Ask Greg McMillan
We ask Greg:
What role do you see dynamic simulation playing in the future of developing the best Flow Measurement Specifications?
Greg's Response:
Lack of recognition of the adverse effects of changes in process stream composition and operating conditions, upstream and downstream piping, and velocity profile in differential pressure (d/p) measurements and vortex meters being mistakenly pervasively used. The extensive problems with impulse line fill and length are additional problems for d/p measurements. Dynamic simulations that include the effect of installation on 5Rs noted last week and first principle relationships to show the effect of stream composition and operating conditions on flow measurement can lead you to the best solution and avoid the following common mistakes:
Top Ten Mistakes in Flow Measurement Specification
Lack of recognition of adverse effect on accuracy and rangeability of
- Density changes
- Viscosity changes
- Low flows
- Large meter size
- Large flow span
- Low Reynolds Number
- Erratic nonuniform velocity profile
- Insufficient upstream piping straight run
- Insufficient downstream piping straight run
- Accuracy in % span instead of % reading
All items apply to differential pressure (DP) flow measurements and vortex meters. DP flow measurements have the significant additional problem of impulse lines and capillaries detailed in the Control Talk columns “Prevent pressure transmitter problems” and “Your DP problems …”. A vortex meter rangeability is less than expected because a line size meter often has a maximum velocity that is much greater than the velocity associated with the vortex meter span max flow.
The rangeability and accuracy of magnetic flowmeters and Coriolis flowmeters is typically 5 and 25 times better, respectively than DP and vortex flowmeters. Plus, their accuracy is in % reading instead of % span, greatly helping accuracy at low flows. For more knowledge, see the Control Talk column “Knowing the best is the best” and the ISA book Essentials of Modern Measurements and Final Elements in the Process Industry (use promo code ISAGM10 for a 10% discount on Greg’s ISA books).