Introduction
The 3500/25 is a Keyphasor Module from the Bently Nevada 3500 Machinery Protection System, with 128718-01 being a related assembly or kit number.
The Keyphasor® is a fundamental and critical component for machinery diagnostics. It is not a monitor for vibration level, but a timing reference. Its primary function is to generate a once-per-turn pulse signal by detecting a physical keyway or notch on a rotating shaft. This pulse is used by other monitoring modules in the 3500 system (like the Vibration Monitor - 3500/42 or the TDI - 3500/22) to measure rotor speed and, most importantly, to provide a phase reference. Phase information is essential for determining the angular location of a vibration imbalance or other fault on the rotor, enabling powerful diagnostic capabilities such as balancing and orbit analysis.
Key Parameters
The following table outlines the key specifications for the 3500/25 Keyphasor Module.
| Parameter Category | Specification |
|---|---|
| Part Number | 3500/25 (Module) / 128718-01 (Assembly) |
| Function | Dual-Channel Keyphasor Module (Speed and Phase Reference) |
| Compatibility | Bently Nevada 3500 Series Racks |
| Input Channels | 2 independent channels (can be from proximity probes or magnetic pickups) |
| Input Signal | • From Proximity Probes (Standard) • From Magnetic Pickups (Optional) |
| Keyphasor Signal Output | A sharp, digital pulse for each revolution of the shaft. |
| Measured Parameter | Shaft Rotational Speed (RPM) and Phase Angle (Degrees) |
| Speed Range | 0 RPM to over 60,000 RPM (dependent on probe type) |
| LED Indicators | Per channel: Keyphasor Signal, OK, Bypass, Alert, Danger |
| Power Supply | Powered from the 3500 rack backplane |
| Operating Temperature | -30 °C to +65 °C (-22 °F to +149 °F) |
| Dimensions (H x W x D) | Approx. 193 mm x 57 mm x 200 mm (7.6 in x 2.25 in x 7.87 in) Standard 3500 module frame size |
| Weight | Approx. 0.7 kg (1.54 lbs) |
| Mounting | Slides into a slot in a 3500 series rack |
Application Scenarios
The 3500/25 Keyphasor Module is indispensable in any scenario requiring detailed understanding of rotor dynamics and is a mandatory component for most machinery protection systems on turbomachinery. Its primary applications include:
Phase Reference for Vibration Analysis:
Orbit Plots: The Keyphasor pulse is used to create orbit plots from XY displacement probes, showing the shape and direction of the shaft's motion within the bearing clearance. The pulse provides the "dot" on the orbit, indicating the shaft's orientation.
Synchronous Vibration: It allows other monitor modules to filter and display the vibration component that is exactly synchronous (1X) with running speed, which is crucial for identifying unbalance.
Rotor Speed Measurement and Overspeed Protection:
Speed Indication: Provides a reliable and accurate measurement of rotor speed (RPM) to the control system.
Overspeed Detection: Can be integrated into the system's logic to trigger an emergency shutdown if the rotor speed exceeds a pre-defined safe limit, protecting the machine from catastrophic failure due to overspeed.
Startup and Coast-down Analysis:
Critical Speed Identification: During machine startup and shutdown, the Keyphasor data is used with vibration data to identify and plot the Bode and Polar plots, which are essential for determining the rotor's critical speeds and its vibration response through these speeds.
Balancing Operations:
Field Balancing: The phase angle provided by the Keyphasor is the single most important parameter for performing in-field rotor balancing. It tells technicians exactly where to place balance weights on the rotor to correct unbalance.
Typical Machinery:
Steam Turbines & Gas Turbines
Centrifugal & Axial Compressors
Large Pumps & Fans
Generators & Electric Motors
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