Positive Displacement Flowmeters |
| How Positive Displacement Flowmeters Work |
| Positive displacement flowmeters repeatedly entrap fluid to measure its flow. It 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 represents the flow. Many positive displacement flowmeter geometries are available. |
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| 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 electronic 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. |
| 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. In liquid service, increasing viscosity decreases slippage and increases the pressure drop across the flowmeter. A large pressure drop across the flowmeter can prematurely wear and/or damage bearings and/or seals. Note that flowmeter size may increase to reduce the pressure drop in these applications. |
| Be careful because damaging the sealing surfaces by will 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 measurement taken with bubbles will be higher than the true liquid flow because the bubble volumes will be measured as if they were a volume of liquid. A gas eliminator may be required to remove bubbles. |
| 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. |
| Corrosive liquid applications are commonly found in the chemical industry processes, and in chemical feed systems used in most industries. |
| 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 fail catastrophically --- they 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 slip exhibited by different fluids can cause measurement error. |
