How Differential Pressure Flowmeters Work
Differential pressure flowmeters use Bernoulli's equation to measure the flow of fluid in a pipe. Differential pressure flowmeters introduce a constriction in the pipe that creates a pressure drop across the flowmeter. When the flow increases, more pressure drop is created. Impulse piping route the upstream and downstream pressures of the flowmeter to the transmitter that measures the differential pressure to determine the fluid flow.
Bernoulli’s equation states that the pressure drop across the constriction is proportional to the square of the flow rate. Using this relationship, 10 percent of full scale flow produces only 1 percent of the full scale differential pressure. At 10 percent of full scale flow, the differential pressure flowmeter accuracy is dependent upon the transmitter being accurate over a 100:1 range of differential pressure. Differential pressure transmitter accuracy is typically degraded at low differential pressures in its range, so flowmeter accuracy can be similarly degraded. Therefore, this non-linear relationship can have a detrimental effect on the accuracy and turndown of differential pressure flowmeters. Remember that of interest is the accuracy of the flow measurement system --- not the accuracy of the differential pressure transmitter.
How to Use Differential Pressure Flowmeters
Differential pressure flowmeters inferentially measure the flow of liquids, gases and vapor in pipes, such as water, cryogenic liquids, chemicals, air, industrial gases, and steam. Be careful using differential pressure flowmeters for fluids with high viscosity, such as some hydrocarbons and foods, because their accuracy can be degraded when Reynolds number is low.
This flowmeter can be applied to relatively clean fluids. With proper attention to materials of construction, the flow of corrosive fluids, such as are found in the chemical industry, can be measured.
Somewhat dirty fluids can be measured by purging the impulse piping with an inert fluid. Be careful when using differential pressure flowmeters in dirty services because the dirt can plug the impulse piping and cause incorrect measurements. Diaphragm seals can sometimes be applied in these applications, however one should remember that diaphragm seals can degrade the performance of the differential pressure transmitter system, and hence, the degrade the performance of the flow measurement system.
Differential pressure flowmeters are generally applicable to many flows in most industries, such as mining, mineral processing, pulp and paper, petroleum, chemical, petrochemical, water, and wastewater industries. Other flow measurement technologies may perform better than differential pressure flowmeters in many applications, however differential pressure flowmeters are still used extensively due to long-standing user familiarity with the technology.
Application Cautions for Differential Pressure Flowmeters
Because of the nonlinear relationship between flow and differential pressure, the accuracy of flow measurement in the lower portion of the flow range can be degraded. Plugging of the impulse piping can be a concern for many services. For slurry service, purges should be used to keep the impulse piping from plugging.
For liquid service, impulse piping should be oriented and sloped so that it remains full of liquid and does not collect gas. For gas service, impulse piping should be oriented and sloped so that it remains full of gas and does not collect liquids. In vapor service, vapor may be allowed to condense in some of the impulse piping to form a liquid seal between the hot vapor and transmitter in order to protect the transmitter from heat.
Be careful because the calibration of the differential pressure transmitter can be affected by the accumulation of liquid or gas in the impulse tubing. In addition, the accuracy of the flow measurement system can be degraded when varying amounts of liquid can accumulate during operation.
Calibration issues can be important to the successful application of this technology. For example, differential pressure transmitter removal for calibration exposes the transmitter to multiple sources of potential problems that can affect the measurement, not the least of which is the extent to which the transmitter tubing is re-tightened after calibration. Calibration should generally be performed in-situ when possible and provisions to do so should be addressed during the design phase. For example, the differential pressure transmitter can be purchased with an integral valve manifold that allows easy calibration without disconnecting impulse tubing.
Gas applications should be designed carefully because changes in operating pressure and operating temperature can dramatically affect the flow measurement. In other words, the gas density can vary significantly during operation. As a result, the differential pressure produced by the flowmeter can also vary significantly during operation. Failure to compensate for these effects can cause flow measurement errors of 20 percent or more in many applications. In these applications, a flow computer can be used to calculate the corrected flow measurement using actual pressure, temperature and flow measurements.