Often overlooked, pumps give facilities much-needed opportunities for energy savings.

When it comes to providing comfortable temperatures energy efficiently, cost savings don’t begin with air-handling units or end with chillers and boilers. To think so would overlook a significant amount of energy used in pumping systems that deliver chilled or hot water to air-handling units. Maintenance and engineering managers tend not to give much thought to pumping systems’ energy demands, as long as systems meet room temperature requirements and chillers or boilers hum along. Yet up to one-half of energy used in commercial and institutional facilities for pumping systems can be wasted if facilities rely only on control valves to regulate flow.
Managers often are far more concerned about the energy needed to move air than water, and that is a mistake, say engineering experts. Many inefficient pumping systems are operating today, and they cost organizations a great deal of money in operating costs. Strategic improvements in pump system design and operation can improve overall efficiency of the system substantially — with a quick payback.

Efficient design
The place to begin improving efficiency is to recognize that a system’s component efficiency is not equal to its overall efficiency. Energy efficiency benefits of even the most efficient pump or motor can be wiped out by inefficiency elsewhere in the system. And the culprit need not be another inefficient component. Sometimes, an otherwise efficient pump might be unnecessarily oversized.
“It is important to understand that in such inefficient systems, the pump and motor may be operating at relatively high efficiencies,” says Don Casada, a consulting engineer based in Knoxville and former researcher at the Oak Ridge National Laboratory. “But they are having to provide more hydraulic energy to the system than is ideal because of artificial losses caused by the valve throttling and/or bypass operation.”

Among the basic design changes that have taken hold in recent years is the switch from constant-flow systems to variable-flow systems that use primary and secondary loops, especially in chilled-water systems, says Mike Maybaum, P.E., executive vice president at Cosentini, an architectural and engineering firm based in New York City.
Primary-secondary systems generate efficiency gains because the system is split in two. The chilled-water flow around the chiller is generally constant, but the secondary and sometimes tertiary loops use variable-speed drives (VSDs) to throttle back flow instead of relying entirely on control valves.
“The switch from an all-constant-flow system to a primary and secondary loop system can cut pumping energy costs by 30-50 percent,” Maybaum says. This arrangement can help make even oversized systems run more efficiently by providing better control of oversized pumps, he says.
Using VSDs for pumping systems is a relatively new idea, even though the drives have a good track record of varying chiller operation, and their cost effectiveness has been very positive so far, Maybaum says.
Control valves are and have been standard in facilities, but control valves can waste about 25 percent of a pump’s energy efficiency.
They do this by creating frictional losses, and it takes a great deal of energy to overcome those losses. Casada equates the situation to driving a car but using the brake to control the speed. In HVAC systems, it is much better to size a pump correctly or use an alternative means of control, such as VSDs. While VSDs initially are more expensive, the energy cost savings are quite good, and the payback on VSDs can be as short as one to two years.
Besides VSDs, managers can reduce energy waste by sizing pumps to minimize valve losses.

Case in point
For example, Casada says, assume that in a water pumping system, a pump has a discharge pressure of 250 pounds per square inch per gallon (psig) at 3,000 gallons per minute (gpm). Also, assume the system only needs 210 psig downstream of the control valve, and pressure is reduced 40 psig at the control valve.
If the motor is operating at 95 percent efficiency and the pump is operating at 85 percent efficiency — both are very good values in such an application — the system would use about 65 kilowatts of electric power to cover the valve losses. If the facility operates 90 percent of the time at an average electric rate of 5 cents per kilowatt-hour, the annual cost of the valve losses would be more than $25,000.
If a manager changes the pump, trims the impeller or slows the pump down so it only develops 220 psig discharge pressure at 3,000 gpm, the loss across the valve would be reduced to 10 psig. This move saves about $19,000 per year if pump and motor efficiencies don’t change.
Of course, the valve frictional loss will cost more than $6,000 per year, Casada says.
A third way to vary the flow is to use multiple pumps. But unless the system is made as efficient as possible, turning on another pump will not double flow. Instead, it will provide only partial support, due to frictional loss in the system. The change would double the energy used to pump, but it would not double the flow.
Finally, even if managers are committed to control valves, they should understand how different types of valves have different effects on system efficiency. According to Casada, globe valves provide significant losses, even when they are fully open. Considering the amount of energy wasted and the rising cost of energy, replacing globe valves with butterfly valves could be cost effective. The return on this investment could be four months to one year.

Maintenance
There’s an additional benefit to changing out control valves in a system: less valve maintenance. Maintenance can play a critical role in ensuring the pumping system operates as efficiently as possible.
There are four general signs of system inefficiency: heavy valve throttling, open recirculation lines in cooling towers, cavitation or rapid phase changes in the system, and multiple running pumps. All four situations affect efficiency and maintenance.
For example, cavitation could lead to pipe and valve deterioration. Technicians can identify this problem by listening for a sound like gravel being run through the valve. Control valves also could be stuck, requiring that another pump be turned on to increase head pressure. Valves need to be checked regularly and replaced as needed.
Another maintenance challenge is scale build-up, which can occur on filters, pipes and valves, decreasing flow rates and increasing system friction. Technicians should clean filters and heat exchangers regularly.
Pumps also have wear rings that should be periodically checked. Wear rings provide the minimum clearance — usually a few ten-thousandths of an inch — between the pump housing and the impeller. Excessive wear of this ring leads to reduced pump efficiency.
The key to preventive maintenance on pumping systems is monitoring their performance, Casada says.
“Install a few flow meters and pressure gauges that you can trust, and read them regularly,” he says. Casada prefers to use temporary gauges, since permanent gauges are not always accurate. For example, he says he has pulled a gauge out of a system that still read 50 pounds of pressure.
Casada also recommends that maintenance technicians track system performance by reading flow rates every day and taking pressure readings every 6 months to one year.
Managers can rely on manufacturers’ pump head or pressure ratings for sizing the system, but they should be careful about using those numbers in place of field measurements; too many field conditions can affect the pumping system efficiency.
Performing even basic maintenance and changes to a pumping system can produce significant savings.
“It’s not hard to find ways to increase efficiency by 15 percent in most systems,” Casada says.
Above article appeared previously in Maintenance Solutions



First published October 2001