PSV Inlet and Outlet Pressure Drop Calculation Question

[Pavan Kumar]

Hi All,

I sized a PSV downstream of a Control Valve for its wide-open failure case and got a 6"Q8" PSV. The required relief rate is 254107 lb/hr with Oxygen as the fluid. The rated capacity of the PSV is 294,851 lb/hr with 10% overpressure. I tried to size the inlet/outlet lines to this PSV. The PSV inlet line is 2 ft long and the outlet pipe is 4 ft long with an elbow. The PSV is to be installed on the 6" line downstream of the control valve with option to move it to the 12" line.


View attachment 10578

My questions:

1. The PSV Inlet pressure drop should be calculated from the source of pressure, in this case from the control valve outlet flange or can I calculate it from the reducing inlet to PSV Inlet?.

My Calculations : (i) When I used 6" inlet line taking off 6" line the inlet pressure drop is 13 psi, which is 4.8% of the PSV Set pressure. I used AFT Arrow to do these calculations. If install the PSV on the 12" line and use 8" inlet with a 8"X6" reducer at the PSV inlet I still 4.8% inlet DP. Also this calculating the pressure drop from the TEE inlet and not the Control Valve outlet flange. Is there an option that you can suggest to reduce this DP to less than 3%. I can go with a Pilot Operated PSV option whose capacity is not reduced by the inlet line pressure losses. But that is an expensive option and I want to avoid it if possible as the body and trim will be made of Monel for Oxygen service.

2. For the outlet side, I used a 8"X12" expander, followed by 12" Sch 10 pipe that is 4 feet long and calculated the pressure drop to be 7.2% of the PSV Set pressure. The velocity is sonic across the PSV, becomes sub-sonic in the outlet line after the 8"X12" expander and reaches sonic at the pipe exit. I want to know if this acceptable. If I use 10" pipe then the pressure drop goes to 9.5% of set pressure. My AFT Arrow model screens shot for the 12" pipe option is shown below. You can see that even though I want 0 psig at the pipe exit, the model achieves only 0.42 psig. If I use 10" pipe then it reaches 6.7 psig (result pasted as well).

I also did calculations using my spreadsheet using isothermal compressible equation and I get 4% for the Inlet pressure drop and 22% for the outlet pressure drop. My excel calcs are close for the inlet side(4% vs. 4.8%) but there is large difference for the outlet side ( 22% vs. 7.2% ). My spreadsheet is attached also. I did this to cross-check AFT Arrow calcs. I wanted to understand why these are so different.

12" Outlet Pipe Option



10" Outlet Pipe Option:




Thanks and Regards,
Pavan Kumar

 

[Snickster]

Your attachments won't open.

 

[georgeverghese]

PSV inlet line : Standard practice would be to include the control valve exit into the inlet line dp calcs.

PSV exit line : Standard practice is to keep exit line velocity at <0.7Mach for brownfield applications, preferably <0.5Mach for greenfield, even if your exit line dp < 10% of PSV set pressure at process design relief flow.

 

[Pavan Kumar]

Your attachments won't open.

Please check attached.

 

[Pavan Kumar]

PSV inlet line : Standard practice would be to include the control valve exit into the inlet line dp calcs.

PSV exit line : Standard practice is to keep exit line velocity at <0.7Mach for brownfield applications, preferably <0.5Mach for greenfield, even if your exit line dp < 10% of PSV set pressure at process design relief flow.

Thanks George.

Pavan Kumar

 

[Snickster]

My Calculations : (i) When I used 6" inlet line taking off 6" line the inlet pressure drop is 13 psi, which is 4.8% of the PSV Set pressure. I used AFT Arrow to do these calculations. If install the PSV on the 12" line and use 8" inlet with a 8"X6" reducer at the PSV inlet I still 4.8% inlet DP. Also this calculating the pressure drop from the TEE inlet and not the Control Valve outlet flange. Is there an option that you can suggest to reduce this DP to less than 3%. I can go with a Pilot Operated PSV option whose capacity is not reduced by the inlet line pressure losses. But that is an expensive option and I want to avoid it if possible as the body and trim will be made of Monel for Oxygen service.

If you install a 12" x 12" tee for the branch take-off then you will only have a 0.5 psi pressure drop in the connection since the tee branch connection pressure drop will be based on the velocity in 12" branch not 8" branch. Then with 12" x 8" reducer (12 ft equiv. L) and 2 feet of straight 6" pipe without need for reducer you will have less than 4.5 psi drop in this section for a total of 5.0 psi drop in inlet not counting the pressure drop due to increase in velocity from 12" to 6".

Also I am not getting 13 psi drop in inlet based on your configuration. I only get about a 6 psi drop with 12"x8" branch tee then 8" x 6" reducer plus 2 ft of 6" pipe. This is using 10 feet equivalent length of 12", 40 feet of 8" (for 12"x8" branch tee) and 10 feet of 6" pipe (8"x6" reducer plus 2 feet straight pipe). Maybe I have a mistake somewhere but I can't find it.

 

[Snickster]

I was just looking at your inlet calculations. I think what you did wrong is that you included all pipe pressure drop under one calculation based on 6" line size. If you do this then the tee branch does give about 9 psi drop as you show. However, if you do a separate calculation for tee branch based on 8" size then the pressure drop through the tee is only about 3 psi. Then a do a separate calculation for the pressure drop through the reducer and 2.5 ft. of 6" pipe based on 6" pipe size and velocity.

 

[Snickster]

Looking at your outlet calcs I see you made a mistake by including one 90 deg elbow in calculation of the 8" pipe segment - it is already accounted for as 12" in calc sheet 2. I also see some mistakes in your calcs for acoustic velocity value = (gkR(2/(k+1))T)^0.5 not what you show. Also for 12" the outlet pressure is not 14.7 psia as you assumed since velocity is sonic at exit so pressure will be above atmospheric at tip of pipe (and keep mach no greater than 0.8 for vent to atmos.). I will continue checking tomorrow if I have time.