Water Well Journal
Issue link: http://read.dmtmag.com/i/529509
Disadvantages include susceptibility to excessive abrasive wear on wearing surfaces such as wear rings and im- pellers (which can cause a rapid drop in efficiency and head), unbalanced thrust conditions, susceptibility to internal cavitation damage, susceptibility to air- locking, and inaccessibility to internal surfaces needed to polish and remove casting imperfections which results in lower impeller efficiencies. Thrust balanced impellers are modi- fied enclosed or semi-open impellers with openings provided from the bottom to the top of the impeller. This bypass port arrangement provides a balanced thrust condition since the thrust forces imposed on the suction side of the im- peller are essentially the same as the discharge side. This feature is important for pumps operating at high speeds, high pressure, or deep settings. Semi-open impellers, although they have some distinct advantages, are the least used type of impeller and are not available from every manufacturer. The advantages in a semi-open im- peller include high efficiencies due to low disc friction and ability to polish in- dividual vanes and surfaces; ability to fine-tune the vertical impeller settings to obtain optimum performance; ability to handle stringy and fibrous material; ability to handle entrained gases better than the enclosed type; and the ability to compensate for wear through adjustment. Disadvantages include high thrust values, critical adjustment requirements, danger of overloading the driver if not adjusted properly, rapid decrease of per- formance with wear, and the need for constant monitoring of performance and adjustment in abrasive applications. "Wear rings" are often used on either or both of the suction and discharge sides of impellers and bowls to limit the leakage that occurs between the lower pressure suction side and the higher pressure discharge side of the impellers as well as provide future repair surfaces. These rings are usually constructed from bronze, but a bronze-rubber com- bination is sometimes used in abrasive conditions as the rubber component will flex and adapt to the worn surfaces. Basic Components The basic components of a vertical turbine pump include the bowl assembly (which includes the impellers and bowls), column and lineshaft, discharge head, and driver (Figure 2). The driver for a VTP is usually an electric motor, but right angle gear drives and belt/ pulley drives are often used in areas without available power supplies or where diesel or natural gas fuel is cheaper than electricity. The driver, regardless of type, must adequately provide or convey the total horsepower required by the entire pump assembly—including the bowl assem- bly, lineshaft, and internal losses. The driver, through its thrust bearing, must also be able to handle all the vertical thrust imposed from the rotating assem- bly and not allow excessive downward movement that could result in rubbing or scraping of the impellers on the bowls. Thrust and inner stage leakage are components of virtually every vertical turbine application, although they are not significant factors in most. Thrust is present in two forms: axial and radial (or lateral). Axial (up and down or vertical) downthrust is the downward force im- posed onto the driver from the sum of the hydraulic thrust originating from the bowl assembly added to the weight of the entire lineshaft and impeller/bowl shaft assembly. While there are other additional axial thrust loads present during operation, they are minor loads that can be disre- garded. In most cases, axial thrust loads add only a few tenths up to 1-2 of addi- tional horsepower requirement onto the driver, but in deep wells or applications at high pressure, axial thrust and the commensurate added horsepower can be significant. In rare cases with pumps at shallow settings, axial upthrust can also be pres- ent. This is especially true when using semi-open impellers on shallow set- tings. In extreme cases, excessive up- thrust can result in the entire pump assembly momentarily rising off the support base, leading to the possibility of extreme damage. Radial thrust is the result of lateral (right to left or side to side) force ex- erted by the water discharged from the impeller onto the bowl diffuser. This thrust value does not usually represent a significant impact to the pump due to the relatively close spacing of bearings within the bowl assembly and the fact that radial thrust around the impeller is reasonably balanced due to the identical diffuser pattern encircling the impeller. Inner stage leakage sometimes oc- curs on older, worn-out pumps as well as high-pressure applications. Again, wear rings are often used to control this problem. The industry term column is used for the steel pipe that suspends the bowl assembly, conveys the water to the surface, and encloses/stabilizes the line- shaft. In most applications it is used in 10-foot lengths to coincide with the lineshaft lengths, but it can be used in lengths as long as 20 feet when used with enclosed lineshaft construction. Lineshafts The industry term lineshaft is used to describe the steel (typically round carbon steel) vertical driveshaft used to rotate the impellers located in the lowermost turbine bowl assembly. Line- shaft is most commonly available in di- ameters from 1 inch up to 2.5 inches. Vertical turbine pumps are driven through lineshaft using one of two lubrication methods: open (or product) lineshaft lubrication construction, or enclosed (oil) lineshaft lubrication construction. In open lineshaft construction, the lineshaft bearings, usually constructed from fluted rubber, are lubricated by the product being pumped—which is usu- ally water. These bearings are typically spaced vertically no farther apart than 10 feet. Due to the lack of lubricating WATER WORKS from page 57 DACUM Codes To help meet your professional needs, this article covers skills and competen- cies found in DACUM charts for drillers and pump installers. PI refers to the pumps chart. The letter and number im- mediately following is the skill on the chart covered by the article. This article covers: PIA-4, PIA-5, PIB-4, PIC-5 More information on DACUM and the charts are available at www.NGWA.org. 58 July 2015 WWJ waterwelljournal.com