Calculate Air Flow through 12 DN SS pipe

[rai asif]

Hello everyone,

I was wondering if anyone could help me with calculating the flow of air through a 12 DN SS pipe. The compressed air header is at 9.5 barg. I am not completely familiar with the calculations needed for this and would appreciate some guidance. Any advice or resources you could share would be greatly appreciated.

 

[LittleInch]

Well you need to know

ID of the pipe,
Length of the pipe
Pressure at or needed at the end of the pipe.
Does the header pressure change when the is flow through your pipe?

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

 

[rai asif]

ID of the pipe,
The ID is 16.45 mm

Length of the pipe
Pipe length from 50 DN header is 10 MTR till the discharge.

Pressure at or needed at the end of the pipe.
Pressure at the discharge is unknown. We are using this air to remove blockage of coal.

Does the header pressure change when the is flow through your pipe?
No, the header flow doesn't change as we have compressors running on the basis of header pressure. They compensate for any loss in pressure.

 

[LittleInch]

Range of pressure at the end?

If basically open then you'll have sonic flowand a very loud noise.

What's at the end of the pipe?

Without a pressure you can't calculate flow.

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

 

[rai asif]

The pipe opens into large surface area with cylindrical shape of 2m diameter.

The air is used to remove blockages from restricted area in Boiler house return leg.

 

[Latexman]

rai said,

A well dimensioned, labelled sketch would have been more efficient.

Good Luck,
Latexman

 

[LittleInch]

Well it's complicated by the large pressure drop in such a short distance. This would seem to be critical flow and end up as sonic flow. So will be very loud.

If this is limited by sonic flow at atmospheric pressure, then you're looking at 340m/sec at the end point.

For that Id, its about 4 to 5 scm/min or about 150 scfm.

But it could be less as this is essentially a restriction orifice.

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

 

[georgeverghese]

To maintain constant pressure at this measuring device, install a backpressure control valve downstream, which senses upstream pressure, and keeps this pressure at a min of say 9barg. You can regulate the desired flow through some globe or needle valve downstream of this backpressure CV. Size the PCV for the max required flow at a dp of say 3.5bar.
Am not a field instrumentation engineer, but would suspect a corner tap or flange tap orifice plate with DP cell will do for DN15 pipe- avoid low points in the piping upstream and downstream to prevent condensed water accumulation.
Since this setup suggested protects your compressed air system pressure by low limiting at 9barg, do you still need to measure flow ?

 

[rai asif]

We have a manual valve on this line as well. I want to optimize the flow, need the maximum flow so that I'm trying to quantify it first.
The flow values are to be quantified just for information and optimization.

 

[katmar]

Ignoring the valve, my estimate would be around 580 kg/h or 0.125 Nm3/s. But as others have said, the exit velocity would be sonic and very noisy. Using the valve you would be able to reduce the flow to some lesser quantity that would still clear the blockages.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[rai asif]

@katmar: How did you come to this estimate?

 

[LittleInch]

Go with kalmar.

The difficulty is estimating the pressure just before it exits the pipe.

But you will hear this in the next factory.

Suddenly expanding to 2m diam will reduce your velocity a lot.

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

 

[katmar]

I used the isothermal compressible model and simply increased the pressure drop until the exit velocity became sonic. This occurs with a total pressure drop of about 8.1 bar. Increasing the pressure drop to the full 9.5 bar available will not increase the flow rate because of the sonic limit.

If you use the adiabatic model (instead of isothermal) you may get a few percent higher flow. The unknowns such as the air temperature, exact pipe diameter and length etc do not justify quibbling over a few percent in the estimate.

The calculator that I used is linked in my signature below.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[rai asif]

Thankyou for the help.

 

[pierreick]

Hi,
Following the same methodology, I got 622 kg/h for adiabatic model.
P2 1890.00 mbars
m 622.28 kg/h
Qv1 0.014 m3/s
U1 65.10 m/s
C1 343.02 m/s
Mach 1 0.190
T2 258.54 K
T2 -14.61 C
ρ2 2.55 kg/m3
Qv2 0.07 m3/s
U2 318.97 m/s
C2 322.13 m/s
Mach 2 0.99

1 refers to inlet
2 refers to exit




Pierre

 

[TiCl4]

FWIW, I got ~600 kg/hr using adiabatic calculations with a Ma of 0.99 on exit.

NOTE! I played around with surface roughness values,k, and at this small of an ID the assumed pipe roughness (I assumed 0.1mm for the value above) plays a much larger role in pressure drop and flow capacity than larger pipes due to the D in the formula roughness = k/D being so small. For kicks and giggles, the smooth pipe estimation results in a much higher flow capacity of a bit over 900 kg/hr of air.

 

[georgeverghese]

Max air flow you could allocate to this plant air demand at this PCV (and the downstream manual valve) would be total output from running compressors minus base load instrument air demand minus any other intermittent plant air you think may be operating simultaneously with this operation. The backpressure PCV is standard for all intermittent non critical plant air demand users.