Sunday, September 23, 2018

Principle of Operation of Positive Displacement Flowmeters

Positive displacement flowmeters have a cyclic mechanism designed to pass a fixed volume of fluid through with every cycle.
Every cycle of the meter’s mechanism displaces a precisely defined (positive) quantity of fluid, so that a count of the number of mechanism cycles yields a precise quantity for the total fluid volume passed through the flowmeter.
Example of Rotary Displacement flowmeter is shown below:

Rotary Displacement Meter


Each shaft revolution in a Rotary Displacement flowmeter represents a certain volume of fluid that has passed through the meter.
Besides, Rotary Displacement flowmeters, we have other types like Diaphragm meter (Bellows type), Liquid sealed drum (wet gas meter), Pistons etc.

Advantages of using Positive Displacement Flowmeters

Positive displacement flowmeters are immune to swirl and other large-scale fluid turbulence, and can be installed anywhere in a piping system. There is no need for long sections of straight of straight-length pipe upstream or downstream as with the case of Ultrasonic Flowmeters. Positive displacement flowmeters are also very linear, since the mechanism cycles are directly proportional to fluid volume.

Limitations of Positive Displacement Flowmeters

The sealing surfaces of rotating mechanisms are subject to wear and accumulating inaccuracies over time. The finely machined construction of a positive displacement flowmeter can suffer damage from abrasive materials like grit present in the fluid, meaning that these types of flowmeters are only used for clean fluid flow streams.


Applications of Positive Displacement Flowmeters
Positive displacement meters are commonly used for custody transfer of gas. Positive Displacement meters can measure high viscosity clean liquids as high as 1 Million centipoise, they also find application in water flow measurement.
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