Turbine flow meter back pressure – a few points

A turbine flow meter primarily works with the principle that a fluid flows into a pipe and rotates a turbine which provides readings in order to provide analytical data regarding the flow. But there are some precautions which always need to be maintained in order to make the flow meter work properly. One of the primary conditions with regards to obtaining an accurate measurement of the flow meter is to take care of the turbine flow meter back pressure. The next question that arises in this context is what the back pressure is. For this answer, let us consider a pipe on which a turbine flow meter is installed in order to take measurements. Now let us say that there is a bend in the pipe just before the flow meter is encountered. This might not seem like a big factor, but it affects the reading by a huge amount. This is due to a pressure drop caused in the cross section of the transportation pipe due to the change in the flow of the fluid.

The process of dealing with turbine flow meter back pressure is to normalize the pressure of the fluid before it reaches the flow meter, even if the flow on a whole is not fully straight. This can be achieved only if there is at least ten times the diameter of the pipe in straight length section before the fluid encounters the flow meter. But it is not enough to take into account the amount of length required upstream alone. It is just as much important to consider the minimum amount of downstream pipe length as well. This needs to be a minimum of 5 diameters in order to prevent back pressure from occurring.

Preventing back pressure is one thing and another to prevent cavitation. Cavitation refers to entrapped air in the fluid, causing damage to the turbine and a serious amount of error in the reading. This is caused by too less back pressure on the flow of the fluid. It can usually be corrected by providing an extra pressure which is 1.25 times the vapour pressure and 2 times the pressure drop through the flow meter. It can be accomplished by providing for the minimum amount of flow requirement before the discharge after the flow meter.

As can be seen from the above discussion, a flow meter loses its accuracy even when faced with something as simple as the turbine flow meter back pressure. Hence, necessary precautions should always be included in order to provide for the best of results.