Flow Measurement in Q&A (Part3/3)

Continuing in this last part with the used flow measurement techniques ..

Review Part-1 & Part-2, then we could continue ..

Starting with "Turbine" flowmeter from the below categorization:

Q13. What is the operating principles for "Turbine Flowmeter"?

Turbine flowmeter uses a free-spinning turbine wheel, with a rotating blade velocity proportional to fluid velocity (gas or liquid). Blade tip velocity is sensed using a magnetic sensor, generating a voltage pulse each time one of the turbine blades passes by. Frequency of this signal is proportional to the fluid velocity, and therefore volumetric flow rate.

We may express this relationship in the form of the following equation:

The following video is simplifying the turbine flowmeter principles in less than 9 min - Enjoy watching:

Q14. Check the following for True/False:

14.1 "Coasting" is one of the "Turbine Flowmeter" limitations. (True/False)

14.2 "Turbine Flowmeter" maintenance is very minimum. (True/False)

14.3 AGA report #7 regulates "Turbine Flowmeter" usage for "Custody Transfer". (True/False)

Answers for Q14:

14.1 True - Flow turbine keeps coasting for a while after fluid sudden stop, which leads to inaccuracy. (Workaround solution is to make the control system ignore the flowmeter signals after "shutoff valve" reaches the shut position)

14.2 False - One of its critical maintenance is turbine bearing lubrication. Bearing is needed to be lubricated frequently in scheduled maintenance, which is critical to maintaining the flowmeter accuracy.

14.3 True - American Gas Association has published AGA #7 report specifying installation and flow rate calculation - including (turbine speed, pressure and temperature) - for using turbine flowmeter in a "Custody Transfer". Report is published on the internet.

Switching to "Vortex Flowmeter" ..

Q15. What is the operating principle for "Vortex Flowmeter"?

Vortex flowmeter is based on "Vortex shedding" phenomenon, where the moving fluid past a "stationary object (bluff body)" and form a series of alternating vortices on either side of the object. Frequency of these "alternating vortices" is detected by a pressure sensor and it is directly proportional to the fluid velocity, and therefore volumetric flow rate.

We may express this relationship in the form of the following equation:

The following video is simplifying the vortex flowmeter principles in less than 4 min - Enjoy watching:

Q16. Check the following for True/False:

16.1 "Vortex Flowmeter" maintenance is very minimum. (True/False)

16.2 "Vortex Flowmeter" is not preferred to be used with erratic flows. (True/False)

16.3 "Vortex Flowmeter" stop working below a specific flowrate. (True/False)

Answers for Q16:

16.1 True - Vortex flowmeter has no moving parts, so it does not suffer the problems of wear and lubrication that facing turbine meters.

16.2 False - As there are no moving elements in vortex flowmeter that could “coast” as in a turbine flowmeter if fluid flow suddenly stops, so it is better suited to measuring erratic flows.

16.3 True - This is one of the vortex disadvantages, that it has a low flow cutoff - where the flowmeter simply stops working below a certain flow rate. This makes vortex flowmeter unsuitable in applications where the desired flow measurement range extends all the way down to zero.

Switching to "Magnetic Flowmeter" ..

Q17. What is the operating principle for "Magnetic Flowmeter"?

"Magnetic Flowmeter" is based on "electromagnetic induction" principle, as when "conducting fluid" moves perpendicular to a "magnetic field" a voltage is induced in that conductor perpendicular to both "magnetic flux lines" and "direction of motion". This induced voltage is proportional to the conducting fluid velocity - The following formulas are used:

The following video is simplifying the magnetic flowmeter principles in less than 5 mins - Enjoy watching:

Q18. Check the following for True/False:

18.1 Magnetic flowmeters are valid for conductive liquids only. (True/False)

18.2 Semi-filled pipeline flow is valid with Magnetic flowmeters. (True/False)

18.3 Magnetic flowmeter must be properly grounded during installation. (True/False)

18.4 Magnetic flowmeter utilizes electromagnet coils to generate the magnetic flux instead of permanent magnet. (True/False)

Answers for Q18:

18.1 True - Fluid must be conductive.

18.2 False - Pipeline must be completely filled to ensure contacts between both probes across the pipe.

18.3 True - This is to avoid stray current in the liquid.

18.4 True - If permanent magent is used with the flowmeter, this will cause "polarization", so electrically charged molecules (ions) will tend to collect near poles of opposite charge ( flowmeter electrodes). This "polarization” would soon interfere with the detection of the motional EMF.

Switching to "Ultrasonic Flowmeter" ..

Q19. What is the operating principle for "Ultrasonic Flowmeter"?

We have 2 types: Doppler Flowmeter & Transit-Time Flowmeter.

For "Doppler Flowmeter", it is based on "Doppler Effect" principle, where flowmeter is sending a sound wave and receiving the reflected wave from a bubble or a particle in the flow stream, measures the "frequency shift" and get the flow velocity - The following formula is used:

For "Transit-Time Flowmeter", it is using pair of opposed sensors to measure the "time difference" between 2 sound pulses traveling with/versus the flow stream - The following formula is used:

The following video is simplifying the ultrasonic flowmeter principles in less than 4 mins - Enjoy watching:

Q20. Check the following for True/False:

20.1 Temperature compensation is mandatory with "Doppler Ultrasonic Flowmeter". (True/False)

20.2 Temperature compensation is mandatory with "Transit-Time Ultrasonic Flowmeter". (True/False)

20.3 "Ultrasonic Flowmeter" is sensitive to fluid flow profile. (True/False)

20.4 AGA report #9 regulates "Ultrasonic Flowmeter" usage for "Custody Transfer". (True/False)

Answer for Q20:

20.1 True - As temperature is affecting the density which - in turn - affects the medium sound speed (parameter in flow metering calculation).

20.2 False - As per its above equation in Q19, there is no sound speed in the medium at all .. which makes "Transit Time Flowmeter" immune to the density change.

20.3 True - Flow profile changes with Reynolds Number as per below illustration:

A popular way to mitigate this issue is to use multiple sensor pairs, sending acoustic signals along multiple paths through the fluid (i.e. a multipath ultrasonic flowmeter), and to average the resulting velocity measurements.

20.4 True - Pressure /Temperature measurement here is mandatory as ultrasonic flow metering o/p in volumetric and it is needed to be in Mass.

Will not cover "Variable Area Flowmeters" or "Positive Displacement Flowmeters" in this series. Switching now to "Coriolis Mass Flowmeter" ..

Q21. What is the operating principle for "Coriolis Flowmeter"?

Coriolis flowmeter is based on "Coriolis Effect" principle. The flowmeter is simply consists of two U-tubes, one positioned directly above the other, shaken in complementary directions by a common electromagnetic force coil. There are 2 "magnetic sensors" monitor the relative motions of the tubes and transmit signals to an electronics module for digital processing.

When there is no flow through tubes .. the measured frequency of tubes shaking is at their mechanical resonant frequency.

When the flow starts through tubes .. the inertia of the moving fluid adds a slight twisting to the tubes instead of just shaking back and forth. This twisting motion is directly proportional to the mass flow rate.

We may express this relationship in the form of the following equation:

The following video is simplifying the Coriolis flowmeter principles in less than 4 mins - Enjoy watching:

Q22. Check the following for True/False:

22.1 Coriolis Flowmeter elements are interchangeable, so we could fix one by installing parts from another spare flowmeter. (True/False)

22.2 Density could be obtained directly from Coriolis Flowmeter. (True/False)

22.3 Internal RTD of Coriolis Flowmeter is optional, to measure the fluid temperature. (True/False)

22.4 AGA report #11 regulates "Coriolis Flowmeter" usage for "Custody Transfer". (True/False)

Answers to Q22:

22.1 - False - Every Coriolis flowmeter element (the tube and sensor assembly) is unique, with no two identical in behaviour. We cannot interchange elements and transmitters without re-programming the transmitters with the new elements’ physical constant values.

22.2 - True - Measured frequency "f" from the above equation is a direct indication for density as the Coriolis tube volume is fixed.

22.3 - False - Internal RTD is mainly used to monitor the tubing temperature continuously (With its internal running fluid) to compensate for the resulting elasticity and dimensional tubes changes based on prior modelling of the tube metal characteristics for exact/accurate runtime flow metering.

This way, Coriolis flowmeter is giving o/p signals for Mass Flow, Density and Temperature.

22.4 - True

Switching to "Thermal Flowmeter" ..

Q23. What is the operating principle for "Thermal Flowmeter"?

Thermal flowmeter is based on "Wind Chill" phenomenon. It consists of a flow tube with two inside temperature probes, one is heated up and the other is a temperature sensor. The "heated probe" is cooled down as the flow rate increases while the other temperature sensor gets the "differnetial temp." and compensates by increasing current through the "heated probe" to bring its temperature back up to temperature setpoint. The amount of electrical power is a direct function of mass fluid flow rate.

The following video is simplifying the thermal flowmeter principles in less than 5 mins - Enjoy watching:

Q24. Check the following for True/False:

24.1 Thermal flowmeter is sensitive to fluid specific heat. (True/False)

24.2 Thermal flowmeter is sensitive to fluid flow profile. (True/False)

24.3 "Hot Wire Anemometer" is a simple form of Thermal flowmeter, and it is used to measure the air speed. (True/False)

Answers for Q24:

24.1 True - “Specific heat” is a measure of heat energy amount needed to change the temperature of a substance by some specified amount. This means that fluid specific heat value must be known and remain constant. For this reason, thermal mass flowmeter is not suitable for measuring the flow rates of fluid streams whose chemical composition is likely to change over time. This is an instrument limitation.

24.2 True - Turbulent flows are more efficient at convecting heat than laminar flows. Therefore, a change in flow regime (from turbulent to laminar, and vice-versa) will cause a calibration shift for the thermal mass flowmeter, Which is another limitation.

24.3 True - It consists of a metal wire through which an electric current passes to heat it up. If air speed past the wire increases, more heat will be drawn away from the wire and cause its temperature to drop. The circuit senses temperature change and compensates by amount of electrical power proportional to air speed.

Coming to the end of this flow measurement series .. Hope to be useful.