Orifice flowmeter design and calibration


Thread Starter

R. Manoj

In our plant, we are facing a problem with the measurement of steam flow. Presently we are told that the readings from the flow meter are too high. Can you help us out of this problem.
Check out the calibration of your flow transmitter. I presume you are using a DP type one. Check also your impulse lines on whether they are filled with water to the top(both sides - HP and LP. Remember to isolate the system first. If you can have a line shut down then remove the orifice and examine the orifice 's edges for sharpness and correct size. If the problem is there from the begining on a newly commissioned system you need to check whether the orifice has been correctly sized in the first place.
Dear J. Zafar,
Our company designs and manufactures orifice plates as well as other differential pressure devices such as ASME flow nozzles. Such meters can
prvide highly accurate flow readings if properly designed and installed. Frequently if the errors are large (>10%), then it typically is a major
error in plate size, transmitter range, installation, or pressure &/or temperature compensation (calculation errors). If it is a relatively small error (<10%) then it typically can be related to the exact device such as
rounded edges, eccentric installation, etc.
Not surprisingly because of all of the potential for errors we offer consulting services to resolve such issues.

John Catch
[email protected]

David W Spitzer, PE


This could be caused by any number of issues, the first of which may be what you are told. Perhaps you can better define the problem.


David W Spitzer, PE
Copperhill and Pointer, Inc.
http://www.icu.com/spitzer845.623.1830 (phone/fax)
Is it just talk-talk, push the buck, put the ball in the other court or is steam flow really high.

1. Steam flow calculation:
Get your process engineer-chemical engineer or production engineer or utility engineer to calculate on theory the steam flow. The theories used should allow for practical losses encountered in an old plant.

2. Steam flow checks:
Check the orifice plate data.
Are your operating at the designed pressure.
If not then steam pressure compensation needs to be applied to your flowmeter reading.
Is it superheated steam.
Calculate the flow for the pressure and temperature that it is presently operating at.
If there is some error due to pressue or temperature variation, then you have the culprit, put a good compensation algorithm in your
PLC/Controller/DCS or PC, or use multiplier, square root extractor etc. in pneumatic or electronic systems.

If the pressure and temperature correction does not yield results, the next reason could be that your DP transmitter is out of calibration. Recalibrate your transmitter.

If this too does not yield results, countercheck your orifice by opening the pipes/gas cutting/taking shutdown and go to basics. BS-1042 is a wonderful ally as is LIPTAK or Considine.

Please visit http://business.vsnl.com/deneb_solutions and follow the trail of fun section.

Happy hunting.

Are you compensating to flow indication for the actual line pressure?

If not, then substantial error can be introduced in the flow indication if you are using orifice plate and dP transmitter for the measurement. Check to see if the steam is flowing at the design pressure in the orifice plate calculation. If it varies from design by more than 5 %, compensation is in order.

Although you don't mention your display HMI, in some cases it is possible to perform the square root function of the orifice flow equation in several places, including in the transmitter (with most smart transmitters), at the A/D conversion step, and at the display step. Check each one of these to make sure that there is
ONLY ONE square root extraction occurring in the loop.

Good luck finding the problem.

John Beck
Pennzoil QuakerState Refinery
Shreveport, LA.