Q & A > Question Details
Is it safe to consider back pressure of 50-70 kg/cm2g when my PSV set pressure is at 229 kg/cm2g? Why are we limited to 3-5 kg/cm2g back pressure maximum when we are designing the HP flare? API 520 part 1 says that I can consider up to 50% of set pressure of balanced PSV, so can I consider up to 100 kg/cm2 g when my PSV is set at 220 kg/cm2g? If not, then what is the reason?
20/03/2010 A: KAARTHIK RAMACHANDRAN, BECHTEL, rckaarthik@yahoo.com
Flare headers are typically 150#.The problem is if you size the PSV for a backpressure of 100 kg/cm2g this can result in 900# headers which is highly expensive for a huge Flare network. At such high design pressures Flare headers would also have to be hydrotested which would increase the cost of the structures.
24/10/2009 A: Manish Tiwari Kumar, RIL, manish.chem4u@gmail.com
I think your question is: why the back Pressure value is not been considered as 50% of the set pressure (as described in API)?
The reason is that in designing a PSV there is a trade off between Allowable back pressure and Flow (PSV capacity). If the back pressure increases the flow will decrease through the PSV.
So it is not cost effective to design a PSV which can provide the required flow when back pressure is very high (this will require very very big PSVs). On the other hand we know that normal back pressure in header will be of the range of 3-4. So every designer wants to design a small psv (low area).
No doubt from safety point of view those PSVs (set pressure 220 kg) can withstand against the 50% back pressure but in that case the flow will reduce.
Normally PSV vendors provide the back pressure value for that required flow.
22/10/2009 A: Prakash Pimparkar, Environmental Consulting Services , prakash.pimparkar@yahoo.in
Please refer my reply earlier
Forgot to address the issue of high back pressure
Back pressure of 50 -70 appears to be too high as this would require a very heavy flare header unnecessary and will not be essential as it is result of pressure drop across flare tip/molecular seal/water seal/KOD/and across flare header up to the point of release.
Back pressure in flare header should normally normally not exceed 1.3 to 1.5 kg/cm2g or so. Please recheck the figure.
22/10/2009 A: Prakash Pimparkar, Environmental Consulting Services , prakash.pimparkar@yahoo.in
I think this question was posed earlier also and since you have not received any answer let me try to explain.The set pressure is the pressure at which the safety valve opens up to release the pressure to protect the equipment or the system you want to protect. And therefore is decided by the pressure vessel operating and design pressure.
The back pressure on the other hand is the pressure the PSV would experience on its downstream side when it tries to open up. And is the pressure in the flare header and is due to the flow of other releases in the flare header.
To arrive at the back pressure you will have to do calculations to arrive at the pressure at the point where your PSV is located backwards from the flare stack tip.There is commercial software to do these complex calculations such as "Flarenet".
If you are not in a position to use this software or arrive at appropriate back pressure calculation yourself do not use arbitrary values as these could compromise safety of your plant. Mind you when you give the back pressure this will be used by the manufacturer to set the valve. If unfortunately the actual system back pressure is below the pressure guessed by you the valve may release premature or at lower than the set pressure compromising the safety and design conditions.
Use of balanced PSV does not help to that extent it will also fail if you are not specifying correct back pressure.
Hope this clarifies your doubts. You should consult a good process engineer even to interpret the API codes as they can be wrongly interpreted.
I hope some of experienced process engineers will help you out since this is a question of safety.