A Guide to Turbine Flow Meters

Turbine flow meters belong to a larger class of flow meters that can be classified as “rotating vane flow meters”. This type of flow meter technology includes turbine flow meters, paddle wheel flow meters, and Pelton wheel flow meters. Each version offers a slightly different variation on the shape of the element that is put into rotation by flowing media. If interested, visit our article on paddle wheel flow meters and Pelton wheel flow meters to learn more about these similar rotating vane flow meter options.

Turbine flow meters utilize the motion of the flowing media to mechanically turn an angled, twisted, or bladed rotor that essentially faces the flow “straight on”. The flow is parallel to the axle. Some common examples to visualize the concept are outboard boat motors, windmills, airplane engines, and simple house fans.

As the media moves through the pipe, the turbine begins to spin, and the motion is detected electronically by a magnetic pick-up, an inductive pick-up, or optical detection. The rotational speed of the turbine within the flow is a direct function of volumetric flow rate. A frequency output is then either used directly or converted into a proportional analog signal.

Turbine flow meters offer a lot of advantages.

• Economical price compared to other technology types
• Fast response time and highly repeatable
• Easy to install and maintain
• Universal mounting positions
• Provides direct volumetric flow measurement
• Many can be coupled with monitoring electronics
• Minimal pressure-drop
• Can sense low flow rates

Turbine flow meters have some elements that must be considered before deciding whether they are a suitable choice for an application.

• Over-ranging (running above max flow rate) causes excessive wear
• Ferrous particles must be filtered out upstream of the flow meter for some models
• Magnetic particles in the water can impede the output signal for some models
• Pipes must be consistently full for accurate readings
• They are not typically bi-directional, although some models can be
• They are primarily best suited for clean, low viscosity media
• They have straight piping requirements to reduce flow turbulence, which affects accuracy

For a mechanical means of flow measurement, turbine flow meters offer great accuracy. The only way to guarantee better accuracy than a turbine meter would be to choose one of the flow measurement technologies that do not use a mechanical means of measurement, like magnetic or Coriolis flow meters. Average accuracy from turbine flow meters is 0.5% to 1.5%.

They are relatively more accurate than their other rotating vane counterparts and other mechanical flow meter technology types. Paddle wheel flow meters offer a general accuracy of 2.5% to 5% and Pelton wheel flow meters offer an accuracy of 1.5% to 3%. To see a full explanation of flow meter accuracy for all the flow meter technology types, visit our article.

Turbine flow meter straight pipe requirements depend on some of the variables of the flow profile created by the piping. Standard straight runs for turbine flow meters are usually 10 to 15 times upstream and downstream. However, obstruction or changes in pipe direction increase the straight length requirements. 90-degree pipe shifts, like elbows or tees, require 20 times. Partially open valves require 25 times, and for two elbows or any spiraling flow, it is 50 times. To learn more about straight upstream and downstream pipe requirements, visit our in-depth article.

Because turbine meters are best suited to measuring clean and low viscosity liquids, turbine flow meters are almost always suitable for water flow measurement. They offer a higher-than-average accuracy for a mechanical water flow meter. When considering a turbine flow meter for water, make sure that the water does not have large particulate matter that will build up behind the turbine or damage the turbine mechanism. Also evaluate whether that water is free of magnetic particles and iron.

If the liquid meets the requirements of not being too viscous or full of particulate matter, turbine flow meters are suitable for a wide variety of liquids. Media compatibility can depend on the material of the flow body. Some are constructed of metal like stainless steel and some are constructed of plastic. Plastic models offer a cost savings over metal models and are an economical solution for many simple, lower pressure liquid flow applications.

Inline turbine flow meters are the more common installation type of turbine flow meters. For smaller line sizes, they are generally the installation type of choice. Turbine flow meters are also available in insertion models for larger line sizes. Because they do not contain an integral flow body, they are able to measure flow in larger pipes with higher flow velocities for a much more economical price.

Insertion meters are installed into a piping system by creating an insertion hole. The measuring probe is then inserted into the flow and the electronics above the probe are affixed to the pipe with an integral fitting. When using an insertion turbine flow meter, make sure that the probe is located well within the highest velocity part of the flow stream and ensure that it is not inserted too far, where the turbine mechanism would be impeded by the side of the pipe.

Turbine flow meters are available with electronic outputs and digital displays. Certain digital displays only display the flow rate and others offer process control capabilities such as batching and totalizing.

Turbine flow meters sometimes offer integral or optional flow switch capabilities. They are not typically manufactured as stand-alone, switch only, no flow rate reading models.

Turbine flow meters deliver a great value when better than average accuracy is desired from a mechanical means of flow measurement. They are cheaper than other flow meter technology types without moving parts that deliver similar accuracy, like ultrasonic or vortex flow meters.