Hi Sirs
I´m discussing with some friends about PSV´s discharge line sizing during individual discharge (for instance, closed outlet case). Some of them are saying that it´s necessary to design discharge lines considering as back pressure the minimum back pressure, that means something near to atmosferic pressure.
In my point of view, this approach is not realistic and can lead to oversized diameters. Let´s try to explain it considering the figure attached.
At the instant t=0, when back pressure is almost atmosferic (minimum back pressure), we have the highest velocity (green curve). Imagine that in this case we have "plugged" flow (Mach=1) in which is not possible to reach the flow through PSV. What will happen? The pressure immediately downstream PSV starts to increase and consequently system pressure will increase too (yellow curve). As the pressure increases, the velocity decreases (considering the same flow rate). This intermediary condition takes place until the rated flow be reached. At this point the flowing through the system reaches the equilibrium and stabilizes itself. So, but how much this pressure can increase? Till reach the maximum pressure of the piping (red curve). In this case I would have the maximum pressure in the base of the flare (already definied in the project)
So, trying to summarize, if I design PSV discharge lines considering as back pressure the maximum at base of flare, evaluating if I reach or no piping allowable pressure I would have a more realistic case leading to smaller diameters.
What do you think about that?
Thank you!!
Edited by Amanda Ferrari, 16 April 2014 - 01:51 PM.