CENTRIFUGAL PUMP OPERATION
Centrifugal Pump Protection
A centrifugal pump is dead-headed when it is operated with no flow through it, for example, with
a closed discharge valve or against a seated check valve. If the discharge valve is closed and
there is no other flow path available to the pump, the impeller will churn the same volume of
water as it rotates in the pump casing. This will increase the temperature of the liquid (due to
friction) in the pump casing to the point that it will flash to vapor. The vapor can interrupt the
cooling flow to the pump's packing and bearings, causing excessive wear and heat. If the pump
is run in this condition for a significant amount of time, it will become damaged.
When a centrifugal pump is installed in a system such that it may be subjected to periodic shutoff
head conditions, it is necessary to provide some means of pump protection. One method for
protecting the pump from running dead-headed is to provide a recirculation line from the pump
discharge line upstream of the discharge valve, back to the pump's supply source. The
recirculation line should be sized to allow enough flow through the pump to prevent overheating
and damage to the pump. Protection may also be accomplished by use of an automatic flow
Centrifugal pumps must also be protected from runout. Runout can lead to cavitation and can
also cause overheating of the pump's motor due to excessive currents. One method for ensuring
that there is always adequate flow resistance at the pump discharge to prevent excessive flow
through the pump is to place an orifice or a throttle valve immediately downstream of the pump
discharge. Properly designed piping systems are very important to protect from runout.
Gas binding of a centrifugal pump is a condition where the pump casing is filled with gases or
vapors to the point where the impeller is no longer able to contact enough fluid to function
correctly. The impeller spins in the gas bubble, but is unable to force liquid through the pump.
This can lead to cooling problems for the pump's packing and bearings.
Centrifugal pumps are designed so that their pump casings are completely filled with liquid
during pump operation. Most centrifugal pumps can still operate when a small amount of gas
accumulates in the pump casing, but pumps in systems containing dissolved gases that are not
designed to be self-venting should be periodically vented manually to ensure that gases do not
build up in the pump casing.
Priming Centrifugal Pumps
Most centrifugal pumps are not self-priming. In other words, the pump casing must be filled with
liquid before the pump is started, or the pump will not be able to function. If the pump casing
becomes filled with vapors or gases, the pump impeller becomes gas-bound and incapable of
pumping. To ensure that a centrifugal pump remains primed and does not become gas-bound,
most centrifugal pumps are located below the level of the source from which the pump is to take
its suction. The same effect can be gained by supplying liquid to the pump suction under
pressure supplied by another pump placed in the suction line.