2 replies to this topic

[PME]

For PSV following are noted.
Outlet size > Inlet size
Outlet rating < Inlet rating
Whether the above are mandatory requirements. If so , please provide a reason for this.

Thanks

[joerd]

For PSV following are noted.
Outlet size > Inlet size
Outlet rating < Inlet rating
Whether the above are mandatory requirements. If so , please provide a reason for this.

Thanks

No, is there a problem with it?

[djack77494]

PME,
As joerd says, these are certainly not mandatory requirements. The situation you describe, though is quite common. Let's see why.

The pressure at the suction flange of the PSV will be up to the relief pressure setpoint (plus accumulation). This could be quite high. The pressure at the discharge flange of the PSV is normally quite low. It would be just above the relief header pressure in most situations. So you shouldn't be surprised to see the Outlet rating < Inlet rating. If the PSV relieves a gas or vapor, then the volumetric flowrate upstream of the PSV will be much smaller than the volumetric flowrate downstream of the PSV. This is due to the pressure differences between up and downstream. Since the line sizing is often done based on velocity, it takes a larger line downstream of the PSV to have the same velocity as upstream.