Water Well Journal
Issue link: http://read.dmtmag.com/i/668983
I n the last installment of The Water Works we talked about designing a deep well pump for a subdivision of 100 resi- dences. It was determined in that column the well and well pump needed to produce a design condition of 500 GPM against a total dynamic head of 269 feet. Here and in the next installment of the column we will ex- amine the design elements associated with a vertical turbine pump (VTP). First a disclaimer. The proper application of vertical tur- bine and submersible pumps is a specialized engineering task requiring consideration of many individual factors. The design principles and application guidelines are those I have applied and used over the course of more than 40 years. While I believe the engineering and design principles men- tioned here to be sound, there is never just one way of doing anything. You are therefore cautioned to exercise appropriate judgment in the use and application of the following informa- tion; seek technical assistance whenever warranted; follow all of the manufacturers' guidelines, especially when considering deep well applications; and follow all specific application and installation specifications. Design of Vertical Turbine Pumps The design of a vertical turbine pump must consider sev- eral elements unique to this method of pump construction. At a minimum these include: capacity, total head, horsepower, NPSH (net positive suction head), submergence, thrust, suction conditions, well diameter, water levels (static and pumping), method of lubrication, driver type and speed, and abrasive (sand) conditions. Each of these parameters must be considered, although not all of them will apply for each individual application. Capacity Obviously, as is the case with any pump, the primary consideration is the required capacity and discharge head. Actually, the required flow capacity of the pump is usually the primary design criteria; the required head comes in second. In a deep well application, the limiting factor of maximum capacity will generally be related to the well diameter. Typi- cally, the maximum efficient capacity of a turbine bowl will be related directly to the outer diameter of the bowl assembly. Therefore the designer must consider the well diameter before the actual bowl selection can be made. For general guidelines, the information in Table 1 can be used to approximate the maximum capacity for a typical bowl assembly (based on the outer diameter of the bowl assembly). Well Diameter Generally, the well should provide a diameter at least 2 inches larger than the nominal bowl diameter. In other words, the required flow in our example mentioned at the very begin- ning of this column is 500 GPM, necessitating at least an 8- inch bowl diameter. Therefore the well diameter should be at least 8 inches + 2 inches = 10-inch minimum. This allowance is needed to compensate for any well deviation or misalign- ment and to permit the pump bowl and shaft to operate straight and plumb and without any deflection or binding. In our case the well with a 12-inch casing size is of ample size for the required flow of 500 GPM. It is usually desirable to select a bowl with the largest diameter that is also efficient for the required flow. In our example of 500 GPM, if the actual well diameter was 14 or 16 inches, a 10-inch or even a 12-inch-diameter bowl should be considered as well as an 8-inch-diameter bowl. This is because larger diameter impellers (and bowls) tend to be higher in efficiency, less prone to premature wear and ED BUTTS, PE, CPI THE WATER WORKS ENGINEERING OF WATER SYSTEMS Part 13—Vertical Turbine Pump Design (1) WATER WORKS continues on page 42 In a deep well application, the limiting factor of maximum capacity will generally be related to the well diameter. Table 1 Maximum Design Capacity at 1760 RPM (Enclosed or Semi-Open Impellers) Bowl O.D. Maximum Flow (GPM) 6 inches 200 GPM 7 inches 400 GPM 8 inches 900 GPM 10 inches 2000 GPM 12 inches 3500 GPM 14 inches 4000 GPM 16 inches 5000 GPM waterwelljournal.com 40 May 2016 WWJ