Part 6: Improving efficiency in wells and pumping plants
By Ed Butts, PE, CPI
So far, this series has explored the causes, methods, and procedures of identifying declining well and pump efficiency and production. This month, we outline the methods and procedures to raise or maintain efficiency in each element of a pumping plant, along with the technical and economic reasons to do so.
Well Efficiency: Final Words
Throughout this series I have touted well efficiency and its role in a successful pumping plant. Unfortunately, precisely defining what a well’s “efficiency” is or should be can be problematic.
Beyond using quality materials along with well drilling and development techniques to deliver an abrasive-free well with the lowest pumping lift possible—there often isn’t much else we can do to improve a given well. If there is one thing I’ve learned during my career, it is this: Favorable results are generally superior to a favorable plan.
This simply means: For an application requiring 1000 GPM, delivering a well with an efficiency of 90% producing only 500 GPM is generally not as favorable to the client as a 60% efficient well with 1500 GPM of yield. The high efficiency we always plan and strive to obtain as professionals must be tempered with the one overriding primary duty to our client—provide a functional and safe water well they can use for their purposes.
For this reason, when designing or planning for a new well, I initially stress and outline to the customer the proper siting, anticipated construction method, proposed well and seal depth, preliminary well casing and screen design and materials, planned development time, and estimated life-cycle costs.
I do this even if the final cost is somewhat higher than initially estimated, with an eye toward the inevitable service and rehabilitation the well will ultimately need. This helps demonstrate to future users and owners of the well you did your job right “way back when” and ensures the well can be serviced and rehabilitated in the future to retain yield.
For those who must try to define a well’s efficiency, the following simplistic relationship has been historically recognized:
Well efficiency (%) =
Theoretical drawdown in feet
Actual drawdown in feet
Obviously, determining a theoretical drawdown can be a laborious and difficult task which requires several assumptions. For this reason, don’t be as hung up on delivering a well with a high efficiency today as much as using your hard-won judgment and years of experience to provide the highest quality product to your client possible for their use over many future years.
It may not be in your lifetime, but wouldn’t it be rewarding to your legacy if the customer’s son remembered and cited the good work you did to your own son?
General Guidelines for Improving Plant Efficiency
There are several specific types of improvements and changes that can be made to pumping plants to increase the overall plant efficiency. Some are relatively easy and low-cost while others require a more detailed assessment to determine if the proposed improvements are worth the investment. Some of these additional methods follow.
1. Optimizing daily operation through control modifications
Considerable energy savings can often be realized by optimizing the day-to-day operation of the pumping plant. This means using a system analysis plus control techniques or settings to modify and limit pump starting and cycling and ensure all pumps operate at or close to their specific best efficiency point (BEP). When a hydro-pneumatic pressurized system is used with multiple identical units, pressure switches and controls should be set to provide alternated cycles for two units or rotation of three or more units.
Use a system demand/head and pump curves chart (Figure 1) to determine the best start and stop settings for a multiple pump installation that uses a lead pump (one pump), lead + first lag (two pumps), and lead + first lag + second lag (three
The proper sequence and operation of the available combinations that use a single (lead) pump as an element of multiple pumps allow the system and units to operate using an operating overlap to efficiently match system demands with the best pump performance.
With this type of control logic, each unit generally operates within the best efficiency window commensurate with the current water system demand. Excessive cycling is avoided by operating each combination of pumps throughout a design range, but eventually at a lower combined flow approaching system shutdown. This usually results in continued operation by the progressively lower flow combination (trading two units for one unit).
When using a ground-level or elevated water storage reservoir, use water level control settings to provide reasonably long operational (run) and rest (off) periods between cycles of 10-15 minutes on average or more.
Next, for large-volume water storage systems used with electrical utilities charging a higher power rate for peak demand periods or events, consider modifying pump operation with sequenced timers to permit what is often referred to as off-peak or load-shedding operation. This type of control is ideal for most SCADA (supervisory control and data acquisition) types of control systems. Also, examine the possible installation of reduced voltage starting equipment to lower the motor’s instantaneous inrush starting current to lessen utility peak demand charges.
Lastly, shutting down larger horsepower pumps at lower demands when not needed in order to allow smaller units to continue operation and satisfy system demand at their peak efficiency band is another effective method of control modification.
These procedures can not only help with lowering energy costs, but will also extend system, motor, and control life by lessening pump cycling and associated water hammer. These adjustments can lower energy consumption by 1%-4%.
2. Using interim water storage with multiple booster pumps for varying flows or duties
In water systems with a wide range of demands, using interim water storage to smooth out and create a uniform source flow, along with re-pressurization by booster pumps or designing/retrofitting a pumping system that employs multiple pumping units with each separate pump’s BEP at varying flows, can pay dividends as well as provide redundancy.
For example, a water system with a wide disparity of demand (an average demand of 50-75 GPM along with a peak demand of 225+ GPM) can benefit by using three total units with two matched alternating low service pumps that each display BEP at flows between 50-75 GPM plus a high service pump with a BEP closer to 225 GPM.
Conversely, this can also be done by using three to four matched units that can each efficiently produce the lower demand with peak demands provided from two or three of the units combined (Figure 1).
This not only provides desired unit redundancy for lower demands, but also avoids continuous operation of a unit designed for best performance at 225 GPM or higher to run at a reduced flow rate of 50 or 75 GPM. This type of change alone can result in 2%-5% of
3. Using a variable speed device or control valve for variable demands
Obviously, using a variable speed device (VSD) on an appropriate pumping plant can lower energy consumption through the affinity laws. These laws simply state three basic tenets:
- Capacity and efficiency varies directly with the speed.
- Head varies with the square of the speed.
- Brake horsepower varies with the cube of the speed (shown graphically in Figure 2).
Therefore, a speed reduction of just 10% will usually result in a proportional reduction of brake horsepower (BHP) of up to 27%. A VSD does not necessarily mean the exclusive use of a variable frequency drive (VFD) motor control since similar types of pump speed modification
devices, such as specialized motor designs or hydraulic slip-clutch drives, can accomplish the same desired pump speed reduction as a VFD without many of the disadvantages to existing three-phase electrical installations or motors. These include online harmonics, radio or television signal interference, noise, stray currents, excessive heat, and possible bearing failures.
Generally, a speed reduction device will be the most efficient and cost effective if a minimum reduction of pump speed between 10%-15% is possible. Along with a direct proportional flow reduction between 10%-15% and head reduction between 19%-28%, the BHP will reduce 27% to as much as 39% from the original conditions.
When considering using any VSD, it is imperative the designer verify the pump curve for the intended pump demonstrates enough head rise (steep curve) to provide sufficient speed reduction for energy reduction while also maintaining the operating head needed to overcome the minimum system static and frictional head. In addition, many types of pumps, particularly deep well vertical turbine pumps and submersible
units, can vibrate at certain speeds due to the revised speed matching the natural frequency or critical speed of the application. In these cases, the VSD should be equipped with a speed band rejection setting to preclude prolonged operation at these speeds.
The potential energy savings in horsepower with a VSD are obviously dependent on the type and specific application of the pump, pump curve shape, original BHP, and degree of speed reduction. However, an input power reduction of up to 30%-40% is not uncommon, with up to 50% possible.
When using a VFD, the system designer must also consider all potential operating issues and use appropriate devices such as line and load filters, cooling fans, shielded cable, and other devices to mitigate these potential anomalies.
In addition to the possible use of a VSD on certain water systems, many other systems can potentially benefit from the alternate installation of a pressure regulating (reducing) control valve to control and modulate downstream system pressure at varying flows. These devices must be selected carefully as the inherent pressure drop through the valve creates frictional head, and thus an added energy loss to the overall system.
However, oversizing a valve can also have a negative impact to the system through a possible loss of finite control at lower flows. In addition, the use of one on a pump with a steep H-Q curve will not necessarily lower energy consumption to a satisfactory level since the lower capacity is essentially exchanged for higher head—in essence trading HP for HP. Therefore, the pump’s BHP may not decrease appreciably. When used with a pump with a relatively flat curve, however, a control valve can offer a viable alternative to a VSD with up to 10%-25% of energy saving benefits possible at a relatively low cost.
4. Retrofitting an older pump with a new pump
Countless older and relatively inefficient pumping units from the 1950s into the 1980s are still in operation today. These units operate using pump and impeller design, technology, and efficiency from their earlier period of manufacture that are almost always inferior to pumps of current design. These older units can usually be easily replaced with a new pump, often with one using the same manufacturer and model but with new construction, current metallurgy, and an updated design.
Due to design and wear differences, it is quite common for a new pump to display up to +10%-+15% or more in hydraulic efficiency than an older unit, with a pump-only payback occurring often within three to five years or less.
In addition, many new multistage styles of low-flow to mid-flow (10-250 GPM) range pumps are now available that can directly replace older lower efficiency single-stage units for the same duty. This is particularly true for lower-capacity (≤100 GPM), higher-head (≥100 feet TDH) applications commonly used in small water systems.
Current multistage pump designs exhibiting a BEP of up to 65%-75% are now available and can directly replace an older single-stage unit that was originally capable of only 40%-50% efficiency. For example, consider a common water system condition of 100 GPM at 150 feet TDH. The potential improvement in pump efficiency alone can reduce the brake horsepower from 7.5 to 5.4 BHP, a reduction of 28%. Depending on operating hours and pump size, the ultimate savings of energy consumption and cost can range from 1% all the way up to 10% or more.
5. Engine rebuild or repairs
Many pumping plants use an internal combustion engine as the prime or backup driver to a pump or generator. For these applications, maintaining the engine in peak performance and highest efficiency possible is critical, especially since an engine is inherently low in overall efficiency to begin with.
Although not much can be done to raise the relatively low efficiency of an engine using a fossil fuel for mechanical energy conversion, there are various application and maintenance procedures that can help.
For a new or replacement engine, select one that will operate within the manufacturer’s recommended range and guidelines for torque, speed, and output brake horsepower. Typically, for most multiple cylinder diesel and gasoline engines, this means operating the engine within an output speed range of 1400 to 2800 RPM. Many smaller two- to four-cylinder engines actually produce peak performance at even higher speeds, up to 3600 RPM.
Also, try to load engines around the maximum torque output and lowest BSFC (brake specific fuel consumption) curve rating. Both values are commonly available from the manufacturer’s performance data curve.
The final application tip pertains to the use of a larger (more cylinders or greater displacement) engine at a lower speed to produce the required horsepower for a given job over a smaller engine with or without turbocharging at a higher speed. Although not always the case, use of a smaller or turbocharged engine or selectively shutting down unnecessary cylinders on larger engines will often be less in capital cost and more fuel efficient for the application. Always check with the engine manufacturer and refer to the applicable selection curves first.
Regularly observe, track, and record engine performance, operating hours, timing, and vibration levels. Replace engine mounts or rebuild the engine if or when vibration cycles exceed recommended values. Routinely clean and replace all air filters and verify inlet louver operation at required intervals or duct inlet (combustion and cooling) air to the outside to ensure the engine has an unimpeded access to free air and can
never operate within an air-starved or negative atmospheric pressure environment for those engines enclosed within a structure or room or at elevations considerably above sea level.
Make sure all exhaust has a low friction loss path to the building exterior. Use an auxiliary cooling fan to provide the additional free flow of air across the engine surfaces. Replace all other fluids and filters and perform tune-ups and timing checks according.to manufacturer’s guidelines and at the recognized service intervals or operating hours.
Finally, I recommend a highly valued engine be equipped with the means to determine fuel consumption, such as adding a fuel tank gauge and hourmeter or inline fuel flowmeter, to register and monitor the fuel consumption against operating hours and conduct an engine tune-up if the fuel consumption increases more than 10%-15% over expected levels.
6. Pump rebuilding: impeller and wear rings
Internal pump components, even the finest ones, inevitably wear or fail due to prolonged operation, fatigue, cavitation, and the presence of abrasives. In fact, several studies have shown a typical new pump in potable water service routinely loses up to 5% efficiency in the first five years of operation and a severely worn pump loses both head and efficiency with a resultant increase of power draw (Figure 3).
For this reason, routine periodic monitoring of pump performance should be conducted to determine when rebuild/repair of the pump will eventually be required.
For single-stage units, maintaining suction and discharge wear ring and impeller eye tolerance and clearances are critical to prevent recirculation and preserve the pump’s performance. Depending on the pump speed, type, material, pumpage, and impeller and wear ring style and diameter, these clearances can range from between one to three thousandths (.001-.003) up to five to 10 thousandths (.005-.010) of radial clearance.
Applications that are NPSH compromised will often display internal damage and pitting to the impeller due to cavitation. When possible, an examination of the impeller eye and vanes should be performed to see if this damage has occurred.
As with many submersible pumps, stacked multistage units for various current models of horizontal or vertical pumps are often manufactured from stainless steel or thermoplastics. Therefore, restoring these clearances is generally impractical and the impeller
stacks must generally be replaced. Many larger (≥6-inch bowl diameter) vertical turbine and submersible pumps are provided with bronze or cast/ductile iron impellers, so restoration of the running surface clearances is generally feasible.
For all types of obsolete pumps using bronze or cast or ductile iron components, oversized metal respraying of surfaces along with lathe or mill machining to the proper dimensions followed by dynamic balancing often provides a reasonable cost option to trying to locate obsolete parts. This can shave energy usage between 0.50% -2% or more.
7. Pump revisions: impeller (stage) removal, trimming, or pump/component replacement
Many existing single or multistage pumping units originally designed for a specific duty are now operating under a revised and completely different service condition. This can occur for a variety of reasons, most notably changes in system design resulting in capacity or head revisions; higher or lower flow rate requirement; upgrading or changes to piping or valving; or the addition or deletion of special equipment such as inline control valves and water treatment devices including screens, sand separators, air strippers, or filtration.
In these instances, existing pumping equipment may now be operating far from the intended BEP at a much lower or higher capacity or total head. For these applications, a careful analysis of the revised service conditions and an energy usage audit should be initially performed.
If this analysis indicates a favorable payback will result from trimming of the existing impeller (for lower head service at the same capacity) or that a complete changeout of the impeller or pump (for a revised flow rate or higher head) is warranted, a further evaluation should then be conducted.
In some cases, a new or replacement impeller or other pump component can be manufactured from composite materials at a lower cost than a full pump replacement. This not only can restore and upgrade an existing unit to the revised service conditions, but when manufactured
from lightweight and low friction materials, save substantial energy over the original unit.
Obviously, the cost to disassemble and reassemble a pump and trim or replace an impeller to retain or modify the capacity at a lower head will usually be considerably less than changing out the entire pump, especially since the suction and discharge piping would not require modification.
Lastly, many multistage pumps, such as vertical turbine and submersible pumps, no longer operate at their original service conditions due to changes in the design. This is particularly true for irrigation applications that may have been converted to low pressure sprinklers or drip irrigation systems. For these pumps, removal of one or two impellers or stages can lower the operating head while maintaining the same
or reduced flow at a relatively low cost. These modifications typically lower energy use by 0.50%-5%.
8. Using or replacing an existing motor with a premium (energy) efficient or VFD-rated motor
This suggestion is one of the most common methods for three-phase electrical motor energy reduction and conservation today.
When you consider an approximate 3% increase of motor efficiency between a high efficiency, fully loaded 150 HP motor can potentially save more than 7600 kW-hours of energy consumption each year with only 2000 hours per year of operation (gross savings = $914/year at $0.12 per kW-hr) over a standard efficiency motor, the savings in real dollars and the extra cost payback becomes evident quickly.
This is the case for virtually any three-phase motor from 15 HP and up with sufficient operating hours per year. Obviously, this will not apply to all applications. Most submersible motors are inherently lower in efficiency due to their compact construction. Some existing
motors with lower HP or marginal or infrequent operation—such as those generally used for emergency or backup use or specialized types such as a wound rotor, part winding, or synchronous motor—may not be available in premium efficiency construction or generate the
needed operational hours to pay back the changeout cost.
In addition, since higher efficiency motors are wound with low resistance wire and construction, the starting current will generally be higher and running current lower than that for a standard motor. This is an important consideration when retrofitting a premium efficiency motor
with a standard motor since the short circuit protection may be more prone to circuit breaker tripping or fuse blowing due to the higher inrush motor current, plus the overload protection may need modification due to the lower running amperage.
In applications where a variable frequency drive will be installed with a new or replacement motor, most motor manufacturers now build motors specifically designed for operation on a VFD without many of the risk factors previously outlined. Depending on operating hours and
motor HP, energy savings can range from 2%-15%.
9. Consider using a lower pump speed or larger diameter bowl assembly
This is often overlooked, but remains a viable type of energy reduction method that can definitely pay dividends. In many single-stage applications, particularly those with 200 feet or less of operating head, the automatic selection of a 3600 RPM pump may not represent the best energy usage or economic choice since many lower speed pumps, especially those with 1800 RPM speeds, often exhibit a higher operating efficiency flow for flow.
This is due to inherent pump geometry differences and the use of a larger diameter impeller to produce the same comparable rim speed, and therefore, head as a 3600 RPM impeller. Certainly the lower speed pump will generally cost more in capital outlay than a comparable higher
speed pump, but the potential savings in energy can often pay back this difference within a year or two.
Refer to Figure 4 (1800 RPM) and Figure 5 (3600 RPM) for two examples of a 900 GPM at 160 feet TDH application. This is an extreme example of this case as higher-speed single-stage pumps generally begin to overtake the efficiency of slower pumps at heads above approximately 150 feet. This is also the case with a vertical turbine or submersible pump since a larger diameter impeller (bowl assembly) will generally be more efficient than a smaller diameter bowl at the same rotational speed (the efficiency of a typical 10-inch bowl is greater than an 8-inch bowl, flow for flow).
10. Perform regular maintenance and monitoring
This specific procedure to conserve energy and prolong unit life may appear to be the most obvious, but is often ignored as a primary method of energy conservation.
This task, while associated with several of the previous tips, is actually a category unto itself. This type of work includes routine vibration and alignment checks and verification; voltage, current, and thermal heat monitoring of motors, motor conductors and connections, and controls; power monitoring devices, circuit breaker, fusing, and overload protection settings; critical speed rejection settings (when used on a
VFD); bearing replacement and lubrication; inspection, adjustment, and replacement of belts; excessive leakage from packing or mechanical seals; and regular monitoring of all other aspects of a pumping plant, including flow and pressure.
Although the amount of energy that can be saved varies with the scope, type, and frequency of maintenance and monitoring, studies have shown periodic and regular maintenance and monitoring can restore close to original efficiency, extend unit life, and conserve 2%-7% of gross energy consumption (see Figure 6).
11. Proper system, pump, and pipe design and sizing (avoid bypass, recirculation, or throttling of flow)
Obviously, designing a system to the appropriate service conditions, pump, motor, and pipe size is important to the overall efficiency of a pumping plant. During design of a new system or selecting a replacement pump, choosing a unit that will consistently operate at the BEP or
within the best efficiency window (BEW) will usually present the highest efficiency and lowest operating energy consumption and costs for the given flow and head.
Try not to use a pump at more or less than 25%-30% of the BEP or BEW flow or head to avoid flow wasting, bypass, or recirculation of excess capacity or manual or automatic throttling of head along with unbalanced or excessive radial or axial thrust. When flow recirculation or
throttling is required, the choice to throttle down a pump rather than bypass or recirculate excess flow will generally result in the lowest energy penalty since the capacity element of a pump design typically consumes more proportional power draw than head.
Check valve sizing requires selecting a valve with the lowest practical head loss (1 psi or less at design flow is preferred), but which will fully open at the lowest projected flow rate, an important consideration when using a swing or disc check valve or with a variable flow (VSD or
control valve) system.
Select well pump riser and column pipe to limit friction loss to between 4-10 feet per 100 feet of setting and limit uphole velocities to 7-8 feet per second and transmission and distribution piping to 4-5 FPS. However, don’t overlook the possibility of possibly using a larger pipe size
as there are cases where a lower velocity translates to much lower friction loss, operating head, and energy consumption and costs.
For example, for a flow rate of 440 GPM, a 6-inch-diameter pipe would often be selected for a half-mile (2640 feet) pipe run since the velocity is right at 5 FPS and the friction loss totals 39.6 feet. However, increasing to an 8-inch pipe lowers the velocity to 2.7 FPS and the total friction loss lowers to 10 feet, 25% of the original 6-inch loss (both pipes H-W C = 140). This results in a drop of 4.38 HP from this
friction loss value alone at P.E. = 75%.
12. Think outside of the box!
Consider all the available improvements of current technology as well as good old-fashioned common sense when evaluating each potential step toward improving a pumping plant’s efficiency.
By way of example, as I grew up in this business beginning in the early 1970s I was constantly told anything good had to be made in the United States. For many years I religiously followed that admonishment and tried my best to use pumps and motors made only in the United States.
During the 1990s, however, I began to notice a subtle shift from this tradition as many international pump and motor manufacturing firms were steadily gaining recognition along with a local increase of project specification and use. Firms such as Flygt, Wilo, Pleuger (now Flowserve), Hitachi, and others slowly gained respect and notice from many of our local engineers, distributors, and suppliers.
I finally decided to research an application for a new 300 HP municipal submersible unit, and to my surprise not only was the water end (pump) 3% higher in efficiency at the same COS than the closest comparable U.S. pump I could find, but the motor from the same manufacturer was also 1.5% higher in motor efficiency. Altogether, a 300 HP pump and motor unit from an internationally based manufacturer was 4.5% higher in overall efficiency (savings = 17.02 IHP or 12.7 IKW = $1.52 less per hour in operating cost at $0.12/kW-hr or $4572 per year at 3000 operating hours/year).
We must all realize we function in one world, so we no longer have the luxury of discounting a product made somewhere else. Consider first and foremost our job is to provide the most efficient and durable product available for our clients. Unless there is an embargo, severe tariff, technical or other contrary reason not to, I feel if something happens to be made overseas, then so be it!
I am not proposing we disregard American manufacturers, I am suggesting you maintain an open mind and do your job. Examine, evaluate, and compare all aspects of all available options, including capital and operating costs and efficiency, with the client before making any final decisions.
This concludes this installment on well and pump rehabilitation and efficiency. I sincerely hope it has been informational and beneficial and will be helpful to you in future years. Next month we will go over some more methods as well as examples.
Until then, as always, work safe and smart.
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 email@example.com.