In process control systems, it’s simple (and smart) to incorporate vibration data into an existing monitoring program. Either 4-20 mA sensors or IEPE sensors can be used to do this, and each setup has its own advantages.
4-20 mA sensors
The most common option in plants with PLC/DCS/SCADA systems is using a 4-20 mA sensor which outputs directly to the control system, just like any of the temperature, pressure, or flow sensors already in place in these plants. 4-20 mA sensors provide continuous trend data, making it easy to track changes in overall vibration levels. This approach is the most cost-effective option for implementing vibration monitoring, at a lower relative cost per data point than standard accelerometers.
For most machinery, 4-20 mA trend data is sufficient to understand machine condition. The majority of faults on rotating machines are in the mid-frequency range and appear as rising vibration levels over time. Imbalance, misalignment, and looseness – some of the most common causes of machine failure – tend to have fault frequencies between 30-60,000 CPM and can generally be detected with this method. The downside is that dynamic vibration data isn’t available, meaning portable data collection isn’t an option, and trend data may not pick up signs of other impending faults.
IEPE sensors
Outside of the mid-frequency range, an IEPE sensor can be a better option than a standard 4-20 mA sensor. In this setup, an accelerometer or velocity transducer outputs to a vibration transmitter, which conditions the dynamic data into a 4-20 mA signal and sends it to the control system.
This approach also provides continuous trend data, with the added benefit of retaining dynamic vibration data. It does come at a higher upfront cost and is advised for applications that require access to dynamic data – essentially, equipment on which you expect to measure very low or very high frequencies, or portable data collection. IEPE sensors are also available with a wider variety of hazardous location certifications, which may make them a better choice for certain applications.
Complex machine faults
To identify pump cavitation, bearing fatigue, gear mesh faults, or impacts from loose components, it helps to look at more than just the machine’s overall vibration level. For example, on critical pumps where cavitation is a concern, you'll want to have the detailed information from an IEPE sensor available for analysis because early signs of cavitation won’t be captured in the overall trend data. In this case, using a low sensitivity/high frequency accelerometer in combination with a vibration transmitter will help you detect cavitation early and address the issue before the equipment fails.
Bearing wear also commonly occurs at high frequencies (4x-20x running speed), and critical bearings are most effectively monitored with IEPE sensors. On slow speed machinery, a low frequency accelerometer will provide more useful low frequency information than a 4-20 mA sensor.
Quick comparison
A 4-20 mA sensor that outputs to the control system:
- Has a lower setup cost
- Provides simple trend data for continuous monitoring
- Is easily incorporated into existing plant infrastructure
- Is best used for general machine monitoring – frequencies between 30-60,000 CPM (0.5 Hz-10 kHz)
An IEPE sensor and a vibration transmitter:
- Has a higher initial cost
- Enables access to dynamic data for portable data collection or detailed analysis
- Provides 24/7 trend data for continuous monitoring
- Wider sensor selection for a more varied range of applications
- Is best used for detecting high frequency faults, monitoring slow speed machines, or route-based data collection
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