Understanding how a Positive Displacement Flow Meter Works

Positive Displacement flow meters are the only flow measuring instrument that can directly measure the volume of liquid passing though the flow meter. These meters achieve this by trapping small amounts of fluid between rotating components within a high precision chamber. Repeatedly filling a beaker with liquid, and pouring the contents downstream, while counting the number of times the beaker is filled, is a comparable method.

As the flow of fluid is causing the rotation, the rotational velocity is directly proportional to the flow rate.

Electronic flow meters contain magnets within the rotating components, which can activate various sensor options outside of the fluid chamber. On the other hand, mechanical positive displacement flow sensors use the rotation to drive a magnetic coupling, or a direct gear train, connected to a mechanical counter.

PD flow meters do not need a power supply to operate, nor do they need straight upstream and downstream pipe runs to be installed. Positive displacement flow meters range in sizes from in to 12 in and operate with turndowns up to 100:1, although ranges of 15:1 or lower are normal. As slippage between the flow meter components becomes reduced, the metering accuracy increases with the increase in the viscosity of the process fluid.

The process fluid must also be free from contaminants. Particles greater than a hundred microns in size have to be removed using filtering methods. Positive displacement meters operate with minor clearances between their precision-machined parts and wear can rapidly destroy their accuracy. For this reason, PD meters not often recommended when measuring slurries or abrasive fluids. However, in clean fluid services, their precision and wide range make them an excellent choice for custody transfer and batch charging. They are commonly used as household water meters and millions of these units are produced every year. Positive displacement flow sensors are commonly used across industrial and petrochemical applications for batch charging of liquid and gaseous matter.

Even though the slippage through the positive displacement flow meter decreases (i.e. the accuracy increases) as fluid viscosity increases, there is also an increase in the pressure drop through the meter. As a result, the maximum (and minimum) flow capacity of the flow meter is reduced as the viscosity is increased. In short, the greater the viscosity, the lower the amount of slippage and the lower the measurable flow rate becomes.

Are DP Meters Accurate?

One of the key benefits of using a PD flow meter is the level of accuracy that can be obtained, and the high precision from the internal components means that clearances between sealing faces is minimal. How small these clearances are, is relative to how high the accuracy will be. Only the fluid that can bypass this seal is not counted, this is known as ‘by-pass’ or ‘slippage’.

Rotating Positive Displacement flow Meters

Another benefit of these flow meters is their ability to process a wide range of viscosities. It is not uncommon to experience greater levels of accuracy when processing more viscous fluids, simply because of the by-pass. When comparing flow meter accuracies, it is crucial to be knowledgeable of both ‘linearity’ i.e. the positive displacement flow meters ability to accurately measure across the complete turndown ratio, and ‘repeatability’ – the ability to maintain high accuracy over a number of cycles. This is another area where positive displacement flow meters excel, with a repeatability of 0.02% and 0.5% linearity being standard.

Reliability

If the right flowmeter has been chosen for a given application, it is expected to perform without error for many years. PD flow meters are often sent for servicing and recalibration, with many instruments having been in the field for 10, and in a lot of cases, 20 years continuous use. This reliability is largely due to this proven technology being in use for over 60 years and has allowed for the significant advances to be concentrated within the fields of tribology, as well as achieving the required precision at an affordable cost.

Low Maintenance

The required amount of maintenance is heavily reliant on the application. For example, if a flow meter processes a fluid that displays lubrication properties, i.e. oil, then the requirement for maintenance is virtually non-existent. However, if the fluid has poor lubricating properties, then the maintenance requirements will need to be discussed with your distributor and/or expert. It is not common that maintenance to a positive displacement flow meter would be more frequent than other instruments within the same system. Any such maintenance can be scheduled to be carried out at the same time as the other components, thus reducing the downtime.

Positive Displacement Flow Meter for Industrial Processes

Positive Displacement Flow Meter for Fuels and Oils

Benefits Summary

• High level of accuracy (0.5% as standard) higher level of accuracy available upon request
• Ability to process a large range of fluid viscosities <1 cP – >500000 cP
• Low maintenance
• High pressure capabilities
• Electronic and mechanical versions available
• Spare parts availability

Limitations and Considerations

Although positive displacement flow sensors are very robust, they do possess some downsides. Firstly, they should not be used for fluids containing large particles, unless they undergo a pre-filtering process before entering the measurement chamber. They are also unsuitable for applications where large amounts air are present within the fluid; however, air eliminators are available for such applications. One factor that requires serious consideration, is the pressure drop caused by the PD meter; even though it is minimal, they should be factored in for system calculations. As mentioned above, when using processing fluids with poor lubricating properties, it is recommended that advice is first sought from your distributor; various options are available for these applications.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.