The rising cost of electrical power has caused many industrial plants to shift their focus to energy consumption. Plants often run pumping equipment continuously, and much research has pointed to opportunities for cost savings by optimizing pumping equipment.
When evaluating the potential for energy savings, end users cannot consider a pump in isolation. The suitability of the pump for the system within which it operates is vital. Even the best designed and most efficient equipment offers power-saving potential if it is run off its best efficiency point (BEP) in a system for which it is ill-applied.
Many plants have been in operation for more than 40 years, and their operating philosophies have changed over time. Plant improvements have enabled higher throughput, often increasing production by as much as 125-150 percent. Unfortunately, little is done to improve or increase the performance of the support-service pumping equipment, such as cooling water pumps.
As system flow demands increase, the duty point of the pumps is forced to shift far to the right of the BEP, well outside the acceptable operating range (AOR). This causes efficiency and pump reliability to decrease dramatically.
Casting tolerances, surface finishes, and impeller/volute or impeller/diffuser geometry have all dramatically improved during the last 40 years. But because many pumps were installed when the plants were commissioned, the existing pumps were manufactured using techniques that would be considered obsolete today. The result is higher energy costs and reduced reliability and availability, which often cause production delays.
The Starting Point
Pumps react to changing system conditions. System demand (or system resistance) determines the flow and pressure at which a pump will operate. As system flow demand increases, the flow throughput of a pump also increases, causing it to operate further on the right-hand part of the performance curve.
The system demand is graphically represented by plotting the system resistance curve as a function of flow. This curve enables the end user to quickly determine system flow for a given pump since the pressure and flow are determined by the intersection of the pump performance curve (red) with the system head curve (green). A process design engineer would ideally select a pump with an operating point that would have coincided with the BEP. This could yield a pump efficiency of 80 percent, as shown in Figure 1.
However, many support pumping systems have exceeded their original design and have much higher flows to support the higher plant production. This is particularly common in cooling water applications, condenser water pumps, descale pumps or any application where water usage is proportional to production.
While the original design may have called for two-pump operation, present-day requirements may require 2 1/2 pumps online, with two pumps being insufficient and three pumps too many. As flows increase, the result is usually that system requirements have exceeded the AOR of the pumps (see Figure 2).
Original Duty Point
The original system design for one processing plant's service water pumps was to have three pumps operating in parallel with an installed spare as a standby. The total system requirement was 105,000 U.S. gallons per minute (GPM) (23,864 cubic meters per hour) at a pressure of 190 feet (57.9 meters) total dynamic head (TDH). Each pump was rated for 35,000 GPM (7,955 cubic meters per hour) at 190 feet (57.9 meters) TDH.
As production increased, more service water was required, causing the existing pumps to operate further out to the right of the performance curve. This caused the net positive suction head required (NPSHR) to exceed the NPSH available (NPSHA), leading to severe cavitation issues. To reduce cavitation problems, the plant ran four pumps in parallel and throttled each pump to keep the individual pump flows low enough to prevent cavitation.