Vel. Hd. losses 0.00
Total fit. VH losses 0.33
================================================================
Lastly, I modelled the PSV in my program HEM to double check there is essentially no flashing at the exit of the PSV’s flow nozzle:
PSV Sizing for Two-Phase Flashing Flow
Estimate of flow rate with 3 data states (Model F).
Areas and coefficients for nozzles.
Reservoir pressure = 94.45 psia
Surrounding pressure = 14.70 psia
Nozzle area = 1.287 in2
Nozzle coefficient = 0.6500
========================== PROPERTIES ==========================
DATA STATE A B C
========== ======= ======= =======
Pressure, psia 94.45 28.80 14.70
Wt. fraction gas 0.00000 0.00001 0.03757
Gas density, lb/ft3 0.22690 0.07003 0.03731
Liquid density, lb/ft3 58.890 58.880 59.830
================================================================
REAL NOZZLE RESULTS:
Mass velocity = 97042.0 lb/hr/in2
Mass flow rate = 124893 lb/hr
Stored energy = 219.16 Btu/ft3
NOZZLE THROAT:
Pressure = 28.80 psia
Thrust / area = 70.2 psi
Velocity = 67.0 ft/s
Wt. frac. gas = 0.00002
I call 0.00002 wt. fraction gas, essentially no flashing. So, everything looks good to me. Any questions?
Hi Latexman,
Thank you very much for taking time in performing the HEM calculations for me in your software. I am really very grateful to you.
I have some questions regarding the results. Kindly help me understand the results when you have some time.
-
Under piping results: I see the upstream pressure as 27.35 psia, and the pipe exit pressure as 28.81 psia. That means P1 at pipe entrance is 27.35 psia. Then what is pressure at the pipe exit. Under the pipe exit I see pressure as 28.81 psia. Is this pipe exit pressure?. If so how can this be higher than pipe upstream pressure. What is the PSV outlet piping DP?. Also I forgot to mention there are 90 degree LR elbow - 4 Nos in the pipe. Can you please add these into the outlet piping and re-run the program. My calculation per API 521 5.5.10 showed upstream pipe pressure as 37.22 psia while the downstream pressure as 26.37 psia, which seems to wrong compared to your results.
-
Under PSV Sizing : I see that the nozzle/throat pressure as 28.80 psia. Is this pressure at PSV nozzle exit. And you say that the pipe entrance pressure is 27.35 psia. How come both these pressures are different?. Shouldn’t they be same?.
I am sorry I could not understand how to interpret the results. My questions may sound silly but it would be really helpful if you explain the results to me.
Thanks and Regards,
Pavan Kumar
- Under piping results: I see the upstream pressure as 27.35 psia, and the pipe exit pressure as 28.81 psia. That means P1 at pipe entrance is 27.35 psia. Then what is pressure at the pipe exit? 28.81 psia. Under the pipe exit I see pressure as 28.81 psia. Is this pipe exit pressure? Yes. If so how can this be higher than pipe upstream pressure? Remember, there is a 34.5 ft. drop in elevation from PSV exit nozzle (pipe entrance) to pipe exit. What is the PSV outlet piping DP? The pressure gain at the pipe exit due to elevation only = 58.88*34.5/144 = 14.1 psi. This means the frictional losses in the pipe are = 14.1 - (28.81-27.35) = 12.64 psi. Also I forgot to mention there are 90 degree LR elbow - 4 Nos in the pipe. Can you please add these into the outlet piping and re-run the program. See the “Total turb. fitting VH losses = 1.20”, that’s 4 x 90 degree LR elbows, or 4 x 0.3 = 1.2. Also, the exit loss is built into my program, so I did NOT add in a K = 1 for that. My calculation per API 521 5.5.10 showed upstream pipe pressure as 37.22 psia while the downstream pressure as 26.37 psia, which seems to wrong compared to your results. I don’t think you corrected for elevation.
- Under PSV Sizing: I see that the nozzle/throat pressure as 28.80 psia. Is this pressure at PSV nozzle exit. Yes. And you say that the pipe entrance pressure is 27.35 psia. How come both these pressures are different?. Shouldn’t they be same?. Yes, and no. First, I calculated backpressure on the PSV using HEM. BP = 27.35 psia = 12.65 psig. Secondly, I inputted this fixed BP = 12.65 psig (program rounds to 12.6 psig) into SafSiz, with no tailpipe, because I calculated BP of tailpipe already with HEM. So, the PSV calculation (SafSiz) and the BP calculation (HEM) are converged. The third is just a double check / sanity check that I did to see if HEM (two-phase calculation) and SafSiz (single-phase calculation with warnings if two-phase envelope is close) agreed on the PSV calculation only. Notice on the third, BP was not entered. So, on this third, given that HEM (two-phase calculation) predicted a more water-like fluid, higher pressure, at the PSV nozzle exit (28.80 psia) than Safsiz (single-phase calculation) was told by the BP calculated by HEM (#1 and 27.35 psia/12.65 psig) , I am confident with SafSiz handling the PSV calculation. Therefore, for converged and consistent results, use the first and second program results. The third just confirms two-phase did not develop in the PSV nozzle.
Hi Latexman,
I forgot to ask you if Safsiz estimated the required orifice area using the required relief rate of 119362 lb/hr at 79.75 psig relieving pressure and 248 Deg F relief temperature. Did it estimate that “J” orifice with API orifice area = 1.287 in2 is required?.
Also it will be really helpful if you can share a copy of the paper “Easily size relief devices and piping for two-phase flow” by J.C Leung.
Thanks and Regards,
Pavan Kumar
No, the “Sizing” section of SafSiz uses API areas and defaults for flow coefficients, not PSV specific/certified numbers, and it selected a “K” orifice. I did not copy the sizing section to the website, as I did not want to confuse. The “Installation” section uses your PSV specific/certified numbers that I manually entered, and the installation is adequate, 125987 > 119362.
I’ll look for that article when back in the office Monday.
Hi Latexman,
Can you please help me the paper “Easily size relief devices and piping for two-phase flow” by J.C Leung, CEP, December 1996, pages 28-50. The copy that I have the pages from 36-40 missing. I want to use this paper to calculate the PSV outlet line pressure drop.
Easily Size Relief Devices and Piping for Two Phase Flow - JC Leung.pdf (2.0 MB)
Sure, I asked our Information Resources Manager to obtain it for me today. I want it for my library too.
Did you correct for elevation?
If not, then a comparison of our numbers with elevation correction is:
| Backpressure | Exit Pressure | |
|---|---|---|
| psia | psia | |
| pxyarala | 37.22-14.1 = 23.12 | 26.37 |
| Latexman | 27.35 | 28.81 |
Not too bad, IMO. Both say the PSV is adequate.