What Are Your Vibration Monitoring Goals?
Identifying goals before starting is key to designing a process tailored to specific needs. What are you hoping to accomplish by monitoring vibration? How would you like to acquire data? What are you going to do with the data? These important questions should be addressed before moving forward.
Users typically take two different paths when it comes to acquiring data: Fast-Fourier Transform (FFT) and overall vibration.
An FFT plot shows the vibration spectrum as a function of frequency. These plots are typically acquired via a portable industrial data collector or online vibration monitoring system. Similar to sound and noise measurement, an FFT plot allows the user to diagnose the source of vibration and begin the troubleshooting process.
Overall vibration is more simplistic and typically measured with PLC, DCS or SCADA systems that already exist in the plant. Simple displays can also be used. The overall vibration is similar to a running average and is plotted against time for trending purposes. This measurement indicates if vibration has increased over time. Trouble lurks as vibration levels increase. An overall vibration signal can help save on downtime by allowing the user to predict impending failures and schedule maintenance on plant machinery.
What Are Your Vibration Monitoring Goals?
Identifying goals before starting is key to designing a process tailored to specific needs. What are you hoping to accomplish by monitoring vibration? How would you like to acquire data? What are you going to do with the data? These important questions should be addressed before moving forward.
Users typically take two different paths when it comes to acquiring data: Fast-Fourier Transform (FFT) and overall vibration.
An FFT plot shows the vibration spectrum as a function of frequency. These plots are typically acquired via a portable industrial data collector or online vibration monitoring system. Similar to sound and noise measurement, an FFT plot allows the user to diagnose the source of vibration and begin the troubleshooting process.
Overall vibration is more simplistic and typically measured with PLC, DCS or SCADA systems that already exist in the plant. Simple displays can also be used. The overall vibration is similar to a running average and is plotted against time for trending purposes. This measurement indicates if vibration has increased over time. Trouble lurks as vibration levels increase. An overall vibration signal can help save on downtime by allowing the user to predict impending failures and schedule maintenance on plant machinery.
A vibration sensor on a pump in a water application.
Industrial Accelerometers-The Key Specs
Analog industrial accelerometers are used with portable data collectors or even 24/7 online monitoring systems. They are capable of providing both FFT and vibration trending data.
Sensitivity
Sensitivity is one of the most important factors when selecting an industrial accelerometer. The "industry standard" is 100 mV/g. This sensitivity will cover most applications, and data collectors are commonly supplied with 100 mV/g accelerometers and pre-configured with 100 mV/g as their default input sensitivity.
Slow speed rotating machinery running at less than 60 cpm may require a more sensitive accelerometer. Models marketed for "low frequency" applications are typically 500 mV/g. An increase in sensitivity creates a trade off as measurement range is reduced. Vibration levels in slow speed machinery are low. However, the increased sensitivity and resolution are an acceptable trade-off versus measurement range.
High speed machinery running at more than 60,000 cpm or impacting applications often use a 10 mV/g accelerometer.
Fit, Form and Function
Of course, any accelerometer needs to be able to fit into and survive the application. A common theme with industrial accelerometers for most every manufacturer is case isolation, hermetic sealing and the use of stainless steel housings. Industrial accelerometers are meant to survive in paper mills, refineries, steel mills, water-treatment plants, etc., but there are some nuances to consider.
Accelerometers with an integral cable are typically submersible. Be sure to pay special attention to the cable jacket's material. For example, polyurethane cable can degrade in a caustic application.
The industry standard for sensors with separate cables and connectors is to use a 2-pin MIL style connector. This rugged connector is threaded with an internal keyway and hermetically sealed onto the accelerometer. When paired with the right cable connector, these sensors work well in outdoor applications where they will be subjected to the elements.
Low-profile accelerometers with integral cable, also known as side-exit models, are popular for downhole pump applications. These models allow for limited clearance and submergibility.
Temperature Requirements
The common IEPE industrial accelerometer has a high temperature limit of 250 deg F. A quartz or ceramic sensing element is inside the accelerometer. This element, when subjected to vibration, outputs a unit of charge referred to as a pico-coulomb. This charge is then converted to a measureable voltage signal by the sensor's internal amplifier. This amplifier will fail when subjected to higher temperatures. High temperature IEPE accelerometers are commonly available that use a military grade amplifier to extend the sensor's temperature range to ~325 deg F.
For applications above ~325 deg F, an accelerometer design where the voltage amplifier is contained inside a separate housing and installed in a cooler area must be used. Charge mode accelerometers are industrial accelerometers without an internal amplifier. Typically sold in kits that include a high temp Teflon cable and a separate charge amplifier, these sensors are available to suit applications up to ~900 deg F.
Quartz versus Ceramic Sensing Elements
Ninety percent of the industrial accelerometers on the market today use a ceramic element, which allows for a superior signal to noise ratio versus quartz. Signal resolution is better with a ceramic element.
Quartz, however, is naturally piezoelectric and more stable versus ceramic over time. Quartz experiences less thermal shift in sensitivity and is recommended for applications where temperatures fluctuate.
Overall Vibration Sensors (4-20 mA)
Most plants have PLC, DCS or SCADA systems already in place that interface with sensors that monitor temperature, pressure, etc. Using the acquisition system already in
