Flow Measurement

What is a Paddle Wheel Flow Meter?

A variation on the theme of turbine flow measurement is the paddle wheel flow meter, a very inexpensive technology usually implemented in the form of an insertion-type sensor.

In this instrument, a small wheel equipped with “paddles” parallel to the shaft is inserted in the flow stream, with half the wheel shrouded from the flow.

Paddle Wheel Flow Meter

A photograph of a plastic paddle wheel flow meter appears here:

Paddle wheel flow meter principle

A surprisingly sophisticated method of “pickup” for the plastic paddle wheel shown in the photograph uses fiber-optic cables (optional) to send and receive light.

One cable sends a beam of light to the edge of the paddle wheel, and the other cable receives light on the other side of the paddle wheel.

Principle of Operation

As the paddle wheel turns, the paddles alternately block and pass the light beam, resulting in a pulsed light beam at the receiving cable.

The frequency of this pulsing is, of course, directly proportional to volumetric flow rate.

The external ends of the two fiber optic cables appear in this next photograph, ready to connect to a light source and light pulse sensor to convert the paddle wheel’s motion into an electronic signal:

A problem common to all turbine flow meters is that of the turbine “coasting” when the fluid flow suddenly stops.

This is more often a problem in batch processes than continuous processes, where the fluid flow is regularly turned on and shut off.

Problems of Flow Meters

This problem may be minimized by configuring the measurement system to ignore turbine flow meter signals any time the automatic shutoff valve reaches the “shut” position.

This way, when the shutoff valve closes and fluid flow immediately halts, any coasting of the turbine wheel will be irrelevant.

In processes where the fluid flow happens to pulse for reasons other than the control system opening and shutting automatic valves, this problem is more severe.

Another problem common to all turbine flow meters is lubrication of the turbine bearings. Frictionless motion of the turbine wheel is essential for accurate flow measurement, which is a daunting design goal for the flow meter manufacturing engineers.

The problem is not as severe in applications where the process fluid is naturally lubricating (e.g. diesel fuel), but in applications such as natural gas flow where the fluid provides no lubrication to the turbine bearings, external lubrication must be supplied.

This is often a regular maintenance task for instrument technicians: using a hand pump to inject light-weight “turbine oil” into the bearing assemblies of turbine flow meters used in gas service.

Process fluid viscosity is another source of friction for the turbine wheel. Fluids with high viscosity (e.g. heavy oils) will tend to slow down the turbine’s rotation even if the turbine rotates on frictionless bearings.

This effect is especially pronounced at low flow rates, which leads to a minimum linear flow rating for the flow meter: a flow rate below which it refuses to register proportionately to fluid flow rate.

If you liked this article, then please subscribe to our YouTube Channel for Instrumentation, Electrical, PLC, and SCADA video tutorials.

You can also follow us on Facebook and Twitter to receive daily updates.

Read Next:

You've successfully subscribed !
Share

Recent Articles

  • PLC Tutorials

VFD Simulator Download – Free Yaskawa V1000 Software

VFD simulator download: Master the online tool from the Yaskawa V1000 & programming software for…

2 days ago
  • PLC Tutorials

Conveyor Sorting Machine PLC Program with Calculation Function

The conveyor sorting machine is widely used in the packing industries using the PLC program…

5 days ago
  • PLC Tutorials

Example of Flip-Flop PLC Program for Lamps Application

Learn the example of flip-flop PLC program for lamps application using the ladder logic to…

6 days ago
  • PLC Tutorials

STAR DELTA Programming using PLC Controller

In this article, you will learn the STAR DELTA programming using PLC controller to start…

6 days ago
  • Instrumentation Design

Single and Three Phase Immersion Electrical Heaters Wiring Circuits

Lube oil consoles of rotary equipment packages in industrial process plants are usually equipped with…

3 days ago
  • Instrumentation Design

Immersion Heaters in Lube Oil Consoles

Rotating equipment packages such as pumps, compressors, turbines need the lube oil consoles for their…

5 days ago