Part 15(c)—Well Pumps as Booster Pumps
By Ed Butts, PE
Although the majority of vertical turbine and submersible pumps are used for deep well applications, they can also be used and are often preferred as booster pumps for many applications including parallel and series service as outlined in the last two columns of The Water Works.
This month, we’ll examine this specific use along with the related design process and pitfalls for open sumps, that is, sumps open and exposed to atmospheric pressure.
As with most water system applications, there are usually different approaches that can be used to accomplish the same goal. The following information and design criteria, although based largely on my personal experience and past project success, is not intended to demonstrate the only method available for the application and design of a well pump for use as a booster pump.
As always, for any questionable or difficult projects, I suggest you enlist experienced and qualified individuals or firms to assist you with your specific application. Please note the following guidelines are intended for informational purposes only and to help you gain an understanding of general sump arrangement, configuration, and dimensions. Therefore, I cannot guarantee the success or adequacy of any sump design, layout, or configuration based on any use of this information.
Well Pump vs. Booster Pump
Besides their most common uses as well pumps, vertical turbine pumps and submersible deep well pumps, as well as ordinary centrifugal pumps, are also used as booster pumps. Generally, this is done for singular or series/parallel performance in both inline pressurized applications using a direct inline feed for centrifugal pumps or pump sumps, cans, or barrels for subs and VTP types—using an open or atmospheric pressure fed source from a nearby reservoir, lake, or river that is directly fed into an open sump, commonly referred to as a “wetwell.”
I have used centrifugal, vertical turbine, and submersible pumps numerous times as booster pumps. Once you accept and work around the issues and limitations associated with NPSH, submergence, and clearance design requirements, proper sealing of the discharge or electrical cable when using a pump can under pressurized service, proper grounding of electric motors, and observing the appropriate approach and
annular velocity for either application, you’ll see they make excellent and efficient booster pumps.
Whereas a vertical turbine pump is limited to operation in a vertical orientation only, a submersible pump can safely function in a vertical or horizontal position and at virtually any angle in between. For purposes of this column, the common terms applied to a centrifugal, vertical turbine, or submersible pump will all be referred to simply as “booster pumps.”
Open Sumps: Submergence
When suction problems arise in pump cans or sumps exposed only to atmospheric pressure, more often than not the issues are associated with the configuration and design of the sump or the need to maintain adequate pump submergence as often as with the pump itself—especially if the pump region is circular and operates with minimal submergence.
It is important to remember the pressure exerted by atmospheric pressure results from the weight of the atmosphere at the appropriate elevation above or below sea level—disregarding all potential frictional losses, vapor pressure, and any additional energy losses generated from the flow of fluid within the pump can or open sump.
Depending on the type, specific gravity, and viscosity of the fluid being pumped—and potential interference caused by the number and spacing of pumping units along with the geometry and layout of the sump and pumps—these losses can be minor, insignificant, or severe enough to accumulate and generate a sufficient loss of inlet head or flow disruption, resulting in a negative impact to the operating pump.
In addition, even if the loss of inlet head within the pump can or sump is not excessive, there may not be an adequate depth of water maintained over the pump’s inlet to prevent a cyclonic action of the water leading to the pump inlet during pumping conditions—creating a vortex or whirlpool, similar to the draining of a bathtub or basin.
Vortexes have been known to form in pumping applications over vertical distances of 20 feet or more. Therefore, it is vital to recognize the potential for a vortex to form and methods to prevent or dissipate their formation.
This required operational factor, known as submergence, is also a critical design element and must be included over and above NPSH concerns as a separate and independent consideration in every open and some closed sump arrangements.
Open Sumps: Hydraulic Configuration and Pump Spacing
Where multiple pumps are used, and in addition to submergence, the proper geometry and configuration of the sump and the inlet pipe are also important. If the appropriate spacing and baffling between pumping units or sump walls or barriers is not observed in an open sump arrangement, severe problems can occur, including severe interference between units during operation. It can also result in reduced capacity, vibration, overload, and pumping conditions that can vary from the intended design or system head curve.
The design of a pump sump and intake for a large or geometrically challenged multi-pump application is usually a compromise between the allowable space and structural and physical constraints combined with the factors involving specific hydraulic and pump requirements. They are usually dictated by various site, constructability, and economic factors mutually determined by the client and design limitations.
Therefore, this information should be regarded as general information only, as it is beyond the scope of this column to provide enough specific criteria for the design of all sumps and intakes without appropriate scale or computer model tests. Space doesn’t permit a full treatment on all the possible sump configurations and layouts that can be used for the multitude of applications. However, the following numbered general guidelines can be used with confidence for the most typical open sump arrangements.
Single Pump Applications: Minimum Requirements
The following are recommended minimum requirements. Refer to Figure 1 and Figure 2 for term definitions.
1. The approach velocity (Vs), floor or inlet pipe gradient (slope), and inlet flow patterns are the most important characteristics for the sump. An approach velocity of 1 foot per second (≤1 FPS) or less is preferred with an allowable maximum velocity of ≤1.5 FPS at the maximum design rate (Q), with an evenly distributed flow to the pump’s inlet or inlets for multiple units without obstructions or directional changes that could result in turbulence or flow disruption or distortion.
An unsatisfactory sump configuration or inadequate submergence should use a Vs of .5 FPS or less to compensate for any possible turbulence caused from geometric anomalies or obstructions. Note the sump approach velocity, Vs, is different than the pump’s inlet velocity of Vd.
2. The clearance between the pump’s centerline to all adjacent walls (B) of the sump and adjacent units should be equal to approximately 1.00 of the pump’s inlet bell diameter (D) or (1.0D) with an allowable range of 0.85D to 1.5D. If possible, the greater minimum distance of 1.0D should be used for circular sumps.
The backwall clearance can greatly impact the required pump performance since inadequate clearances can result in vortex formation and interference between units.
3. The clearance (C) between the suction bell or screen to the sump floor should be equal to .5D with a permitted range of .33D to 1.0D. This clearance affects pump head, efficiency, and submergence requirements. Excessive clearance between the pump inlet and sump floor above 1.5D can lead to dead zones where stagnant liquids are not removed.
4. If possible, submergence (S) over the top of the bowl’s discharge case should be equal to at least two times the bell diameter (2.0D) with a range of 1.5 to 5 times the inlet bell diameter, based on the approach flow velocity and degree of uniformity. As a rule, submergence should be at least equal to or greater than the pump’s NPSHR at the design flow rate. Surface vortex formations are generally less with increased submergence over the pump suction. However, in cases with pump inlet or screen, or a flow inducer plate on the pump column
may be indicated to lessen the potential of vortex formation or break up pump-induced vortexes once they occur.
5. The minimum area of flow for a sump inlet cell width (W) by the normal operating depth of water or equivalent pipe diameter area should be equal to two times the inlet bell diameter (2.0D) with an allowable range of 1.5 to 2.5 times the bell diameter. A circular sump (Figure 2) should possess a minimum diameter ratio of 2.0 × (D).
6. Pump inlet (Vd) velocity should be limited to 2–3.5 FPS at the design flow but no more than 5 FPS under all flow conditions. If the pump is to be placed inside a cylindrical can or sump, the diameter should either match the size of the inlet or approach pipe or centered to the sump to prevent radical changes of flow direction that could develop into premature flow acceleration and possible pre-rotation as the water enters the sump or pump inlet.
7. The chart shown in Figure 3 reflects the desired ratio between submergence to inlet diameter (S/D) as well as the approach velocity into the sump, and can be used to approximate when inlet conditions are adequate or should be modeled.
Example: For a single pump application with a maximum design flow (Q) of 1200 GPM, an associated pump inlet or bell diameter (D) of 12 inches and overall bowl length of 60 inches would have the following recommended sump configuration:
- Find inlet and approach velocity (Vs): Q in CFS: 1200 GPM/448 = 2.678 CFS
- Find minimum sump size (diameter) = 12-inch bowl inlet diameter × 2.0D = 24-inch minimum pump can diameter
- Find inlet pipe size: 2.678 CFS/1 FPS = 2.68 ft2/3.14 = √0.853 = 0.92 ft. radius × 2 =1.84 ft. × 12 = 22.17 inches Use 24 inches
- Check corrected inlet pipe velocity: 2.678 CFS/3.14 ft2 (for 24-in. ID area) = 0.85 FPS < 1 FPS (OK)
- Verify Vd ≤ 3.5 FPS at maximum Q: 2.678/0.7854 ft2 (12-in. bowl inlet area) = 3.41 FPS < 3.5 FPS (OK)
- Find minimum submergence: 2(12-in.) = 24 in. + 60 in. bowl length + (0.5)(12 in.) bowl clearance = 90 inches from sump floor
Finally, determining the length of column and overall pump setting will be based on the required sump depth to provide the required submergence and use of the full pond depth, less a factor for sedimentation and any screening devices along with an energy loss of less than 2 inches (0.167 feet) from the water source to the sump.
Normally, limiting the maximum head loss between the source and sump to 2 inches of head will go hand in hand with the desired velocity as these two values are interdependent.
Depending on the specific application, raw water quality, and shape and configuration of the pond or source, I generally use a minimum sedimentation allowance of 6 inches between the bottom of the inlet pipe and storage body (pond or impoundment). Therefore, if the normal water surface was 6 feet (72 inches) below ground in a 12-inch-deep pond and the source was 20 feet away from the sump’s intended site, the overall minimum sump depth will be: 90 in. + (72 in. – 2 in. hf) = 160 inches (13.33 feet).
Typically, in this case if I was planning to use a concrete sump, I would default up to the next depth of standard lengths of precast concrete pipe sections of 4 feet or use 4×4 sections = 16 feet and adjust the pump setting length and inlet pipe depth accordingly.
Depending on the specific type and size of offset inlet pipe and related hydraulics, the invert (bottom) of the inlet pipe projection in the pond will be: 12-inch pond depth – 0.5 feet (allowance for pond sedimentation) = 11.5 feet. So the invert of the inlet pipe entering the sump would then be placed at: 11.5 ft. – (20 ft.)(0.02) (for a 2% pipe slope) = 11.1 feet (use 11 feet), which places the top (crown) of the inlet pipe at: 11 feet + 2.04 feet (for a 24.5-inch OD pipe) = 13.04 feet, say 13 feet.
With a 16-foot overall sump depth, the bottom of this inlet pipe will enter the sump with approximately 5 feet of water depth below the invert, which provides an excellent relationship of submergence to velocity for most single or multiple pump installations.
The pump setting should be adjusted to provide 6 to 12 inches of clearance between the pump’s inlet screen to the floor of the sump to ensure submergence while avoiding the potential for excessive buildup of sediments on the sump floor. As a check on the energy losses for inlet pipes ≤ 30 feet in length (culverts), the following relationship is used (inlet pipes greater than 30 feet in length should include adding the head loss from the pipe itself):
Total hf = 0.0056 ft. + 0.0089 ft. = 0.0145 ft. ≤ 0.167 ft. or 0.0145 ft. × 12 in. = 0.174 in. ≤ 2 inches. Either approach is OK.
Even if the inlet pipe’s frictional losses were added, the total head loss would amount to only 0.017 feet or 0.2 inches, well below the maximum recommended value of 2 inches. This is an example demonstrating all of these factors must obviously be weighed together to ensure the sump is deep enough for the intended application while maintaining adequate clearance and inlet submergence.
All the above recommended minimum requirements rely on a reasonable uniform velocity distribution in the sump bay immediately upstream from the pump. Failure to achieve this uniform velocity distribution could result in occurrences of adverse flow phenomena, including possible vortex formation.
Multiple Pump Applications: Minimum Requirements
Figure 4 will provide a few basic tenets of good sump (wetwell) design for multiple pumps in open sumps as well as various illustrations of sump configurations not recommended.
Typically, the information contained in this figure is applicable to a wetwell with multiple pumps with a combined capacity of 10,000 GPM or less. Applications with greater capacity should be properly modeled and simulated through the application of either computer or Froude models.
The design of vertical turbine or submersible pumps in an open sump arrangement is largely based on personal preference and local availability of wetwell material and installation procedures and limitations; pump bowl diameters and capacities; and the planned size, depth, and geometry of the application. Many designers prefer to use sumps with a square or rectangular plan for pump support and suction supply as sumps while others, including myself, prefer to use circular corrugated steel or precast concrete wetwells for open sumps.
Not only is a circular wetwell generally easier to design and construct, but limiting the typical lengths (rings) of precast concrete sections to 3–5 feet per section permits custom depths of sumps to be created.
Typically, the diameter or cross-sectional dimension of a wetwell is controlled by the number and bowl diameter of pumps needed, factored with the required spacing between units and the barrier wall. Unless the units are intended for singular operation, adequate spacing and clearance must be provided between each unit to avoid possible interference during parallel operation.
In most cases under 10,000 GPM, wetwells are designed by using the bowl or inlet diameter (D) as the controlling factor. In cases with combined or singular unit flow rates exceeding 10,000 GPM, minimal submergence, or unusual sump configurations, hydraulic or computer modeling should be used to confirm and match the best possible sump design to the pump selection.
Figure 5 illustrates a typical wetwell design for three pumping units using a 6- to 10-inch-diameter circular concrete wetwell for an irrigation application.
This column has been primarily directed toward the use of vertical turbine or centrifugal pumps in an open sump arrangement. However, the relatively recent advancement of submersible pumps warrants including them. Standard deep well submersible pumps can be just as efficient and cost effective as a VTP or centrifugal pump in an open sump—but there are three basic factors each designer should consider before commitment.
- By design and construction, the motor on a deep well submersible pump is placed below the pump. Based on comparable water levels required for each style of pump, this requires a deeper overall sump or an individual pump sump than would conventionally be needed for a VTP or centrifugal suction pipe.The added depth must not only accommodate the motor length, usually 4 to 5 feet, but also enough depth to provide for sand and silt sedimentation and water to pass alongside the motor to maintain adequate cooling. A shroud, generally at least 2 to 4 inches in greater nominal diameter, encompasses both the pump and motor and is usually employed to route passing water down and around the motor on the way to the pump’s inlet.
- Submersible pump motors are designed to operate inside of a water well where the inherent grounding capabilities of the well provide a safe operating environment. Use in an open sump or pump can change this dynamic, though, as the motor is no longer grounded from the earth’s reference ground and is more exposed to dangerous shortages and possible electrocutions to personnel.Even though specifically required by the National Electric Code (NEC), some installers still do not see the need to provide a separate grounding conductor to the motor on a water well installation, but it is imperative when this type of motor is used in a sump arrangement. Failure to extend a properly sized and fully connected grounding conductor to the motor frame can result in a dead short to the motor without opening the protective fuse or circuit breaker. If the shorted motor comes into contact with humans or animals, the resulting electrical current can be enough to result in serious injury or death.
- Submersible pumps and motor are generally designed to operate at 3600 RPM, which is twice the rotational speed of a conventional vertical turbine pump at 1800 RPM. In identical operating environments, this factor of speed usually results in four times the wear to components. This increased wear factor can result in frequent repair or rebuild of running surfaces if the sump is exposed to sandy or excessively silty water that has not been adequately prescreened.
Potential Design and Construction Issues
Although beyond the intended scope of this column, a buried sump—whether circular, rectangular, or square in plan—will be exposed to lateral loading from the backfill soil and any groundwater surrounding it.
Generally, a circular shape of sump will safely handle these stresses since the well-distributed hoop strength gained from the circular shape is uniform and predictable. However, the system designer should always use the services of an experienced geotechnical or structural engineer to verify the strength of the intended sump layout. This is particularly true for deep (>20 feet) sumps with surrounding groundwater or those using corrugated steel.
Just as any groundwater around the sump will expose it to greater lateral loads than soil alone, if the wetwell is provided with a lower floor, typical for wastewater applications, the potential uplift that may be created from this groundwater surcharge must also be evaluated with preventive measures to avoid any uplift to occur which can result in the sump popping up from the ground or the “swimming pool effect.”
Support of the pumps at the top of a wetwell varies with the type of wetwell, horsepower, weight of pump, and degree of protection required. Most wetwells using concrete for the sump will also use a precast or cast-in-place concrete slab at the top to support the pumps.
This type of design must be planned carefully to ensure adequate spacing between units and the wetwell sidewalls have been provided as well as a large enough opening at the top slab for bowl clearance.
Sumps constructed from corrugated steel, common for irrigation applications, generally use a fabricated steel support structure to support the pumps. In this case, verify the support structure is stiff enough and adequately bolted to and resting on sufficient mass to resist the pump and motor vibration, but not from the wetwell itself.
This conclude this column on using booster pumps in open (atmospheric) sumps. The next column of The Water Works will wrap up this series on parallel and series pumping systems with an overview on the design and application of pressurized pump cans.
Until then, keep them pumping!
Ed Butts, PE, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at firstname.lastname@example.org.