Dry running
Dry running is usually undesirable in a centrifugal pump it occurs in the total absence of the liquid component of the fluid handled (e.g. following the ingress of air in the suction line) or if under normal operating conditions gas bubbles (see Formation of air pockets) attach themselves to normally wetted rotating components (partial dry running).
During normal operation of a well-designed centrifugal pump, the fluid handled completely fills the flow space inside the pump, including narrow controlled gap seals on impellers, gland packings and mechanical seals. The liquid helps to cool and lubricate the components in contact with each other and exercises a centring action in the clearance gaps of the impellers and shaft passages (see Multistage pump) so that long and slender ring-section pumps for example are able to run without the rotor touching the casing.
In the absence of liquid, dry running can occur in certain areas because of insufficient cooling and centring action. The consequences are overheating, abrasion, seizure of the materials, vibrations and other phenomena which may in due course lead to the complete disintegration of the pump.
If the pump operator cannot avoid such instances of absolute or partial dry running, it may be necessary to invest in the optimisation of the centrifugal pump's design. An improved pump design should include reinforced shafts which prevent radial contact between the rotor and the casing, specially designed clearance gaps (controlled gap seal) and mechanical seals, gland packings and bearings which are supplied with external lubricating or barrier fluids rather than using the fluid handled.
Pumps fitted with a hydraulic axial thrust balancing device must be equipped with an additional thrust bearing to prevent rubbing contact or seizure. In the event of incipient dry running, the pump can also be stopped by a dry running protection device.
Self-priming centrifugal pumps must always be filled to a certain liquid fill level prior to start-up in order to be able to self-prime. During the self-priming phase they operate under partial dry running conditions.
