For the correct application of any centrifugal pump there needs to be a flow of liquid through the pump which will give a cooling flow at the mechanical seal, or in the case of magnetic drive pumps through the bearings and magnetic coupling area. Running a pump dry, or with a closed valve, will stop this flow and heat will be gnerated in these areas.  In addition, only part of the power transmitted by the shaft is converted into useful work by the pump - this defines the efficiency of the pump.  The inefficiencies of impeller design and surface finish means that waste energy will be converted to heat which will heat any liquid in the pump casing.

This diagram shows a pump running normally with full suction pipework, NRV completely open, with a good flow of cooling liquid through the pump.  Current and power absorbed is normal.  Pump runs smoothly and cool.

At the start of dry run, the suction pipework begins to empty.  Initially there is still cooling flow through the pump and the NRV is still open.

As liquid level approaches suction level, vortex forms in the pipework drawing air into the pump impeller.

Motor power and current reduces - this is measurable and important in prevention of dry run.

Vibration and recirculation begins, eventually the impeller eye is vapour bound and flow through the pump stops.  The NRV closes.

Circulation to the mechanical seal or magnetic coupling and bearings stops.

Mechanical seal begins to run dry and starts to heat up.  Alternatively the bearings start to heat up due to friction, the containment shell (if metallic) and magnets heat up due to eddy currents.

Any liquid in the casing is forced to the outsides of the pump and away from impeller eye. This liquid heats up quickly and can vaporise - potentially a safety problem where chemicals are concerned.  The air/liquid mix acts similarly to cavitation and can damage the pump casing and impeller.

Continuous vibration with potential damage.  Mechanical seal or bearing breakdown is likely with potentially catastrophic pump failure.

Some magnetic drive pump manufacturers offer specially coated bearings which they say will run dry for a while without damage.  These will still suffer from vibration and potential damage due to heating liquid in the casing and air/liquid swirl causing damage.

Dry running any centrifugal pump is like running a car without oil - not recommended.  There are several simple methods of avoiding dry run - why not use them?