Shaheryar
Chemical
- Sep 21, 2001
- 15
Hi,
I want to confirm the exact location where the 10% back pressure rule needs to be satisfied. For the sake of explanation lets take a an example:
a) Single PSV is protecting a Section VIII vessel with 100 psig design pressure
b) PSV set at 100 psig and reaches it relieving capacity at 110 psig.
c) For the sake of this example, let assume the reveling fluid to be compressible so that we can match up with NON MANDATORY APPENDIX M INSTALLATION AND OPERATION section M-7 c which requires use of Rated or stamped capacity for back pressure calculation for PSVs in compressible flow.
d) say the PSV is 2xHx3
e) Inlet pipe size if 3-inch and then a 3 to 2 reducer comply with 3% inlet pressure loss requirement.
f) Outlet tail pipe is 6-inch. Calculated equivalent length of pipe to include the PSV discharge expansion, in terms of 6-inch pipe. The system complies with 10 % back pressure at stamped capacity.
The problem is that it the hydraulic calculation is based on all equivalent lengths (including those of the reducers) converted to 6-inch, but if I add a hypothetical 3-inch flange to represent the tiny bit (~120mm) of 3-inch tail pipe before the reducers are welded on, the back pressure exceeds 10%.
So the question is that, where is the 10% back pressure defined ? Does it include the outlet flange that is connected to the PSV or is it for the tail pipe and fittings only?
Thank you in advance for input.
I want to confirm the exact location where the 10% back pressure rule needs to be satisfied. For the sake of explanation lets take a an example:
a) Single PSV is protecting a Section VIII vessel with 100 psig design pressure
b) PSV set at 100 psig and reaches it relieving capacity at 110 psig.
c) For the sake of this example, let assume the reveling fluid to be compressible so that we can match up with NON MANDATORY APPENDIX M INSTALLATION AND OPERATION section M-7 c which requires use of Rated or stamped capacity for back pressure calculation for PSVs in compressible flow.
d) say the PSV is 2xHx3
e) Inlet pipe size if 3-inch and then a 3 to 2 reducer comply with 3% inlet pressure loss requirement.
f) Outlet tail pipe is 6-inch. Calculated equivalent length of pipe to include the PSV discharge expansion, in terms of 6-inch pipe. The system complies with 10 % back pressure at stamped capacity.
The problem is that it the hydraulic calculation is based on all equivalent lengths (including those of the reducers) converted to 6-inch, but if I add a hypothetical 3-inch flange to represent the tiny bit (~120mm) of 3-inch tail pipe before the reducers are welded on, the back pressure exceeds 10%.
So the question is that, where is the 10% back pressure defined ? Does it include the outlet flange that is connected to the PSV or is it for the tail pipe and fittings only?
Thank you in advance for input.
