If you use a flowmeter to measure flow rate, you must consider the effect of back pressure. In some processes, sufficient back pressure will exceed maximum operating pressure causing some component of the process to fail. This translates to undesirable downtime and potential cost implications.
Anything connected to the flowmeter, including valves, elbows, and other structural elements, will create back pressure. In many cases, this is fine as long as the pressure generated does not exceed the maximum. For some low-pressure applications, such as an in-house air flow supply or a cylinder tank, back pressure may be minimal. For higher pressure uses in process control and research, such as in a reaction vessel pressurized system, the lines could burst from additional back pressure unless there is a relief valve. To determine what your back pressure is or will be, use a pressure gauge.
Causes of Back Pressure
- The flowmeter itself is undersized. The accuracies provided by manufacturers and suppliers are full scale. It is best not to get too close to this specified maximum when choosing a flowmeter for your process application. For example, if the accuracy is listed as 5 liters per minute and your actual flow rate is 4.9 liters per minute, this proximity to the maximum could cause pressure buildup. Selecting the appropriate flowmeter size from the start is a worthwhile investment, even if other models are more economical. Installing an undersized flowmeter will ultimately prove more costly.
- A blockage in the line produces excessive pressure. Anything that impedes the flow in the line will cause a surge in pressure, whether the blockage is from material buildup, a high level of particulates or sludge, or some other obstruction. Detecting blockages as the source of pressure changes can be challenging. Of course, removing the blockage should normalize back pressure.
- The fluid is too viscous (heat it, if possible) or the velocity is too high (increase the piping diameter/re-size the meter).
- The design of the plumbing setup. If you are experiencing consistent back pressure issues and your flowmeter is the right size for your process, it may be time to take a critical look at your setup. If elbows are positioned too close to the meter itself or to one another, back pressure will increase. Avoid creating unnecessary restrictions in the line by including a fair amount of straight runs in the setup.
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