Make sure you consider all the pertinent items pertaining to a pump when designing a well system.
By Marvin F. Glotfelty, RG
Water wells should be designed to meet the needs of the pump equipment’s size, depth setting, and planned operation schedule for the well. The pump equipment is also a critical consideration when evaluating or troubleshooting older existing wells.
As I laid out in my book The Art of Water Wells (NGWA Press, 2019), the first thing that should be considered when designing a new water well is the intended purpose of the well, which provides a determination of the pump size required to meet that intended well function.
The well design sequence (Figure 1) is applicable to essentially all well uses—from small household wells to large municipal or industrial wells. All the subordinate well design decisions will rely on the selected pump size since the casing diameter, screened interval, etc. must accommodate the pump equipment and its planned use.
The flow rate of a pump is generally controlled by the diameter of the pump bowls (and impeller size within each bowl), whereas the pump’s ability to lift water to the land surface (the total dynamic head or TDH) is generally controlled by the number of pump stages. There are numerous pump types used for groundwater withdrawal (such as jet pumps, hand pumps, windmills, centripetal pumps), but two of the more common pump types used in water supply wells are line shaft vertical turbine pumps and submersible pumps.
Line Shaft Vertical Turbine Pump Considerations
Line shaft vertical turbine pumps (Figure 2) operate by pulling the groundwater into the pump intake (Figure 2E) and up through the column pipe (Figure 2B) as the pump impellers (Figure 2D) are rotated by the inner shaft (Figure 2C), which is powered by the pump motor (Figure 2A) at the land surface.
Since there is a long rotating shaft in this type of pump, it is important to have good well alignment when a line shaft vertical turbine pump is used to accommodate proper operation of the pump’s line shaft. Even though the well casing and screen may have some degree of horizontal drift, the inner shaft of a vertical turbine pump must remain in a straight line (Figure 3). Otherwise, the pump will develop excessive vibration during its operation and the bearings in the line shaft will wear out prematurely.
Many well designers confuse the well’s plumbness requirements with alignment requirements or simply consider “plumbness and alignment” to be a single attribute of water wells. Actually, the plumbness of a well (the vertical orientation of the well toward the center of the Earth) is not critical, as long as it has adequate alignment (the positioning of the well in a straight and uncurving path).
A few years ago, the general manager of a major pump manufacturer told me if a line shaft vertical turbine pump is properly shimmed (wedged at an angle) at the land surface to align the motor with the line shaft (Figure 3B), an out-of-plumb well with a dip of up to 30° can still operate properly. However, if the well has doglegs and poor alignment (Figure 3C), the pump’s line shaft will experience wear and tear that will reduce the longevity and efficiency of the pump.
While it is appropriate to put a greater emphasis on alignment than on plumbness for a well, there should still be consideration of the well’s plumbness in cases where the pump must be set within the screened interval. In those cases, the vibration and torque of the pump upon startup may result in abrasion of the pump bowls or well screen as shown in Figure 4.
Submersible Pump Considerations
If silent operation of the pump is needed (such as near residential neighborhoods) or if the pump is to be used in a crooked well, a submersible pump is preferred. Submersible pumps are also commonly used for smaller household wells or monitoring wells.
Submersible pumps (Figure 5) don’t have the challenges of a long rotating shaft as with line shaft vertical turbine pumps because the pump motor (Figure 5E) is within the well near the pump bowls. The submersible motor rotates the pump impellers (Figure 5C), which draw water into the pump intake (Figure 5D) and lift it to the land surface through the column pipe (Figure 5B).
Electrical power is provided to the pump motor by an electric cable (Figure 5A), which will have an increased gauge thickness as the motor horsepower and pump depth settings are increased. Thus, the width of the electrical cable must be included in the well’s dimension considerations to assure that there is adequate room for the pump equipment within the well casing.
Wells are generally designed for the pump equipment to be placed well below the pumping water level, but still above the top of the well screen. In some cases, however, the hydrogeological conditions or structure of a well necessitates placing the pump within the screened interval. This condition calls for some additional well design considerations.
For submersible pumps, the motor generates a lot of heat during its use, so it is important to have an adequate flow of water pass the motor to cool it (Figure 5F). If much of the groundwater contributed by the well screen is produced from a depth interval above the pump intake, the flow of water passing the motor may be inadequate to prevent overheating of the motor.
To alter the water’s flow path and protect the submersible pump motor, a pump shroud (Figure 5G) can be used to direct the pumped groundwater past the motor (Figure 5H) even if the water is produced from above the pump’s depth setting. So, in cases where a submersible pump is used, a larger-gauge electrical cable or a pump shroud may be required, and thus the casing and screen diameter should be increased accordingly.
Whether a line shaft vertical turbine pump or a submersible pump is to be used, if the pump intake is located within the screened interval, the well designer may want to include a pump gallery, a 10- to 40-foot-long blank section of casing within the well’s screened interval (Figure 6). Pump galleries provide a non-perforated location for the pump intake, where the higher flow velocities in that area of the pump intake won’t cause sand invasion to increase.
Setting the pump directly adjacent to the well screen can result in pump bowl or screen abrasion (Figure 4), or cause formation sand to be produced because of the high entrance velocities of the pumped groundwater. The well designer should strategically select a depth for the pump gallery that it is neither too shallow nor too deep to accommodate the projected depth setting for the pump. An overly shallow pump setting may limit the available water-level drawdown (and therefore the potential pumping rate). An overly deep pump setting will increase the costs of column pipe and electrical cable.
Pump Performance Curves
The designed operation range of a pump is indicated by its performance curve (Figure 7). A performance curve is a plot of total dynamic head (TDH) on the left vertical axis versus the gallons per minute (GPM) on the horizontal axis.
The head capacity plot shows the feet of lift that the pump can achieve at various discharge rates. The right-hand vertical axis on the performance curve has other values, representing the pump efficiency, brake horsepower, or net positive suction head required (NPSHR).
A pump should be operated within its recommended operating range (vertical dashed lines in Figure 7) to facilitate efficiency of the pump equipment. For purposes of water well design, our focus is primarily on the planned pumping rate (GPM) and lift capacity (TDH) along with the necessary pump submergence (NPSHR).
The NPSHR is defined as the suction pressure at which the pump’s hydraulic performance degrades by a certain amount (generally 3%), as designated by the pump manufacturer. More simply put, NPSHR represents the distance that the pump’s first impeller should be submerged below the pumping water level to prevent cavitation.
Cavitation results from the condition where the water is exposed to low pressure as the water enters a pump impeller, which can make it essentially boil and develop water vapor bubbles. As the water moves to areas of higher pressure while it is flowing through the impeller, the bubbles collapse and implode. The imploding bubbles sound like a batch of marbles
or gravel is being circulated through the pump, and the cavitation will cause significant damage to the pump impellers.
Cavitation can be prevented by maintaining enough submergence of the pump to create adequate back pressure at the impellers. The absolute suction head pressure available at the first stage impeller (minus the water’s vapor pressure) is termed the net positive suction head available (NPSHA). As long as NPSHA remains greater than NPSHR, cavitation will not occur.
The pump may emit sounds and vibration that are like cavitation conditions if the water being pumped contains entrained air. Entrained air can result from such conditions as cascading water within the well, leaks in the pump system valves or piping, or air introduced from neighboring recharge facilities.
A simple test to differentiate between cavitation and air entrainment is to throttle down the pump’s discharge valve, which will make the pump operate farther to the left on its performance curve (Figure 7). If this results in reduced noise and vibration, the root cause of the problem is likely cavitation. However, if the noise and vibration remain unchanged, the problem is likely entrained air.
Pump Equipment Consideration Through the Entire Operational Life of a Well
When an older existing well is being evaluated to assess declines in its performance, the pump equipment needs to be included in that analysis. The first thing we need to do is identify the general source of the well’s drop in water production. It could be due to a problem with the well itself, such as a clogged screen that has become blocked with biofilm and scale, or the well could have been developed incompletely when it was drilled, such that residual drilling mud remains in the borehole.
The problem could also be with the pump equipment and not the well—such as worn impellers, a hole in the column pipe, or an electrical problem. A third possibility is degradation in the regional aquifer, which would be a distinct problem from either well or pump issues. Aquifer characteristics don’t generally change over time (although there are some exceptions), but a drop in the regional water table is not unusual and such an occurrence will definitely impact the well’s water production.
Figure 8 illustrates the signals that will enable us to generally categorize the source of a well production problem. If the static water level remains unchanged but we see a drop in the well’s water production and also a drop in the pumping water level, we have a well problem.
If the static water level and also the pumping water level remain constant, but we still see a drop in water production, it signals a pump equipment problem.
If the static water level is dropping over time, we will also experience a decline in the pumping water level and the production rate of the well. This indicates a problem with the regional aquifer due to overdraft conditions and/or an interruption of recharge sources.
The pump bowls and motor will occasionally be replaced during the life of the well, and the specific equipment can be adjusted as needed along with the pump’s setting depth. These pump equipment adjustments can be done over the years of well use in response to the changing conditions that may occur with the groundwater system.
However, we cannot change the well’s structure during its operational life without undertaking difficult and expensive rehabilitation efforts. The well’s casing diameter, screened interval, pump gallery placement, etc. should be selected with both current conditions and future conditions in mind, so that the well will operate efficiently for many years.
None of us have a crystal ball that will tell us exactly what the future holds regarding the supply and demand of the water system. However, we can make reasonable predictions of future water requirements and sensible projections of the likely well/pump/aquifer system conditions, so that throughout the operational life of the well (about 25 to 100 years) we will meet our water production goals with an efficient and reliable groundwater supply.
Marvin F. Glotfelty, RG, is the principal hydrogeologist for Clear Creek Associates, a Geo-Logic Associates Co. He is a licensed well driller and registered professional geologist in Arizona, where he has practiced water resources consulting for more than 35 years. He is author of The Art of Water Wells (NGWA Press, 2019) and was The Groundwater Foundation’s 2012 McEllhiney Lecturer. Glotfelty can be reached at firstname.lastname@example.org.