Gas velocity through pipes

assuming this isn't a trick question...

velocity = actual volumetric flow * 4 / (pi * D^2)

700000 * (273+45)/273 * 10.2/(10.2+0.8) = 756084 Am³/h [10.2m being the Normal atmospheric pressure in meters water column - adjust if required)

v=4*756084/pi/2.5^2 = 154028 m/h = 42.8 m/s

Phil

No Tricks Phil - many thanks for a swift answer.

It just sounded a high velocity to me.

G

Ignoring viscosity, scaling factors (like Reynolds et al) let's start with a rough n ready estimate:

700000 m^3/hr = 194.44 cu m/sec If the 2.5 m pipe were frictionless and laminar, the uniform speed would be 194.44/(pi*2.5*2.5/4) = 39.61 m/sec

Like a low speed wind tunnel? I've misread your numbers, I expect.

Brian W

Phil,

based on fairly recent work that I have done on compressible flow simulations, there is a small caveat to add. As gas flows experience pressure drop, the gas density goes down and the gas velocity goes up. In effect, there is a small acceleration effect for gas flows. If the original poster doesn't need high accuracy, this effect should be ignored. However, if there is some need to accurately know the gas velocity at different points in the pipe, this effect will need to be addressed.

Phil Thomps> >

Charlie:

To the best of my knowledge, compressible effects can be ignored for gas velocities at Mach numbers below approximately 0.3. In other words, gas flow may be considered to be incompressible if the Mach number is below 0.3. The original posting in this thread didn't state what gas was involved ... but assuming it was air:

S = speed of sound = (kRT/M)^0.5 = {(1.4)(8314)(318)/29}^0.5 = 357 m/s M = Mach number = V/S = 42.8/357 =0.12 which is well below 0.3 and so we can ignore compressible effects

where: S = speed of sound in the specific gas k = specific heat ratio = cp/cv = 1.4 for air R = gas constant in appropriate units = 8314 T = gas temperature = 318 degrees K M = molecular weight = 29 for air V = gas velocity = 42.8 m/s per Phil's calculation

Milt Beychok (Visit me at

Phil,

not really, its about 80 mph so a bit of a strong wind but if you were designing a pipeline on an economic basis such velocities would not be uncommon. There's a trade-off between buying a bigger pipe and pressure losses - higher pumping costs.

Steam systems at low pressure often use velocities at this level or higher to keep the pipe size down. 2.5m is a bit of a mother to start with, drop the velcoity and it gets bigger !

Phil

agreed. I assumed a pipe of negligible length :-)

Phil

This sounds analogous to the delivery of electric power. It's more efficient to user high voltages: the current-carrying requirements of the medium are reduced and losses due to resistance are minimized.

The other reason steam systems should be run at high velocity is to minimise cooling and hence condensation.

John