Page 105 - Root Cause Failure Analysis

96 Root Cause Failure Analysis

the viscosity increases, the pump must work harder to deliver a constant volume of fluid
to the discharge. As a result, the brake horsepower needed to drive the pump increases to
keep the rotating speed constant and prevent a marked reduction in the volume of liquid
delivered to the discharge. If the viscosity change is great enough, the brake horsepower
requirements may exceed the capabilities of the motor.

Temperature variation is the major contributor to viscosity change. The design specifi-
cations should define an acceptable range of both viscosity and temperature for each
application. These two variables are closely linked and should be clearly understood.

Rotating Speed
With positive-displacement pumps, output is directly proportional to the rotating
speed. If the speed changes from its normal design point, the volume of liquid deliv-
ered also will change.

Suction Supply
To a degree, positive-displacement pumps are self-priming. In other words, they have
the ability to draw liquid into their suction ports. However, they must have a constant
volume of liquid available. Therefore, the suction-supply system should be designed
to ensure that a constant volume of nonturbulent liquid is available to each pump in
the system.

Pump performance and its useful operating life are enhanced if the suction-supply
system provides a consistent positive pressure. Pumps required to overcome suction
lift must work harder to deliver product to the discharge.

Installation
Installation requirements for positive-displacement pumps are basically the same as
those for centrifugal pumps. Those requirements were discussed previously in this
chapter.

Special attention should be given to the suction-piping configuration. Poor piping
practices in hydraulic-system applications are primary sources of positive-displace-
ment pump problems, particularly in parallel pump applications. Often the suction
piping does not provide adequate volume to each pump in parallel configurations.

Operating Methods

If a positive-displacement pump is properly installed, there are few restrictions on
operating methods. The primary operating concerns are bypass operation and speed-
change rates.