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The Flaws of Flow Meters

Introduction

Pipelines in every process industry are outfitted with flow meters to detect the actual amount of fluid in the system. All flow meters are utilized for automatic control of the flow to process equipment in this age of automation. Variable area (Rotameter), differential pressure (orifice, venturi), electromagnetic, turbine, vortex, and other types of flow meters are available on the market.

In this blog, we will look at how flow meters function and how process characteristics, circumstances, and practices affect flow detection. It will also provide some suggestions for making them precise and valuable.

Problems and Guidelines

  • If a change in molecular weight is foreseen, make a molecular weight adjustment. A change in the molecular weight of the process fluid exposes an inconsistency in flow measurement. When the fluid's molecular weight (MW) is reduced, flow metering equipment frequently detects less volumetric flow than is present, and vice versa. A compressor's suction flow measurement provides surge protection, and furnace fuel gas is used to heat at the correct fuel gas flow rate to get an appropriate temperature. When the MW of the fuel gas is reduced, it detects less and admits more fuel gas into the furnace, resulting in a drastic change in furnace temperature.
  • In general, as time passes, the orifice plate erodes and the area grows, detecting fewer delta P, but the real flow is extremely high. Therefore, use proper venturi nozzles instead of orifice plates where there is a risk of erosion, such as steam.
  • Compensate the flow with greater and more varied temperatures and pressures, such as steam and hot gases of compressor discharge.
  • Always double-check and install orifice flow in the correct direction. Higher than average flow is detected during reverse installation.
  • To avoid liquid vaporization during normal operation, always position the flow measuring equipment at the highest pressure point (less pressure drop and no flashing).
  • Open the equalization or bypass valve twice a year to check the zero in the flow metering device. Check the flow measuring device tapping and flush in congealing service/hydrate/rust/salting areas where there is a high risk of plugging. Even if the real flow is larger, upstream tapping plugging will detect a lower flow and downstream tapping plugging will detect a higher flow.
  • Always give some purge of compatible fluid to maintain the clean flow measuring device tapping (where process fluid is not clean).
  • Always keep an eye on the velocity of the purge fluid. The flow measurement would be affected if the purge flow or velocity is higher. The device detects the velocity of the purge fluid, also.
  • Provide adequate support to avoid any flow-induced or equipment-related vibration. These vibrations are picked up by the vortex and coriolis meters and generate errors.
  • Check the completely developed flow condition and accordingly mount flow measuring devices at all times; completely developed entry length is determined by Reynolds number, which varies with fluid velocity. It means that fully developed length varies with throughput/process flow.
  • Avoid any dual phase flow in the flow measuring device. The vortexes, as well as magnetic detectors, are affected by air or gas bubbles.
  • Wherever there is a possibility of reverse flow, always install a bidirectional flow measurement instrument that detects it.
  • Always install flow measuring equipment in the pipeline where fully formed volume presents and check elbow, tee, and any obstacles in line.
  • Wherever the flow measurement instrument is positioned on a vertical loop, it must be compensated for a vertical head loss.
  • In the horizontal liquid line, eccentric orifice should be used to make it free draining and avoid any accumulation of liquid.
  • The flow measuring device taps for the transmitter must be used in accordance with the service fluid. Lower tapping is required for liquid service, while top tapping is required for gas delivery.

“Learning from your own experience is good but learning from the mistakes of others is wise. It will help to avoid any perilous event before it happens.”

 

References

Lieberman N.P., Lieberman E.T., “A Working Guide to Process Equipment,” McGraw-Hill Education (Fourth edition 2014). 

McMillan G.K., Douglas M.C., “Process/Industrial Instruments and Controls Handbook,” McGraw-Hill Fifth Edition (1999).

Abhishek Sharma
Abhishek Sharma
Abhishek Sharma is a process engineer at an ethylene plant. He has more than four years of experience in the steam cracker unit. He completed his B.Tech in chemical engineering from the National Institute of Technology Raipur, India with honors and completed process equipment design and process plant design courses from IIT Roorkee and IIT KGP, respectively. He is an active professional member of the American Institute of Chemical Engineers and an associate member of IChemE. He is currently involved in the smooth operation and troubleshooting of the plant.

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