Gas flow in pipeing with changing diameter (bottleneck)

[engr2GW]

Hi,

In liquid flow (probably because of incompressibility) when flow in a pipe that goes, say, from 2" to 1" and back to 2". The Q=VA ensures that the flow rate is the same in each segment. Is it fair to say that the flow max is equal to the max capacity flow of the smallest diameter?

If this is true, does it apply to gas?: I have a 150' 2" pipe that changes to 1" for just about 3' and back to 2" for another 100'. At a supply pressure of say, 100 PSIG, does the 1" segment limit max flow? I also noticed that pressure after the 1" segment dropped to about 90PSIG

Thanks for any input, answer, experience, or pointer as to how I can figure.

As much as possible, do it right the first time...

 

[mk3223]

You may check both liquid and gas systems that the flow rate Q is the same for each segment of the system. So, the velocity V is changing at different segment as the pipe cross area A changed. But, as you observed, the pressure of the segment will be dropped along the lines.

 

[LittleInch]

Lets go back to basics here:

In a steady state system the mass flow at all locations will be the same, gas or liquid.
In a liquid system due to it's virtual incompressibility, the volume flow will also be constant
In a gas system the actual volume flow will vary with pressure / density which changes as it goes down the pipe

Is it fair to say that the flow max is equal to the max capacity flow of the smallest diameter? Not really. There is no such thing as "max capacity flow" until you're flowing at sonic velocity of the fluid ( probably several hundred feet per second)

At a supply pressure of say, 100 PSIG, does the 1" segment limit max flow? Again not until you approach the speed of sound in your gas.

At the diamters you quote the frictional losses in your 1" pipe will be in the order for 2^4 to 2^5, or 16 to 25 times what it is in your 2" pipe (d/D)^4 to 5 if all other things stay the same. Now there will be other losses if the change form 2" to 1" is a sharp edge, but in essence your 3 foot bit of 1" pipe is like adding 50 to 60 feet of 2" pipe.

Now if you put the 1" pipe at the start rather than the end it would have a lower impact as the density is greater, but that's fine details.

So unless your actual velocity of gas through your 1" pipe is becoming critical (near mach1) then you're just adding extra frictional losses.

I don't actually understand what you're trying to "figure" so a bit more data wouldn't go amiss...



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

You'll find all you need to know in Crane TP 410 - Flow of Fluids, or any other college or uni text on the same subject.