Tackling pump energy efficiency is not always easy—no two pumping systems are alike, and many are unique to a particular industry. Identifying energy efficiency opportunities may require data logging of power, flow and pressure, but opportunities will reveal themselves if end users look for indications of potential problems within a system. Personnel seeking efficiency improvements should ask smart questions of plant staff and follow their intuition as they examine the system to identify problems.
I recently conducted pumping system assessments at two pulp and paper mills that had one common denominator—oversized pumps. Most often, process or emergency flow rates dictate the initial pump-size criteria. With time, those requirements may change, moving the operating point away from the intended design parameters and the pump’s best efficiency point (BEP). Consolidating pumps and resizing failed pumps are among the options to consider when optimizing pump systems to meet current process flow requirements.
Out of Sight, Out of Mind
Our team conducted a scoping study in a pulp and paper mill to identify energy efficiency opportunities for several pumping systems. In a large industrial plant such as a pulp and paper mill, hundreds of pumps could be investigated. The first question was, “Where do we start?”
Fortunately, plant staff had already identified 12 pumps in six different systems to investigate. Savings opportunities with some pumps were obvious, such as using variable frequency drives (VFD) and changing control strategies. With those approaches, along with operating and maintenance measures (O&M), the team assumed all the bases were covered. This was a mistake.
The final pump examined was located in a hard-to-reach barge that was rarely visited by plant staff. The unit was a 200-horsepower, end-suction centrifugal pump that provided cooling water for the mill’s wastewater treatment system. The pump seemed to be operating continuously but at an unstable operating point. It was throttled heavily and had notable recirculation and vibration issues.
After locating the pump curve, the team learned that the pump was rated at 7,000 gallons per minute (gpm), which provided the required flow to meet the cooling needs of the wastewater treatment system when it was originally commissioned. When the process and instrumentation diagram was reviewed, a flow meter installed in the pipe a few hundred feet away indicated an average flow rate of 487 gpm, although the pump was rated for 7,000 gpm.
Equipped with this knowledge, the team had specific questions for plant staff and learned that the mill had established a successful water recycle and reuse program that reduced its cooling water requirements. As a result, the pump was heavily throttled, wasting energy. Obviously, the pump was oversized, operating at a dismal 10 percent hydraulic efficiency at the time of the study. The team recommended that the mill resize this pump to a 40-horsepower VFD-operated pump with a premium efficiency motor, which could save more than 980,000 kilowatt hours per year, valued at $38,000 annually.
The team had examined all the pumps on the plant staff’s list, but they were not done yet, locating another pump of identical horsepower just 10 feet away from the oversized pump. It was not on the list. Like its neighbor, this pump was making strange noises and vibrating violently. From the frequent surging and vibration, the team determined that this pump was also oversized.
The team proposed that the plant consolidate the two systems because the pumps were collocated, pumping from the same water source and experiencing similar vibration and surging problems. This approach would require some piping and flow control changes but would be worth it—especially if 200 to 300 horsepower of pumping energy use could be eliminated.
|Image 1. Oversized pump with a failed motor|
Take Advantage of Misfortune
Unexpected downtime and lost production because of equipment failure are expensive. Plant managers will do almost anything to get the system back online as fast as possible. However, beware of replacing equipment that has reached the end of its service life with a unit of identical size and performance without first evaluating the process requirements—which may have changed since the original installation—and investigating if a more energy-efficient alternative is available. A plant could save thousands each year if some time is invested to find out if the pump was meeting or exceeding the system needs.
The plant staff at a second pulp and paper mill requested that the team determine if a burned-out pump motor should be rewound or replaced with a new premium efficiency motor. The team responded, “It depends.”
The pump in question—driven by a medium voltage 250-horsepower motor (see Figure 1)—was used to pump “white water” (which is filtrate from the wet end of the paper machine) from a large tank to several points in the paper-production process. The motor failure was an unexpected burnout, but the pump had a backup motor so downtime was minimized. With the system back online, mill staff asked the team to investigate the next step for the spare equipment.