How Positive Displacement Flowmeters Work
Positive displacement flowmeter technology is the only flow measurement technology that directly measures the volume of the fluid passing through the flowmeter. Positive displacement flowmeters achieve this by repeatedly entrapping fluid in order to measure its flow. This process can be thought of as repeatedly filling a bucket with fluid before dumping the contents downstream. The number of times that the bucket is filled and emptied is indicative of the flow through the flowmeter. Many positive displacement flowmeter geometries are available.
Entrapment is usually accomplished using rotating parts that form moving seals between each other and/or the flowmeter body. In most designs, the rotating parts have tight tolerances so these seals can prevent fluid from going through the flowmeter without being measured (slippage). In some positive displacement flowmeter designs, bearings are used to support the rotating parts. Rotation can be sensed mechanically or by detecting the movement of a rotating part. When more fluid is flowing, the rotating parts turn proportionally faster. The transmitter processes the signal generated by the rotation to determine the flow of the fluid. Some positive displacement flowmeters have mechanical registers that show the total flow on a local display. Other positive displacement flowmeters output pulses that can be used by a secondary electronic device to determine the flow rate.
How to Use Positive Displacement Flowmeters
Positive displacement flowmeters measure the volumetric flow of fluids in pipes, such as water, hydrocarbons, cryogenic liquids, and chemicals. Some designs can measure gas flow although liquid flow applications are much more prevalent. In liquid service, increasing viscosity decreases slippage and increases the pressure drop across the flowmeter. Surprisingly, accuracy can actually improve at low flow conditions in a given positive displacement flowmeter when viscosity increases and slippage decreases.
A large pressure drop across the flowmeter can prematurely wear and/or damage bearings and/or seals. Therefore, most positive displacement flowmeters have a maximum pressure drop specification that is intended to limit positive displacement flowmeter bearing wear to reasonable levels. Operating the flowmeter above the pressure drop limits of the flowmeter can result in premature bearing wear and catastrophic flowmeter failure. Note that flowmeter size may increase to reduce the pressure drop in these applications. This can increase cost significantly but failure to adhere to this specification can be even more expensive in some applications.
Be careful because damaged sealing surfaces can increase slippage and degrade measurement accuracy. Using positive displacement flowmeters in abrasive or dirty fluids can cause maintenance problems because of potential damage to the sealing surfaces, damage to the bearings, and/or plugging of the flowmeter. A filter may be required to remove dirt.
Be sure that gas bubbles are removed from liquid flow streams when using positive displacement flowmeters. Flow measurements taken with bubbles present will be higher than the true liquid flow because the bubble volumes will be measured as if they were a volume of liquid. Therefore, the presence of gas bubbles and (especially) the presence of a varying amount of gas bubbles can adversely affect the flow measurements associated with positive displacement flowmeters. A gas eliminator may be required to remove bubbles and mitigate this problem.
This flowmeter can be applied to clean, sanitary, and corrosive liquids, such as water and foods, and some gases. Materials of construction are important because small amounts of corrosion or abrasion can damage the sealing surfaces and adversely affect measurement accuracy. In addition, consideration should be given to all wetted parts, including the body, rotating parts, bearings and gaskets.
Many positive displacement flowmeters are used in municipal water districts to measure residential water consumption. Considering an installed base of millions of houses and apartments with metered water service, this application represents perhaps one of the largest number of applications of positive displacement flowmeters worldwide.
Corrosive liquid applications are commonly found in the chemical industry processes, and in chemical feed systems used in most industries. However, other flowmeter technologies may be more suitable for these services.
Application Cautions for Positive Displacement Flowmeters
Avoid using positive displacement flowmeters in dirty fluids unless the dirt can be effectively removed upstream of the flowmeter. Operating these flowmeters in dirty fluids can cause plugging and increase maintenance costs. Be careful when selecting bearings because the non-lubricating nature of some fluids, impurities, and dirt can increase bearing wear and maintenance costs. Note that bearings usually do not necessarily fail catastrophically --- they can slow down and adversely affect accuracy before they stop working.
Avoid liquids with gas bubbles unless the bubbles can be effectively removed. As viscosity increases, be sure to ensure that the pressure drop across the flowmeter is acceptable. Make sure that the viscosity of the operating fluid is similar to that of the calibrated fluid, because the different amounts of slippage exhibited by different fluids can cause measurement error.