While modelling PSV, in my company we normally consider following Forces.

1) F1 in horizontal direction
2) F2 in Vertical direction

where
F1= 2*F (Assuming DAF=2)
F2= 1*F
F= Reaction force from Vendor

Can anyone explain the logic of considering 1F in vertical direction ?

Aashumon,

I do not utilize this particular analysis method, but I will speculate on their reasons for this decision.

I do not know what "DAF" stands for, but I call it "DLF," or "Dynamic Load Factor."

First, the presumptions:

1. There is an object that a PSV is protecting.
2. All things considered, the PSV is not physically capable of moving that object around; it is effectively of infinite rigidity.
3. The PSV is connected to that object via a pipe of known non-infinite rigidity.
4. That pipe is oriented in the vertical, is "short," and has high axial stiffness.
5. The tail pipe does not exist. The only pipe is the vertical pipe.

ASME B31.1 discusses how to calculate DLF for this simplistic scenario, but you need to understand that it's based on natural frequency. But your pipe has either 4 or 6 natural frequencies. X,Y,Z,RX,RY,and RZ, or in the case of symmetry, Translation Axial/Radial and Rotation Axial/Lateral.

For the purpose of this discussion, your scenario only has 2 natural frequencies that matter: Translation Axial and Rotation Lateral. They say a force acts vertically (axially) and a force that acts at the top of the pipe, laterally. Thus, the DLF of each applies for each case.

Except because the natural frequency in the vertical (axial) is assumed to be infinitely high, the DLF is presumed to be 1.

The question then is, should it always be 1? If our PSV inlet piping comes out horizontally and turns vertically before the PSV, does it still make sense to make it 1? Probably not.

But the actual vertical loads from modern relief valves opening is generally very low, and they are often written off altogether.