Zoran Savovic, Editor PipeFlowCalculations.com

The following Q&As are based on recent forum entries at PipeFlowCalculations.com, a website featuring calculators and a bulletin board for fluid flow applications.

Defining Maximum Gas Pipe Velocity
Q:
What is the typical natural gas pipe velocity, and what is the maximum gas pipe velocity?

A: For underground installations, 20 m/s is normal. Maximum velocity in a pipeline, on the other hand, is defined by available pressure, but in control and safety valves, it can be allowed up to 100 m/s.

Q: Thanks for the information. I noticed from the calculator on PipeFlowCalculations.com that for two-inch pipe with 1,500 feet length, 90 PSI in and 10 PSI drop, it gives a result of 337 CFM gas flow with a 44 FPS and 50 FPS velocity. Shouldn’t the velocity of this be 257 FPS (based on that 337 CFM flow and two-inch pipe).

A: It would only be 257 FPS if pressure is at standard conditions at 1.013 bar, but since the pressure is 90 PSI (cca 6 bar), it is about six times less (257/6 = 44 FPS – approx.)

Q: So it’s acceptable to have a 20,000 CFH gas flow in a two-inch pipe with 90 PSI with a drop of 10 PSI? (yielding m/s pipe velocity)?

A: Yes, it is just about perfect.

How to Calculate Gas Consumption
Q:
We have a batch fryer in our food processing plant. There is a gas flowmeter to record the flow of gas. We use LPG that is stored in a sphere, underground. The gas supplier bills us for LPG in MT (assume in liquid form). How do I calculate consumption and cost of the LPG per pound of cashews/nuts roasted. The flowmeter readings in cubic M/hr are recorded at the start of fryer and after shut down of fryer, during which time “X” pounds of nuts would have been roasted. Obviously, we have to trace the consumption in vapor form to liquid form, to find out the costs.

A: First you need to know if the flowrate of the gas mixture is measured on real conditions with some pressure that is above atmospheric. Or is it already converted to standard conditions with volume corrector? If the flowrate that is measured is already converted to standard condition, calculation of mass flowrate is straight-forward. You must know the composition of the mixture — propane butane ratio — like 30-70 percent or so. Then the density of that mixture can be calculated based on the density of every single component and its participation in mixture.

For the theory on how to calculate fluid mixture density, see: http://flwctrl.com/iF932N

Component density should be used on standard conditions, and you can view the table for it at:
http://flwctrl.com/l0Ni0e

When you have the fluid-mixture density, then the measured flowrate should be multiplied with the density to determine weight flowrate:

G=Q x rho

where:
G = weight flowrate — which is the same for gas and liquid
Q = measured flowrate – volume
rho = mixture density

If the measured gas flowrate is not converted to standard conditions, then the density of every single component should be used on the pressure and temperature that is on the point of flowrate measurement. This means you should use component density on standard conditions using the gas state equation. Calculate density on a given pressure.

p1/rho1=RT1
p1/p2 * rho2/rho1 = T1/T2
if 1 is for standard conditions then
rho2 = rho1 * T1/T2 * p2/p1

Relationship Between Reynolds Number & Flow Through an Orifice

Q: Hi, I was wondering about your software — I was looking for something like it for years gone by, but it looks wrong that in changing the viscosity of the fluid from a light oil to a thicker one, like from 10 to 100 cSt (or mm^2/S) the flowing liters passing by the orifice increases instead to decrease. Can you explain if I’m wrong, or not?

A: It may seem incorrect, but if you look at the equation for flowrate through an orifice, you can see that only in terms of discharge coefficient C, through Reynolds number, viscosity is treated. Also, in the equation for discharge coefficient, Reynolds number is in the denominator, which means if viscosity is higher, Reynolds number is lower, and the coefficient of discharge is also higher, which results in higher flowrate for the same pressure difference in front of and after the orifice.

That is a mathematical solution and confirmation for this problem, but what about the physical? I would say that due to higher viscosity, the vena contracta effect is smaller, and the contraction of flow stream after the orifice is not as big as it is with a less viscous fluid. As contraction of the flow stream after the orifice is smaller, the flow cross section is bigger and, correspondingly, the flowrate is also higher.

To review the theory from ISO for orifice flow, see: http://flwctrl.com/jyqIci

Zoran Savovic a mechanical engineer with more than 10 years of experience in the fluid transport systems design and engineering. He is also the owner and editor of pipeflowcalculations.com.