Vulture860
Mechanical
- Oct 25, 2012
- 7
I'm calculating the safety valve reaction forces using the equations out of B31.1 Nonmandatory Appendix II (for a steam PSV) and out of API 520 Part 2 (for a flare gas PSV). I have a few questions regarding these calcluations:
1. My understanding is that B31.1 Nonmandatory Appendix II is for steam applications only while API 520 Part 2 can be used for both steam and for all other vapors. Is this a correct statement?
2. When looking at open discharge systems, what does the P component in the API 520 reaction force equation physically represent? The definition is "the static pressure within the outlet at the point of discharge". I've always understood that the pressure component of these equations was your delta P at the discharge (conservatively using set pressure minus atmospheric), which results in some pretty substantial pressure thrusts at higher set pressures. However from just thinking about this, I feel that the end of a discharge piping likely won't see the set pressure at the outlet considering the gas is expanding through the PSV orifice into the larger discharge header. It seems B31.1 outlines an approach to calculate P1 (assumed at very edge of discharge piping before it encounters atmoshperic pressure). However, I don't have a good understanding of what they're physically calculating and I assume this approach only works for steam applications.
3. I've seen arguments for not having to account for reaction forces in closed discharge systems. Is there any truth to this, I simply don't have a good understanding of how the relief vapor would act physically on a closed discharge system.
4. I typically apply the B31.1 dynamic load factor value (max = 2.0) to all PSV applications, including reaction forces caluclated out of API 520. Does anyone see an issue with this approach (too conservative, not conservative enough, etc)?
Any advice or guidance to any these items would be greatly appreciated. I am attempting to develop a standard approach to analyzing PSV's from a pipe stress analysis standpoint and would like to hear how others approach these applications.
Thanks.
1. My understanding is that B31.1 Nonmandatory Appendix II is for steam applications only while API 520 Part 2 can be used for both steam and for all other vapors. Is this a correct statement?
2. When looking at open discharge systems, what does the P component in the API 520 reaction force equation physically represent? The definition is "the static pressure within the outlet at the point of discharge". I've always understood that the pressure component of these equations was your delta P at the discharge (conservatively using set pressure minus atmospheric), which results in some pretty substantial pressure thrusts at higher set pressures. However from just thinking about this, I feel that the end of a discharge piping likely won't see the set pressure at the outlet considering the gas is expanding through the PSV orifice into the larger discharge header. It seems B31.1 outlines an approach to calculate P1 (assumed at very edge of discharge piping before it encounters atmoshperic pressure). However, I don't have a good understanding of what they're physically calculating and I assume this approach only works for steam applications.
3. I've seen arguments for not having to account for reaction forces in closed discharge systems. Is there any truth to this, I simply don't have a good understanding of how the relief vapor would act physically on a closed discharge system.
4. I typically apply the B31.1 dynamic load factor value (max = 2.0) to all PSV applications, including reaction forces caluclated out of API 520. Does anyone see an issue with this approach (too conservative, not conservative enough, etc)?
Any advice or guidance to any these items would be greatly appreciated. I am attempting to develop a standard approach to analyzing PSV's from a pipe stress analysis standpoint and would like to hear how others approach these applications.
Thanks.