cswosu
Mechanical
- Jun 25, 2003
- 9
After perusing this forum and debating with my coworkers, I think I understand the approach to discharge pipe sizing...maybe. Can you guys let me know if this is right?
For discussion's sake, let's assume:
1) PSV is 6M8
2) Flow medium is air
3) PSV rated flow is 264,000 lb/hr
4) Back pressure will not be high enough to hinder PSV from reaching rated flow
5) Air temp at valve outlet is 95 F
I'm using Hysys to obtain my results, FYI.
Let's say I use a 10' section of 8" Sch. 40 as my outlet piping. In order to achieve the rated flow and not exceed Mach 1 at the pipe outlet, I need 43 psia at the PSV outlet/pipe inlet. The pipe dP in this case is 5.2 psi, and Hysys gives my pipe outlet pressure as 37.8 psia. Therefore, I have a shock at the end of my pipe where the gas transitions to atmospheric pressure.
Now, if I care to avoid reaching Mach 1 anywhere in the outlet piping, I would have to go up to 14" pipe. I obtained this result by specifying 95 F and 14.7 psia at the end of the pipe and increasing the pipe size until the maximum velocity in the pipe stays under Mach 1.
Questions:
1) Are both of those scenarios correct?
2) 14" outlet piping seems awfully large to keep the flow from a 6x8 valve under Mach 1. I've typically never seen much more than an increase to the next size (10") in practice, but this won't keep me from reaching sonic velocity in this case (and maybe it's just a function of my assumptions). In any case, should I always strive to avoid reaching Mach 1 in the outlet piping even if a large increase in pipe size is required?
Thanks in advance for your help. This has been confusing me for a while.
For discussion's sake, let's assume:
1) PSV is 6M8
2) Flow medium is air
3) PSV rated flow is 264,000 lb/hr
4) Back pressure will not be high enough to hinder PSV from reaching rated flow
5) Air temp at valve outlet is 95 F
I'm using Hysys to obtain my results, FYI.
Let's say I use a 10' section of 8" Sch. 40 as my outlet piping. In order to achieve the rated flow and not exceed Mach 1 at the pipe outlet, I need 43 psia at the PSV outlet/pipe inlet. The pipe dP in this case is 5.2 psi, and Hysys gives my pipe outlet pressure as 37.8 psia. Therefore, I have a shock at the end of my pipe where the gas transitions to atmospheric pressure.
Now, if I care to avoid reaching Mach 1 anywhere in the outlet piping, I would have to go up to 14" pipe. I obtained this result by specifying 95 F and 14.7 psia at the end of the pipe and increasing the pipe size until the maximum velocity in the pipe stays under Mach 1.
Questions:
1) Are both of those scenarios correct?
2) 14" outlet piping seems awfully large to keep the flow from a 6x8 valve under Mach 1. I've typically never seen much more than an increase to the next size (10") in practice, but this won't keep me from reaching sonic velocity in this case (and maybe it's just a function of my assumptions). In any case, should I always strive to avoid reaching Mach 1 in the outlet piping even if a large increase in pipe size is required?
Thanks in advance for your help. This has been confusing me for a while.