Listen in tutorial: Positive displacement flowmeters
Mechanical technology still thrives in demanding applications.
While positive displacement (PD) flowmeters may represent an old-tech solution when compared to electronic sensing approaches, in some specialized applications, they still provide high precision when working with difficult liquids. ( Listen in: Click here to hear comments from Daniel Turek, product line manager for Max Machinery .)
The PD family includes a group of designs, so there is more than one type. The designs are similar to positive displacement and metering pumps, and in many respects, they are like driving a pump with pressurized liquid.
One of the most popular configurations is a gear meter, which is much like a gear metering pump. The pressurized liquid causes the gears to rotate, and each revolution indicates that a specific amount of liquid has passed through. There also are piston, helical, and rotary vane configurations. Each has its corresponding pump design. The shaft rotation can drive a mechanical readout (such as your home water meter), but more commonly an electronic device counts turns and partial turns of the shaft, which is translated into volume units. PD designs provide a true volumetric reading and are immune to most changes in viscosity.
PD flowmeters have several key advantages:
High accuracy, with
Stable output with relatively infrequent calibration necessary;
Very wide turndown ratios, with 100:1 common and even 500:1 available;
Able to work with difficult, high viscosity liquids, non-conductive liquids, etc.; and,
Scalable over a very wide range of capacities.
Since they are mechanical, they are subject to wear, corrosion, etc. Moreover, if a unit fails and locks up, it can block liquid flow almost completely.
They have relatively high pressure drop, which is the price you pay for wide turndown. The highest turndown designs usually also have the highest pressure drop due to tight internal clearances and more complicated mechanisms.
They have low tolerance for solids. This varies according to design and manufacturer, but debris caught in the wrong place can damage or jam the mechanism.
These points are generalities, so make sure you discuss specifics with your supplier. Given the tradeoffs of advantages and disadvantages, PD designs offer some specialized advantages that are difficult to achieve with any other flow meter technology approach.
Positive displacement flowmeters of various designs are available from a number of suppliers, including:
You can also search online at the Control Engineering Supplier Search .
—Peter Welander, process industries editor, PWelander@cfemedia.com ,
Process Instrumentation & Sensors Monthly
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