Top 5 submersible pump killers

There are many factors that affect the life and performance of a submersible pump. As with most things, proper selection of equipment for the application is fundamental to ensuring long and efficient operation. Careful evaluation of the system conditions and operation needs is critical to making the right selection. However, not all aspects of a well system are easy to assess or predict.

Dry running

Centrifugal pumps require water flow not only for motor cooling, but also to keep the rotating elements lubricated and to prevent cavitation. Without water, pumps will see dramatic levels of friction and heat, causing mechanical wear and motor failure.

Well location and water demands are major factors in dry-running problems. In certain geographical areas, water table levels are dropping due to reduced aquifer recharge and heavy usage. This can result in well loss, a measurement of diminishing static water level or yield. As well loss increases, pumps previously sized for higher well yields may produce large drawdowns, resulting in partial dry-running. Over time external conditions may cause the water level to drop below the functional limits of the well.

Things to consider: When a well is no longer functional due to low water levels, further drilling and deeper pump setting may be required. Alterations to the well will result in changes to the system’s total dynamic head and required pump performance. It is important to ensure that system equipment is resized to determine if it’s compatible with the operation requirements of the re-drilled well. Pump protectors can also be installed to monitor the electrical conditions of a submersible motor. When low amp draw (load) is detected, the motor protector will shut the pump down for a given period to protect the system and allow for well recharge. The protector will not eliminate the problem of reduced yield; however, it will guard against costly motor replacements due to dry-running.

Cavitation

Commonly arising during partial dry-run conditions, cavitation occurs when pressure on the suction side of the submersible pump drops below the vapor pressure of the liquid, resulting in the formation of small vapor bubbles. When the pressure rises during pump operation, these bubbles collapse or implode and release shock waves, damaging the impeller.

Pit marks left by cavitation reduce pump efficiency and decrease flow and head, resulting in a reduction in overall pump performance. Furthermore, cavitation can increase operation noise and vibration, which can damage bearings, seals and welds, resulting in a shorter pump lifespan.

Things to consider: Since cavitation often occurs during partial dry-run or conditions with excessive entrained air, a pump protection unit can be used to help eliminate the risk. Symptoms of cavitation include decreases in pump yield and irregular noise during operation; however, cavitation cannot be confirmed until the pump is disassembled.

Corrosion

Stainless steel is naturally corrosion-resistant, but it is not fully immune to rust. While it is less susceptible than conventional steels, exposure to chlorides or other chemicals can speed up reactions and break down stainless steel over time. Extended periods of exposure to heat can also act as a catalyst in chemical reactions, speeding up corrosion.

Narrow openings and spaces between surfaces on the pump typically have stagnant conditions that make them particularly vulnerable. Crevices, such as those found at flange joints or at threaded connections, are thus often the most likely spots for rust formation and corrosion.

Corrosion causes degradation in the shape and integrity of pump components, leading to seizing, slipping, deformation and component failure.

Things to consider: Corrosion will occur naturally over time on almost all metal materials. To ensure that you get the longest life possible from your pump, it is important to understand the chemical makeup of the liquids you are pumping and select products for these characteristics. It is also important to consider how heat and abrasives affect your application. If these factors are also present, it may be best to invest in higher-quality alloys.

Power

There are several aspects of power quality as well as unexpected power events that can severely hamper the operation of a submersible motor:

There are several aspects of power quality as well as unexpected power events that can severely hamper the operation of a submersible motor.

Voltage. When a motor is operating on voltage lower than its nameplate rating, it will draw more current to create the power needed to operate. Since power equals current multiplied by voltage (P=I*V), if the voltage is low the current must increase to supply the power required. When excessive current is used by the motor, it dissipates in the form of heat. The rise in temperature of the motor winding will start to deteriorate the insulation until it finally fails and the winding shorts out. In most cases the damage from increased heat takes time to cause failure. This process could take months or even years depending on the severity and frequency of the undervoltage events.

Based on the effects of low voltage and the formula for power calculation, one may expect that if the motor is operating on voltage higher than its nameplate rating, the current would be lower. This, however, is not the case. Overvoltage, beyond the manufacturer’s tolerance, can cause the magnetic portion of the motor to be pushed into saturation where it will draw more current. As with low voltage, the increased current in an overvoltage situation will create increased heat in the motor and ultimately premature failure.

Things to consider: Standard electrical protections including fuses, circuit breakers and adjustable overloads should be included in your pumping system design to safeguard the motor. It is also important to confirm that the transformer supplying the location can support all loads that are connected to it. Contact the local utility and have professionals check the voltage at the transformer. In many cases the transformer feeding the site can be adjusted up or down if needed. It is also important to ensure that the correct wire size is used to connect the service panel to the motor. Undersized wire will cause voltage to drop through the cable just like pressure drop in an undersized pipe.

Standard electrical protections including fuses, circuit breakers and adjustable overloads should be included in your pumping system design to safeguard the motor.

Asymmetry (voltage and current). Asymmetry is the lack of balance of voltage or current in 3-phase electrical loads. Voltage asymmetry (or imbalance) occurs when the electrical load is unequally distributed across the three legs of the power supply. This can occur because of the varying demands of each leg’s connections.

Even a relatively small voltage asymmetry can have a significant effect on a pump motor. As the voltage imbalance increases in a system, the temperature in the motor winding increases. At a 2% voltage imbalance, the motor winding temperature will increase by 8%. This 8% increase in winding temperature can reduce the motor life by 50% compared to a motor running at a normal temperature.

When voltage supplied to a motor is not balanced, the individual windings will consume current in an unequal manner. This is referred to as current asymmetry or imbalance. A voltage imbalance of 1% can affect the current imbalance by up to 10%. When a 3-phase motor consumes current in an unbalanced manner, one winding will “pull” harder on the rotor. This can cause the motor to have increased vibration, uneven wear on the shaft and bearings and increase heat rise in the windings. The excessive heat and vibration will lead to motor failure.

Things to consider: Voltage and current asymmetry should be calculated at the time of installation to determine the best connection to the incoming power. The National Electrical Manufacturers Association recommends that 3-phase motors should not operate with a voltage imbalance greater than 1%. If the voltage imbalance is greater than 1% the installer or owner should have a dialogue with the local utility to find a solution. For current imbalance, the target should be at or below 5% for best operation. If the voltage asymmetry is within acceptable limits and the current asymmetry remains above 5% for all connections, it is important to determine if the largest difference in current consumption is consistently drawn from the same incoming power leg (L1-L2-L3) or, if it follows the same motor winding with each connection. If the largest difference is consistently from the same incoming leg, the issue may be from the utility side of the system. If the higher current is consistently on the same motor winding, the motor should be evaluated for a potential issue.

For detection of and protection from these power quality issues, the use of an advanced motor protection device is recommended. These types of devices are available from several suppliers and in most cases will not only monitor the system for power issues, but they will also stop the motor when significant problems are detected. An advanced motor protection device may also record voltage and current in real time and either store this data or send it to a cloud server. Additional advanced motor protection device functions may include monitoring for loss of phase, phase sequence change, motor temperature, power factor and more.

Surges and lightning. An unexpected increase in transient voltage can be caused by changes or malfunctions within the power grid, an individual system or by a lightning strike. These voltage spikes are often random and difficult to predict and can wreak havoc on an unprotected pump motor.

Things to consider: Surge arrestors are a critical, and in many places mandatory, component in helping to prevent catastrophic equipment failure due to grid or system surges. A surge arrestor should be mounted as close to the motor as practical. The location is usually in the pump panel, but sometimes it is placed at the well head in a separate electrical box. It is imperative that the system be connected to an acceptable grounding system. A surge arrestor works by redirecting the power surge and sending it to ground. This surge will take the path of least resistance. If the grounding system has higher resistance than other devices in the system, the surge will flow to those components and cause damage. The National Electric Code suggests that the resistance in a grounding system should be no more than 25 ohms. For systems with sensitive electronic controls like a variable frequency drive, the resistance to ground should be approximately 5 ohms for proper operation and protection.

Sand and sediment

Sand and particulates are one of the greatest enemies of rotating equipment. Sand and gravel are common in aquifers and it is difficult to prevent these particulates from entering a well. Turbulence created from the running pump can also stir up and suck in debris that settles on the bottom of the well. Once these materials enter the pump, they are set in motion at high speed through the impellers. These particles act as abrasives to the metal and can wear away at components over time. This can result in loss of efficiency and even cause the pump to seize.

Things to consider: Typical limits for sand content in domestic well water according to the National Ground Water Association should not exceed 2.5mg/l. Screens and gravel pack or sand filters should be installed in the well to prevent excessive sand inclusion and prolong the life of the pump.

By taking the time to assess and plan for each of these risk factors, in addition to the fundamentals of proper pump sizing, you can help to ensure the longest and most efficient operation life for your pump.

Bobby Fischer is a technical sales engineer for Grundfos, Brookshire, Texas.
Scott Byfield is a market development manager for digital and controls for Grundfos, Brookshire, Texas.
Alan Bixler is a field sales manager for Grundfos, Brookshire, Texas.