We’ve all had it happen. Either ourselves or our neighbors have had their basements flooded during a major rainfall. To make matters worse, a lightning strike has taken out the neighborhood power grid, rendering the basement sump pump inoperable. It’s a helpless feeling to watch as the water rises up through the foundation. Whether it’s a small basement foundation or a major flood control levee, pumps are critical to the performance of hydraulic control structures. Except for those rare cases in which small-scale manual pumps are still employed, pump operation depends on the available power supply. For potentially critical situations, a ready backup power supply should be available to take over if the main power grid fails. But before these backup power systems can be appraised, the basic question of how much power a pump requires has to be answered.
PUMP TYPES, MECHANICS, AND POWER REQUIREMENTS
Pump operations are defined by two sets of curves: the operation or system head curve and the pump performance or characteristics curve. The first curve is a function of the physical characteristics of the piping system that the pump is discharging into. This operational head curve is a line that plots the relationship between pump flow rates and the pipe system’s resistance or head (typically measured in equivalent feet of water column, 1.0 foot of head being equal to 62.428 psf [pounds per square foot] or 0.434 psi). This resistance head is a sum of the pipe system’s static head from elevation differences, the friction head losses incurred while flowing through the pump, and the velocity head losses which derive from the flow rate—GPM or cubic feet per second (cfs)—and the pipe’s cross-sectional flow area.
The first component of the operational head value is static head (Hs). This is calculated by subtracting the elevation of the pump inlet point (Ep) from the elevation of the pipe system discharge point (Ed): Hs = Ed – Ep. For example, a sump pump with an inlet elevation of 100 feet pumps into a pipe system whose ultimate discharge point occurs at an elevation five stories higher at 150 feet. The resultant static head is equal to 50 feet (equal to 31,121 psf or 21.7 psi).
The first step in determining velocity head is to calculate the flow velocity of the liquid flowing through the pipe. This is determined by dividing the flow rate (Q, cfs) by the cross-sectional area of the pipe interior (A, sf) as determined by the formula for a circle: A = pi * r2. So, a pipe system consisting of 3-inch nominal diameter pipe segments (1.5-inch radius) would have a cross-sectional flow area of about 7.07 square inches (0.05 square feet). If the pipe carried a flow rate of 100 GPM (or 0.223 cfs), the flow would have a velocity of 4.54 feet per second.
With the flow velocity known, the reality velocity head loss can be determined by dividing the velocity squared by twice the acceleration due to gravity (g, 32.17 feet per second squared): Hv = V2 / (2 * g). In this case, the resultant velocity head would be only 0.32 feet. In most cases, compared to static head, velocity head is not significant.
What are potentially significant are friction head losses. These are the result of the liquid losing energy due to friction with the interior surface of the pipes as it flows through the pipe system. Using the flow velocity again, a Reynolds number (Re) can be determined for the pipe flow. This value is calculated by multiplying the flow velocity (V, fps) by the pipe diameter (D, feet) and dividing the result by the viscosity of the liquid (v, square feet per second): Re = (V * D)/ v. Viscosity is a physical characteristic describing the extent to which a fluid resists a tendency to flow. Liquids with high viscosity do not flow as easily as liquids with low velocity. Viscosity is a result of both the chemical characteristics of the liquid (molasses, for example, having a higher viscosity than water) and the temperature. Viscosity tends to decrease with increased temperature of the liquid. This results in a lower Reynolds number and a higher resultant friction factor. The viscosity of water is often specified as 0.0000141 square feet per second at 50°F. The resultant Reynolds number is 80,488.
The Reynolds number is then used to determine the friction coefficient of the flow by cross-referencing its value to the laminar flow, smooth-walled pipe curve of the Moody chart. In this case, the friction factor is 0.019.
The friction head loss is determined by multiplying the friction factor (f) by the length of the pipe (L) by the velocity and dividing the value by the result of multiplying twice the diameter by acceleration due to gravity, as follows: Hf = (f * L * V) / (2 * D * g). Assuming a pipe length of 1,000 feet, the friction head loss would be 24.3 feet. Adding this value to the friction head and the static head gives a total discharge head of 74.64 feet.
The second curve is a result of the pump’s mechanical design and operation. This is actually a family of three curves: brake horsepower (BHP, typically measured in watts), efficiency (measured in percent), and head (measured in feet). These are also measured in regard to the pump’s flow rate. Different pump types (centrifugal pumps, axial-flow or propeller pumps) have significantly different operational designs and will have widely different operational characteristics.
The head curve gives the pump’s feet of head for a given flow rate. The pump generates less head as the volume of flow increases. The efficiency curve compares the pump’s operational efficiency for given flow rates. The curve reaches a peak efficiency point for a given flow rate based on the pump’s design and operation for a given impellor diameter. The Best Efficiency Point (BEP) is the operating head that coincides with the flow rate associated with peak efficiency. Break horsepower (BHP) is the input power that is actually delivered by the motor to the pump and converted into mechanical movement and fluid flow to perform useful work; it is the useful power that the pump can develop. In short, BHP is the mechanical horsepower available at the shaft at specified rpm. The resultant pump efficiency is derived from the difference between the brake horsepower and hydraulic power.
There is a fourth performance curve typically shown with the other pump data. This is related to the pump’s Positive Suction Head (NPSH). There are actually two definitions of NPSH: available head (NPSHa) and what is required (NPSHr) by the pump without being subject to potentially damaging cavitation and resultant reduction in pumping output.
By cross-referencing the pump’s performance curve (which relates operating head with flow rate) to the system resistance curve (which shows the pipe system resistance head resulting from applied flow rates), the actual operating point can be determined at the point where these two curves cross. For efficient operation, the operating point should lie somewhere within the pump’s recommended operating range in order to maximize efficiency and minimize operating costs.
Now that pump characteristics have been defined, it is time to categorize the various types of pumps by the mechanical actions they use to lift and move fluid. There are two broad categories of pumps: centrifugal and positive displacement. The centrifugal category is further divided into sub-categories of submersible (designed to be placed below water level) and extraction (which operate from above the level of water being pumped). Within these major categories are pumps designed to perform specific tasks or operate in harsh environments (water with high turbidity and total suspended solids content, groundwater extracted from well points, water with large soil particles or other objects, raw sewage and landfill leachate, industrial water pollution, highly viscous fluids and oils, slurries and sludges, oils, and other hydrocarbons).
Pumps that handle fluids containing large particles or significant contaminants are classified by the size of the suspended particles they are designed to handle: sump pumps (up to 0.375 inches, equivalent of fine gravel; effluent pumps (up to 0.5 inches, medium gravel); and sewage pumps (up to 2.0 inches, the size of cobbles). And then there are grinder pumps. These don’t handle large objects per se, but crush and grind these objects into a fine slurry for pumping. The operational characteristics of these pumps are modified to let them handle these large solids. For example, sewage pumps will operate at higher flow rates needed to move larger than average objects. Pumps managing smaller objects will rely on higher head. Grinder pumps, of course, would require very high levels of horsepower.
A positive displacement pump could also be referred to as a “pusher” pump in that it physically pushes the liquid by means of a cyclically operating mechanical device. There is a wide variety of these pumps depending on the displacement mechanism they utilize, such as: a piston moved back and forth by a rotating cam, a reciprocating bellows cavity, vibrating diaphragm or flexible liner, peristaltic tubing squeezed by roller, or syringes operating at high pressures but low flow rates.
In contrast to displacement pumps, centrifugal pumps use a set of rotating impellor blades spinning on an axis to “fling” water to the outer casing of the pump as the blades rapidly spin. This centrifugal force causes the water to exit the casing at a pipe connection located on the outer circumference of the pump housing. The result is high flow rates with low operating pressure heads (and lower operating costs in terms of required energy to operate the pump). Centrifugal pumps also come in a wide variety. One interesting design uses flexible impellors rather than fixed vanes. These impellors essentially trap water and sweep it toward the discharge pipe. Pumps utilizing rigid rotary vanes (both fan-shaped and lobe-shaped) are also used, with the second type being used for highly viscous flows. Rotating gear pumps act in a method similar to that of peristaltic pumps in that these squeeze water forward into the discharge tube, only these designs apply the squeeze force with intermeshing gear teeth. Submersible centrifugal pumps typically utilize drum impellors that utilize driving motors attached to a vibrating drum and can operate underwater without the need for electricity.
As seen above, pumps can be simple or very complicated in design. Each application often uses its own unique style of pump for optimal performance under particular flow conditions. Each has one thing in common—the need for an exterior power source, either mechanical or electrical—in order to operate. Should the power supply fail, the pump ceases to operate and flooding occurs. But how much power does a pump require?
Determining the hydraulic power needed to drive a pump is a straightforward calculation that depends on: the pump’s mass flow rate (cfs or GPM), the density of the fluid (62.4 pcf [pounds per cubic foot] for water), the differential head (feet) and acceleration due to gravity (g, 32.17 feet per second squared). These values are multiplied together and divided by a conversion factor depending on whether the outcome is in SI or English units. Using the values from the example above (75 feet of head, and a flow rate of 100 GPM or 0.223 cfs), the pump’s resultant shaft power requirement would be 2.36 kilowatts (kW) or 3.16 horsepower (hp). Assuming a 60% operating efficiency, the applied hydraulic power becomes 1.14 kW or 1.90 hp.
TYPES OF BACKUP POWER SYSTEMS FOR PUMPS
Backup power is a necessity for any pump system, whether it is servicing a homeowner or a Corps of Engineers flood control area—in the first instance because an isolated home may have to wait a long time for power to be restored and in the latter, because of the damage that can be done to key infrastructure by excessive flooding even over limited time periods. But when evaluating backup power systems for pumps, it must be remembered that there is a difference between standby and emergency power systems. There are three broad categories of backup power systems: emergency power systems, required standby power systems, and option standby power systems as defined by NFPA 110, “Standard for Emergency and Standby Power Systems.”
Emergency power provides automatic power when normal power grids fail. The national code requires that emergency power systems provide power in less than 10 seconds after a power outage to all essential safety and health related electrical systems (fire alarms, elevators, emergency lighting, smoke and gas ventilation, hospital patient life support, surgery and emergency room operations, police communications, etc.). Emergency, by definition, means “a serious, unexpected, and often dangerous situation requiring immediate action.” Emergency backup power exists to respond in emergency situations caused by the failure of normal power supplies. In order to function in an emergency situation, these backup systems need their own dedicated conduits, control, electrical panels, and more to ensure that they are not physically damaged by the same event that takes down the power grid.
What types of pumps fit into the category of critical systems requiring immediate emergency power supply backup? At the large scale are pumps designed to dewater flood control levees during storm events. An excellent example of the failure of such a pump system—with disastrous consequences—occurred in New Orleans during and after hurricane Katrina in 2005. Multiple levee failures let in floodwaters from the storm surge, but a critical component to the subsequent disaster was the failure of flood control pumps in the Lower Ninth Ward. This was followed by a series of pump failures, starting with the Duncan and Bonnabel Pumping Stations, which ceased functioning after roof damage to their pump building, followed by failure of the Hayne Boulevard Pumping Station, and the failure of all pumping stations in Jefferson and Orleans parishes later that night.
National codes and local ordinances also require that pumps and other critical systems have standby power systems. While emergency backup systems have to engage in less than 10 seconds, standby power has a full minute under code regulations to start up in the event of a power failure. Also, unlike emergency backup power systems, standby power does not need its own independent operating and transmission system isolated from the damaged grid. Standby power provides the next, higher level of protection. It is designed to service systems that enhance and support lifesaving operations. These can include heating, ventilation and air conditioning (HVAC) systems, phone and internet systems, television and radio broadcasting systems, building mechanical and automation systems, and most hospital equipment.
Industrial and sewage pump systems would qualify as needing at least standby power systems. Though not an immediate threat to human health, the resulting damage from pollution and chemical overflow is a grave threat to long-term environmental health. An example of such a failure would be the West Point Water Treatment Plant near Seattle in 2017. Four critical pumps designed to remove treated wastewater from the plant had clogged just when maximum flows were being discharged from the Seattle metro area after days of steady rainfall. 15 million gallons of untreated wastewater and raw sewage overflowed the plant, flooding work and office areas and damaging millions of dollars of equipment. To relieve the buildup, the plant dumped 30 million gallons of raw sewage into Puget Sound.
Optional standby systems, though not required by national code or local ordinance, are typical utilized by owners to protect very valuable or critical operating systems. These can include financial data, government records, data centers, and corporate files. Optional systems can be used to ensure the comfort and security of customers or residents during a power outage.
Like the example of the homeowner with the flooded basement, office buildings and commercial shopping centers would not normally be required by law to have standby power systems for their drainage pumps. The failure of sump pumps can result in the flooding of basement and lower building floors. HVAC system pumps can fail, resulting in the spoilage of perishable goods from too much heat or, conversely, the bursting of pipes from too much cold. In all cases, however, electrical backup power of all types should be limited to only those areas deemed critical by either law and regulation or by the property owner. The rest of a building or facility can be ramped down or even shut down completely in an emergency in order to conserve resources and power to focus on critical systems.
Backup power systems come in three varieties: backup generators, uninterruptible power supplies (UPS), and backup batteries. Generators, either reciprocating engines or turbines, convert the potential chemical energy in fuels (typically diesel, but gasoline, biogas, and natural gas are also used) into mechanical energy to turn a rotor to generate electrical energy. For emergency situations, reciprocating engine generators are typically preferred since they have a quicker startup than turbines. Their operational readiness is ensured by testing with loadbanks and a strict regime of care and maintenance.
Energy from an uninterruptible power supply (UPS) is instantaneous, but short-lived. A UPS acts as a bridge, providing power long enough after a grid failure to allow for the true backup power system to engage or to allow the operator to safely shut down the protected system. As such, they are often associated with computer systems and data centers, as well as critical health facilities and key communications systems.
For pump systems, the primary means of providing UPS are batteries. Though flywheels (a.k.a. rotary UPS) and other types of UPS systems are applicable for larger loads and currents, for most pump energy requirements, batteries are the preferred source of backup power. Not only are the primary pumps supported by backup batteries, but there is typically a system of backup pumps powered by a UPS. There are two types of sump pumps: pure DC-powered and hybrid AC/DC. When the main power grid fails, emergency sump pumps can operate directly off the DC current from a storage battery. Hybrid pumps can utilize AC current while the grid is operating without having to deplete a backup battery. Types of back up batteries include deep cycle batteries that require little or no maintenance, standard lead-acid “wet-cell” batteries, and advanced lithium-ion batteries.
Crane Pumps is a world-class manufacturer of pumps and accessories, as well as a provider of pump-related services. Its products and solutions are provided to municipal, commercial, industrial, residential, and military market segments. A recent innovation is their SITHE submersible chopper pumps. A solution to clogging in wastewater flows laden with large solids, SITHE pups are available in 7.5 hp to 60 hp sizes operating at a maximum head of 200 feet and a maximum flow of 1,750 GPM. SITHE chopper pump includes highly efficient Barnes hydraulic designs, dual/tri voltage plug-n-play quick connect cords, liquid cooled motors, class H insulation, 416 stainless steel shafts with a tapered fit, readily available premium quality mechanical seals, heavy duty bearings, large lifting bails, and stainless steel hardware. Similarly, their new BLADE grinder pumps are available for grinder applications that require both high operating heads and high flow rates. BLADE models are designed for municipal, commercial, and light industrial applications needing operating heads of 240 feet and flows of 155 GPM. BLADE XGV grinder pumps are equipped with the Barnes Slicerator grinding technology, and they can grind and pump even the most troublesome solids in wastestreams like wipes, cloth, diapers, plastics, ropes, and elastic nylon materials. Their NSF 61/372 Weinman-Deming Split Case Pumps meet exacting standards for the supply of clean drinking water. To meet these standards, they are made of lead-free and zinc-free aluminum bronze and cast-iron construction. They are available in horizontal and vertical designs and provide heads up to 460 feet and flows up to 7,000 GPM.
Global Pump/Mersino designs and installs complete bypass pumping systems for sewer bypass, water supply systems, or fracking operations. In addition to their pumps, they supply a matching series of backup generators. This includes generator rentals from 10 kilovolt-amperes (kVA) to 2000 kVA, as well as auxiliary power accessories to ensure site safety and reliability. Their generators are used for backup power and emergency power, as well as job site use and special applications. They include 10 kVA to 2000 kVA diesel, natural gas, and liquid propane gas units. They come equipped with NEMA 1 or NEMA 3R enclosures/weather enclosure or sound-attenuated enclosures with ratings as low as 50 A-weighted decibles (dBA) at 50 feet. Mersino’s pump systems really shine in dewatering applications for construction sites and building foundations and include deep well, well point, and eductor system dewatering. Their pumps operate at flow rates ranging from a few GPM to 70,000 GPM.
Grundfos Pumps is a global leader in submersible groundwater pumps. Their SP and SQ pumps feature state-of-the-art hydraulic design combined with energy-efficient motors, providing high operating reliability for all applications. Operationally, they are available in a wide range of sizes and materials and are serviced by advanced monitoring systems and controls to ensure system optimization.
Thompson Pump & Manufacturing has provided high-quality, heavy-duty pumps to the construction market for the past 48 years. They offer services for well point installation, sewer bypass, wastewater, lift station, stormwater, and water supply pumping. Additional services are provided by their in-house Applications
Engineering Department, which creates design solutions for complicated and special application pumping projects. The design submittals are followed up with hands-on field support by field personnel specifically assigned to the job site. Thompson is used to working in harsh environments ranging from mine dewatering, hazardous waste cleanups, and contaminated groundwater remediation. Thompson’s hydraulic submersible pump sets eliminate suction lift limitations that occur when pumping out excavations exceeding 25 feet deep.
Thompson’s expertise in tailor-made pump services is illustrated by their work with Southwest Florida municipalities. A Southwest Florida municipality had a problem. While attempting to provide emergency backup power to their various wastewater lift stations, a Southwest Florida municipality and a neighboring city experienced frustration with diesel-driven generators during inclement weather or power outages. They identified four problem areas in their current setup:
- Generator accessories failed to indicate power loss during critical situations.
- Efficiency losses of generator and electric pumps caused excessive fuel consumption.
- Generators were often oversized, causing insufficient load on the engines.
- Inflow and infiltration problems arose as rising groundwater and floodwaters entered collection pipelines.
To improve this situation, municipality officials decided to explore the option of permanent, diesel-powered, dry-priming backup pumps as a more reliable and cost-effective replacement to the generators. Thompson Pump illustrated all the advantages of replacing their current system with a stationary system for lift station backup. This meeting involved engineering consultants, contractors, and municipal managers, who ultimately decided the best way to move water is with a pump rather than a generator. “We just needed a chance to prove ourselves,” says David Perry, municipal sales manager for Thompson Pump. “We have a competitive advantage when you look at our product design, knowledge, experience and the unsurpassed level of service we provide.” Thompson Pump’s proposal was tailored to meet and exceed the municipality’s needs at each lift station. It highlighted Thompson Pump’s product performance during a time of need, the reduced cost of operation when compared to generators, and the basic fact that pumps move water whereas generators do not. This detailed proposal explained all the different ways in which Thompson Pumps would help prevent Sanitary Sewer Overflows (SSO) at each lift station.
The Thompson Pump system was put to the test and proved its worth during Hurricane Irma when the municipality lost power to all their 840 lift stations. Of those 840, approximately 11 Thompson Pumps had already been installed. Each performed perfectly, automatically bypassing all the flow coming into each station, resulting in zero Sanitary Sewer Overflows (SSO). In addition, the pumps used far less fuel than the generators—important not only from a cost standpoint, but also because fuel was difficult to obtain during the hurricane.