Sizing Vent Pipe

[MechMad1]

Hi all,
we have a vent pipe that we use to warm up the main pipe (steam to the turbine).
This vent pipe is just a pipe with an isolating valve and it discharges to the atmosphere (we are thinking about getting a silencer).
I'm trying to calculate the flow that will be evacuated trough this line (and also the pressure drop), taking into account the conditions in the main pipe.
I have never done this and I'm not sure that the common formulas will apply in this case (with such high velocities).
Any suggestions will be welcome.
Thank you!

 

[Latexman]

Buy Crane Technical Paper 410. It's one of the best, concise references there is on fluid flow.

Good luck,
Latexman

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[LittleInch]

You will need first off to obtain the mass flow rate of the steam and the pressures at various points. By the soounds of it you have a low pressure, high veleocity steam vent which is controlled from the far end?

I am a bit confiused as to what exactly you are trying to calculate as you say have this pipe but only now you're trying to calcualte the flow and velocity - why? If the pipe is open to atmosphere and has a simple isolating valve, the flow control is then somewhere else, i.e at the valve from the boiler to the main pipe. If there is a real velocity problem, just go up a couple of line sizes which makes a huge difference in pressure drop and velocity for the same flow rate.

What sort of sizes are we talking here?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MechMad1]

Hi,

will have a look at the Crane. Thanks.

LittleInch,

I have a presurized system (40 bar) which is the pipe (8") from a boiler to the steam turbine (ST). The ST end of the pipe is blocked. To warm up this main line, we install a vent to the atmosphere (2"~4"). This vent has just an on/off valve (no control valve). I assume the flow through this line will be directly proportional to the difference of pressure between the main line (40 bar) and the atmosphere. The Boiler will produce the amount of steam that can be evacuated through this vent line (this flow remains unknown and I want to calculate it).
I don't really have a request for the flow. But the more, the better (the warming up will take less time).

Hope you can see the image here

http://img703.imageshack.us/img703/1397/sinttulozzu.png

So, to summarize, my data is a pipe of 2" (or 4"), that connects 40 bar to 1 bar. How do I calculate the flow? What happens if the velocity is near critical?

 

[Latexman]

You will probably have Mach 1 at the exit. What happens? An expanding jet of steam and lots and lots of noise. I would suggest modelling the 8" pipe as an unlimited supply of 40 bar steam and the small line as adiabatic, compressible flow. You could muddle thru it using a handbook or an old college fluid flow textbook. Crane would help you make short work of this little task.

Good luck,
Latexman

Need help writing a question or understanding a reply? forum1529

 

[LittleInch]

I think you may need to consider other options such as - work out the mass of steam in the 8" pipe at 40 bar from the boiler to the vent location. I don't know how long your pipe is but allow say 5 minutes to get hot steam from the boiler up to the vent and multiply the steam mass by a factor (1.5?) to allow for the "warming up" to occur. This is then your mass flow rate. in kg/sec or some mass over time unit that needs to be vented.

Plug that into an orifice calcualtion and find the biggest vent pipe you can. Attach the orific plate immeadiately downstream of the valve. This will give you some sort of control over what currently sounds like a fairly brutal and very noisy venting system. Your vent pipe design in the current scheme would be subject to huge loads and thermal shock and also needs to be rated at the header pressure as the pressure d/s of the isolation valve will be close to or at header pressure. Your valve also probably won't last very long as isolation valves are not much good at opening and closing against flow

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[macmet]

If you feedback on the position of the valve, could you use the position and the manufacturer's data package to determine the Cv?

Maybe then you could use the Cv and the pressure drop across the valve to find the flow.