In a Oil and Gas treatment facility

[asghar]

In a Oil and Gas treatment facility the tanks drains and vessels blow downs are connected to a single skimming tank. The skimming tank is open to atmosphere. A number of vessels operate at 1100 psig.
Considering the safety aspect a two (HP and an LP) header policy has been adopted. Th HP header will be connected to a flash tank, from where the gases will go to flare and the liquid will be drained to a Skimming Tank. The LP header will directly be connected to Skimming Tank. The Max. drain size on HP (1100 psig) vessel is 2" through which 200 gpm blowdown is assumed.

Assuming that blowdown valve remaines open for (let us say)1 minute, what will be the pressure in 8" HP header? I have used P1V1/T1 = P2V2/T2 equation for determining the pressure and came up with less than 170 psig pressure, assuming that P1=1100, V1=200gpm, T1=100oF, T2= 85oF, header size 8" and 500 ft long. The flash drum pressure is controlled by Flare K.O drum, which is maintained at 4-7 psig.

Question 1) Am I on righ track?
2) would not there be high pressure (localized)
at the point where the blow down is connected?
3) Based on 200 gpm the velocity in 8" pipe is
around 1.5 ft/sec.
4) The liquid will travel a max. of 90ft at this
velocity and my calculations are based on 500
ft. long pipe. should not the V2 be based on
90 ft rather than 500 ft.

 

[SooCS]

No you are not in the right track.

First you calculate what is the max flow of fluid based on the CV of the valve when it is fully open. Might consider the fluid is totally gas.

Calculated the back pressure in the piping system. The pressure will well exceed the piping system design pressure.

Check the temperature drop due to JT effect. The temperature of the blowdown gas may well bellow the brittle temperature of the system material.

For high pressure service, it is common practice to have two valving system on the drain outlet from high pressure equipment, usually one globe valve and another ball valve in series. Some company, even requires to paint this drain valves with different colors from other equipment and locked it or at least CSC.

 

[asghar]

Thanx SooCS,
Since we are draining therefore mostly it will be liquid.
Following ur recommendations the full open, full port Cv of 2" valve is 288. Assuming 7 psi delta P across 2" valve and using Q=Cv SQRT(delta P/Sp.Gr, the flow is 1000 gpm. Based on P2 = 10 psig (flash drum pressure, the calculated P1 = 15 psig at point A, which is 500ft upstream of the drum and where the 2" dia drain is connected.
I am confused which pressure should I take at point A, 15 psig or (1100-7)=1093 psig (1100 psig is vessel pressure and 7 psi is the valve delta P)? If I take delta P = 1093 then the calculated Q through 2" valve will be 10,712 gpm, which is very very high.

 

[SooCS]

What you are thinking, normally is correct. But when you are designing something you should think what can go wrong.

Normally, before the operator drain the vessel, he or she will have to depressurise the vessel prior opening the drain valve. Unfortunately,like Murphy Law says, there is always someone who doesnt quite follow procedure.

In your case, under normal operation the delta p across the valve and the piping system of 15 psi may be adequate depending how long the operator is willing to wait for the vessel to drain.

But I will check if the drain system can withstand the back pressure if the operator open the drain valve prior depressuring the vessel. For blowby case that's what you should consider, p1 = 1100 psig initially flashing liquid followed by gas.