How an Oversized Pump Can Harm the Motor & Increase Operational Costs

Last month's column (Pumps & Systems, February 2014) reviewed the pump selection process and described how many users oversize pumps by adding design margins to account for unknown conditions present during the pump specification process. Design margins are similar to insurance, providing protection for unknown conditions expected in the life of the plant. This column will explore how oversizing a pump affects the motor driving the pump. It will also examine the adverse effects that occur when a pump is no longer operating at its best efficiency point (BEP) for extended periods of time and situations in which a design margin increases cost of ownership.

Drive Operations

Induction motors used to drive pumps are efficient in converting electrical energy into mechanical energy. A motor is 92 to 96 percent efficient depending on its classification and frame size. The efficiency is relatively constant from 50 to 100 percent of full load, with only a slight drop off when operating down to 25 percent of full load. Figure 1 shows a typical efficiency graph for a totally enclosed fan-cooled (TEFC) motor at a range of sizes operating at varying loads. The problem associated with lightly loaded alternating current (AC) induction motors is their effect on power factor. To understand the way electrical energy is converted to fluid energy using motors to drive a pump, consider the components and operation of a motor. An electric motor consists of a stator and a rotor. The AC power supplied to the stator generates a rotating magnetic field within the motor's stator windings. When the rotor's windings are exposed to the stator's rotating magnetic field, an electric current is generated in the rotor's windings. This results in a magnetic field within the rotor. The interaction between the stator's rotating magnetic field and the rotor's magnetic field causes the motor shaft to rotate. The motor shaft is connected to the pump shaft, resulting in rotation of the pump's impeller. Increasing the flow rate through the pump requires more power from the motor. A rise in power requires increased current to the motor stator, resulting in a rise in the motor's electrical load. The amount of electrical power used in the motor to rotate the pump can be determined by measuring the voltage across the stator and the electrical current through the stator winding. The amount of power supplied to the motor is referred to as real power (P) and is measured in watts. Because the source of electrical power is alternating current and the motor's stator consists of a conductor wound around a stator, an induced load in the motor develops. The result of the induced load is referred to as reactive power (Q) and is measured in volt-amperes reactive (VAR). The reactive power caused by the stator coil lags in relation to the real power by 90 degrees. This reactive power produces no useful work. The reactive current simply moves between the generator and the motor across the electrical grid. The product of the real power and the reactive power is called apparent power (S) and is measured in volt-amperes. The power factor of the motor is defined as the ratio of the real power over the apparent power and is expressed as the cosine of the angle φ between the real power and apparent power. Power factor ranges between 1, all real power, and 0, all reactive power. The value is unitless. The power factor of a fully loaded motor is indicated on the motor's nameplate. The real power varies with the load on the motor, but the reactive power is based on the construction of the motor. Figure 2 shows that a reduction in real power causes an increase in the angle φ. As a result, a lightly loaded induction motor has a reduced power factor. The real power is used to drive the motor. The reactive power simply flows between the motor and the generator supply electricity from the utility. The resulting apparent power flows through the power grid. The electric utility must design the transmission lines depending on the apparent power, but the generator only measures the real power. Figure 3 shows how a motor's power factor varies depending on the motor frame size and percent of full-load horsepower. Figure 3 shows that a lightly loaded motor has a very low value of power factor. An industrial customer with many lightly loaded motors will either have a power factor surcharge added to their utility bill or will need to purchase and install hardware to correct the plant's low power factor. An increased power expense is one example of the added costs of excessive design margin.

Pump Operations

A pump is designed for a specific range of flow rates and head values to meet the needs of the market. The pump's peak performance and maximum service life for flow rates will occur around the BEP. A pump operating at maximum efficiency results in a uniform flow of fluid through the pump. Figure 4 shows the pump curve used in last month's column. The pump's BEP of 79 percent occurs at 1,070 gallons per minute (gpm). In addition, the efficiency value quickly drops off from the BEP to the minimum and maximum recommended flow values. Unlike a motor, the pump operates away from its BEP. The flow through the pump is no longer uniform, resulting in recirculation within the impeller. This non-uniform flow creates uneven pressure distribution in the pump, causing an increase in vibration and hydraulic load as well as a reduction in efficiency. The price here results from a combination of energy costs, mechanical wear and operational inefficiencies. In Figure 4, the process flow rate through this pump for the first year is only 200 gpm, with the flow rate increasing to 400 gpm in the second year and 800 gpm by the fifth year. The design flow rate of 800 gpm is only 75 percent of the pump's BEP flow. A standard developed by the Hydraulic Institute, the ANSI/HI 9.6. Rotodynamic (Centrifugal and Vertical) Pumps Guideline for Allowable Operating Region, provides recommendations on pump operation. The standard can identify a variety of potential problems when a pump is not working at its BEP. The ANSI/HI 9.6. standard defines the preferred operating region (POR) as a range of flows either side of the BEP in which the hydraulic efficiency is not degraded. The standard defines the allowable operating region (AOR) as 70 to 120 percent of the BEP flow rate for pumps with specific speeds less than 4,500 and from 80 to 120 percent of the BEP for pumps with specific speeds greater than 4,500. The pump displayed in Figure 3 has a pump specific speed of 870, resulting in a POR between 750 and 1,285 gpm. The AOR represents a wide range of flow rates outside the POR acceptable to the service life of the pump. The pump manufacturer defines the AOR on requirements other than energy efficiency. The AOR is used extensively in the pump selection process. The pump manufacturer should be consulted before the pump is operated outside the AOR. In Figure 4, the manufacturer specified the pump's AOR to be between 350 and 1,650 gpm. The standard outlines concerns that the pump manufacturer considers when specifying the AOR. Operating outside the AOR could result in reduced bearing life, reduced shaft seal life, internal mechanical contact, shaft fatigue, thrust reversal, excessive temperature rise through the pump, increased vibration and noise, insufficient net positive suction head and suction recirculation. The pump should operate within the AOR specified by the manufacturer to avoid potential long-term mechanical damage to the pump. These issues lead to maintenance and energy costs as well as unscheduled shutdowns.

Evaluating the Pump

In Figure 4, the 200 gpm of flow rate for the first year of operation is well below the minimum AOR flow rate of 350 gpm recommended by the manufacturer. If the pump were to operate at this flow rate for an extended period, extensive mechanical degradation would occur. To achieve the manufacturer's minimum flow of 350 gpm, a recirculation line will need to be installed to recirculate 150 gpm back to the supply tank. When the preliminary sizing was performed, design margins for pump head were added. Unknown conditions were factored into the preliminary design including the elevation of the tanks, tank levels and pressures, pipe length, and number of valves and fittings. Now that the system is built, the conditions are known. In Figure 4, the pump head at the design flow rate of 800 gpm was calculated at 211 feet, but the system as built only requires 190 feet of head. The additional 21 feet of head required by the design margin is excess head. Because the full value of design margin is not needed and the pumps are already in place, the additional head (the 21 feet) is absorbed across the control valves to eliminate excess energy.

Assessing the Cost of the Piping System

As demonstrated, assigning design margins of head and flow are an integral part of every pump selection. They provide designers and engineers with the required flexibility to select long lead items early in the design process. Just like any expense, the amount of insurance and resulting costs should be evaluated and re-evaluated. Oversizing pumps increases the system's operational, maintenance and capital cost. Without knowing the true cost of owning and operating a pump, the process cannot be improved. The purpose of the pump system assessment is to review how the installed and operating system is performing. The system is reviewed, and margins are adjusted for optimization. The initial design phase can only supply a best-case scenario. Determining costs associated with each component in a piping system helps calculate actual operating costs, and performing a post assessment allows engineers and operators to evaluate what can be done to improve system performance further in full operation.