Water Well Journal
Issue link: http://read.dmtmag.com/i/529509
the suction capability of an ordinary centrifugal pump. Many of these early applications were in water wells, but in most cases the lifts were less than 100 feet. The vertical turbine pump is cur- rently the most efficient type of multi- stage centrifugal pump available. Bowl assembly efficiencies as high as 90% have been recorded for many commer- cially available pumps. The term "turbine" is somewhat a misnomer as most vertical turbine pumps use equipment and operational theory applicable to most centrifugal pumps. The use of the term though is derived from the fact most VTP im- pellers either resemble or are identical in appearance to the runners used in many power generation turbines. The impellers used in vertical turbine pumps are classified as either centrifu- gal or mixed flow design, depending on the geometry and specific speed of the impeller. A third type of dynamic de- sign, the axial (propeller) type, is not frequently used in water well applica- tions and will not be discussed in this article. The fundamental theory of a vertical turbine pump is based on the premise a rotating impeller will receive the re- quired energy through a rotational force from some method of horsepower trans- mission (a driver) such as an electric motor or diesel engine, and transfer the needed energy to a liquid (usually water) and—depending on the design, diameter, and speed of the impeller— move a specific quantity of fluid against a specific value of resistance and lift (usually referred to as feet of head). How a Vertical Turbine Pump Works In a turbine pump, the impeller de- velops a force known as kinetic energy (energy imparted due to motion) to the pumped fluid, and the bowl (or diffuser) converts this kinetic energy force to a pressure force. The relative capacity of an impeller mostly depends on the width, amount of vanes, and internal area of the impeller, while the head mostly depends on the speed and diameter of the impeller. The unique characteristic of an im- peller used in a vertical turbine pump is primarily related to the angle of the discharge plane (rim) of the impeller as opposed to the diffuser in the bowl assembly. To optimize efficiency and capacity in an impeller with a relatively small diameter, the pumped fluid is released from the impeller at the same approximate angle as the receiving diffuser vanes. The diffuser vanes and outlet ports are constructed with uni- form gradual turns at a full 360° pattern totally circling the impeller rather than the limited sharp turns associated with most centrifugal pumps. These construction features con- tribute to the relatively high internal ef- ficiencies observed in most turbine bowl assemblies. Impellers are usually con- structed from some type of bronze alloy, although stainless steel or cast iron are also used. Bowls are generally con- structed from gray cast iron, but cast steel is often used on high-pressure applications. Bowls are generally con- nected together through flanged and bolted arrangements, although 8-inch and smaller diameter bowls are usually threaded together. Optional metallurgy to optimize per- formance, wear, or efficiency is avail- able as well as internal coatings such as porcelain or epoxy. Bowl shafts are al- most always constructed from a strong and wear-resistant form of 400 series stainless steel. Intermediate, suction, and discharge case bearings are typi- cally bronze or a combination of a bronze-rubber arrangement for use in crooked or sandy wells. Impellers There are four basic impeller types used in turbine pumps: enclosed (stan- dard), thrust balanced enclosed, semi- open, and thrust balanced semi-open (Figure 1). Each type has inherent ad- vantages as well as disadvantages that must be considered for each application. Enclosed impellers are the most com- mon and are used in most applications due to their predictable performance, adjustability, ability to install wear rings on top and bottom running surfaces, lower axial and radial thrust values, and relative ease of performing future rebuilding on wear surfaces. Figure 1. Vertical turbine pump impeller types. WATER WORKS from page 54 56 July 2015 WWJ waterwelljournal.com