PSV Total back pressure calculation 2

[Rajjil]

I am in a process of sizing the PSV for Crude oil -Liquid relief. This PSV is to be located on a Crude pipe line located in Petro-Canada Edmonton Refinery. This PSV will use to protect some part of pipeline inside the refinery. In recent operation they observe some spike in Pressure up to 373 psig in pipe line while change over. This PSV will protect some part of pipe line which is designed for 270 psig. The required relief flow is 1030 m3/hr. I have done a preliminary sizing but have concern about the back pressure and selection of type of PSV.

There is a very high back pressure(>70%) in one case when there is a simultaneous flow in the line where PSV discharge is connected. To calculate the total max. back pressure I considered the PSV dischrging to other line when there is a flow. My question is how to calculate superimposed and built back pressure in this case.
I calculated the total back pressure on PSV outlet flange, the problem is we are combining the PSV relief flow with existing line flow. The back pressure in this case is really very high and velocity is just crossing the erosional velocity allowed.
Although the PSV relief scenario will last for only few minutes (the time taken to divert the flow to other consumer in block outlet case).

I need your advise in sizing of this PSV and the outlet line back pressure calculation.

 

[CMA010]

You say the "spike" occurs during change over, what kind of spike is this, do you need a PSV or a surge relief valve?

Your relief load (1030 m³/hr) seems to be a bit high considering the 8" pipeline.

A 2000 ft discharge line (excluding the tail pipe to the 12" pipeline) is rather much. How would this work and can you be sure that the valves located in the inlet of the tank are always open?

 

[Chance17]

Are you aware that ANSI pipe is allowed to operate 133% of design for short duration (10-hr per year)?

 

[DSB123]

Yitbos,
Are you aware that 133% of 270 psig is 359 psig which is less than the value of the pressure spike (373 psig) therefore your comment is not applicable. Also what is an ANSI pipe? Do you mean ASME B31.3 ? It is important to be precise and clear.

 

[Rajjil]

I am aware about surge protection devices used in pipe line like Dan Flo. This is not the Surge protection design.
The valve located at tank inlet will be made locked open.
Now there is change in design, instead of PSV there will be a control valve(Sur Flo) will be provided (Sur Flo type control valve is capable of handling very high back pressure and used in pipe line application) and in parallel to control valve there will be a rupture disk for over pressure protection. The control valve will take care of spikes and rupture disk will serve the over pressure protection.
There will 2 cases for rupture disk
case-1: When there is no flow through 21P500-12", the total back pressure at rupture disk outlet flange will be 97.3 psig with rupture disk already burst.
case-2: When there is simultaneous flow through 21P500-12" and rupture disk is bursting, the total back pressure = 238.1 psig.

My question is how to decide the burst pressure of rupture disk to include these two cases. The control valve set pressure will be 25-50 psig lower than the set pressure of rupture disk to take care of pressure spikes.