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Basics of Pump Selection

October 01, 2013
Richard Burkhalter
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When it comes to selecting a pump for a given process, the choices seem endless, rendering the effort somewhat confusing and time consuming.  However, if the primary objectives in pump selection are kept in mind, the road to selecting a pump that meets the pumping needs and is both operator and maintenance friendly can be made somewhat smooth.  The objectives are most likely to be achieved when three issues are carefully considered and evaluated.  They are:

  1. Application
  2. Installation
  3. Operation

The fluid to be pumped and the conditions under which the pumping will be done define the pump application. Furthermore, operating conditions, such as, the chemistry of the fluid, or the temperature and pressure of the operating system, must also be carefully considered when defining the application.  For example, low viscosity fluids like many mineral and synthetic base oils are easily pumped and can be readily handled by a centrifugal or a sliding vane pump.  Additionally, the temperature and pressure of the fluid to be pumped not only affects the flow characteristics of the fluid, but also impacts the construction of the pump; i.e., clearances, materials of construction, and seals.  On the other hand, high viscosity fluids, such as high viscosity base oils and additives, are generally pumped with a positive displacement pump.  Again, physical and chemical characteristics of the fluid can heavily influence the pump selection.  Therefore, defining the fluid properties can narrow the field of options substantially.

Unlike Newtonian fluids that have fairly definable and consistent flow characteristics, non-Newtonian fluids behave differently, thus making them more challenging to pump.  At rest, their “apparent” viscosity is very high, giving the appearance of nearly being like a semi-solid.  However, when the non-Newtonian fluid is in motion, the “apparent” viscosity is generally reduced.  The degree of motion within the fluid, or, turbulence, is measured in terms of a shear rate.  The exact definition of a shear rate is somewhat too complex to cover in this paper in detail.  However, as a practical approximation, it can be thought of as a difference between the moving boundary and a stationary boundary, and then divided by the distance between these boundaries.  Lubricating grease is an example of a non-Newtonian, pseudo-plastic, fluid that behaves quite differently than a viscous base oil like Bright Stock.  Deciding which “apparent” viscosity and flow rate to provide the pump vendor requires an understanding of the non-Newtonian characteristics of each grease being pumped.  Though many greases are similar based on their thickener type and NLGI grade, some greases have very different fluid properties.  Therefore, there can be significant risk to proper pump selection when assuming grease fluid (rheological) properties.

The installation is another factor that affects pump selection.  Pipe size and configuration, as well as the anticipated operation of the pump can greatly affect pump selection.  When considering a non-Newtonian fluid, such as lubricating grease, larger pipe size does not necessarily correlate to increased or improved throughput.  Furthermore, piping configuration can also be critical in that the number and type of fittings in the discharge line can increase the overall pressure requirements to a point beyond the capabilities of many positive displacement pumps, not to mention the increase in power requirements.

Though often not given sufficient consideration, the pump suction configuration can also have a dramatic impact on pump selection as well as performance.  High speed, low cavity volume pumps are severely affected by suction line configuration whereas large cavity volume, slow rotation speed pumps tend to be more forgiving, especially when pumping lubricating grease.  The most significant effect of low suction pressure is the propensity for creating a low vacuum and cavitation.  At low pressure, a pump may start to cavitate which results in the sealing chamber being filled with a mixture of fluid and vapor (or even all vapor).  This does not maintain a good heat rejection rate away from its seal chamber. When surrounded by a mixture of liquid and vapor, the seal overheats quickly, and can fail prematurely.  In addition, cavitation can induce air into the grease that is subsequently finely dispersed through the shearing action within the pump.  Generally, suction piping design is the same for Newtonian and non-Newtonian fluids; i.e., positive net suction pressure, usually provided through an elevated source and low pressure drop piping configuration.  Again, with grease, low pressure drop suction lines do not necessarily translate into larger diameter pipe.

The type of operation must also be carefully considered when selecting the pump.  Will the pump operate continuously, or will it be used in a utility type function, running only intermittently?  Will the operation be critical to the process? The answers to these and other questions will have a direct impact on the pump selection process.  For example, when pumping lubricating grease, there is often substantially greater power required in the initial start-up of the pump as compared to that required when the pump is continuously in operation.  The reason for that is a combination of the thixotropic behavior of the fluid, and the change in the coefficient of friction from static at rest, to dynamic at motion.  Therefore, frequent, intermittent pumping, can lead to early pump fatigue, mechanical seal failure, and excessive power requirements.

Summary: This article does not attempt to cover all of the aspects of pump selection.  However, it is intended to generally relate the issues in to be considered and developed during the pump selection process.  Pumping of Newtonian and non-Newtonian fluids have definite nuances, which must be understood for proper application of pumps and their sizing.  Common to both types of fluids are the effects on suction pressure, discharge pressure and power consumption.  Creating a design that mitigates the adverse effects of the three issues listed above will generally ease the process of selecting the appropriate pump for the process.

Editor's Note: definitions for Newtonian vs. non-Newtonian fluids can be found in STLE's glossary.

R.L. Burkhalter works at Covenant Engineering Services, LLC.

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