In most facilities, pumps are critical pieces of equipment and are monitored routinely as part of a predictive maintenance program. Imbalance, misalignment, looseness, lubrication issues, cavitation, roller bearing faults: there's plenty that can go wrong with a pump. By utilizing vibration monitoring, developing faults can be identified early and addressed before serious damage (or worse, complete failure of the pump) occurs.
But, if you are using the wrong accelerometer for the job, distorted signals might mask early indications of a developing problem. These are three of the warning signs that often show up in vibration data from pumps that indicate your accelerometer isn’t capturing the right measurement range.
Signal corruption from low-frequency noise
If a vibration measurement is close to the electronic noise floor of the sensor, the signal becomes distorted. This often looks like ski slope data. A corrupted signal doesn’t capture the low-frequency readings that could indicate destructive surge pulsations or problems with the pump’s reduction gears or rotors.
If you’re seeing signal distortion, using low-frequency accelerometers or velocity sensors in place of general-purpose accelerometers is a good option. Low-frequency sensors with responses down to 0.1 Hz are available, which will be able to handle the lowest pump fault frequencies and minimize the impact of the sensor’s inherent noise.
Sensor overload from high-frequency noise
Cavitation, bearing impact noise, and high-pressure leaks are common sources of very high-frequency vibrations (above 600,000 CPM / 10 kHz) which are outside the measurement range of a general-purpose accelerometer. Low-frequency, high-sensitivity accelerometers are particularly vulnerable to interference from high-frequency noise. This noise can overload the sensor and obscure useful low-frequency data. Overload often appears as a clipped signal or ski-slope measurements.
The worst part? Since the frequency of the vibration is higher than the average sensor’s measurement range, the high-frequency data that would help you detect pump cavitation or bearing impacts won’t be recorded. If you’re experiencing sensor overload, use a lower sensitivity sensor to capture high-frequency measurements. Check out our tech tip on sensor overload for more information.
Increasing vibration levels in the mid- to high-frequency range
Usually, pump vibrations are between about 450 and 300,000 CPM, including the harmonics of the drive motor, some bearing faults, and vane pass, gear mesh, or reciprocation impact frequencies. Misalignment and improper mounting, two common causes of pump failure, also produce vibration levels in this range — so developing faults will appear as rising vibration levels over time but won’t exceed alarm levels.
Analyzing long-term vibration trend data is the recommended approach to detect a variety of pump faults in the mid-frequency range, and this requires continuous monitoring. 4-20 mA sensors or standard 100 mV/g accelerometers are used for 24/7 online monitoring and have a low signal-to-noise ratio, making it easy to see increasing vibration levels and identify problems early. Access to long-term trend data has the additional benefit of providing a more complete picture of machine condition and makes detailed fault analysis possible when necessary.
What to do with your data
Important fault frequencies for pumps can be anywhere from 0.1 to 10,000 Hz (or higher), so deploying the right accelerometer to monitor your pump is critical for effective condition-based maintenance. When analyzing vibration data, look out for these signs that you might not be seeing the complete picture about your pump’s health:
- Sensor overload
- Signal clipping
- Signal distortion
- Ski-slope measurements
- Rising vibration levels over time
Wilcoxon’s knowledgeable and dedicated application engineers are here to help you select the right sensor for effective machinery health monitoring — just contact our application support team at [email protected] or +1 301 330 8811.
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