Water Well Journal
Issue link: http://read.dmtmag.com/i/529509
thing you should know about pumps is how to read a pump curve. Although I have worked with many people who would dispute this, working with pump curves (also called perform- ance curves) is really not hard to do. The primary consideration of reading pump curves is to re- member almost all pump curves are based on the principle of plotting data on a curve using the "x" and "y" axis. Remem- bering back to our basic geometry, the x axis of a curve refers to a line in the horizontal direction and the y axis refers to the vertical direction. With this information in mind, curves are typically plotted using a pump's capacity or design flow rate on the x axis, and head on the y axis (although efficiency, power input, and net positive suction head [NPSH] can also be plotted on the y or vertical axis). The most common type of pump curve in the water well industry is called the "head-capacity" curve, in which a pump's capacity is plotted on the horizontal axis and the pump's available head is plotted at the corresponding point on the vertical axis. An example of a typical pump curve is shown in Figure 1. In this case, the plotted condition of service (COS) point or the design condition (indicated by the red hash mark) is 650 GPM at 230 feet TDH with an 8.5-inch impeller diameter. In addition to the capacity and head, many pump curves also in- clude additional data relevant to the specific pump at various flow rates along the curve—such as the net positive suction head required (NPSHR), efficiency, and power input, shown usually in brake horsepower (HP) or kilowatts (kW). In some cases, notably for irrigation types of centrifugal pumps, the maximum total dynamic suction lift (TDSL) of a pump is shown instead of the engineering-preferred value of NPSH. However, either term is acceptable and used. This provides for an easier computation of a pump's ability to lift water, but corrections for altitude and other factors are required for full accuracy. Curves are not usually created by plotting each individual data point on a curve, but rather by plotting several data points on a graph and then extending a continuous line between and connecting the data points. Thus, a curve with reasonable ac- curacy is created for all operating conditions between the two most extreme plotted points. Without these extension lines, the values for each parameter would have to be plotted in rows of long columns of numbers. These columns would have hundreds of entries, would be difficult if not impossible to understand and use, and would not accurately represent the way a centrifugal pump actually works. Understanding Suction Problems and NPSH The next pump factor everyone should know is how to deal with suction problems. Suction problems can really put a damper on what was once a nice day, especially with pumps under a suction lift. But even well pumps can present a chal- lenge on this basis. In most cases, understanding the pump's NPSH is a vital operating condition and one that can help in figuring out why a pump is not performing as it should. In our sample pump curve, the NPSHR is 15 feet at the de- sign flow rate of 650 GPM (denoted by the intersection of the blue line at 15 feet at 650 GPM). This basically means this specific model and brand of pump requires a positive inlet head of 15 feet at the pump's inlet or suction. This is a funda- mental design factor of all centrifugal pumps, although it is a critical factor for most centrifugal pumps under a suction lift, or where the source of water is situated below the pump's centerline. In our sample pump selection, we would want to verify the atmospheric or other source of head is at or above the mini- mum of 15 feet of inlet head the pump requires to prevent cavitation or vapor formation after deducting all losses. Cavitation is a common condition to centrifugal pumps many people equate to the sound of "gravel passing through the pump." It is actually the sound that occurs when vapor pockets, formed by the pressure of the fluid being pumped, falls below the appropriate vapor pressure for the temperature of the fluid. When this occurs, these vapor pockets travel through the pump until they encounter a region of higher pres- sure, usually at the pump's discharge zone. The higher pres- sure results in collapsing of the vapor pockets, resulting in the familiar sound. This can cause severe pitting of the pump components as well as reduced head and abnormal operating conditions. However, this problem can often be avoided with some simple calculations. This is determined by starting out with a value of 33.9 feet. This is the value of the weight of atmos- pheric pressure (air) at sea level converted to head (14.7 psi × 2.31 feet/psi). Elevations above sea level should deduct 1 foot for every 1000 feet of rising elevation so as to account for the reduction of atmospheric pressure at higher altitudes. From the remaining value we then deduct .59 foot for the vapor pressure of 60° water, then the 15 feet of NPSHR by the pump. At this point we insert the calculated friction loss value for the suction pipe, foot valve, and other piping right up to the pump's inlet, which we determine to be 6.0 feet. From all of this we can conclude: 33.9 feet – 0.59 foot – 6.0 feet – 15.0 feet = 12.31 feet. This is the maximum lift this pump is allowed under these conditions. Any change in any of these factors will modify the others as well. This relationship is shown graphically in Figure 2. By understanding a pump's NPSH and how it works with a pump's suction capability and friction loss, a designer or field technician can troubleshoot field conditions and adjust one or two of the design factors, such as the piping friction loss or suction lift, to prevent suction problems. In addition to NPSH issues, some problems are simply caused by a misapplication of a pump. Many people don't real- ize pumps do not have universal suction abilities. Some pumps are designed and therefore can lift water 20 feet, while other units struggle to even pull water 5 feet from a well or stream. It is vital each pump's performance chart is consulted and verified it is capable of the conditions it is being asked to do. Another often overlooked factor are suction leaks. Suction leaks can occur anywhere on the suction assembly including ENGINEERING from page 43 ENGINEERING continues on page 46 44 July 2015 WWJ waterwelljournal.com