Chemical Equipment Update
Positive displacement meters: pros, cons and selection
Charles Wemyss sets out to differentiate between some of the most popular types of positive displacement meters and provide some guidance to selection.
P
ositive displacement flowmeters, sometimes known as PD meters, have been around for more than 100 years. They are commonly used in a wide range of applications from domestic water measurement to measuring
ultra flow rates of chemical at high pressures subsea. First off – what is a ‘positive displacement’ meter? Well,
as the name suggests it involves the positive displacement of a volume of fluid – this is usually a liquid but there are some units suitable for gas. There is a chamber and inside the chamber, obstructing the flow, is a rotor. The shape of the rotor and chamber
vary greatly with each meter type but they all provide an output for each rotation. Most meter designs therefore lend themselves to being totalisers. Most can have the flow rate calculated from this primary data.
Advantages
An accurate PD meter will have minimal ‘leakage’ across the rotor seal. This is generally minimised with the use of more viscous liquids and accuracies of ±0.1per cent are sometimes quoted. On the other hand rotary piston flowmeters are used by the water industry in the UK for measurement of water over a normal flow range to accuracies of ±2 per cent. Because they measure a volume
precisely it does not matter if the flow is pulsing. They will follow the increase and decrease of flow found in reciprocating pumps of all types. With higher viscosities the turndown
Fig. 1. Helical Screw – outstanding long term accuracy and rangeability. Copyright KRAL.
ratio can be high. Even with water 100:1 is not uncommon and 3000:1 is possible at 250cSt. Few applications require this but it does enable measurement of ultra low
flow rates without miniature parts or normal flow measurement at minimal pressure drop. Most meters are simple to maintain as they have only one or
two moving parts and are coupled with simple readouts that are easily understood in the field. There is no requirement for straight pipe lengths like that
Fig. 2. Primary measurement by Wet Gas meter ±0.2per cent. Photo courtesy of Ritter.
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might be needed for electromagnetic or turbine devices. They can be connected directly to elbows or valves and in most cases in a variety of orientations. Designs are relatively easy to adapt for high pressure applications eg over 100 bar.
Disadvantages
All PD meters require clean fluid so a filtration level of 100µm is usual. Some meters can actually block the flow if a larger particle is trapped in the wrong place. Many meters are not made in high specification materials
and therefore corrosion can be a concern. An all plastic or all 316SS meter is the exception rather than the rule. As the application flow rate increases the size of the PD
meter seems to increase by a square law! It is rare to find meters over 12-in in size although they exist at these elevated sizes for the prime reason of accuracy – frequently being utilised for custody transfer reasons. In the author’s opinion, the most common PD meters are as
follows:
❒ Rotary Piston: As mentioned above these form the basis of domestic water measurement but the design of the rotary piston that oscillates in a circular chamber with a fixed web has been modified and extended to ultra low flows and high
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