upscale venturi tube pressure drop

[Dainuri]

Hi All,

I have a problem with Venturi Tube,
The venturi was designed with pressure drop 0 - 2500 mmH2O. Actually when the process online, the Flow Indicator showed always the max flow rate where theoretically the flow rate was only 80% from the maximum. I have checked all of the instrumentation device, and all is okay. so, the problem is the venturi has upscale pressure drop.

my question, if we assume that there is no mistake in the design,
what things can cause the upscale problem?

Thanks for reading, and I am waiting for your help.

Best Regards,
Akhmad Dainuri

 

[danw2]

Note that changes in geometry error drive Orifice Plate DP lower/downwards (along with the indicated flow rate), but changes in venturi geometry does the opposite: the error drives venturi DP error higher/upwards.

What is the Normal Change in Accuracy?

Experience has shown the discharge coefficient of the Venturi meter generally drops with a change in the internal surface condition of the meter and/or any change to the cross-sectional tolerance of the meter from its original “as built” geometry. In addition, a change from a sharp edge piezometer to a rounded edge will also result in a drop in the value of the coefficient.

Because of its position in the flow formula for Venturi meters, if the coefficient drops due to change in tolerance, tap effect or surface irregularities, the indicated flow rate will be higher than it actually is — this is called “over-registration.” If annular chambers are in use, the pressure produced by the high-and low-pressure rings may or may not be impaired due to internal plugging; depending on what the result is (lower high pressure, lower low pressure or both), the differential produced by the comparison of the high-and low-pressure signals internally to the differential pressure transmitter will be in error and the effect can be either under-registration or over-registration.

http://www.primaryflowsignal.com/im...ers/PFS_whitepaper_Venturi_Meter_Accuracy.pdf

 

[Dainuri]

thanks danw2,

for additional information, the venturi tube is installed vertically, and the flow is from up to down.
do you have any recommendation for vertical installation?

Best Regards,
Akhmad Dainuri

 

[Dainuri]

thanks danw2,

for additional information, the venturi tube is installed vertically, and the flow is from up to down.
do you have any recommendation for vertical installation?

Best Regards,
Akhmad Dainuri

 

[roydm]

I am assuming this is liquid you are measuring

Flow down is always a problem, any entrained air has the chance of getting trapped in the impulse lines.
I would suggest you have the transmitter located below the Venturi with the impulse lines sloping down and toward transmitter with no flat spots where air can accumulate. If there are any entrained solids run the impulse lines down several feet below the transmitter diaphragm with a blowdown valve at the bottom tee into these and slope back to the transmitter with the air bleeds at the top.

If it's gas of course everything will be in reverse.

If you are measuring the venture with a DP cell did you remember to set the transmitter OR the DCS input for square root extraction?
One, not both, don't ask me how I know

 

[danw2]

Roy has a point about double square rooting.

The square root of 64% max DP will report 80% of max flow rate. If that 80% of max flow rate signal is then square rooted, the result is a fictitious almost 90% (89.44%) of max flow rate. (I don't have to ask, because it's happened to me, too.)