By Jim Rosner, Lead Industrial Applicati on Engineer, Motor Control and Protecti on, ICD, Eaton
If a pump operates with no flow through the pump, due to a closed discharge valve or line blockage, a dead- head will occur. The pump re-circulates the same water, causing water temperature to continually rise. If the pump continues to run in a dead-headed condition for too long, excessive heating can damage the seals and reduce the life of the pump.
As there is a significant amount of energy being added to the liquid in a pump in a dead-head state, dead-heading in a centrifugal pump can lead to explosions. Hydraulic overpressure and possible chemical reactions in the pump can also be caused by the overexertion of pressure. The same results can be caused by running the pump dry for an extended period; this can lead to cavitation.
Although it may seem like a ‘no brainer’ to address these concerns and fix them, dead-heads are extremely hard to detect. A pump float switch will not detect a dead-head because the water level does not decrease. The key to accurate pump protection against dead- head damage is monitoring the motor load to see if it decreases. Two methods are available to determine motor load: motor current and input power.
Monitoring current for underloaded conditions is not effective because the current is relatively constant up to 50% of the motor load. Even at light loading, the current and the power factor is high, as current is flowing to the motor but it is not doing useful work (power). As the input power varies linearly across the motor load range, low power detection is an extremely reliable method for catching even small decreases in motor load. Therefore, a protective device looking at motor current may not be able to distinguish between ‘normal’ and dead-head conditions, but a device using low input power detection can.PE_jun_V3.indd
So, how can a user protect their pump? It is ideal to detect the development of an underloaded condition, like a dead-head, before the damage occurs. Traditional pump diagnostic methods, like a float switch, are ineffective in detecting a dead-head condition because water levels do not decrease. The key to fast and accurate detection is being able to identify decreases in motor load.
The best method for detecting underloaded conditions is using tools that can monitor power. This is because of the difference between how the power varies across the load profile compared to current, making low power protection a reliable method for quickly detecting even small decreases in the load due to a dead-head. For light loads, input power is up to 10 times more sensitive than current-based methods of detection.
For example, tools like motor management relays provide industry-leading configurable protection options for current, voltage, and power conditions. The low power protection can accurately detect a dip in power when a dead-headed condition occurs, taking the motor offline before costly damage occurs. Low power protection can be set to the desired percentage of rated power and provide separate warning trip levels and delays. The motor management relay therefore provides a single, compact solution for pump protection, as well as thermal overload, ground fault and other current or voltage based protections.
Figure 1. Current and input power vs. motor load. Image courtesy of Eaton.
How to Protect a Pump
To help shield assets from costly damage, here are 11 tips and reminders for protecting a pump system.
1. Expand monitoring beyond pump float switches: Because the water level does not decrease with the use of a pump float switch, dead-heads are commonly overlooked with this detection method.
2. Do not monitor motor current alone: In dead- head conditions, motor current remains high even at light loading. The power factor may also be high, as current is flowing to the motor, but not doing useful work. Therefore, a protective device looking at current may not be able to accurately distinguish between ‘normal’ and ‘dead-head’ conditions.
3. Monitor the pump motor load carefully: Accurate discovery of motor load decreases can be critical for identifying dead-head conditions.
4. Explore possible underload conditions: While monitoring current, observe whether the motor current remains nearly constant (up to 50%) of the motor load. Input power should vary linearly across the motor load range.
5. Always monitor for low power: Small decreases in motor load may be significant; but even at light loads, power is up to 10 times more sensitive than amperes. This makes low power detection an extremely reliable method for catching even small decreases in motor load.
6. Leverage global motor management relay for optimal protection: Motor management relay provide current, voltage and power-based monitoring and protection – including low power protection – for the most comprehensive defense against harmful dead-head and underloaded conditions.
7. Take motors offline as soon as a dip in power is detected: Look for global motor management relay products that offer reliability and response without the addition of separate relays. When a pump dead-heads, these products will more quickly and accurately detect the dip in power; but they will also prevent further damage by automatically taking the motor offline.
8. Avoid or upgrade separate ‘pump-off’ or dedicated dead-head protection relays: Quality global motor management relay products now embed low power detection capabilities directly with the device’s connectivity options. Leading solutions can be set to the desired percentage of rated power and offer full customization for separate warnings, trip levels and delays.
9. Invest inflexible solutions with advanced protection features: Configurable protection options provide critical pump protection while also providing additional critical motor protections such as thermal overload and other current and voltage- based protections.
10. Optimize equipment lifecycles by choosing solutions with predictive analytics: Durable pump protection solutions should help optimize equipment lifecycles. Solutions that effectively leverage remote monitoring and troubleshooting enable automated maintenance tasks to occur. These tasks reduce the need for and extent of scheduled maintenance periods, while also minimizing required manpower and travel to remote locations.
11. Explore paired solutions to achieve greater efficiency, reliability, and protection: Some vendor solutions also offer easy integration with other protective products that ease commissioning while improving protection and extending uptime. For example, a global motor management relay paired with a variable frequency drive enables regenerative energy management. This feature allows the circuit to recognize changes in bus voltage and then store that power to keep the pump motor operating optimally. The drive frequency will dynamically adjust to discharge any added voltage rise on the DC bus.PE_jun_V3.indd
By providing insight and opportunity for local engineering and service, quality pump protection products can offer greater stability to operations, while improving uptime and reducing costs. In harsh environments especially, protective devices that offer preventative diagnostics, enhanced stability and control, endurance, and longer life cycles are not only highly advantageous, but also beneficial to the safety of infrastructure, people, and environment.
About the Author
Jim Rosner has worked as an application engineer in Industrial Controls and Factory Automation for over 41 years. His focus has been on PLC applications, industrial networks and motor control and protection. He has worked with and helped many customers over the years with their application issues and questions.