Reliability in Water and Pumping Systems

Part 3. Failure modes for motors and ancillary components.

By Ed Butts, PE, CPI

We discussed the failure modes associated with a typical centrifugal pump in the last installment of Engineering Your Business (September 2022).

In addition to the pump, there are obviously other elements needed to create a total pumping system. These additional components are typically vested in the driver, usually an electric motor or internal combustion engine, and ancillary components such as a source of power to the driver (electric power or fuel), motor starters, supervisory controls and instrumentation, power transmission means, lubrication and cooling systems, hydraulic components (valves and piping), safety controls, and auxiliary systems.

Some of these elements are illustrated in Figure 1 within the boundary that constitutes a pumping system.

This column will detail failure modes and reliability factors for drivers, specifically electric motors and internal combustion engines, as well as ancillary components. We will conclude with a brief discussion on water system reliability.

Electric Motor Failure Modes

An electric motor is one part of a pumping system that includes its source of voltage, mounting assembly, shaft coupling, and the pump.

Most motors are repairable, so they are evaluated using the mean time between failure (MTBF) rationale discussed in Part 1 (August 2022). However, many smaller motors, particularly fractional and low HP single-phase motors, are not generally economically repairable and are instead included in a Mean Time to Failure (MTTF) analysis.

Figure 1. Elements of a pumping system and its boundary.

The typical operating environment of an electric motor includes the ambient temperature, airborne or water contamination exposure, mounting to pump, support method, shock, and vibration factors.

The typical failure causes and percentages of a three-phase motor are shown in Table 1.

As excessive heat is a common metric, motor reliability is usually associated with bearing life and the life of the windings before they require rewinding.

Bearing failures are normally caused by poor maintenance practices such as allowing contaminants to enter the motor during lubrication, using the wrong grease or oil, applying too much grease, and not allowing the bearing cage to relieve the volume of excess or expansive grease through the drain plug.

Bearing failures with submersible motors are usually the result of failure of the thrust bearing situated in the bottom of the motor. Excessive or repeated thrust loads imposed by the pump’s transfer to the thrust bearing causes wearing of the thrust plate or shoes. The upper bearing, called a radial bearing, does not generally exhibit the same failure action or rate as the thrust bearing.

Lateral loading of the shaft with some frame-mounted motors is also a common bearing problem, which can occur from incorrect belt tension, dynamic overloading, or misalignment.

Another prominent failure mode for a motor is winding to ground or dead shorting of the motor windings. The temperature rise is a function of the amount of heat generated in the rotor and stator per unit of time with the efficiency of the heat transfer system. Controlling temperature rise of the windings is critical to motor reliability and life since the insulating materials’ age and degrade over a period of operating time and heat. This degradation process is directly related to the operating temperature. Eventually the materials lose their insulating properties, resulting in a short circuit of the motor.

Winding temperatures are also related to the power losses of the motor (copper and iron). Copper losses occur due to the resistance of the winding and current flow, while iron losses (eddy current losses) are formed in the core of the motor.

As these losses increase as a function of time and operating hours, the winding temperature also increases. The permitted temperature rise of the windings is dependent on the class of insulation and associated temperature limits.

Motor overheating is also caused by motor overloading, usually from too high of an ambient temperature, excessive load (current), inadequate cooling flow, or incorrectly applied voltage. This makes operating the motor within the permissible full load and voltage ratings a critical factor towards reliability.

Finally, excessive frequency of cycling (starting and stopping) of the motor can also contribute to winding temperature rise as the motor cannot adequately cool between short cycles. Manufacturers generally provide the recommended maximum number of starts and stops per unit time as a function of load and speed. Limiting the frequency of cycles to the manufacturer’s specifications will provide adherence to the predicted failure rate.

For general purposes, the information in Table 2 provides guidance on maximum cycles per day for various motor horsepower, phase, and speeds.

Other modes of motor failure are primarily due to mechanical issues including imbalance, resonance, and rotor deflection. A common mechanical problem with the motor shaft is often due to a worn or deformed shaft, incorrect coupling, shaft alignment, or overhung loads.

Lastly, since electric motors rely on a source of electrical power to function, reliability of the electrical power source should also be considered as it is often the leading cause of low system reliability. This variable is usually available from the serving utility or public utility district that supplies the power or by reviewing past operating and failure logs.

Reliability in hours of available electrical power per year is generally the most meaningful metric to use when determining water system reliability.

Internal Combustion Engine Failure Modes

Internal combustion engine failures often result from a complex set of conditions, effects, and situations. Engine failures between the MTBF, and thus reliability, are impacted by several variables.

These include the hours of operation; frequency of starts; HP loading vs. rated engine output HP; duty type (severe, normal, or light); speed (RPM); lubrication practices; age; fuel rating and cleanliness; presence of vibration; and type of engine (compression or spark ignition).

If power boosting is used (turbochargers, aftercoolers, etc.), maintenance frequency and practices and engine cooling method are also variables.

Although there is some overlap among the following causes, the top 10 causes of stationary internal combustion (diesel and gas) engine failure beyond the break in period are in no particular order:

  1. Overheating: particularly damaging to the block, head, intake and exhaust valves, and manifolds
  2. Lubrication: inappropriate (wrong oil grade) or dirty/inadequate (lack of complete oil circulation)
  3. Detonation (spark knock)
  4. Oil or coolant leaks
  5. Component metal fatigue/cyclic loading connecting rods, bolts, pistons, valves, crank/camshaft)
  6. Sensor (sending unit) failure: failure to shut down engine from low oil pressure or high heat conditions
  7. Inadequate coolant fluid level/flow (bad water pump or thermostat) or low airflow (air-cooled engines)
  8. Wrong fuel for the engine type and construction
  9. Sustained or excessive conditions of overload, overspeed, or severe underload
  10. Bearing failure.

In addition to this list of causes of engine failure, other causes for rebuilt or recently activated new engines include assembly errors, incorrect or misaligned components, inadequate or improper break in period, and improper clearances.

Excessive heat is one of the most common and severe causes of engine failure since it can cause serious problems as heat causes metal to expand. Thus, the hotter the engine gets, the tighter the clearances or interference become until there are no more remaining clearances, causing valves to gall and stick and pistons to scuff and seize.

Excessive heat can also cause cylinder heads to swell, warp, and crack. Aluminum heads are especially vulnerable to warpage and cracking as aluminum has a much higher coefficient of thermal expansion than cast iron. Consequently, when a bimetal engine with an aluminum head becomes excessively hot, the head tends to expand in the middle the most, causing it to warp and result in a blown head gasket.

If the engine has an overhead cam design, the resulting misalignment in the cam bores created by this warpage can gall or seize the cam bearings or even break the cam. Anytime a warped or cracked aluminum head is encountered, or an overhead cam head has a seized cam, chances are the damage was caused by overheating.

As opposed to electric motors with more predictable operation and failure modes, engines are more subject to the variables of the application and type of service conditions. Short of a total failure or seizing, engines are almost always repairable through overhaul, and thus subject to the reliability factors associated with the service.

The time between overhauls for piston engines vary, but usually fall somewhere between 1200 to 2000 hours of service, with turbocharged and overloaded diesel engines frequently failing at the lower end.

Many properly loaded and lubricated engines under infrequent or standby service may operate for up to 10,000 to 15,000 hours between major rebuilds, while some under severe duty, overloaded, and excessively dirty conditions may not last 1000 hours.

Relatively speaking, combustion engines are very inefficient; most diesel engines do not even possess a thermal efficiency of 40%. This means that out of every gallon of diesel burned by a combustion engine, less than half of the energy generated by the fuel becomes mechanical energy, with the rest converted to heat from friction and combustion losses.

Spark ignition (gas) engines are even less efficient, usually down to 25% to 30%. This means adequate levels of heat removal in the form of water cooling with a radiator and fan or water jacket cooling, convection and forced air cooling for aircooled engines, and supplementary over-engine fan cooling is one of the most critical factors towards maintaining adequate engine reliability and availability.

Lubrication is another important element in maintaining the reliability and extending the life of an internal combustion engine. The frictional loads generated from the tight tolerances emanating from the constant sliding action of the pistons within the cylinder walls, repeated and rapid opening and closing actions of the intake and exhaust valves, rotation of several bearings and crank and camshaft, and the other frictional losses resulting from the close tolerance interaction of metal against metal results in severe levels of radiated heat.

This friction must be reduced to allow the metal contact surfaces within the engine to continue to slide across each other in as much of a frictionless state as possible and to prevent the expansion of metallic components to the point of seizing or failure.

Lubrication is typically provided from an oil pump by using a proper weight of motor oil and appropriate filter, both changed at regular intervals or when the oil becomes dirty or loses its viscosity.

Intake and exhaust valves are also important engine components that are used to control the rapid flow and exchange of gases in internal combustion engines. They are used to seal the working space inside the cylinder against the intake and exhaust manifolds. Rapidly opened and closed, they cause repeated cyclic loading by means of what is known as the “valve train mechanism.”

Such valves are also preloaded by spring forces and subjected to thermal loading due to the high temperature and pressures within the cylinder. Repeated loading and heat cycling often results in the material failing well below their associated yield strength.

When the material is subjected to repeated heat stress and fatigue, one or more tiny cracks will usually start propagating in the material. These tend to increase in size until complete component failure ultimately occurs.

The balance of air to fuel is also critical to maintaining proper engine performance and reliability. Observing proper replacement of the air filters is a vital task towards this goal. In addition, the fuel itself must be kept clean and in the proper proportions with air to create a combustible vapor.

This not only requires a clean fuel filter and regular carburetor/fuel injection adjustment but maintaining fuel storage vessels to prevent condensation and water mixing with the fuel. Regardless of the fuel type, even a minor mixture of fuel and water will dramatically reduce the fuel’s energy potential and resulting engine performance.

Generally, observing and practicing proper engine maintenance using the appropriate materials and filters at the specified intervals of operating hours and service duty—along with periodic tune-ups, timing checks and adjustments, fuel turnover and replacement, appropriate battery charging and maintenance, and regular engine exercising and operational testing at both idle and dynamic levels of loading under simulated starting and running conditions—provides the highest degree of engine reliability.

Figure 2. “Typical” groundwater supply system.

Ancillary Components

Although the pump and driver comprise the major components of a pumping system, ancillary components such as motor starters for electric motors, lineshaft for a vertical turbine pump, drop cable for a submersible pump, shaft coupling for a frame-mounted or SAE-fitted centrifugal pump, and valves and piping are also important components that contribute to the system’s overall reliability.

Each of these components possess their individual reliability factor that must be included to determine an overall pumping system reliability. Generally, ancillary components are 98% or greater in component reliability, often resulting in a pumping plant reliability of 95% or greater.

Water System Reliability

Just as a pumping plant possesses a unique reliability determined by the type of pump and driver and individual reliability of the various components, a water system also functions at an overall reliability.

The reliability of a water system is a calculated value that is carefully analyzed and ascertained from the reliabilities of each individual component that functionally contributes to the operation of the system.

For smaller water systems, reliability of a water system is generally determined from a series configuration. This is where water is often delivered to customers using a single source, pump, storage, treatment, and piping network and where the loss or interruption of any of the components will result in widespread disruption of the entire process, just as an entire chain is only as strong as its weakest link.

Larger water systems also typically function in a parallel configuration. This means multiple sources or booster pumping capacity are potentially available and delivered from different units or sites.

This is an important distinction and the water system definition of redundancy. Many water systems of all sizes function as both a series and parallel system where failure of a single major component without an alternate or “bypass” will result in a total or partial loss of water service from that site.

This necessitates multiplying each relevant factor together as shown in Table 3 to obtain an overall water system reliability (series system). However, this must be tempered by the possible presence of source and equipment redundancy from several different production sites, which significantly impacts the well and pumping system reliability factors (parallel system).

As an illustration, the following represents the overall reliability for a typical water system using groundwater sources exclusively with multiple wells, reservoirs, and booster pumps at different sites and routes to deliver water to the distribution system to create sufficient source, storage, pumping, and transmission redundancy. Refer to the example in Figure 2.

The stated reliability of 95.14% indicates the water system can expect to incur a partial or total disruption of water service, usually occurring at irregular intervals, for 425.74 total hours (17.74 days) out of a given year, primarily due to electrical outages. This is a low and generally unacceptable water system reliability, which may require consideration of adding elevated water storage or backup power to select wells or booster pumps.

Obviously, determining the reliability of a structured water system is generally much more complicated and includes other salient variable factors such as financial resilience; availability of a local backup source; ascertaining and verifying the individual component reliabilities; natural, man-caused, and physical hazards; and the age and condition of each water system component. These are often called the “intangibles.” However, this example does provide the basic concepts behind water system reliability.

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This concludes this three-part series on water and pumping system reliability. Next month, we will embark on another topic with a discussion on possible downsides to excessive control automation.

Until then, work safe and smart.

Learn How to Engineer Success for Your Business
 Engineering Your Business: A series of articles serving as a guide to the groundwater business is a compilation of works from long-time Water Well Journal columnist Ed Butts, PE, CPI. Click here for more information.

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 epbpe@juno.com.