Positions of static pressure sensors for orifice pressure drop meas.

[Feldmann359]

Dear colleagues!

I have to measure the pressure drop of orifices.
Now I am reflecting on my meausrement method.
In the vicinity (up- and downstream) of an orifice,
the flow is disturbed.
After the orifice, the static pressure increases again
while the streamlines are attaching again at the pipe walls.

Now, where, in diameters, I have to measure the pressures,
if I want to determine the correct pressure drop over
the orifice?

Is there a method to calculate this and the pressure
increase after the orifice?

Thanks and greetings,
Stephan

 

[FredRosse]

ASME Book, Fluid Meters, Their Theory and Application, details the unrecoverable losses of orifices, as a percentage of the maximum differential (at the Vena Contracta) pressure. They also define the location of the Vena Contracta taps.

The flow distance for complete recovery (to measure the overall loss) is not defined in the ASME reference, and is complicated by the fact that wall frictional losses grow with increasing distance from the orifice. However, for all orifices the unrecoverable loss approaches 100% of the Vena Contracta signal as the area ratio of the installation (Aorifice / Apipe) approaches zero. Considering your rather small orifices, you can probably meet the condition of the area ratio approaching zero. Also be aware that the ASME publication has special considerations for installations below two inch diameter.

 

[JLSeagull]

For flow measurement in piping we normally use flange taps and a paddle type orifice. Quick change fittings accomplish the same measurement for pipelines etc. Another technique for measuring the dp is using pipe taps instead of orifice flanges.

Both API and ASME standards include factors depending upon the location of the static pressure tap. API MPMS 14.3 reflects the downstream tap for orifice flange connections. The pressure measurement can be absolute or use a gauge pressure transmitter and adjust for atmospheric pressure. I did not check the standards for using pipe taps or the ISO standard.

 

[hacksaw]

Is this the 1/2 to 2 mm orifice of a previous post?

closup taps or even "flange taps" would be almost meaningless without special calibration.

i don't think paddle orifice plate are available for this bore size

regardless any d/p meter of that size requires custom calibration. There are flow labs that do this.

I've dealt with critical flow nozzles with bores of such dimensions and those are available with traceable calibrations at non critical flows.

 

[Feldmann359]

Thank you very much for the helpful posts!

But, I am not going to perform flow measurements, I have to measure the dp of the orifices. The orifices will later be uesed as trimming/calibration devices in a hydraulic application.

Hacksaw, yes, it is still the same issue

Today I again looked at the discharge coefficient formula:

cd/sqrt(1-ß**4)=(dm/dt)/(A0*sqrt(2*rho*dp)) and the fact that I get cd values larger than one, what is physically nonsense.

So, is it possible that the dp in the formula above is the orifice dp WITHOUT the pressure recovery? So if I would place the downstream static pressure tap directly after the orifice (and not at 8 * diameter downstream, I would measure the a higher dp and the discharge coefficient would be lower ?

Thanks in advance for your comments!
Greetings,
Stephan

 

[FredRosse]

I believe your basic equation is incorrect. The term (dm/dt)is representing mass flow? This term should actually be “Q”, the volumetric flowrate.

Your equation must be dimensionally consistent, without exceptions, and it will prove itself dimensionally consistent in any rational units system, Metric, English, etc.

When checking your dimensions, be sure to differentiate between force and mass, we always label pounds mass as LBm, and pounds force as LBf, they are not interchangeable.

In the English system, they are related by Newton’s
F = M * A

1 LBf = 1 LBm * 32.174 ft / sec2