Last year’s energy blackout crisis was a shock to many of us. It reminded us how dependent the water and wastewater industry is on the energy supply and the importance of equipment reliability and efficiency.
Pumps are major consumers of energy and by making them operate more efficiently, we can save a substantial amount of money and reduce operating energy bills by a significant amount.
Even though we take pump efficiency for granted, most pumps actually operate inefficiently. A significant impact on pump energy consumption is where it operates on its head-capacity curve with regard to Best Efficiency Point (BEP) flow. Consider the typical example presented in Figure 1.
Over time, plant operations change. A pump sized for a certain flow many years ago may no longer be required to deliver the flow that was required in the original design. For example, a pump sized and installed to operate at the peak capacity of 3,000 gpm may now be required to provide only 1,500 gpm in response to a change in demand.
When a pump capacity is throttled, or bypassed, the energy to move the “unneeded flow” is wasted.
For example, if the initial efficiency at the BEP was 80% at 3000 gpm, therefore at 1500 gpm the efficiency may be only 40%, according to pump efficiency curve. Even if a pump operating point has not changed significantly, the wear of the internal components (impellers, rings and bushings) would still reduce the efficiency.
Money-saving formula
Have you ever wondered how much energy a typical 100-hp pump consumes in a year?
Well, the answer may surprise you. One hundred horsepower (75 kW) delivered to a fluid 24 hours per day, 365 days per year, at $0.07 per kilowatt-hour is:
75 x 24 x 365 x 0.07 = $45,990 per year
If, as in the example above, a pump is operating to the left of the BEP at, let’s say, half of the original efficiency, then only half the energy is consumed for useful purpose, and the other half is wasted.
Which means that $22,995 is lost every year—and that is only for one pump.
Studies have shown that more then 50% of the plant pump population no longer operates at the original design efficiency, for various reasons—sometimes knowingly, and sometimes not.
Every problem has a solution. But the question is at what cost?
Clearly, replacing a big inefficient pump with a smaller one sized for proper flow is one way. But a complete pump costs a lot of money, and that is only part of the problem. Changing piping could be even more expensive. Removal of the pump from piping, cost of modifications and associated logistics can run-up cost many times more than the cost of the pump itself. In some cases, it is almost impossible to even think of changing the piping.
A better solution is to modify the internal hydraulics by fitting the existing casing with a new impeller, specifically designed for the new required flow. That may be a much more economical, expedient and practical way.
There is also another benefit of the impeller retrofit, which can come in the form of an upgrade in the material used in constructing the pumps. Lighter materials can have an impact on cost savings from the efficiency viewpoint while also improving reliability. (See Figure 2.) If the impeller is made from a lighter material, a significant reduction in shaft deflections can be accomplished.
A similar example using significant weight reductions also can be accomplished by incorporating pumps made of titanium, which is 40% lighter than steel, or composite, which is 80% lighter than steel. These types of materials have excellent resistance to corrosive and abrasive attack by water run-offs, salt water, brine, brackish, raw water or sewage intake.
The materials are also good for circulating pumps, waste and stormwater applications, condensate pumps, and others involving water and wastewater applications.
The benefits of such dramatic weight reduction are lower shaft deflections and, therefore, longer life of seals, bearings and couplings. Lower deflections would allow tighter wear ring clearances and thus substantial reduction in leakage across the wear rings, which can result in additional energy savings.
Analysis and fine-tuning of each application may reveal several ways to improve reliability while saving on energy costs.
Depending on each particular case, these savings can be very substantial and can directly contribute to a facility’s operating bottom line.
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