Part 15(b)—Booster Pumping–Series Pump Systems
By Ed Butts, PE, CPI
We introduced parallel and series pumping systems in the January column of The Water Works with a discussion on parallel pumping installations and possible pitfalls. This month we continue the discussion with series pumping installations.
As opposed to a parallel pumping configuration, a series (often mistakenly referred to as a real booster pump system) pumping installation implies the entire discharged volume (flow rate) of source water working against a proportionally lower amount of the total head will be delivered from a water supply source (a source pump or pressurized water system) and immediately delivered to the inlet (suction) of an inline booster pump to elevate the combined system head (pressure) to the total dynamic head (TDH) in tandem.
An idealized scenario where two identical pumps are used to effectively double the discharge head from 150 feet to 300 feet at a common flow rate of 1000 gallons per minute is shown in Figure 1 (TDH: 1 + 1 = 2). In some cases, up to three to four pumps at different levels are used to generate the TDH in stages and at different intervals, with each pump adding its unique and individual value of head toward the resulting total head, all at a uniform flow rate.
Although the head delivered by each pump may be dissimilar, it is critical that all pumps used in a series pumping configuration are designed for and capable of accepting and producing the same flow (discharge) rate or range, preferably within the best efficiency window.
As an example, referring to the original water well design example using a submersible pump we have used throughout this series:
Design conditions: primary conditions of service (COS): 500 GPM at 260 feet TDH; well lift component of TDH: 90 feet
Alternate COS: 156 GPM at 240 feet TDH; well lift component of TDH: 62 feet
Even when using a variable frequency drive in many actual applications, depending on the use with each service condition, this type of wide variance in flow rate (more than 25%) between the primary and alternate conditions presents itself as a potentially ideal type of situation to consider using a series pumping installation.
This permits the source pump to be designed for a wider range of flow with a higher efficiency at the more frequently used alternate (usually lower) flow condition and with a lower operating efficiency at the less used primary condition. So long as the source pump is capable of the higher capacity primary condition flow rate from the well and well pump with sufficient head into the suction of the second stage, or booster pump, the system can be an efficient alternative to using a single pump.
As a rule of thumb, I have found a well/booster pump system is most efficient when the well pump is required to produce 50% to 75% of the total head, with the booster pump producing 25% to 50% of the total head at a common flow rate. This percentage range in a two-step pumping arrangement generally allows for the possibility of using the source pump only for the lower system demands.
It is also a valid scenario when an existing or lower horsepower well pump is present and desired to be retained which may be capable of producing the required higher flow rate but is not capable of the required higher head without boosting, the power supply is limited or not capable of starting a single large (50 HP for example) motor, or the client does not wish to use a variable frequency drive in the installation.
In this case, the well pump would generally be designed as a single speed, lower HP unit that would be capable of delivering the primary flow of 500 GPM into the booster pump and efficiently pumping at a reduced flow rate of 156 GPM at the required lower head (approximately 240 feet).
In our revised design example using a submersible well pump and end-suction centrifugal booster pump combination, we begin by examining and deducting the 90 feet well lift (pumping water level, PWL) added to the riser pipe and discharge friction loss (7 feet), which at 500 GPM totals an adjusted pumping water level (APWL) of 97 feet. This means, at a minimum, enough of the well pump’s head must be reserved to lift the water to the top of the well and into the booster pump under maximum flow conditions.
Typically, in these cases I try to add a minimum safety factor of no less than 10 psi (23 feet) to this value of head. I actually prefer to add 20 psi (46 feet) of reserve head to the APWL, when available, to account for a possible future decline in the PWL and to provide adequate inlet head into the booster pump at all times, which ensures the booster pump will not cavitate from inadequate inlet pressure or NPSHA.
In our case, the required head from the well (source) pump as a portion of the total head is:
Well lift (PWL) and riser pipe friction loss: 97 feet (90 feet + 7 feet riser pipe hf = 97 feet APWL) + 20 psi of booster pump inlet head: 46 feet = required minimum well pump head: 143 feet (55% of the total of 260 feet).
The alternate submersible well pump we have now selected is a 9-inch-diameter × five-stage bowl assembly (81.5% efficient) with a 25 HP motor, instead of the 50 HP sub motor originally selected. The revised design condition for this pump is now 500 GPM at 152 feet TDH, 9 feet more than required. The well pump curve is shown in Figure 2.
The remaining needed head must now be provided from the booster pump:
Originally required total system head (TDH): 260 feet (at 500 GPM) – less head from well pump: 152 feet (57.4% of 265 feet) = 108 feet + added inlet head for booster losses: 5 feet = required (net) booster pump head: 113 feet (42.6% of 265 feet).
The booster pump required COS of 500 GPM at 113 feet TDH can easily be met from an end-suction centrifugal pump as the required head may be too low for many multistage VTP or submersible units. The booster pump curve (Figure 3) indicates the booster pump will deliver 500 GPM at 116 feet TDH, 3 feet more than required (the booster pump impeller can be trimmed to fit precise conditions).
Unless otherwise determined, I routinely recommend adding 5 feet of head to the booster pump head requirement to account for the energy losses associated with the pump and manifold assembly.
In our example, an 18.60 BHP demand is calculated for the booster pump; therefore, a 20 HP motor is needed. The combined brake horsepower from both pumps is now: 23.55 HP (well pump) + 18.60 HP (booster pump) = 42.15 BHP, only 1.54% higher in BHP than the original single 50 HP submersible pump BHP of 41.50, indicative of an efficient pumping tandem.
Although the alternate condition of 156 GPM for the well pump is slightly less than the original design head of 240 feet (230 feet actual TDH or about 4 psi less pressure), after consultation with the client this small difference in delivery pressure is not felt to be important.
Series Pumping Red Flags
Just as we noted in the January column with a parallel pumping installation, there are also potential issues in a series pumping configuration.
Incorporate a check valve bypass for primary pump operation.
While this may seem a no-brainer, I have witnessed numerous examples where a system was designed with the primary or “source” pump always pushing water through and out the booster pump, even when the booster pump is disabled. In addition to the extreme and unnecessary head loss associated with routing the flow through the booster pump, this type of operation generally results in a slower speed rotation of the pump and motor.
This lower rotational speed of the booster pump and motor does not usually generate sufficient lubrication of the motor bearings. This will be outlined in our next column.
Be aware of possible system or booster pump overpressurization.
This is one of the most critical considerations for a high-pressure (less than 150 psig) series pumping installation since the residual pressure or head developed from the source pump is added to the pressure (head) from the booster pump at a common flow rate to generate much higher finished pressure.
This is particularly true on irrigation applications using hard hose reels with big guns or other high-pressure systems. In extreme cases, particularly if the booster flow rate is throttled or reduced, the combined pressure can reach enough of a dangerous level to result in pipe or pump case rupture.
This type of risk is most apparent in high head applications using an end-suction centrifugal pump with a cast iron or plastic volute or the upper stages of a multistage vertical turbine or submersible pump used as the booster pump, especially when an inline booster pump or pressure-reducing valve is used for on/offline flow transition or pressure control.
As an illustration and using our well example: If the flow rate was totally shut down with both source and booster pumps running, the residual wellhead pressure could reach 255 feet (from well pump) + 145 feet (added from booster pump) = 400 feet TDH – 50 feet minimum SWL = 350 feet (152 psi).
While this pressure may not be immediately injurious to the pumping units or manifold, it could conceivably be higher than the pressure rating of the booster pump case or a lower pressure rated PVC pipeline, potentially resulting in bursting either of them.
The designer must always be aware of this potential, and if necessary, specify a ductile iron or steel volute or bowl assembly whenever the discharge pressure can exceed the maximum working pressure of the well or booster pump. This factor should be examined on every series pump design with effective measures—such as a high-pressure cutout switch or pressure relief valve incorporated into the design if needed to avoid serious equipment damage or personal injury.
Verify that the source pump is not overpumping.
When applying a booster pump to an existing well installation, the well pump and, by association, the well may be asked to pump at a lower pressure and thus a higher flow rate than formerly the case. The designer should verify the well will not be operating above the safe capacity of the well and the well pump will not surge or overload at higher flows.
Use a low-pressure cutout and electrical interlock to operate and protect the booster pump.
Incorporating a low-pressure cutoff switch is an effective and simple way of protecting the booster pump from dry running conditions. Although an inline flowmeter can be used for the same purpose, a pressure switch with a time delay is a much cheaper and more reliable alternative, plus it provides a method of automatically starting and stopping the booster pump, particularly for irrigation systems.
With systems in which the supply water is provided from a separate pressurized water source, a low-pressure cutoff switch is also an effective method of protecting the booster pump and supply from operating during extreme low-pressure conditions occurring with the source or from potentially developing negative pressures within a pressurized water system, such as booster pumps used in cities and water districts.
Using a minimum booster pump inlet operating pressure of 20 psig ensures adequate inlet pressure is always available to the booster pump and is not starved, nor is the water system functioning at potentially low or even negative pressures, which can introduce dangerous contaminants into the system.
In two-pump systems, using an electrical interlock between the booster pump control with the source pump control is also recommended to ensure the booster pump can run only when the source pump is operating.
Do not forget the well lift and head for well pumps.
Once again, this is an important design factor when applying a booster pump to an existing well and pump. It is critical that adequate head developed by the well pump be reserved to lift the water from the well, plus deliver the needed flow rate into the booster pump.
This type of error can occur if the well has been pumping for years at one flow rate and associated water level and is asked (or assumed) to be able to operate at a higher flow rate without determining or verifying the revised pumping water level. If feasible, I recommend adding no less than 20 psi (46 feet) (10 psi, minimum) of head to the maximum pumping lift (in feet) as the minimum well pump design head.
Use extra precautions with well pumps.
Although the procedure shown here for using an end-suction centrifugal pump as a booster pump can also apply to either a vertical turbine or submersible pump, when adding a pressurized pump or piping directly into the suction port of a vertical turbine pump, there are a few additional precautions to be observed.
The primary consideration relates to the axial thrust (downthrust) developed by the pump and resisted by the motor during high head service. In many cases the relatively low values of allowable downthrust in smaller vertical or submersible pump motors may not be adequate to resist the higher thrust developed from the pressure values associated with high-pressure booster (series) pumping.
In order to avoid these potential situations, I suggest the maximum downthrust rating for a motor intended for booster (series) service be initially checked and verified against the actual downthrust for adequacy, particularly if the design calls for prolonged operation at service conditions approaching the shutoff head of the booster pump or a 4- or 6-inch-diameter submersible motor is used as the booster pump driver. If needed, using a 175%-rated thrust bearing may be required for a VTP motor or a larger HP or diameter motor for a submersible application.
Conversely, in other extreme cases, modifying an existing VTP or submersible pump installation to generate a higher flow into a booster pump can lower the well pump’s discharge pressure enough to result in a potential upthrust condition to occur to the well pump or motor. This situation is much more common and sensitive for a shallow set (20 feet to 50 feet), high capacity mixed flow well pump. However, it can potentially occur to any well pump, particularly during startup, so designers are cautioned to verify this possible anomaly and correct by applying higher pressures against the well pump, especially during a pipeline fill or system startup.
Next, when using a submersible pump and motor for an inline booster pump application, it is just as important to generate an adequate velocity past the motor as it is for a well installation. Designers must verify the intended pump design either routes the inlet water into the bottom and past the motor or incorporates a motor-surrounding shroud with a sufficient annular velocity (less than or equal to .50 fps) to maintain adequate motor cooling.
Finally, in other cases, modifying a well pump to a higher flow rate for booster service can negatively impact the NPSHR (requirement) of the pump, especially for applications using a tailpipe with a suction lift upwards to the bowl assembly. In these examples, the designer must verify the well pump will not cavitate or lose prime during higher capacity demands.
In other cases using a high-speed submersible pump as the booster pump, the inlet head or submergence over the pump’s inlet may not be sufficient to avoid cavitation. This can generally be determined through an examination of the bowl’s NPSH and submergence requirements. The actual procedure and methods for using well pumps as canned booster pumps will be outlined in our next column.
Be cautious when using mismatched pumping units.
Although most pumps with dissimilar H-Q curves will eventually find a common ground of flow to function, there are conditions in which a series pumping installation will not be effective using pumps with dissimilar curves. Certainly, the easiest way to avoid this is by using units in which the shape of both H-Q curves match or are similar throughout the range of flow.
However, units in which either pump is asked to produce more than 75%-80% of the total head may not be as efficient as simply using a single larger unit to perform the entire service. In some applications where the rated capacity of the booster pump is much higher than the source pump, errant operating conditions can occur, particularly during startup or line fill.
Surging conditions or a short duration loss of flow between the pumping units can occur where the booster pump can actually develop a severe loss of inlet head, creating a suction lift upon the source pump—causing a sudden but momentary loss of flow from the source into the booster pump, often resulting in air entrainment or vapor lock in the booster pump or piping manifold. This is usually rapidly supplanted by an immediate resumption of pumping.
In water well conditions, this can lead to hydraulic surging resulting in an undulating thrust condition upon the well pump and motor, possibly leading to premature failure. This situation can generally be avoided by making sure both pumps in a series configuration are designed for the same rough capacity range and that adequate discharge head is developed by the well pump under all flow conditions to avoid damaging upthrust.
To be certain, these potential conditions are rare in actual practice. However, designers are nonetheless cautioned to recognize the potential, and when present, ensure the booster pump design condition and flow range closely matches the flow rate range of the source system or pump or to incorporate an automatic control valve to maintain a minimal value of discharge pressure from the booster pump under all flow conditions.
This concludes this month’s edition of The Water Works. The upcoming July and October editions will apply many of these same concepts into the actual design and layout of a booster pump application or station for open or closed applications.
Until then, keep them pumping!
Ed Butts, PE, CPI, 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.