Overpressure and Dry Run Protection for Positive Displacement Pumps

This diagram shows the relationship of flow rate, head (or pressure) and power for a typical positive displacement pump at a given speed.

In any positive displacement (PD) pump there is internal leakage, also known as slip, which together with internal friction losses determine the efficiency of pumping. The slip will increase with pressure as shown. As pressure developed by the pump increases, so does the motor power.

All PD pumps have a maximum working pressure and all manufacturers warn that the pump must not be used beyond this limit. If overpressure occurs, this can cause damage to the pump due to shaft deflection or gasket failure and to the mechanical seal which can quickly fail.

Adjacent pipework and equipment can also be damaged. This can be very dangerous. Overpressure can be caused by closing a discharge valve or simply through trying to pump too much liquid through the pipework resulting in high pressure drops through the system. This can often happen when pumping viscous liquids if for example, the viscosity increases.

There are two methods of ensuring this does not happen. The first is to fit a full bore pressure relief valve. This will lift before the maximum pressure is reached and relieve the flow back to tank.

This however can be costly and in the case of food and pharmaceutical pumps add to the cleaning requirement.

The second method is to use a power control monitor mounted in the control panel. The monitor is set to cut out at minimum "A" with a time delay to prevent nuisance trips during start up or temporary interruption to the flow and also at a maximum "B" which corresponds to a power below that absorbed by the pump at its maximum pressure and below the motor power. It is recommended that no time delay is used at "B" as PD pumps can build pressure very quickly when a valve is closed.

So for only £220.00 you can protect your pump from damage caused by overpressure and dry run.

Compare this with the cost of a mechanical seal! Email or call Epump today to discuss further, or read the technical information provided on this web site.