The Pump Wizard    |    Submersible Pump Overview    |    Feature Comparison Table    |    Cost Comparison Chart Home > Pump Overview > Electric The electric pump itself consists of an electric motor driving high-speed (generally 3750 RPM) impellers to provide flow. Image: Electric Pump The impellers are usually supplied in multiples, called stages, to provide the required combination of flow rate and discharge head. The motor is usually filled with water or oil for cooling purposes, and needs to be sealed from the well environment to protect the bearings and motor. A control box is required to house a power supply switch and the level controls to shut the pump off when the liquid level in the well drops too low; the level control system also requires down-well probes to sense liquid level. A complete electric pump system includes the pump, power supply to the well, control box, well level probes, wellhead assembly, discharge piping and power cable. A check valve at the pump discharge is frequently advisable, to  minimize watter hammer, prevent reverse operation of the pump after shutdown and to prevent possible siphoning of pumped liquid back into the well or riser from the discharge header and/or collection tank.   Image: System Diagram All of these components need to be considered in the specification, installation and operation phases. The design process begins with identifying the specific pump curve, and therefore the pump, which produces the combination of flow rate and discharge head needed. The matching controls, power supply, liquid discharge line and cables can then be specified. Electric pumps can provide higher flow rates and higher efficiency compared to air-powered automatic pumps and piston pumps. Higher flows are easily achieved with electric pumps by increasing motor size and the number of pump stages. The electric pump also has an advantage in power efficiency compared to air-powered pumps because the electric pump's motor drives the impellers directly. In contrast, air-powered pumps require the extra step of compressing the drive air supply for the pump. However, electric pumps' advantages in flow rates and efficiency can be offset by the following factors: Significantly shorter warranty- typically only one year vs. 2-5 years Restrictive manufacturer limits on pumped fluid temperature, solids, corrosives and solvents Higher maintenance and replacement costs Higher overall installed system costs Potential personnel safety issues related to shock hazards and use in flammable and explosive environments Controls A control box governed by float switches or level probes in the well is required for electrical pumping systems. The electrical control system is subject to failure due to lightning strikes, power surges, moisture, and deposits that can form on the level sensing probes. If the level sensing system malfunctions for any reason, the pump will likely suffer catastrophic failure due to motor overheating from running dry. Additional control components can be added to reduce this risk, but at the expense of higher costs, complexity and control box size. At some sites, even finding a safe and suitable place to mount the controls can be a challenge. At landfills, for example, below-grade, confined vaults present the hazard of methane gas presence, and landfill fires are one of the top concerns of site operators. Remediation sites with high concentrations of flammable contaminants face similar concerns. Building an electric pump system to full explosion proof standards raises costs markedly. Emulsification of Free-Phase Liquids A special consideration applies to the use of electric pumps at remediation sites with free-phase hydrocarbons: the high-speed impellers in electric pumps create water/hydrocarbon emulsifications that can cause problems in downstream treatment equipment such as oil/water separators, air strippers and activated carbon beds. Oil water separators are commonly designed and sized on the basis of the desired removal efficiency for the smallest, most difficult to remove free-phase droplet sizes, such as 20 microns. The fluid shearing action in electric pumps creates very small droplets such as these , in effect substantially increasing the required size of the oil/water separator. Electric pump use is so common that their role in oil/water separator sizing and cost is often overlooked. Diameter Generally, electric pumps used for remediation are at least 3.875 in. (9.84 cm) in diameter so any existing, smaller wells can't be used for remediation. Some smaller electric pump sizes are available in specialty versions, but these are limited, more expensive and usually require sophisticated variable frequency controllers. Materials Electric pumps are mass-produced primarily for standard well water supply usage, and are therefore not offered with a wide range of materials choices. Stainless steel is a common material of choice of electric pump manufacturers for some components, but the grades of stainless steel used on different components on the same pump can vary considerably, so it is important to check the details to avoid a "weak link", such as welds and wire connection terminals. The presence of low pH, elevated temperatures, high salinity and dissolved solids can quickly corrode any metals below 304 or 316 SS grade. Image: Corroded Electric Pump Solids Particulate matter in the water or leachate are very common, and pose a serious threat to the longevity of electric pumps. In fact, many landfills face a constant threat of having the pump become silted in place, making removal difficult or impossible. The presence of so much silt and sand can damage the motor shaft and seals, then enter the motor and destroy the bearings and increase friction. The increased friction, in turn, causes the motor to draw more current and overheat, which leads to premature failure. Particulate matter most commonly causes rapid erosion of the pump impellers, causing pump flow and pressure output to decrease accordingly. Image: Impeller Diagram Finally, solids can cause the pump to lock up by jamming between the impeller and the cup, which usually destroys the motor. On-Off Cycling Ironically, electric pumps' high flow capabilities can lead to problems at landfill and remediation sites. Pumping systems are often conservatively designed to provide high maximum flow rates based on the limited information available before system installation. Once a system is in operation the actual fluid recovery rates in the wells can be much lower than expected, leading to frequent motor starts and stops which raise the motor temperature above its intended limits. Such high temperatures dramatically shorten the life of the motor. Pump manufacturers provide guidelines for maximum pump starts per hour, and these should be adhered to. Flow/Pressure Output Electric submersible pumps are designed to deliver flow/pressure combinations defined by a flow curve specific to each pump and motor configuration, as shown here. Conversely, electric submersible pumps are not generally intended to be operated off of their fixed flow curve, unless a more sophisticated, variable speed motor control system is included. For example, at the project design stage, a typical electric pump and motor combination may be selected that delivers 40 gpm against 70 psi discharge head. If site experience later suggests the need for lower flow rates, this can be difficult to achieve with the same pump. Throttling back the pump output with a valve on its discharge line can move the pump performance off of its recommended flow curve, leading to cavitation, impeller erosion and overheating. Temperature Electric submersible pumps have model-specific upper temperature limits that are largely governed by seal materials and motor limits. 104° F is a typical upper temperature limit for common electric submersibles, with higher limits in the 140° F range available on special, higher priced models. It is important to note that the various temperature effects work together: fluid temperature, stop/start cycle rate, and flow/pressure output. For example, a pump submersed in a warm fluid closer to its specified temperature limits will be more susceptible to the incremental impacts of stop/start cycling and lower flow rates due to throttling. Summary of Electric Pump Strengths and Weaknesses Strengths High flow capability Power efficiency Does not directly introduce air contact with the pumped liquid, a source of clogging deposits at some sites Ready availability at local suppliers Weaknesses Warranty coverage may be voided by pumping conditions typical of landfills and remediation sites Higher maintenance costs due to: impeller and motor bearing wear; corrosion; motor failure caused by on/off cycling and/or high fluid temperatures; and run-dry and locked-rotor "hard failure" modes Emulsification of hydrocarbons that hinders treatment efficiency Cost and safety impacts associated with electrical power under hazardous conditions Limited ability to adjust flow rates downward unless advanced variable speed control system is used Upper temperature limits that may be of concern in landfills and steam recovery remediation projects Tel (734) 995-2547 • Toll-Free in North America (800) 624-2026 • Fax (734) 995-1170 © 1982 - 2004, QED Environmental Systems, Inc.   All Rights Reserved.