Water Well Journal
Issue link: http://read.dmtmag.com/i/668983
failure, and generate more head per stage than smaller im- pellers for comparable flows. This decision must be balanced, however, with the knowledge bowl assemblies with less than three stages generally require an efficiency correction (usually lowered). You may also note the selected speed in Table 1 is 1760 RPM. Although other rotating speeds, such as 1200 and 3600 RPM are also available, most well type VTPs are designed to operate at a nominal speed of 1800 RPM (1750–1780 actual speed range). This is primarily due to the inherent speed limitations for the lineshaft and bearing combinations used in most deep well turbine pumps. In our example, I would typically select a 10-inch nominal diameter (9.5-inch actual diameter) bowl assembly for this ap- plication since this size of bowl provides adequate clearance between the bowl and the well casing, plus it represents the best compromise between having enough stages to produce the needed head without any corrections in efficiency. Head Requirements Head requirements for turbine pumps are determined in the same way as for any other type of well pump. However, the selection of a specific turbine pump is unique. Curves for VTPs are developed based on capacity as the primary ele- ment, with head and horsepower per stage shown rather than the total head usually found with many of the pre-assembled submersible style pumps. This difference allows the designer total flexibility during the design of the pump as the bowl can be selected using the required number of stages and impeller trimming if needed for the precise design condition. After calculating the total dynamic head (TDH), the total head is divided by the feet per stage indicated on the curve of the selected bowl assembly to determine the number of stages needed to generate the required head. Trimming of impellers can be performed to fine-tune the selection when the total required head falls between the calcu- lated number of stages. Horsepower The required horsepower needed by the bowl assembly is also determined by multiplying the number of stages by the horsepower line indicated on the bottom of the pump curve. Bowl horsepower can also be calculated by using the follow- ing formula (for water): Bowl horsepower = GPM × TDH (feet) 3960 × Bowl efficiency (%) Design example = 500 GPM × 269 feet TDH = 41.4 bowl hp 3960 × .82* *82% (.82) is the assumed bowl efficiency for a typical 10- inch bowl. Although Table 1 lists maximum capacities for most typi- cal brands of VTPs, the diversity of available bowl sizes be- tween manufacturers can be significant. Some manufacturers also make 9-, 11-, 13-, and 15-inch-diameter bowls in addition to those listed in Table 1. Often this intermediate size will fit an application perfectly where a larger or smaller bowl is not feasible. In addition, bowl efficiencies for common flows between manufacturers can also vary widely. As an example, for our flow of 500 GPM, I was able to compare curves for several 8- inch bowls from different manufacturers with efficiencies as low as 72% and up to a high of 84%. Also, 10-inch-diameter bowl assemblies generally displayed efficiencies for the same flow between 80% up to 85%. When designing a vertical turbine pump, a good designer should investigate several selections from various manufactur- ers and not limit the customer's choices to only those brands he sells regularly. Submergence With the diameter of the bowl assembly tentatively select- ed, the next step will be to determine the required pump setting, the diameter and size of the column and shaft, and the type of shaft lubrication method. The pump setting is mainly determined by well factors— chiefly the diameter, depth, and pumping levels—as well as the need to maintain sufficient water head over the inlet of the pump (technically referred to as submergence) at all antici- pated flow conditions to avoid the introduction of air and pos- sible cavitation during pumping. In some cases, the minimum submergence is measured from the top of the bowl rather than the inlet to provide a safety factor. This minimum submergence is necessary to prevent a vor- tex (also known as swirl) condition in the well, in addition to possible air entrainment. A vortex is the spinning cyclonic type action of water often observed when draining basins and toilets. A vortex condition in a well can cause severe air en- trainment and reduction of pump performance even when the water level seems to be adequately deep over the bowl. Vor- texing can also lead to a phenomenon known as prerotation. In a prerotation hydraulic condition, the water enters the pump inlet with a rotational approach, either the same or counter to the pump's normal rotational direction. During this condition, the pump must stop the rotation of the water and re- direct it into the first stage impeller. Prerotation can cause a severe change (by raising or lowering) in the capacity, head, and horsepower as well as severe vibration and pump damage. Vortex and prerotation conditions are best avoided by main- taining adequate submergence over the bowl. In severe cases, straightening vanes may be needed to prevent prerotation. Net Positive Suction Head NPSH is another pump term that, while usually not a criti- cal design factor for deep well turbine pumps, still requires some limited discussion. Simply stated, as the pump impellers spin, they develop a negative pressure or vacuum (commonly but incorrectly referred to as suction) in the impeller eye. There must be adequate pressure, either from atmospheric pressure (head) or generated water pressure, to push water into the impeller eye and relieve this negative pressure. If in- sufficient head or pressure exists, the liquid will fall below its corresponding vapor pressure, resulting in vaporization of the fluid. This is commonly referred to as cavitation. The sound accompanying cavitation many people describe as "gravel going through the pump" is actually the collapse (imploding) WATER WORKS from page 40 waterwelljournal.com 42 May 2016 WWJ