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Ask The Experts...

PERMANENT PRESSURE LOSS

Permanent pressure loss is a term every system engineer, designer, or technician should be aware of. Whenever a piece of equipment or pipe is added to a flow system, pressure is lost. This pressure loss makes the pump or compressor work harder to generate the same flows in the system. If too much pressure loss exists, the system will simply stop flowing. This may be of concern if you are working with both low and high pressure systems. Every bit of pressure loss is equal to extra energy used (electricity, steam, or natural gas) to pump or compress the fluid, i.e. more money to operate.
In the case of flowmeters, a loss is incurred because a piece of straight pipe would not have as much loss as the flowmeter. The loss is also permanent.
Permanent pressure loss should not be confused with pressure drop. Meters such as differential pressure-types have a pressure drop inside the meter section. The pressure measured upstream of the meter will be greater than the pressure just downstream of the meter. As you move further downstream of the meter, the pressure recovers to a level not quite as high as the upstream pressure. The difference between the upstream pressure and the downstream recovered pressure equals the permanent pressure loss.
Fluid velocity also plays an important role in permanent pressure loss. The faster the fluid is moving, the greater the pressure loss. Therefore, a permanent pressure loss value must always be associated with a certain flow
rate. Meter manufacturers often state the permanent pressure loss at the maximum stated velocity of the meter.
There are many different meter types and all have different characteristics of permanent pressure loss. Some meters have no restriction in the pipe, so therefore no permanent pressure loss. In other words, they incur the same loss as a straight piece of pipe. For example, magnetic meters and ultrasonic meters generally have no permanent pressure loss.
Other meters have a very high loss. These meters have physical restrictions due to the nature of the meter. Examples of high loss meters include curved-tube type coriolis flowmeters and positive displacement meters.
Permanent pressure loss is just one of the characteristics to consider when evaluating a flowmeter. A meter with a low loss is not necessarily better than a meter with a high loss. Every characteristic of the meter technology must be weighed according to the needs of the application.

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Copyright 1999 McCrometer

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