3 replies to this topic

[vadi]

Hi,

Natural gas is to be vented out from a high pressure pipeline to atmosphere the main line is at 74 barg. The venting will take place through a 6" by pass line ( by crack opening 6" blow dowbn valve).

Q- Actual gas flow rate, calculated by

P1=1atm.
T2= 15 Deg c.
V1=X MMSCMD.
P2=?
T=venting temp,Deg C.
V1=y m3/hr(after appropriate conversion)


please advice what will be P2 value wether 1atm or (Initial pressure- pressure drop across blow down valve - line pressure drop).

If i consider P2 to be 1atm flow rate is coming very high and hence velocity.(line size will also be unacceptable )

Please advice.
(Venting is discussed under pressure reliving codes so it has been posted here )

[gvdlans]

You are not clear about what the meaning of the variables P1, P2 etc. is. If I assume that P2 is the pressure upstream the blowdown valve, it is clear that if P2=1 atm (so equal to P1), there will be no flow at all. So I don't understand what you mean when you write that "If i consider P2 to be 1atm flow rate is coming very high and hence velocity."

What you seem to have missed is the phenomena of choked flow (or sonic flow or critical flow). When the pressure upstream an orifice (or control valve etc.) is more than about two times (1.7 to 1.9 to be more precise) the downstream pressure the flow through the orifice is choked and is independent of the downstream pressure.

Check out http://www.air-dispe...om/msource.html for the relevant mass flow equations.

I would use the following procedure:

1) Make an estimate of the pressure downstream the valve
2) Check whether you have sonic flow over the valve
3) Calculate flowrate through the valve using the appropriate orifice equation (sonic or not sonic based on outcome of step 2)
4) Calculate pressure drop over the tail end pipe using pressure drop equation and flowrate calculated in step 3)
5) Calculate pressure downstream the valve (= 1 atm + pressure drop over tailpipe)
6) Compare outcome of step 5) with value estimated in step 1), when there is a big difference, go back to step 2) using outcome of step 5) as new estimate

When you want to see what happens over time (= dynamic simulation), you will have to include a mass balance over the upstream system as well as the gas law (P*V/T=znR). You then calculate flowrate and upstream pressure for small timesteps. Initially there will be choked flow but when upstream pressure has dropped sufficiently flowrate will become non-choked.

This procedure can easily be automated e.g. by using an Excel spreadsheet.

[vadi]

gvdlans:

Thank you very much for reply and Appreciate the time you have spent on this systematic reply and this help me in future to post my querry appropriately.

This information is requested to study Jet dispersion of hydrocarbon gas during venting.

P1 which i have stated in first post is atmospheric pressure. the calculation data was written to convey that gas flow at standard is been converted to gas flow at actaual condition.

Please clarify:

Wether gas Pressure at outlet of the valve will be atmospheric pressure ? as end of the ventline is open to atmosphere or it will be equal to = Initial pressure- pressure drop across the valve or As pressure will decrease pogressively from valve outlet to ventline out let average pressure to be considerd as line flow pressure.

To convert flow from standard conditions (P1=atm, T1=15 Deg C, V1= MMSCMD) to actual condition (P2=, T2= ,V2=CMD) should i consider P2 to be average of pressure at valve outlet and pressure at the ventline exit.
Accepted vent velocity in code is in the order of 155m/s , if i consider P2 to be 1atm then velocity at valve out line will be very high (10 times of accepted value)

It is requsted for line sizing and to control flow velocity.

Best regards,
Vadi,

[gvdlans]

1) Pressure downstream the valve will not be atmospheric, since you need a pressure difference for a gas to flow... Pressure there will be atmospheric plus pressure drop over tail pipe. In case you have sonic flow this pressure is not really relevant.

2) Just calculate the flow through the valve/orifice. This will be the vented flowrate. If you calculate the mass flowrate you can simply convert it to volumatric flowrate when you know the pressure. I don't know where the 155 m/s comes from so I also don't know at what pressure it has been defined. I assume this is mentioned in the code you write about.