Content area
Full text
Ask the Experts
Q. My pump stops pumping for no apparent reason. It is acting as though there is an obstruction in the suction line because it is not generating any head either. Shutting down and inspecting the system reveals that the pipe is clear. Upon re-startup, the pump works fine, until the next incident How can I prevent this from happening?
When a pump stops producing flow, but resumes pumping after it is shut down and restarted, it has more than likely vapor locked. Also known as "gassing off or "losing prime," vapor locking occurs when a critical amount of gas enters a pump and becomes trapped in the eye of the impeller, thereby blocking the influx of fluid. The gas becomes trapped because the centrifugal forces generated by the impeller vanes cause the fluid with a higher density, the liquid, to be thrown outward from the eye, and the gas to concentrate in the center of the eye. The pump, being a very poor compressor, is unable to push out the gas (i.e., when the impeller spins, there is no movement of fluid and no generation of pressure). However, when the pump is shut off, the gas floats to the top. When the pump is restarted, the liquid that has entered the eye of the impeller pushes the gas out of the discharge nozzle. The pump continues to behave as a pump until more gas finds its way inside.
It doesn't take a great deal of gas to vapor lock a pump - as little as 10% by volume of the fluid entering the system. This article discusses a few ways that gas can enter a pump and how to prevent vapor locking.
Vortexing. Gas can enter the pump through a vortex - a mini cyclone that forms on the surface of a liquid above the drain hole. Just as water draining from a bathtub begins to swirl around the drain and eventually forms a vortex that sucks air down the drain, a vortex will also form in liquid discharging from a tank or sump. A surface disturbance will grow from a slow swirl, to a faster swirl, until the surface dimples. The dimple grows deeper and wider. Once the gas-filled tail of...