Sirius P.Eng.
Chemical
Adequacy check of existing relief devices (2No. 8T10s set @17.7 barg; conventional)for LPG storage spheres confirms they are grossly oversized in the event of governing external fire scenario.
Using API model I calculated relief load of 79,833 kg/hr. Relief fluid z factor is around 0.6691 indicating non-ideal behaviour. However, I used API 521 sizing equations for ideal gas and selected a 1No 4P6 as adequate for overpressure protection.Certified rated capacity is 26.90%. Aspen Flare System confirms back-pressure is over 30% so a balanced type valve was selected. I reckon a dynamic simulation may even result in lower relief load. I am guided by the general acceptable overcapacity is between 25-30%.
I passed my results to a Contractor/Consultant for validation/verification.
Using the superior HEM Method, they calculated relief load is ~89,087.3 kg/hr. Now they proposed 2 options:
1. 1No. 6Q8 with valve lift restriction to about ~44.2% overcapacity.[restrict orifice area up to ~52 cm2 from full lift orifice area of Q (71.29 cm2)].
2. Insulate Spheres
Provide 1” thick foam glass (thermal conductivity is very less ~0.055 W/m∙K) conservatively as a fire proof insulation on listed sphere with environmental factor of 0.3 per Table 5 API 521 to mitigate oversize concern.
o Required Orifice area: (~26,730 kg/hr; 11.96 cm2)
o Selected Orifice area: (~ 41,160 kg/hr ; 18.406 cm2 considering full lift of 4L6 orifice; API area)
o Relief valve is ~54% oversized
In this case also, restricted lift will be required for lower 4L6 size.
Given this, which option should be considered considering a whole bevy of technical safety factors and cost.
Thanks.
Using API model I calculated relief load of 79,833 kg/hr. Relief fluid z factor is around 0.6691 indicating non-ideal behaviour. However, I used API 521 sizing equations for ideal gas and selected a 1No 4P6 as adequate for overpressure protection.Certified rated capacity is 26.90%. Aspen Flare System confirms back-pressure is over 30% so a balanced type valve was selected. I reckon a dynamic simulation may even result in lower relief load. I am guided by the general acceptable overcapacity is between 25-30%.
I passed my results to a Contractor/Consultant for validation/verification.
Using the superior HEM Method, they calculated relief load is ~89,087.3 kg/hr. Now they proposed 2 options:
1. 1No. 6Q8 with valve lift restriction to about ~44.2% overcapacity.[restrict orifice area up to ~52 cm2 from full lift orifice area of Q (71.29 cm2)].
2. Insulate Spheres
Provide 1” thick foam glass (thermal conductivity is very less ~0.055 W/m∙K) conservatively as a fire proof insulation on listed sphere with environmental factor of 0.3 per Table 5 API 521 to mitigate oversize concern.
o Required Orifice area: (~26,730 kg/hr; 11.96 cm2)
o Selected Orifice area: (~ 41,160 kg/hr ; 18.406 cm2 considering full lift of 4L6 orifice; API area)
o Relief valve is ~54% oversized
In this case also, restricted lift will be required for lower 4L6 size.
Given this, which option should be considered considering a whole bevy of technical safety factors and cost.
Thanks.
