Learn the basics of design considerations when specifying a pump

Centrifugal pumps make up nearly three quarters of the industrial pumps in use today and are commonly used in the power generation sector. An understanding of the design principles and an awareness of the latest developments can make a big difference when designing, specifying, maintaining or replacing a centrifugal pump in order to deliver long term reliability.

As a vital piece of equipment within a power generation site, centrifugal pumps need to be correctly specified and maintained to deliver efficient and reliable service. This process of specification begins with understanding the basic terminology, principles and design characteristics to ensure that each pump can meet its full potential and deliver the required performance.

Every pump has a performance graph that should be used to determine the suitability of the pump for a particular application. For a range of flows, the graph indicates the generated head, power requirement, efficiency and the net positive suction head required (NPSHr).

Each pump design has an optimum flow rate, which occurs at the best efficiency point (BEP). In this example, the normal operation conditions (80 to 110 percent of BEP) and the preferred operating region (70 to 120 percent of BEP) are on either side of the BEP.

Pump efficiency is a crucial factor when designing a pumping system, since 95 percent of the lifetime costs for a pump will be the energy it uses.

Expert assistance from guy in blue shirtImage 1.Expert assistance from original equipment manufacturers (OEMs) can reduce downtime. (Images and graphics courtesy of Sulzer Pumps)

It is important to correctly specify any new pump and also ensure that the performance of existing pumps is checked when the operating characteristics of an application change. A comprehensive pump assessment can deliver significant savings as well as reduced payback periods when additional investment is required.

In addition to the pump curves, the system curve is another important tool when defining the correct pump for an application. Every system will incur additional frictional losses from piping, valves, strainers and reducers. These losses are proportional to the square of the flow rate and are measured in feet (or meters) of head.

pump performance graphFigure 1. Pump performance graph

The overall system curve is comprised of the frictional resistance and the static head, which is the net difference in height between the suction liquid level and the discharge liquid level. Pump designers use both the system curve and the pump curves—along with additional information such as fluid specific gravity and viscosity—to select the most appropriate pump for an application.

Parallel or Series Solutions

In some situations, the specification of a centrifugal pumping system will involve multiple pumps, either in parallel or series configurations. This enables a number of smaller pumps to deliver much greater flow or head, depending on the pump arrangement, which may be preferable to a much larger, single pump.

Extensive testing blue shirtImage 2. Extensive testing and refinement improves performance.

When working with pumps in series, it is important to understand that although the pumps may be hydraulically the same, the designs may be different. Any pump located upstream of the initial pump will be operating at a higher pressure and so the castings, shaft diameter and pipework will need to be correctly rated for the higher pressure.

Understanding Specific Speeds

Modern pumps are classified by their specific speed, which is a dimensionless quantity that describes the geometry of the pump’s impeller. Specific speed is a correlation of pump capacity, head and speed at optimum efficiency, which classifies pump impellers with respect to their geometric similarity. The specific speed of an impeller is defined as the revolutions per minute (rpm) at which a geometrically similar impeller would run if it were of such a size as to discharge one gallon per minute (gpm) against one foot of head.

Similarly, the suction specific speed is a rating number, which is also dimensionless, that indicates the relative ability of centrifugal pumps to operate under conditions of low net positive suction head available (NPSHa).
Pumps with a low specific speed, 500 rpm for example, will have radial flow impellers that produce high head but lower flow rates. An axial flow impeller, with a specific speed of 10,000 rpm, will produce high flow rates and low heads, while a mixed flow impeller delivers a compromise between the two.

design principles remainImage 3. The design principles remain the same, regardless of scale.

Avoiding Problems

A correctly specified pump should deliver decades of reliable service, provided that the correct maintenance is performed and the original characteristics of the application do not change significantly. One of the most common faults in a centrifugal pumping system is cavitation.

When a liquid enters a centrifugal pump, the pressure drops as it flows from the suction flange, through the suction nozzle and into the suction eye of the impeller. The amount of pressure drop is a function of many factors, including pump geometry, rotational speed, frictional losses, hydraulic shock losses and flow rate.

If the pressure at any point within the pump falls below the vapor pressure of the liquid being pumped, vaporization or cavitation will occur.

Typically, cavitation is brought about by a reduction in suction head, initially being signaled by the formation of vapor bubbles, causing an increase in vibration levels and a slight increase in pump noise.

Gradually the output head begins to drop, and at 3 percent the drop is considered as a readily measurable data point for the pump suction performance. After this point, the pump becomes very noisy. Full cavitation is initiated and the output head drops off dramatically.

Discovering the Best Solution

Every application will have its own challenges, and it is the task of the pump designer to deliver the solution. Centrifugal pumps will often provide the answer, but their internal design and external dimensions will determine their efficiency.