Pressure after Needle Valve and Rotameter

[VCas]

Hello,

I am currently evaluating the performance of a swirl burner. I got two lines going into the burner rig, (1) a methane line that comes from a pressurized cylinder and (2) a compressed air line. Each going through a needle valve and a rotameter. Currently there are no manometers after the rotameters. I am unsure if I should consider the operating pressure of the rotameter as (almost)atmospheric or if the operating pressure would be different than that. Is a manometer after the rotameters needed? As I turn the needle valve will the pressure in the rotameter increase considerably? Could it possibly reach the pressure from before the needle valve? Unfortunately I do not have the specifications for the needle valves.

I'll give as much information as possible for each line:

Methane line:

The pressure at the outlet of the gas cylinder is at 1 bar gauge, the line runs for about 17 meters of piping and elbows before passing through a needle valve and a rotameter. After the rotameter, the line goes through about 1 meter of piping and mixes with the air line.

Scale in the rotameter is at standard conditions. Volume flow rates through this line (as measured by the rotameter) vary from 3 liters/minute up to 18 liters/minute. Pipe inner diameter is 7.1mm

Air line:

The compressed air pressure is regulated to 5 bar gauge before passing through a needle valve and a rotameter. The lines then goes through about 1 meter of piping and mixes with the gas line.

Scale in the rotameter is at standard conditions. Volume flow rates through this line (as measured by the rotameter) vary from 10 liters/minute up to 100 liters/minute. Pipe inner diameter is 7.1mm

Methane+Air line

After the mixing point, the gas mixture goes to the swirl burner and is ignited. Burner is unconfined (i.e. open to the atmosphere). Pipe inner diameter is 7.1mm

A diagram of the setup is

Any help would be highly appreciated.

 

[VPbris]

The way I see is the following

If there is a flow, then there is a pressure difference. At the end of the piping (at the burner) the pressure will be atm, and until it reaches there, there will be a gradient in pressure along the piping.

So depending on the distance from the pressure source, you most likely will get different readings of pressure along the pipe.

 

[Compositepro]

There is no reason to measure the pressure after the rotameter. If the pressure there is significant then the piping to the burner is not designed correctly. What would you do with this information, anyway? The only adjustment you have is the needle valve. The flow and pressure that results have a fixed relationship.

 

[VCas]

I understand that there is nothing I can do. My concern is regarding the flow measured by the rotameter. If the pressure after the needle valve is significantly higher than atmospheric, I would need to make a correction on the rotameter measurements (I want to know the mass flow rate of the gases going into the burner).

 

[EdStainless]

All of our meters have the needle valves after them. This way the pressure in the meter (and the calibration corrections) are based on the supply pressure.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[LittleInch]

Depends entirely on what level of accuracy you're aiming for.

At low pressures like this it might make 5-10% difference in mass flow so if you can add a tapping point and take a manometer reading - why not.

Be aware though that you seem to be operating at the outer ranges of a rotameter. Depending on what its max reading is, anything less than 10 to 15% of the max reading gets quite inaccurate, so your 10l/min of air supply might be 7 or 13. similarly at 90-100% of readings it will get less accurate. Make sure you understand the uncertainty of the metering device you're using from the manufacturer.

You might need two different range meters in parralell and just use one for low flow and one for higher flow.

As said, all depends on how accurate you need your measurements.

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

 

[VCas]

All of our meters have the needle valves after them. This way the pressure in the meter (and the calibration corrections) are based on the supply pressure.

That is a good suggestion. Unfortunately I don't think I will be able to change the setup too much.

At low pressures like this it might make 5-10% difference in mass flow so if you can add a tapping point and take a manometer reading - why not.

This is what I was looking for. My initial thought was that I could count on the pressure being very close to atmospheric after the rotameter. I was a bit worried that I was missing something and that the pressure would be 2 or 3 bar gauge after the air rotameter instead of being more close to 0 bar gauge. Or in other words, that the needle valve would not generate much of a pressure drop. In fact, I got a question regarding this, what would happen if the needle valve was close to open (especially in the air rotameter), wouldn't that increase considerably the operating pressure in the rotameter and affect the mass flow rate? Or is that just dumb to think and the needle valve will always get the pressure close to atmospheric in this case?

You might need two different range meters in parralell and just use one for low flow and one for higher flow.

Yes, in fact I got rotameters in parallel. I just didn't want to complicate too much the question.

Thanks a lot for the answers!

 

[LittleInch]

For systems like this you need to work backwards from known pressures. If, as we are led to believe, your burner is basically a atmospheric pressure then that is your start point.

I can't be bothered to work out how much pressure drop you're going to get in a 7.1mm ID line at the combined gas and air flows you have, but lets assume it's no more than 100 mbar. That is your pressure at the junction point. Then work along the branches. Hence at B1,2,3 at the flow rates you mention, you're never going to be more than say 200-300 mbarg.

For the air system if your valve was fully open, with 5 bar upstream you'll be a critical flow. Depending on the fully open Cv of your valve you could be getting a lot of air and yes, the pressure will increase, but for 2m of tubing it's difficult to see it getting above 1 barg at which point you won't be mixing any gas as its pressure is max 1 barg, though its flowing pressure might be lower given the number of bends and changes in ID.




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

 

[georgeverghese]

There will be some pressure drop at the burner itself, which is related to total gas flow and gas density at the burner inlet. You can get information on how to calculate burner dp from burner catalogues.
Obviously, the way you've set it up now, the rotameters are measuring actual gas volumetric flow, which in this case, is related to the actual gas pressure at the rotameter.