How to Use Positive Displacement Flow meters

Positive displacement flow meters measure the volumetric flow of fluids in pipes, such as water, hydrocarbons, cryogenic liquids, and chemicals. Some designs can measure gas flow although liquid flow applications are much more prevalent. In liquid service, increasing viscosity decreases slippage and increases the pressure drop across the flow meter. Surprisingly, accuracy can actually improve at low flow conditions in a given positive displacement flow meter when viscosity increases and slippage decreases. 

A large pressure drop across the flow meter can prematurely wear and/or damage bearings and/or seals. Therefore, most positive displacement flow meters have a maximum pressure drop specification that is intended to limit positive displacement flow meter bearing wear to reasonable levels. Operating the flow meter above the pressure drop limits of the flow meter can result in premature bearing wear and catastrophic flow meter failure. Note that flow meter size may increase to reduce the pressure drop in these applications. This can increase cost significantly but failure to adhere to this specification can be even more expensive in some applications. 

Be careful because damaged sealing surfaces can increase slippage and degrade measurement accuracy. Using positive displacement flow meters 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 flow meter. A filter may be required to remove dirt. 

Be sure that gas bubbles are removed from liquid flow streams when using positive displacement flow meters. Flow measurements taken with bubbles present will be higher than the true liquid flow because the bubble volumes will be measured as if they were a volume of liquid. Therefore, the presence of gas bubbles and (especially) the presence of a varying amount of gas bubbles can adversely affect the flow measurements associated withpositive displacement flow meters. A gas eliminator may be required to remove bubbles and mitigate this problem.