Paddle Wheel Flow Meter

Paddle Wheel Flow Meter Working Principle

The working principle of a paddle wheel flow meter is simple. A paddle wheel probe is in contact with the process fluid and, as the fluid flows in the flow meter, it makes the paddle wheel rotate at a speed proportional to the flow rate. A magnetic or an optical sensor detects the movement of the paddle wheel and relays the information to the flow meter which converts it to a flow rate reading.  a paddle wheel in a pipe with a flow stream. The flowing fluid creates a torque on the paddles and makes the paddle wheel rotate. The rotation speed of the paddle wheel is proportional to the velocity of the fluid. The optical or magnetic sensor located near the wheel detects each paddle passage and generates a signal proportional to the flow velocity. The flow rate can then easily be computed using this signal if the inner diameter of the pipe is known.

Some precautions must be taken to ensure the accuracy of flow rate readings when using a paddle wheel flow meter since they are influenced by disturbances in the flow stream caused by valves, pumps, elbows, etc. To minimize disturbances, a straight run of pipe must be present before and after the flow meter. The minimum inlet and outlet pipe length is usually specified in the manufacturer’s documentation.

Ensuring that the flow has the right flow profile is another element essential to obtaining a good accuracy. Most paddle wheel flow meters require a turbulent flow profile to accurately measure the flow rate.

Services :- 

Relatively clean liquids, gases, and vapor.

Sizes :-

Paddle wheel units available for up to 12 in. (305 mm) pipes; insertion turbine probes not limited by pipe size, can also be used in open channels.

Outputs :-

Generally, linear frequency outputs are provided, but 4- to 20-mA DC can also be obtained through conversion.

Operating Pressure:-

Plastic paddle wheel units operable up to 200 PSIG (1.4 MPa) at ambient temperatures.

Pressure Drops :-

Negligible.

Operating Temperature :-

The plastic paddlewheel units operable at up to 220 ° F (105 ° C) if operating pressureis < 25 PSIG ( < 172 kPa).

Materials of Construction :-

The plastic paddle wheel units are made of polypropylene  and metallic parts.

Price :-

The flow element of the plastic paddlewheel units for sizes between 0.5 and 12 in.(13 to 305 mm) costs between $250 and $500. Flow elements can be providedwith analog indicators ($350), digital readouts ($500), recorders ($850), or batchtotalizers ($600).

Paddle Wheel flow meter advantages and disadvantages

Advantage :-  

  • Simple working principle
  • Low cost
  • High repeatability
  • Easy installation

Limitations :-  

  • Require a fully turbulent flow for accurate measurement
  • Not suitable for two-phase flow measurement
  • Subject to wear
  • Poor linearity at low flow rates
  • Not recommended for high viscosity or dirty liquids
  • Require maintenance and cleaning