How to calculate pressure at a given point in the vent header

[rob5377]

Dear All,

I am trying to calculate a pressure at a specified point of the vent system header (see point P1 on the attached draft). The purpose of that is to be sure that during the depressurization of HP section of the pipeline connected to the same header as RD, the disk wont rupture. The set pressure of RD is 3,5 barg. The HP’s pipe pressure is 30 barg and will be depressurized via the vent pipe to the atmosphere.
I am wondering how should I solve this problem. I was thinking about approach:
1.Calculate the maximum mass flow rate after 12” globe valve is full open for critical flow
2. Calculate the velocity in the pipe
3. Calculate pressure drop due to friction starting from the end of the vent (top of the vent stack p=1 atm) to the reducing tee 12”x8”
4. And then I am not sure. Should I keep the same velocity and calculate the friction losses to the RD’s outlet?

Is my approach is OK? At a first glance I supposed that the pressure (only for a moment, at the beginning of the depressurization) will be 30 barg – friction losses of piping from the reducing tee to the RD. But this would be valid only if a RO with small hole was at the end of vent stack.

Can anybody give me a clue how to solve it?

Thanks
rob

 

[Homayun]

I would say try solving this backwards. You should start from the vent stack where the pressure is known, Atmospheric, and work your way back to the point where you are interested in.
Hope this helps

 

[Flareman]

Homayun has the key.
Divide the system up into sections of equal diameter.
Use standard expansion losses and bend losses at section changes. Calculate DeltaP in each section using the isothermal compressible formula.
The static pressure downstream of the outlet is atmospheric. Accumulate all losses as total pressure. Remember the total pressure at any point is the sum of velocity pressure and static pressure (Bernoulli). You will need an iterative routine because the velocity presssure in any section uses density, which depends on static pressure.

When you get to the point you care about, calculate total, velocity and static pressures. If you put a gauge on the pipe you will only read static.
Oh yes, if the density is much different than air, you also need to include static head. Sometimes a low density gas results in a negative staticpressure on the gauge.

David

 

[rob5377]

Thanks Folks.
What I did, I solve this backwards starting from vent stack as suggested by Homayun and API 521 using formulas from API 521 for tailpipe pressure drop. But using this method I was only able to calculate the pressure at the globe valve outlet during the HP depressurization . I assumed that during the depressurization through 8" globe valve I will have the same pressure at p1 like at the 8" globe valve outlet.

regards
R

 

[hacksaw]

At a minimum you need to maintain the relief capacity of the rupure disk to avoid over pressuring the protected equipment with all other likely flows present.

That generally requires P1 max < 0.5(3.5+1) barA

If your R/D disc discharge pressure requirment is exceeded, then you need larger pipe...to keep the discharge header safe