Meeting Increased Demand for Efficient Pump Designs


Written by:
Dr. David Japikse, Concepts NREC

While turbo pumps cover the full range of specific speeds, the overwhelming majority run at low specific speeds and are comprised of an inlet pipe or duct, an impeller and an exit volute. Recent design advances have been few since some producing companies do not have a single hydrodynamicist on their staffs.

By contrast, industrial process pumps, including boiler feed pumps, have a complex inlet, an impeller of any specific speed (and may require special treatment to deal with higher velocities), a diffuser and either a return channel or a crossover to deliver the flow to the next stage, in the case of the multistage machines. (Industrial process pumps include irrigation pumps and deep well pumps for home or business water supply.)

Progress toward assisting pump designers to deliver superior, efficient pump designs for these complex constructions must be predicated on the basis of possible design improvements, the tools to assist such work and the availability of necessary staff. Improving pump design is a worthy goal; according to The Freedonia Group, Inc., a Cleveland-based industry market research firm, "Global demand for fluid handling pumps is forecast to increase at a 4.4 percent annual rate to $47 billion in 2012." To address this challenge, we will look at what is needed technically and then what is becoming available.

Possibilities for Improvement

Figure 1 is a well-known plot from Stepanoff (1948, 1957), showing roughly the breakout of losses in various pumps. To achieve superior designs, we must mitigate these losses. Similar diagrams exist for radial compressors and turbines because they all have the same core issues involved: friction dominates at low specific speeds, and kinetic energy effects dominate at high specific speeds. (For instance, items 5a and 5b in Figure 1 are actually kinetic energy effects also found in other machinery classes.)

Estimated distrubution of loss elements in typical centrifugal pumps

Figure 1 shows that at low specific speeds, the process is dominated by friction (particularly disk friction) and, in some cases, leakage losses. On a direct assault, we will make little headway against such a tough challenge. At high specific speeds, the game changes: one must be careful to deal with kinetic energy effects (disk friction may be a tiny contributor), so we must treat all viscous processes with care and achieve excellent diffusion and overall control of the flow field. This is a different design challenge, and one for which superb tools exist.

Do not forget the middle ground in Figure 1 because the highest efficiencies are found there. Efficiencies can be several points higher than at nearby higher and lower specific speeds. A change in speed is needed to reach this middle ground, at least for many instances where both head and flow may be frozen.

Speed can be adapted quite easily today and carries a small price for gaining efficiency, a price often compensated by improved part load operations and savings. Our traditional reliance on simple synchronous motors needs to be updated, and, in some cases, greater care will have to be taken to avoid cavitation (see below). The good news is that higher rotational speeds lead to smaller diameter machines, which afford greater design opportunity. One of these opportunities is the potential to include a proper diffuser to achieve a more efficient stage, and while these are now used worldwide for high performance engineered pumps, they are not common for mass-produced pumps.

Figure 2 gives an example of a pump stage reported in the past in Holland.

A conventional clear water pump with an added diffuser cascade.

Now, with better design speed, some reduction in diameter and a diffuser included, there is the possibility of a performance gain-one that is worthy of real design optimization. Other possibilities exist as well, including the usage of high performance crossovers for multistage pumps, such as shown in Figure 3.

Vertical pump stage with impeller and continuous crossover, orderly streamlines 

 

These crossovers have been shown to have excellent diffusion and low losses, a fact that can be carried over to many other applications.

Tools to Improve Performance

Clearly, possibilities exist to achieve improved performance. The modern tools to improve the designs include Computational Fluid Dynamics (CFD) and three dimensional (3-D) finite element stress and modal analysis (FEA). Interestingly, however, these tools will not achieve much if we stay locked in the design world of the past and try to tweak low speed designs where disk friction will dominate. An inexperienced designer might pick up a point here or there with such tools, but a better designer probably does not leave such glitches in his designs in the first place.

When modern applications are considered with modern motors and controls, it is tempting to start into new designs with some of the available modern tools available. However, much has been learned from experience that should not be lost at this stage of the process.

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See also:

Upstream Pumping Solutions

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