Proper Use of Proximity Probes and Seismic Sensors: Choosing the Right Tool for Vibration Monitoring

Vibration measurement is a critical aspect of maintaining and optimizing rotating machinery in various industries. Accurate monitoring of vibrations can prevent costly equipment failures, enhance operational efficiency, and extend the lifespan of industrial assets. Two primary tools used for this purpose are proximity probes and seismic sensors, each with unique characteristics and applications. Proximity probes, known for their non-contact sensing capabilities, excel in measuring relative shaft vibration and displacement. In contrast, seismic sensors like accelerometers and velocity sensors are adept at detecting absolute casing vibrations across a wide frequency range. The choice between proximity probes and seismic sensors for vibration measurement is not always straightforward. This decision depends on specific applications, machinery types, operating conditions, and measurement objectives. Understanding the strengths and limitations of each sensor type is crucial for implementing effective vibration monitoring strategies.

Understanding Proximity Probes
Proximity probes, often called eddy current probes, are the go-to non-contact sensors for fluid film bearing machines for shaft vibration measurement. These clever devices create a high-frequency electromagnetic field that interacts with conductive materials - usually the shaft of your rotating and reciprocating machinery.

What makes proximity probes stand out? For starters, they're hands-off. No physical contact means less wear and tear on both the sensor and the shaft. They're champions at detecting relative shaft vibration, giving you the inside scoop on how the shaft behaves within its bearings. Plus, they're great at measuring shaft displacement, both statically and dynamically.

In the world of vibration monitoring, proximity probes shine in tough spots. High temperatures? No problem. Oil-soaked environment? We've got you covered. They're particularly handy for keeping an eye on journal bearings even in high-speed integrally geared centrifugal compressor applications and slower-moving machinery like hydro-turbines, thanks to their knack for dynamic vibration response and static position measurements.

But like any tool, proximity probes have their quirks. They can be picky about the material they're measuring, the system length from the probe to the oscillator demodulator and the probe type being used, so calibration is key. Metrix has provided for these variables through the use of a Digital Proximity System that can be configured in the field. Also note, the proximity system prefers a clean, conductive surface to work their magic, however, just like with seismic monitoring, looking at the vibration and position trend over time provides excellent value even with shaft surfaces that may not be ideal.

Even with these challenges, proximity probes are indispensable in the rotating machinery diagnostics toolkit. When you need to know exactly where that shaft is and how it moves relative to its surroundings, these sensors are often your best bet.

Understanding Seismic Sensors
Let's dive into the world of seismic sensors, the workhorses of vibration measurement. These compact sensors are like tiny seismographs, constantly on the lookout for the slightest tremors in your machinery.

At their core, seismic sensors use piezoelectric crystals that generate an electrical charge when squeezed or stretched. Attach one to your machine, and it'll faithfully report on every shake, rattle, and roll typically for machines that have casing temperatures less than 120° C. For high temperature applications up to 375° C, we typically use a moving coil velocity sensor, in lieu of a piezoelectric crystal, to avoid integrated circuits that usually have much lower temperature limits.

What sets seismic sensors apart? First off, they're all about that direct contact. They hug your machinery tight, feeling every vibration firsthand. This close connection allows them to capture absolute motion on the casing of the machine. When it comes to high-frequency vibrations, above 2,000 Hz, accelerometers are in their element, picking up on those rapid oscillations that other sensors might miss.

Velocity sensors are typically used with machines that operate at 60 Hz or less (≤ 3,600 RPM). They have an excellent signal to noise ratio for the middle frequency range from 10 Hz to 2,000 Hz, which happens to be the range of most rotating machinery with rolling element bearings.

Accelerometers bring a lot to the table. They're versatile gadgets, happy to measure vibration in any direction. Their broad frequency range makes them ideal for catching both low rumbles and high-pitched whines. And let's not forget their portability - perfect for those walk-around vibration checks.

Accelerometers do have their limitations. They can struggle with low-frequency measurements and require secure mounting, which can be tricky in tight spaces. While durable, harsh environments can affect their performance. Despite these challenges, their versatility and high-frequency capabilities make them invaluable in many vibration monitoring scenarios.

Despite these challenges, seismic sensors remain the go-to choice for many casing vibration monitoring scenarios. Whether you're troubleshooting a noisy bearing or keeping tabs on overall machine health, these sensors have got your back.

Comparing Proximity Probes and Seismic Sensors

When it comes to shaft vibration monitoring, proximity probes and seismic sensors each bring unique strengths to the table. Let's break down how they stack up against each other.

Measurement Capabilities:
Proximity probes excel at capturing relative vibration - they're all about the relationship between the shaft and its housing. Seismic sensors, on the other hand, measure absolute casing vibration, giving you the picture of overall casing movement. Together, in certain applications, these sensors can provide absolute motion of the shaft. This combination is used when the machine has a light casing in comparison to the rotor. When the casing to rotor mass ratio is less than 10 to 1, there is a high likelihood that the casing can be moving along with the rotor. The phase relationship between the vibration of the shaft and the vibration of the casing is taken into account when using both a proximity and a seismic sensor to measure absolute shaft motion.

In the frequency game, accelerometers are the clear winners for high-frequency detection, often reaching up to 20 kHz or more. Proximity probes typically tap out around 10 kHz but shine in the low-frequency range, even down to DC for static position measurements.
Amplitude-wise, proximity probes usually measure within a few millimeters, perfect for precise shaft movement. Seismic sensors can handle a wider range of amplitudes, from tiny vibrations to significant shakes.

Installation and Maintenance:
Proximity probes need careful installation to maintain a consistent gap from the shaft. They're often permanently mounted but require minimal maintenance once set up. Accelerometers are more flexible in placement but demand solid mounting for accurate readings. They're also great for portable use in walk-around monitoring.

Data Interpretation:
Proximity probe data directly shows shaft position and movement, making it intuitive for fluid film bearing analysis. Seismic sensor data often needs integration and provides comprehensive spectral analysis.

Cost Considerations:
Generally, proximity probe systems come with a higher initial cost due to their specialized nature and additional required components. Seismic sensors tend to be more budget-friendly and versatile, especially for rolling element bearing machines.

Choosing between these sensors often comes down to your specific monitoring needs, machinery type, bearing type and budget constraints. Many advanced monitoring systems use both proximity and seismic sensors to get the best of both worlds.

Choosing the Right Sensor for Different Scenarios
Selecting the right sensor for shaft vibration monitoring isn't a one-size-fits-all affair. It's crucial to consider the machinery type, operating conditions, and your specific measurement goals.

For large rotating machinery like turbines or compressors, proximity probes often take the lead. They're ideal for monitoring shaft centerline position, which is critical in these fluid film bearing applications. If you're dealing with journal bearings, proximity probes are your go-to choice. They excel at measuring the thin oil film between the shaft and bearing, providing invaluable data on bearing health. Low-speed machinery also benefits from proximity probes, as they can detect subtle movements that might escape other sensors. On the flip side, accelerometers shine in scenarios requiring high-frequency vibration detection. They're perfect for spotting early signs of bearing faults or gear mesh issues in higher-speed equipment. When it comes to measuring casing vibration - think pumps or electric motors operating at or less than 60 Hz – a velocity sensor is the tool of choice. At speeds greater than 60 Hz, accelerometers are used. The versatility of accelerometers makes them ideal for portable vibration analysis, allowing technicians to quickly check multiple points on various machines.

Operating conditions play a big role too. In high-temperature environments or where space is tight, proximity probes might have the edge. But for general-purpose monitoring across a wide range of machines, seismic sensors often prove more practical and cost-effective. Remember, your measurement objectives are key. If you need precise shaft position data with fluid film bearing machines, go with proximity probes. For overall vibration trends and broad-spectrum analysis, seismic sensors are typically the way to go. In many cases, the best solution might be a combination of both sensor types, giving you a comprehensive view of your machinery's health.

Wrapping up, it's clear that both proximity probes and seismic sensors play crucial roles in machine vibration monitoring, each with its unique strengths. Proximity probes excel in relative shaft movement and low-frequency measurements, while seismic sensors shine in absolute vibration detection and high-frequency analysis. The choice between them hinges on your specific machinery, operating conditions, and monitoring objectives.

Remember, there's no one-size-fits-all solution in vibration monitoring. The key is to match the right sensor to your specific needs. For optimal results, don't hesitate to consult with the vibration monitoring experts at Metrix. They can help you navigate the complexities of sensor selection and implementation, ensuring you get the most accurate and useful data for your machinery health monitoring program.