Fluid flow velocity in pipes

[Zonkytonkman]

Hello, I'm tasked with using nitrogen to push some glycol out of a pipe assembly that we have, though there is one single 2" diameter pipe that has some sensitive equipment in it that can't see very fast flows(I'm told) I was wondering if I knew the pressure difference across this length of 2" pipe, knew the length, etc, how could I estimate the velocity across it? The length is only a few feet. Thanks in advance.

 

[eayvjer]

It is possible, but you would need some additional bits of information. The pressure difference down a pipe is due to resistances in the pipe. These come in the form of minor losses and friction losses. Minor losses are due to junctions and obstructions to the flow. Friction losses are due to the shear forces at the side of the pipe. Ignoring minor losses for the moment:

hf = (z1 + p1/(rho*g)) - (z2 + p2/(rho*g)
= change in z + (change in p)/(rho*g)

Once you know the head loss, the velocity can be found:

hf = f * (L/d) * (V^2 / 2*g)

Where, z1 = initial height
z2 = final height
p1 = initial pressure
p2 = end pressure
L = pipe length
d = pipe diameter
V = bulk velocity
rho= denstiy
f = friction factor

The friction factor is found using the Moody chart which you should be able to find on the net somewhere. To find it you'll need to know the relative roughness of the pipe.

I am not sure what effect the equipment you have in the pipe will have on the minor losses but to get an accurate value for the speed of the flow you would have to take them into account.

Hope this is of some help,

James.

 

[Cockroach]

"Eayvjer" is quite right, except for friction factor. It was stated that velocity must be low, i.e. slow flow, so that the fluid condition would be laminar. This means that Reynolds Number is under 2300.

In this case the friction factor is f = 64/ReD, ReD is the Reynold's Number corresponding to the ID of the pipeline. Therefore, average roughness is not required in order to deduce friction factor.

Otherwise, good job on the theory!

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Assumptions]

Would just add a caveat to what Cockroach has pointed out. The friction factor "f" is related to laminar fow in some literature as 16/NRe and in some as 64/NRe. The difference is in the definition of f. All one has to know is which reference is being used.
Otherwise in laminar flow pressure loss is independent of roughness as previously pointed out.

 

[25362]

A ROT says that friction factor values for steel pipes double as a function of the pipe age and the fluid, as follows:

Every 25 years for gases and hydrocarbons
Every 15 years for medium petroleum distillates
Every 10 years for (petroleum) distillation residues