Understanding how a Positive Displacement Flow Meter Works

Positive Displacement flow meters是唯一可以通过流量计直接测量液体量的流量测量仪器。这些仪表通过在高精度室内旋转组件之间捕获少量流体来实现这一目标。可以反复用液体填充烧杯，并在计算填充烧杯的次数的同时将内容物倒入下游，这是一种可比的方法。

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

如果为给定的应用程序选择了正确的流量计，则预计它将在很多年中执行任何错误。PD流量计通常被发送用于维修和重新校准，许多乐器已经在现场使用了10个，并且在很多情况下连续使用20年。这种可靠性很大程度上是由于这项已久经考验的技术已有60多年的历史，并且允许将重大进步集中在摩擦学领域，并以负担得起的成本实现了所需的精度。

低维护

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

燃料和油的正排量流量计

Benefits Summary

• High level of accuracy (0.5% as standard) higher level of accuracy available upon request
• 能够处理大量流体粘度<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 thePD meter;即使它很小，也应该考虑进行系统计算。如上所述，当使用润滑性能较差的加工流体时，建议您首先向分销商寻求建议；这些应用程序都可以使用各种选项。

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