Selecting a hydraulic flow meter

23 August, 2017

4. What effect might the flow meter have on the fluid and vice-versa?

This ‘effect’ can be measured by the energy loss due to the presence of the meter, better known as the pressure drop (∆P) across the device. This has two effects: Increasing the upstream pressure and generating heat.

5. How important is it to measure flow; what is your budget?

A better question might be, ‘What happens if you don’t measure flow?’ If the answer is ‘Nothing; it’s just one of many indicators we look at’, then you already know that you’re working with a very small budget. However, if the answer is, ‘the PLC will think that the machine lubrication has failed and stopped working’, then you know how much you might budget to stop that from happening.

The most common types of hydraulic flow meter are as follows:

Variable orifice flow meters

The idea of flow displacing an object, usually a piston or a ring, forms the basis of simple ‘variable orifice’ flow meters. The momentum of the fluid exerts a force on a piston that is held in place by a spring. Such meters typically have an accuracy of between 2 – 5% of full scale.

Gear type flow meters

These are positive displacement flow meters. On the inside, they look similar to a gear-type motor. Fluid passes around the outside of a pair of intermeshed gears, rotating the gears on their shafts. A transducer mounted above one of the gears generates a pulse each time a gear tooth passes under it. The rotation of the gears is proportional to the flow rate. Sometimes, two transducers are used to measure direction and improve resolution.

Turbine-type flow meters

In a turbine-type flow meter, a turbine rotor is mounted on a shaft between two sets of flow straighteners. The fluid passes through the flow meter and rotates the turbine blade. As for a gear-type flow meter, a transducer is mounted above the turbine and generates a pulse each time a blade passes under it. The frequency from the transducer is proportional to the flow over a limited range.

Other meter types

Oval gear meters are similar to conventional gear meters, but use two elliptical gears that rotate together at 90 degrees to one another inside a housing. The fluid is swept around the chamber by the gears, and the frequency of rotation is directly related to the volume of fluid through the meter.

The above article is an edited and abbreviated version of the 8-page ‘Guide to selecting a hydraulic flow meter’, published by Webtec.

www.webtec.com