Water Well Journal
Issue link: http://read.dmtmag.com/i/573697
T he July column in our continuing series on the basics of water systems engineering covered the fundamentals of vertical turbine pumps. In this column, we introduce the concepts and design features of submersible pumps and motors. The next column, the last in our three-part series, will include a design example for both types of pumps. Background of Submersible Pumps Submersible pumps (Figures 1 and 2) basically refer to any pump designed for operation (continuous or intermittent) while partially or fully submerged in water. They are com- monly referred to as submersibles or more simply subs. Sub- mersible pumps are most commonly associated with dynamic pumps, specifically a centrifugal type of pump, although posi- tive displacement pumps have also occasionally been used as submersible pumps. The history of submersible pumps coincides with the paral- lel development of electric motors designed to operate under submerged conditions. The original submersible pumps date back to the early decades of the 20th century when they were developed and used primarily for deep oil well pumping. They weren't very reliable, but they offered an alternative, especially in very deep oil wells, to the pump jacks that were commonly used then. Early submersible motors, in accordance with their initial use, were oil cooled and lubricated. Today most submersible motors used in water well applications are either oil cooled and lubricated using an FDA-approved food-grade oil, or water cooled and lubricated. As motor technology gradually improved, submersibles gained rapid favor and popularity as well pumps used to pump water from deep water wells during the late 1940s through the 1950s. Continued improvements in submersible pump and motor technology and increased reliability during the late 1950s through the 1960s resulted in the virtually exclusive use of submersible pumps for deep water well applications that continues to this day. In addition to their widespread use as well pumps, sub- mersibles are currently used as booster and dewatering pumps in many projects. Today, under optimum conditions, improve- ments in submersible motor and pump technology have in- creased the average life of a unit from only three to five years in the 1950s to as much as 15 to 20 years currently. Types of Submersible Pumps The term "submersible pump" can actually refer to several different types of pumps designed for distinct and specific jobs. The two most common types of submersibles have com- pletely different applications, but many of the common prob- lems and solutions apply to both styles. A "submersible sewage pump" is a type of pump designed to transfer raw sewage, stormwater, or effluent from one loca- tion to another. A "multi-stage submersible pump" is typically designed to deliver potable water at high heads from one loca- tion to another. Multi-stage submersibles, as centrifugal pumps, are gener- ally circular in construction in small enough dimensions to enable easy installation in circular sumps or wells. They are available in bowl (pump) diameters of 3 inches up to more than 18 inches. These pumps are most commonly used in water wells and currently represent the most popular method of water pumping for domestic, irrigation, and commercial water systems. Multi-stage submersibles—also referred to as "deep well submersibles"—are often associated with their nominal diameter when classifying a pump for a specific application. In other words, a 4-inch submersible pump refers to a deep well submersible pump with a nominal outside diameter of 4 inches, even though the actual diameter is closer to 3.875 inches. The most common motor diameters available today include 4, 6, 8, and 10 inches, although larger slow speed units are manufactured for industrial and municipal applications. Typi- cally, the diameter of the motor for a deep well submersible matches the diameter of the pump with the motor affixed to the pump directly below the inlet. The motor, as the pump, is designed for continuous operation in a totally submerged con- dition with operational water heads often exceeding 1000 feet. A submersible type of sewage or effluent pump more resem- ED BUTTS, PE, CPI THE WATER WORKS ENGINEERING OF WATER SYSTEMS Part 12(b)—Pumping, Part 2 WATER WORKS continues on page 60 Improvements in submersible motor and pump technology have increased the average life of a unit from only three to five years in the 1950s to as much as 15 to 20 years currently. 58 October 2015 WWJ waterwelljournal.com