Pumps – Centrifugal Pumps

Almost every cleaning system incorporates at least one pump.  Understanding how that pump works and how to treat it with the gentle, loving care it deserves is critical to the successful operation and maintenance of a cleaning system.

Pumps are, in general, devices which impart added pressure to a flow of liquid.  Although there are a number of different kinds of pumps, centrifugal pumps make up the overwhelming majority of those used in cleaning systems.  Centrifugal pumps are simple, efficient, reliable, relatively inexpensive, and easily meet the needs of most cleaning system requirements including spraying, overflow sparging, filtration, turbulation and just moving liquids from one place to another using pressure.

A centrifugal pump uses a combination of angular velocity and centrifugal force to pump liquids.  The illustration below describes, generally, how a centrifugal pump works.

The pump consists of a circular pump housing shaped somewhat like a donut with an inlet where the donut hole would be.  The outlet extends tangentially from the diameter of the pump housing.  Inside the pump housing there is a device called an “impeller” which rotates perpendicular to the central axis of the pump housing and is driven by a shaft secured to its center of rotation.  The shaft, frequently powered by an integral electric motor, enters the pump housing through a liquid tight seal which prevents leaking.  Liquid entering the pump through the inlet is swirled in a circular motion and displaced from the rotation center of the impeller by centrifugal force.  The combination of the swirling action (angular velocity) and centrifugal force (radial velocity) push the liquid being pumped out of the pump through the outlet.

Things to Know About Centrifugal Pumps –

Centrifugal pumps are relatively maintenance-free except for the seal on the rotating shaft.  These seals need to be replaced on a regular basis.  The material of construction of the pump and the pump seal need to be selected based on the liquid being pumped.  Liquids containing abrasives (particles shed from grinding wheels and lapping compound for example) often require special consideration for the seal to prevent premature wear.  In extreme cases, a secondary supply of abrasive-free liquid is supplied directly to the wear surfaces of the seal.

Pump seals must be kept lubricated and cooled.  Both lubrication and cooling are normally provided by the liquid being pumped.  Because of this, centrifugal pumps should not be operated without liquid being present in the pump.  Failure of the pump seal due to overheating as a result of friction may happen in less than a minute of operation without liquid in the pump.

Centrifugal pumps must have a “flooded inlet” when liquid pumping starts.  In simple terms, liquid should arrive at the inlet of the pump as a result of gravity.  The pump, which has relatively large clearances between the impeller and the pump body, will not provide a vacuum to lift liquid against the force of gravity.  In some instances, once the pump is “primed” with liquid, it can work against the force of gravity.  “Priming” is accomplished by filling the pump and the plumbing leading to the pump with liquid prior to turning it on.  Operation in this mode is not encouraged as the “priming” step is often overlooked.

The impeller in a centrifugal pump MUST rotate in the correct direction.  In pumps operating from a single phase electrical source, this is not a problem.  In pumps operating from a three phase electrical source, however, the proper phasing sequence of the three wires supplying power to the pump must be correct.  If any two wires are reversed, the three phase motor will rotate in the reverse direction.  Most centrifugal pumps have an arrow indicating the proper rotation direction located either on the pump body or on the motor.  If the pump motor rotates in the direction opposite that indicated on the pump, reverse any two wires in the supply to change the direction of rotation.

The basic centrifugal pump described above takes on many different configurations depending on its intended use.  Further description of these variations of the centrifugal pump as well as other types of pumps will be provided in upcoming blogs.

–  FJF  –