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Steam PSV Inlet / Outlet Pressure Drop Calculation Methodology 4

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Pavan Kumar

Pavan Kumar

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Hi All,

I am performing the inlet and outlet pressure drop for a Steam PSV ( sketch attached) set at 72.5 psig and wanted to discuss the methodology of calculating them using adiabatic compressible flow equations. Your inputs shall be valuable to me.

A) Inlet Pressure Drop:

For the inlet pressure drop I will assume the opening pressure of the PSV to be at set pressure plus 10% over pressure(P2) and then back calculate the pressure(P1) in the equipment protected using adiabatic compressible flow equation for the rated capacity of the PSV ( back calculated using the selected orifice area and vendor kd ). The pressure difference P1-P2 is the inlet pressure drop and has to be less than or equal to 3% of the PSV set pressure.

B) Outlet Pressure Drop Calculation:

The PSV vents to atmosphere through a pipe that is 23 ft long. Since the PSV vents to atmosphere the flow will reach critical pressure and since the velocity cannot exceed sonic velocity, critical pressure would achieved at the pipe exit ( P4 ). I then back calculate the pressure the PSV outlet flange(P3) using the adiabatic compressible flow equation. The difference in these pressures that is P3-P4 is the PSV outlet pressure drop and has to be less than 10% of PSV set pressure for conventional PSVs.

Please let me know what you all think.

Thanks and Regards,
Pavan Kumar


 
 https://files.engineering.com/getfile.aspx?folder=97cea6a6-8ce2-4710-9516-9c4ada99ed11&file=Steam_Reboiler_PSV_Sketch.pdf
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If I may.

A) Inlet Pressure Drop:

For the inlet pressure drop I will assume the opening sizing pressure of the PSV to be at set pressure plus 10% over pressure(P2) and then . . .

B) Outlet Pressure Drop Calculation:

The PSV vents to atmosphere through a pipe that is 23 ft long. Since the PSV vents to atmosphere the flow will reach critical pressure and since the velocity cannot exceed sonic velocity, critical pressure would achieved at the pipe exit ( P4 ).

You should noodle on the above some more.

Critical pressure/sonic velocity will exist at the exit of the PSV’s flow nozzle. There will probably be a shock wave with a pressure discontinuity between the critical pressure at the PSV flow nozzle exit and the back pressure at the PSV outlet flange. The pipe exit will most definitely NOT be at the critical pressure ratio of the sizing pressure.

Good Luck,
Latexman
 
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Comment regarding the statement that the inlet pressure loss "has to be less than or equal to 3%.."
- Understand that the 3% rule is not a sufficiently reliable predictor of instability/chatter. Refer to the discussion on this API 520 Pt II.

Comment regarding the statement that the PSV outlet pressure drop "has to be less than 10%.." for conventional PSVs.
- This statement is only true when the PSV is set at the MAWP and the allowable accumulation is 10%. A better (more generalized) statement is to say that the outlet pressure loss for conventional PSVs is limited to the amount of overpressure. That is, if the overpressure is 10%, then the outlet piping loss is limited to 10%. If the overpressure is 50%, then the limit is 50%. Etc.
 
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Hi Don,

1. For the inlet pressure drop even with pressure within 3% of the set pressure chatter / PSV instability is still cannot be ruled out?.

2. The MAWP of the equipment protected is 210 psig while the set pressure is 72.5 psig. With allowable accumulation as 10% which means the maximum pressure the vessel can see is 1.1*210 = 231 psig. With this as the basis the % overpressure would be (231-72.5)*100/72.5 =218.6%. So you are saying the allowable outlet pressure drop is 218.6% of set pressure?.

Thanks and Regards,
Pavan Kumar

 
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Hi Latexman,

How do I calculate the pressure at Pipe exit if it is not same as critical pressure ( calculated from PSV sizing pressure)?.

Thanks and Regards,
Pavan Kumar
 
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Hi All,

Can anyone provide literature that I can use to calculate pressure drop for two phase flashing flow for Condensate flashing out at the PSV orifice in the outlet line?. Your help will be highly invaluable to me.

Thanks and Regards,
Pavan Kumar
 
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You might want to review the notes and guidance for steam safety valve piping contained in ASME B31.1 - 2020 Particularly Nonmandatory Appendix II - Rules for the Design of Safety Valve Installation. Although B31.1 is primarily a Piping Code for Power Piping, this Appendix is specific to Safety Valves and contains various formulae for piping, including outlet positions. If your steam safety valve is following ASME I, this is recommended, even for ASME VIII, but you have not stated what your application is or to what code your re working to.

*** Per ISO-4126, the generic term 'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***
 
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Hi The Obturator,

The PSV is protecting the shell side of a kettle type reboiler which is generating 10 psig steam for process use using the LP steam in the tube side ( please see sketch attached ). The code of construction of the heat exchanger is ASME Sec VIII Div 1, so the PSV will have to sized per ASME Sec VIII Div 1 rules correct?.

Thanks and Regards,
Pavan Kumar
 
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For the exit loss, I use K = 1.

If exit velocity is > Mach 1, use a larger diameter pipe. (My company guidelines are "maximum discharge velocity should not exceed 75% of sonic velocity". Your company may be different.)

If [Δ]Poutlet > 10% (or > the overpressure), use a larger diameter pipe.

Good Luck,
Latexman
 
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Pavan said:
1. For the inlet pressure drop even with pressure within 3% of the set pressure chatter / PSV instability is still cannot be ruled out?.
Click to expand...

An inlet loss of 3% or less does not rule out the risk of instability. Research has shown, for example, that a PSV with an inlet loss of 2% may chatter, while one with an inlet loss of 7% may not chatter. The 3% rule is technically insufficient for predicting instability. Again, refer to the explanation and guidance in API 520 Pt II.



Pavan said:
With this as the basis the % overpressure would be (231-72.5)*100/72.5 =218.6%. So you are saying the allowable outlet pressure drop is 218.6% of set pressure?.
Click to expand...

Yes, that's exactly what I'm saying. The 218.6 % outlet loss is offset by 218.6% of overpressure. The force balance on the PSV seat is exactly the same as that for a PSV which has 10% overpressure and 10% outlet pipe loss. Engineers commonly cite a 10% limit for outlet piping loss, and that is the limit for typical cases (PSV set at MAWP and 10% allowable accumulation), but it's not an accurate way to state the limit for all cases. Saying that the outlet piping loss is limited to the amount of overpressure is the universally accurate way to state this limit.
 
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Pavan Kumar said:
Can anyone provide literature that I can use to calculate pressure drop for two phase flashing flow for Condensate flashing out at the PSV orifice in the outlet line?. Your help will be highly invaluable to me.
Click to expand...

Yes. Reference API 520, Part I, Appendix C.2.3 "Sizing for Subcooled Liquid at the Pressure-relief Valve Inlet Using the Omega Method"

This section is applicable to saturated liquids as well, and is designed for evaluated PSV capacity when flashing occurs either just upstream or downstream of the throat of the PSV nozzle. The caveat is that the fluid entering the PSV MUST be all liquid.

Pavan Kumar said:
With this as the basis the % overpressure would be (231-72.5)*100/72.5 =218.6%. So you are saying the allowable outlet pressure drop is 218.6% of set pressure?.
Click to expand...

Yes. See API 520 Part I 5.2.2.1.3. In a conventional PRV application, the allowable built-up backpressure is equal to the allowable overpressure

Just be aware of the reduction in PSV certified flow capacity with increasing backpressure.

 
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Hi TiCl4,

API 520 Part 1 provides for the Two phase PSV sizing per Omega method you mentioned. I understood that. I need methodology to calculate PSV outlet pressure drop for flashing flow of condensate in the PSV outlet line. For the inlet side the condensate is subcooled or pressurized liquid so incompressible flow equations are applicable. The fluid is two phase in the outlet side as the saturated condensate at 79.5 psig at 120 Deg C flashes to atmospheric pressure in the outlet line. I have seen literature to calculate pressure drop for two phase flow ( gas-liquid flow) when the gas is different component than the liquid. I do not have Aspen with me to calculate and need to use equations. Any help would be highly invaluable.

Thanks and Regards,
Pavan Kumar
 
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Pavan,

Take a look at API 521 5.5.10.

Daniel
Rio de Janeiro - Brazil
 
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Hi Latexman,

Latexman said:
If exit velocity is > Mach 1, use a larger diameter pipe. (My company guidelines are "maximum discharge velocity should not exceed 75% of sonic velocity". Your company may be different.)

If ΔPoutlet > 10% (or > the overpressure), use a larger diameter pipe.
Click to expand...

The critical pressure ratio for steam at 79.75 psig is 0.53 which means the critical pressure is 0.53*(79.75+14.7) = 50.2 psia = 35.5 psig. Since the atmospheric pressure is less than the critical pressure the velocity of the steam will reach sonic velocity no matter how big the line size is increased to. How do I calculate pressure drop in the PSV outlet line in this case?.

Thanks and Regards,
Pavan Kumar
 
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You are still thinking about this wrong. The flow will choke in the PSV flow nozzle. The flow passes through the PSV flow nozzle before the tailpipe, and its a lot smaller in diameter than the PSV tailpipe. You should simulate (Aspen?) the inlet line, PSV flow nozzle, and outlet line as series connected pipes in adiabatic, compressible flow to see what goes on.

If you can't do that for any reason, characterize the inlet pipe and fittings, the PSV, and outlet pipe and fittings, and I'll be glad to run it.

The tailpipe exit will probably be near atmospheric pressure. To be choked at the exit, the backpressure at the PSV outlet flange would need to be about 14.7/0.53 = 27.7 psia or 13 psig. 13/72.5 x 100 = 18% overpressure. That's way more then the 10% allowance, so the tailpipe exit will not be choked on a successful design, and, yes, the tailpipe exit will be at atmospheric pressure + 1 velocity head.

Good Luck,
Latexman
 
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Hi Latexman,

I don't have Aspen but I have AFT Arrow that can simulate Compressible flow. I will simulate and check.

Thanks and Regards,
Pavan Kumar
 
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Latexman, can you explain your statement saying the 18% was more than the 10% allowance? Backpressure allowance is 218% due to the lower-than-MAWP setpoint.
 
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TiCl4, yes, I saw that discussion between PK and don, but in the post above mine, PK is using a sizing pressure of 79.25 psig (set pressure + 10% overpressure), so that’s what I used.

Good Luck,
Latexman
 
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