Water Well Journal
Issue link: http://read.dmtmag.com/i/668983
For our example, since our motor horsepower will come in somewhere between 40–50 HP, we can see a 1-inch lineshaft at 1770 RPM will work (good up to 57.5 HP). Due to the fact the lineshaft has mass and weight and must be rotated at high speeds to transmit the horsepower from the driver to the bowl assembly, it also stands to reason this shaft will impose an additional load on the driver. This is unquestionably true; the additional horsepower the driver must produce to rotate the lineshaft usually amounts to a fractional value of the actual bowl horsepower. Nonetheless, it must be calculated and added to the total horsepower. Once again, all manufacturers publish shaft horsepower charts for their available sizes of lineshaft. Shaft horsepower is a direct function of the size and speed of the shaft and is calculated using the following formula: Shaft hp = Mechanical lineshaft friction per hundred feet of shaft (from manufacturer's table) × Total lineshaft (including bowl and top shaft) length in 100 feet In our example, for 1-inch lineshaft we will have: .54 HP/c × 1.6 (for 160 feet of total length) = .86 HP loss Even though the actual pump setting is 150 feet, it is cus- tomary for me to add 10 feet of more shaft to account for the added lineshaft length through the discharge head and motor. A convenient chart for determining mechanical friction of lineshaft is shown in Figure 2 on Table 2b. Thrust Thrust, specifically axial (or down) thrust loss, is also pres- ent in all deep well VTP applications and, although a minor value, must be included in the design. Thrust loss is usually carried by the driver, which must have adequate capacity to support the weight of the impellers, hydraulic thrust, and line- shaft weight without excessive downward deflection. The two components of thrust are hydraulic thrust and lineshaft/ impeller deadweight. Hydraulic thrust is an element unique to each bowl assem- bly and is determined by multiplying the total operating (or shutoff) head by a thrust factor (commonly known as a K fac- tor). The K factor is the thrust value of that particular im- peller/bowl arrangement per foot of head. The K value can vary between 1 and 2 up to a high value of more than 20. This value must be obtained from the manufacturer's engineering data, although it is commonly shown on curves. Total thrust, therefore, is calculated from the following formula: Total thrust = Hydraulic = K value × TDH in feet + Lineshaft rotating mass (= lineshaft weight per foot × total shaft length) + Bowl assembly rotating mass (= impeller + bowl shaft weight per stage × number of stages) The total thrust is then applied to a manufacturer's table or chart to verify the thrust will not result in excessive shaft stretch (technically referred to as elongation), which can cause impeller damage due to scraping of the impeller on the bowl. On very deep sets (more than 300 feet) the column stretch must also be calculated and the elongation corrected using this additional value. In all cases, the maximum shaft stretch must be less than the allowable end play listed in the manufacturer's data sheet. The horsepower impact on the driver from thrust is calculated using the following formula: Thrust horsepower = .0075 × Pump RPM × Total thrust 100 1000 This information on thrust is also shown in Figure 2. Elongation of lineshaft can be determined by using Table 3 on Figure 2. Method of Lubrication The selection of the lineshaft lubrication method is the next design element we must examine. There are generally two methods of lineshaft lubrication. There is product (or water) lubrication, where water passing over the lineshaft bearings cools and lubricates the bearings every 10 feet. There is also oil lubrication, which is where the lineshaft is enclosed in a steel or wooden tube and oil is introduced from the top of the well to lubricate bronze (or redwood) bearings. Typical nomenclature for vertical turbine pumps and both methods of lineshaft lubrication and the most common im- peller types are shown in Figure 3. As was indicated in the July 2015 Water Works column, product (water) lubrication is the preferred method, especially if the quality and potability of the water must be considered as in this case. There are other cases, however, where water lubrication is either impractical or risky to use. If the static water levels are routinely lower than about 60 feet. If the pumped water con- tains significant quantities of sand or abrasives. If the pump will operate at speeds greater than 2200 RPM. If the well is crooked or not straight. In all these cases, oil lubrication should be considered. In applications involving potable water, food-grade lubri- cating oils are now available, replacing petroleum-based oils used for decades. In our example, none of these factors are relevant and product (water) lubrication is selected. Driver Type and Speed Finally, the last primary selection is the driver. The horse- power will obviously be determined from the total horsepower required from the pump assembly. This is generally the sum of three elements: (1) bowl brake horsepower, (2) lineshaft mechanical friction, and (3) thrust bearing loss. WATER WORKS from page 44 DACUM Codes To help meet your professional needs, this column covers skills and competencies found in DACUM charts for drillers and pump installers. PI refers to the pumps chart. The letter and number immediately following is the skill on the chart covered by the column. This column covers: PIA-3, PIA-4, PIB-4, PIC-5, PIC-6, PIC-9 More information on DACUM and the charts are available at www.NGWA.org. waterwelljournal.com 46 May 2016 WWJ