Water Well Journal
Issue link: http://read.dmtmag.com/i/573697
tion are feasible, although each potential installation should be judged on a case-by-case basis. 5. Generally, submersible pumps can be reliably operated in a deeper setting than VTPs, up to 15,000 feet in some cases. 6. The past advantages in bowl efficiency enjoyed by VTPs have been largely negated in recent years as many sub- mersible bowls have been either redesigned or can now be equipped with new bowl assemblies built with new materi- als that now rival VTPs in many classes. In most cases, VTP motors still enjoy a superior efficiency, however, as submersible motor efficiency averages up to 10% less in comparable horsepower for the lower horsepower classes. For example, efficiency for a cross-section of the various brands of submersible motors average 82.7% in 6-inch- diameter three-phase motors from 15-60 HP and 87.28% in 8-inch-diameter motors from 40-200 HP. Comparable horse- power vertical motors in the 15-60 HP range average 92.6% efficiency and 40-200 HP average 94% efficiency. If fully loaded at one hour of operation at $0.10 per kWh, a 30 HP VTP motor will cost $3.24 per hour of operation, a typ- ical fully loaded submersible 30 HP motor will cost $3.63 per hour, a difference of 39 cents per hour or 11%. At larger sizes the disparity is not as apparent as the gap in motor efficiencies lessens as the diameter of submersible motors increases and it becomes possible to use larger magnet wire and geometrically design the motor to a higher efficiency. For example, a 100 HP VTP motor will run at $10.64 per hour and a 100 HP submersible motor will cost $11.46 per hour, a difference of 82 cents per hour or 7%. The real test in a "motor to motor" analysis, however, lies in the duty of the motor, or how many hours per year of opera- tion are projected for each class of motor. Although the verti- cal motors can generally tout lower operating costs per hour, for many cases of intermittent or infrequent service, such as irrigation or fire protection, the difference in efficiency is not nearly as much of a factor and the inherent advantages for using submersible motors become more apparent. Disadvantages of Submersible Pumps Although there are some obvious advantages in using sub- mersible pumps and motors for many installations, obviously you cannot ignore the disadvantages present for many poten- tial uses. 1. The advantages stated for the operating speed become a distinct disadvantage when considering pump and motor bearing and component wear. Although it is true most sub- mersible units operate at twice the speed of VTPs, this dif- ferential of speed becomes a significant factor in abrasive situations, such as sandy wells. In certain water wells with serious sandy conditions, the sharp and granular shape of the sand can create a signifi- cant wear factor on bearings, often in less than a single year. Since removal of the sand before introduction into the suction of a submersible pump is both difficult and expen- sive, it is imperative this potential impact on the pump and motor must be carefully considered before proceeding with the installation. 2. As most submersible motors depend on the pumped media (water) for cooling and lubrication, the temperature of the water becomes a prime operating factor. In some cases, specially designed motors or one for higher operating temperatures must be used. 3. As the pump is isolated at the lower end of the pump set- ting, future adjustment of the pump impellers is not possible, as it would be with a VTP; therefore, the pump impellers must be preset for the full range of the projected conditions before installation. This includes all situations for pump and motor thrust, future pump wear, and possible changes in operating conditions. 4. Even though the advantages touted for the lack of a line- shaft is true, much of this potential advantage is offset by the alternate presence of a drop cable. In most cases, the horsepower loss through the drop cable is fairly consistent with the same type of energy loss through a lineshaft. Therefore, there is no significant energy advantage gained through the use of one over the other. In addition, the drop cable for a submersible installation is located on the outside of the drop pipe, exposing the cable to all potential damage from well conditions and negating any possible protection to the cable. Any potential nick or violation of the drop cable insulation that occurs during the installation can eventually result in a motor fail- ure, even if the defect was not initially discovered during the original installation. 5. The potential performance range for a submersible pump installation is much more limited than with a VTP. As a service condition with a VTP can be accommodated using multiple bowl diameters with numerous impeller trims across motor speeds of 600, 900, 1200, 1800, and 3600 RPM, submersible well pumps are typically limited to two speeds and about half of the bowl conditions available to VTPs. 6. The pump and motor used for submersible installations is located at the lowest terminus of the entire installation; therefore, all operating components are inaccessible with- out removal of the entire installation, resulting in signifi- cantly greater downtime. 7. Potential electrical issues involving three-phase electrical motors are even more pronounced with submersible mo- tors. Factors such as high and low voltage protection, three-phase power imbalance/phase reversal, and light- ning/high voltage surges are so critical that each one typi- cally warrants individual consideration. Although this applies to each factor, it is of extreme importance for high voltage surges, particularly lightning. Since a submersible motor is situated at the lowest end of the drop pipe and cable, it is a natural ground rod and all high voltage electrical surges will be directed through the cable and motor windings to ground—the well water. If the surge cannot either be diverted around the motor or these same components are not capable of resisting this surge, the high voltage surge will destroy the motor. Typically, it is not economically feasible to construct smaller motors to absorb these surges, often occurring in multiple values over several years; therefore, bypassing or WATER WORKS from page 61 62 October 2015 WWJ waterwelljournal.com