mengh2
Mechanical
- Nov 12, 2015
- 4
Hello all,
I was asked to run some calculations on a PSV that was suspected to be installed with an undersized discharge line. I will be reviewing with our PE when he's free later in the month but wanted some theory double checking.
Specs -
Conventional spring-loaded PSV
Hydrogen
Set pt 13,500 psig
0.250" orifice
Direct mount on pressure vessel (negligible inlet pressure drop)
Discharge to atmosphere 12.1 psia - total length 92", ID 0.62", 1 elbow, 1 branch-run tee, top terminates with a tee and longhorn vent (5" length in each direction)
Ambient temp on warm day (540 R)
References: ANSI API 521, AlChE "Sizing Pressure Relief Devices"
Calculations: Spreadsheet segmented calculations following API 521 7.3.1.3.3, with graphical table lookup from Fig. 14, recalcuating Z at each
Results - 38,866 SCFM H2 choked flow across PSV at 10% overpressure (matches manufacturer table)
Sonic chokes at both longhorn vent exits (Pcrit=316 psia)
Sonic choke at tee from main vent mast (Pcrit=1507 psia)
4672 psia at PSV outlet
So my questions here are:
The backpressure/(set pressure + overpressure) is at 31%. API 521 states that it should remain at 10% for conventional PSVs but the AlChE manual (Figure 7) does not show any reduction in capacity until about 55% backpressure for conventional PSVs in vapor service, and it is still under the Pcrit for choked flow across the PSV. What would the negative effects of having ~30% backpressure be on PSVs?
Does the secondary choke make sense in this context? I can see how there would be a choke at the vent exits (sonic boom as it exits the mast) but does the calculated result of another choke point at the tee (where the flow splits into two paths) mean that I need to re-iterate the calculations with a lower flow rate?
Cross posted to ChE PSV forum. Thanks all.
I was asked to run some calculations on a PSV that was suspected to be installed with an undersized discharge line. I will be reviewing with our PE when he's free later in the month but wanted some theory double checking.
Specs -
Conventional spring-loaded PSV
Hydrogen
Set pt 13,500 psig
0.250" orifice
Direct mount on pressure vessel (negligible inlet pressure drop)
Discharge to atmosphere 12.1 psia - total length 92", ID 0.62", 1 elbow, 1 branch-run tee, top terminates with a tee and longhorn vent (5" length in each direction)
Ambient temp on warm day (540 R)
References: ANSI API 521, AlChE "Sizing Pressure Relief Devices"
Calculations: Spreadsheet segmented calculations following API 521 7.3.1.3.3, with graphical table lookup from Fig. 14, recalcuating Z at each
Results - 38,866 SCFM H2 choked flow across PSV at 10% overpressure (matches manufacturer table)
Sonic chokes at both longhorn vent exits (Pcrit=316 psia)
Sonic choke at tee from main vent mast (Pcrit=1507 psia)
4672 psia at PSV outlet
So my questions here are:
The backpressure/(set pressure + overpressure) is at 31%. API 521 states that it should remain at 10% for conventional PSVs but the AlChE manual (Figure 7) does not show any reduction in capacity until about 55% backpressure for conventional PSVs in vapor service, and it is still under the Pcrit for choked flow across the PSV. What would the negative effects of having ~30% backpressure be on PSVs?
Does the secondary choke make sense in this context? I can see how there would be a choke at the vent exits (sonic boom as it exits the mast) but does the calculated result of another choke point at the tee (where the flow splits into two paths) mean that I need to re-iterate the calculations with a lower flow rate?
Cross posted to ChE PSV forum. Thanks all.
