Jim Rosner is the lead industrial application engineer, motor control and protection, ICD for Eaton.
Today’s always-on environments demand operational predictivity and proactive engineering strategies to prevent costly equipment damage and downtime. This makes finding an effective means of protection for pump systems critical – and low power protection devices are a great place to start.
What is a Dead-Head?
When a pump operates with no flow through the pump due to a closed discharge valve or line blockage, a dead-head has occurred. 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 expensive seals and reduce the life of the pump.
Dead-heading in a centrifugal pump can lead to explosions, due to the energy being put into the liquid in the pump. 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, which can lead to cavitation.
How to Detect Dead-Heads
It seems like a “no brainer” to address the concern and fix it, but 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 that the motor load decreases. Two methods are available to determine motor load: motor current and input power. However, monitoring current for underloaded conditions is not effective because current is nearly constant up to 50% of the motor load. Current is high even at light loading and the power factor is high, as current is flowing to the motor but it is not doing useful work (power).
The input power varies linearly across the motor load range, making low power detection an extremely reliable method for catching even small decreases in motor load. At light loads, power is up to 10 times more sensitive than amperes. Therefore, a protective device looking at current may not be able to distinguish between “normal” and dead-head conditions while a device providing low power detection can.
So, how you can you protect your pump? It's 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.
How to Detect Underloaded Pump Conditions
The best method for detecting underloaded conditions is using tools that can monitor power. This is because of the difference between the way power varies across the load profile compared to current. While current is relatively flat below 50 percent loading, power varies with respect to load across the load range. This makes low power protection a reliable method for quickly detecting even small decreases in the load due to a dead-head. For light loads, power is up to 10 times more sensitive than current-based methods of detection.
For example, tools like Eaton's Power Xpert C445 motor management relay provide 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.
11 Tips to Protect Your Pumps
To help shield assets from costly damage, here are 11 tips and reminders for protecting your pumps:
- 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.
- 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 just not doing useful work. Therefore, a protective device looking at current may not be able to accurately distinguish between “normal” and “dead-head” conditions.
- Monitor the pump motor load carefully: Accurate discovery of motor load decreases can be critical for identifying dead-head conditions.
- 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.
- Always monitor for low power: Small decreases in motor load may be significant: 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.
- Leverage global motor management relays for optimal protection: Motor management relays 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.
- 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.
- 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.
- Invest in flexible 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.
- Optimize equipment lifecycles by choosing solutions with predictive analytics: Durable pump protection solutions should 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.
- 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.
Pump Motor Fault Predictive Features on the Horizon
There are additional options and enhancements that are in the works to add even more protection features for pump motors. These include:
- Early prediction of pump motor failures.
- Motor Current Signature Analysis is being developed to detect pump cavitation as well as other issues with the motor or connected load.
- This technology can be used along with power measurements and sensor-less flow and pressure estimate techniques to improve cavitation detection and potentially see the onset of cavitation.
These enhancements to existing motor management relays are being developed that can provide detection of pump motor failures before they occur and before they cause an unwanted result. That allows the motor to be repaired or replaced during normal maintenance rather than during a time it is needed most.
This feature will also be incorporated into variable frequency drives (VFD). With drives the speed can be varied while allowing the process controller to control the valve to potentially eliminate the cavitation condition without intervention.
These features are on the horizon and will most likely be available in the next year or two. So, stay tuned, the future of motor protection is coming that will not only protect motors as is the case today, but will also predict motor failures with enough advanced warning to prevent motors from failing while in service.
A Few More Thoughts
By providing insight and opportunity for local engineering and service, 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.
