(14a) Centrifugal Pumps 101 for Process Engineers

This paper will present an overview of important concepts for the process/systems engineer regarding selection of centrifugal pumps from the perspective of the rotating equipment engineer. These concepts were chosen based on the relative control of the process/systems engineer over these items and their impact on pump selection.

The paper will begin with an overview of the theory behind how a centrifugal pump adds energy to a liquid stream to provide the discharge pressure required by the process. In this section, the effect of head and flow rate on the appropriate speed, size and type of pump will be discussed.

The paper will then move into items that affect the performance of the pump, namely net positive suction head (NPSH) and minimum flow. NPSH required by the pump and its association with flow rate and pump speed will be discussed. An explanation will also be given concerning the importance of respecting pump minimum flow requirements because of its effect on pump longevity.

Switching gears slightly, the paper will then consider how the pump selection affects downstream piping. It is desirable that the pump discharge pressure increase with decreasing flow (head rise to shut-off) in order to better control the pump with reasonable precision. Downstream piping must be designed to take this increase in pressure. Methods available to the pump manufacturers for setting the slope of this head rise will be discussed as well as margins imposed by API for motor driven and turbine driven pumps.

Finally, the paper will consider the selection of an Ethylene product pump with these concepts in mind. In addition, end-user preferences on pump orientation for this service will be discussed and why it is important to consider this factor in setting the parameters of upstream process conditions.