What do Pump Pros Know and why do they know it? This series of articles highlight the brilliance of those who work with pumps.
Pump Pros Know Efficiency and Reliability
Focusing on high efficiency and high reliability when specifying a pump or pumping unit is essential to achieving the lowest total cost of ownership for an application.
As with any machine, a pump cannot be 100% efficient. However, pump reliability can approach 100%, but it requires specifying the correct pump for the application, strictly adhering to the manufacturers recommendations, and regularly monitoring and maintaining the pumping unit (pump, driver, and drive) and system components. Furthermore, pump efficiency and reliability are often intertwined. As pump efficiency decreases, the pump reliability often decreases as well, and vice versa. Let’s first examine efficiency.
Efficiency relates to energy consumption: energy output versus energy input. If two machines perform the exact same amount of work, the machine that consumes less energy to preform that work is more efficient. Efficiency is expressed as the ratio of power output divided by power input. For example, if a pumping unit consumes 25 kW of electrical power to produce 21 kW of pumping power, the unit is 84% efficient (21 kW ÷ 25 kW = 0.84 or 84%). But only a portion of the inefficiency can be attributed to the bare pump itself. The rest comes from mechanical and electrical losses in the motor and drive.
Because the combination of the pump and the electric motor driving it has the greatest effect on wire-to-liquid efficiency, selecting the correct and most efficient pump for the application often leaves little room for compromise. However, you usually have a wide choice of motors to drive a pump, and some will be more efficient than others.
Most pump drivers operate at a constant speed, but some applications are best served by variable speed (or variable frequency) drives. These systems use electronic controls to reduce the speed of the motor (and, therefore, the pump) to the required flow or pressure that the system demands. This can complicate the selection of the pump and motor because the motor runs more efficiently at one speed and percent torque load, whereas the pump may be more efficient at another speed or operating condition. The decision of selecting the pump and motor, then, requires careful study to match the pump, driver, and drive to the system requirements.
Regardless of whether the pump will run at a fixed or variable speed, if you choose to spend less money for a less-efficient motor you’ll pay the penalty of higher energy costs year after year for the life of the motor. The additional cost of energy to operate the pumping unit can cost hundreds—sometimes thousands—of dollars more each year in exchange for saving a few hundred dollars up front.
Sources of Inefficiency
People often think of pump efficiency as mechanical efficiency. Again, pump efficiency is the ratio of power output to power input. If a pump has an efficiency of 87%, the 13% loss in efficiency is attributed to mechanical, volumetric, and hydraulic losses within the pump. Mechanical efficiency typically accounts for bearing, packing box, mechanical seal and all disk friction losses including wear rings, balancing disks or drums if present.
Volumetric efficiency accounts for leakage through wearing rings, internal labyrinths, balancing devices and glands.
Hydraulic efficiency accounts for liquid friction losses in all through-flow passages including suction elbow or nozzle, impeller, diffuser vanes, volute casing and crossover passages of multistage pumps.
For example, centrifugal pumps are rated for a theoretical flow based on the pump’s speed, design, and fluid properties (viscosity, density, temperature, etc.). If a pump’s theoretical flow is rated at, say, 100 gpm, it’s actual flow may be 98 gpm. In this case, the pump’s volumetric efficiency would be 98%. The same procedure would apply for the hydraulic efficiency. If the theoretical output pressure of the pump is 50 psi and its actual output pressure is 44 psi, then the pump’s hydraulic efficiency would be 88%.
Pumps must be manufactured so that moving parts do not contact each other. Otherwise, friction between the contacting surfaces would wear rapidly and potential cause premature failure of the pump. Therefore, pumps are made with small clearances between moving parts, such as the gap between a centrifugal pumps impeller eye and its wear ring or volute casing. Without the clearance, the impeller would rub producing additional friction losses and potentially causing a premature failure.
These clearances provide a path for liquid to bypass the pumping chamber. So a small amount of fluid leaks back to the inlet area of the pump rather than out the discharge nozzle. This flow is referred to as internal leakage, a measure of volumetric inefficiency.
Pumps are usually most efficient when they are new, so pump efficiency tends to diminish with age. No surprise here. As a pump’s internal components become worn, efficiency decreases. Likewise with contamination. As dirt and other contaminants block passageways or become trapped between moving parts, the pump can become less efficient.
Efficiency is a key parameter when selecting a pump, but equally important is reliability. Just because a pump has a high best efficiency point (BEP) doesn’t mean it’s reliable. Likewise, a pump can be reliable without being the most efficient. If a pump is chosen for an application based solely on BEP efficiency, the cost of energy to operate the pump might be lower over time, but the higher operating cost of the unreliable pump can incur downtime and maintenance costs that far exceed any savings from low energy consumption.
High reliability results from carefully considering and understanding all parameters of pump application. These include:
Selection of the pump and motor for the application—Pumping units must be specified not only for the fluid being pumped at the required pressures and flow rates, but to be compatible with the surrounding environment. Likewise, the electric motor must be powerful enough to drive the pump at the required speed and deliver the required torque.
Proper installation and startup—As with any rotating machinery, pumps must be securely installed and in proper alignment with the driver and mating components. These components include the foundation, suction piping, discharge piping, and certain accessories. Startup procedures must adhere to manufacturers’ recommendations for pumps, motors, and other components that make up the pumping unit.
Correct operation and flow control—Manufacturers’ recommendations must also be followed regarding operating parameters (minimum flow, perforable operating range (POR), allowable operating range (AOR), and minimal continuous stable flow). In addition, operating the pump at the correct speed is critical because it can have a profound effect on flow, pressure, and power consumption. For example, it’s often preferred to specify variable-speed operation rather than fixed-speed operation to drive a pump. That’s because fixed speed operation drives the pump at one continuous speed and uses throttling valves to increase system losses to achieve different flowrates or pressures required by the application.
This approach typically reduces energy efficiency because the pump draws full speed power at a lower pump efficiency even though the application requires only a portion of total flow. A variable-speed drive, however, would place the operating condition at a higher efficiency point on the pump curve allowing the pump to be more energy efficient and more reliable. Nevertheless, depending on the type of pump and application requirements, a variable-speed drive may not be required or may not be the best choice.
Proper maintenance—As with any industrial machine, maintenance is the key to reliable operation. Even if everything discussed thus far has been executed to perfection, incorrect or negligent maintenance will eventually lead to component or even complete pump or system failure. Proper maintenance goes beyond repairing a malfunction once it occurs and should include preventive maintenance.
Preventive maintenance starts with keeping critical areas clean to promote cooling, changing, or cleaning filters at regular intervals or when needed, and lubricating bearings and similar components. Preventive maintenance also involves regularly inspecting any components that wear or degrade over time so they can be adjusted, repaired, or replaced to prevent catastrophic failure.
Monitoring operation and history—The next level is predictive maintenance, which relies on collecting data on key variables over time. This data may include condition monitoring of pressures, flows, vibration, and other parameters of key components. For example, a pump produces a baseline vibration signature or housing temperature when it is commissioned. Monitoring the vibration and temperature over time can give an advance warning of an impending failure if either value increases suddenly or reaches a threshold value over time. This provides a reliable method of predicting that failure is about to occur. Shutting down the system at an opportune time to inspect and correct the condition, then, prevents a catastrophic failure.
Failure in any of these areas usually results in equipment failure and unscheduled downtime. The only way to ensure that all of these conditions are executed properly and at the right time is establish a pump management program. Doing so will maximize the reliability and efficiency of the pumping system.