Vibration monitoring and predictive maintenance for compressors and turbines: sensors, API 670, retrofit

Vibration monitoring on compressors and turbines does two jobs. Machinery protection watches shaft vibration, axial position and speed continuously and trips the machine through a relay when a set limit is crossed; condition monitoring keeps the same signals, adds spectra and phase, and trends them so that maintenance can be planned before a fault grows into a trip. On monitors such as Rockwell's Dynamix 1444 and Bently Nevada's 3500 one system does both, so replacing an old monitor touches the shutdown logic, the control system and the maintenance team at once.

Maintenance strategies: what triggers the work

The US Federal Energy Management Program (FEMP) O&M Best Practices Guide, Release 3.0 (2010), describes four approaches:

Predictive work follows the machine's actual condition, not a preset schedule. Wikipedia treats predictive maintenance as condition-based maintenance timed from an estimate of how far the item has degraded, with monitoring done periodically (offline) or continuously (online). The two terms overlap, so the figure keeps them side by side.

Vibration analysis is one of several predictive technologies, alongside infrared, acoustic and oil analysis (Wikipedia). FEMP lists faults it helps diagnose on rotating equipment, such as unbalance, misalignment, rotor rub and bearing problems, and warns that signature analysis needs people trained in both the technique and the machine.

Machinery protection and condition monitoring

Bently Nevada describes its 3500 system as protecting machines by tripping them to prevent damage, and, with its System 1 software, as supplying condition monitoring information for a proactive maintenance programme. Emerson draws the line by the speed of the failure: protection to API 670 is meant for fast, dangerous failures, such as an impending bearing failure or an overspeed after a coupling failure, while predictive monitoring suits degradation that develops over hours, days or months. Emerson adds that protection is usually kept for large, expensive machines.

Standards: API 670 and the ISO 20816 series

API Standard 670, Machinery Protection Systems, sets minimum requirements for a machinery protection system: hardware, installation, documentation and testing. Emerson describes it as a purchasing specification for any rotating machinery, covering radial shaft vibration, casing vibration, shaft axial position, speed and critical temperatures, and notes that products cannot be certified to it: applying it is the end user's decision. Emerson's page, updated in 2025, gives the current edition as the sixth, released on 7 July 2025. Rockwell states that Dynamix 1444 is designed in accordance with the relevant sections of the 5th edition and that compliance depends on the components supplied, the options required and the installed configuration. A specification should name the edition and the options it wants.

ISO 20816, Mechanical vibration — Measurement and evaluation of machine vibration:

• ISO 20816-1:2016, General guidelines. Procedures for measuring and evaluating vibration in operational monitoring and acceptance tests, with guidance on setting operational limits. It covers absolute and relative radial shaft vibration but excludes axial shaft vibration and torsional vibration.
• ISO 20816-2:2017 (Amendment 1:2024). Land-based gas turbines, steam turbines and generators above 40 MW with fluid-film bearings and rated speeds of 1 500, 1 800, 3 000 or 3 600 r/min.
• ISO 20816-3:2022. Industrial machinery above 15 kW running between 120 and 30 000 r/min, including rotary compressors, steam turbines and generators up to 40 MW and industrial gas turbines up to 3 MW. Rotary positive displacement compressors such as screw compressors are outside its scope, and it does not cover diagnosis of gear or rolling-element bearing condition. ISO lists ISO 10816-3:2009 as withdrawn and revised by this part, so older specifications citing ISO 10816-3 point to a withdrawn document.

ISO 17359:2018 gives general guidelines for a condition monitoring programme. This article gives no zone values or limits: they belong to the standards and the machine maker's documents.

Sensors on a compressor or turbine train

Proximity probes (eddy current probes). The probe's alternating electromagnetic field induces small eddy currents in the shaft surface, and the driver, which Bently Nevada calls a Proximitor, outputs a voltage proportional to the distance from probe tip to shaft. Mounted in the bearing housing, the probe sees the shaft moving inside its bearing: it measures shaft vibration relative to the bearing, and the same signal carries the average shaft position. The gap is set in the middle of the linear range. Bently Nevada uses these probes for rotor vibration and position on journal (fluid-film) bearing machines.

• X/Y pair. Two probes per bearing, 90° apart and ±45° from vertical, clear of the bearing split line. Together they show the orbit, the path of the shaft centreline.
• Axial (thrust) position. Probes mounted axially measure the shaft's axial position, one of the measurements API 670 lists.
• Phase reference. A probe facing a notch on the shaft gives one pulse per revolution, the reference for phase, speed and timing; Bently Nevada calls it the Keyphasor.

Casing sensors. Accelerometers and velocity sensors on the bearing housing measure casing vibration; Bently Nevada applies them to rolling-element bearing machines. Before proximity probes, velocity sensors on bearing casings were used to measure casing vibration. Amplitude can be expressed as displacement, velocity or acceleration; FEMP says the best one depends on the machine's speed and design. Rockwell's own sensor lines are the 1442 eddy current probe systems and the 1443 accelerometers.

What the monitor measures

• Overall level. One broadband value per channel, such as RMS or peak; ISO 20816 evaluates broadband vibration.
• Spectrum (FFT). The signal split into frequencies, so that a trained analyst can trace the vibration to a component (FEMP).
• 1X amplitude and phase, orders. Vibration at running speed and its multiples, with phase measured against the once-per-turn reference.
• Position. Thrust position, eccentricity, differential expansion and probe gap, the static part of the probe signal.
• Speed, from the phase reference or a tachometer input.
• Transient data. Dynamix 1444 keeps transient and alarm buffers in the module for review after an event.

Rockwell Dynamix 1444, the XM series and Bently Nevada 3500

Dynamix 1444 is described by Rockwell as a general-purpose monitor for machinery protection and condition monitoring. The main module, 1444-DYN04-01RA, has four measurement channels, two tachometer inputs and one relay, and accepts eddy current probe systems, ICP accelerometers, velocity sensors and voltage signals. Expansion modules add a tachometer signal conditioner, four relays per relay module and 4–20 mA outputs. The module is an EtherNet/IP adapter with two ports for linear, star or Device Level Ring (DLR) networks; Rockwell recommends DLR for machine protection. The module cannot be the ring supervisor, so the ring needs a controller with DLR capability or a 1783-ETAP tap to take that role. Setup is in Studio 5000 Logix Designer through the module's Add-On Profile.

XM series (Bulletin 1440). The current technical data (1440-TD001H, 12/2025) covers the XM-124 two-channel module on DeviceNet with 4–20 mA outputs and relays, the XM-220 speed module, the XM-441 and XM-442 relay modules and the XM DYN module, which connects to ControlLogix through a ControlNet adapter. Rockwell writes that the XM-124 brings together most of the functions of the XM-120 to XM-123 and offers basic single-channel thrust measurement like the XM-320.

Lifecycle status on Rockwell's pages, read 2 November 2026:

Rockwell's product pages carry a general notice that their information may be incomplete or inaccurate, so check each catalogue number in Rockwell's Product Lifecycle Status search when ordering. Emonitor (Bulletin 9309) is Rockwell's condition monitoring software for trending, plotting and alarming, working with 1444 and 1440 monitors and Dynamix 2500 portable data collectors.

Bently Nevada 3500 is a rack-based system for continuous online monitoring, with relay modules offering configurable AND/OR logic, gateways for Modbus and Ethernet communication, and a transient data interface to System 1. ASME's history dates the 3300 monitors to 1988 and the 3500 to 1994.

Connecting the monitor to the control system

A monitor has two outputs, and they should be designed and tested separately.

1. Trip path. In all three product lines the protective action leaves the monitor as a relay contact wired to the machine's shutdown logic. Rockwell also describes the 1444 relay module raising an alarm or putting the machine in a safe state if the main module stops responding correctly.
2. Data path. Values, alarm states and module health go to the PLC or DCS: EtherNet/IP for Dynamix 1444, DeviceNet or ControlNet for XM, Modbus gateways for the 3500, or hardwired 4–20 mA outputs. From there they reach the HMI, the alarm system and the historian. Spectra and waveforms go to the condition monitoring software (Emonitor, System 1).

Vibration alarms join the plant alarm system and deserve the same rationalisation as other alarms (ISA-18.2). For the ring network see DLR or PRP; a networked monitor also belongs in PLC and OT security hardening.

Retrofitting an old monitor: a checklist

TDL's vibration monitoring service replaces obsolete Bently Nevada 3300 monitors with Rockwell Automation modules, connected to the existing sensors where they remain in use, with Emonitor and integration with the plant control system and HMI, onshore and offshore. Project pages: at Dinh Co GPP (2015), XM modules on DeviceNet, vibration sensors and Emonitor for gas compressors K-1011A/B/C/D; at the Thanh Thanh Cong Tay Ninh sugar plant (2016), the Bently Nevada 3300 on two turbine units replaced by XM modules on DeviceNet and Emonitor; on the Vietsovpetro LP compressor (2018), Siemens S5 control and Bently Nevada 3300 monitoring replaced by ControlLogix, PanelView HMI and Rockwell Automation vibration monitoring.

Points to settle before the shutdown window:

• Channel list. Machine, bearing, sensor type and model, X, Y, axial or phase reference, monitor channel, alarm and trip setpoints, delays, relay. Take setpoints from the machine maker's documents and the old monitor; agree any change with the machine owner.
• Sensors to reuse. Compare each probe system (probe, extension cable, driver) and casing sensor with the inputs in the new monitor's technical data. Record gap voltages and overall levels before shutdown as a baseline.
• Trip logic. For each trip relay: the alarm that drives it, any voting between channels, its normal energised or de-energised state and the shutdown input it lands on. Write the bypass procedure for the cutover.
• Network. Protocol, controller support (a DLR ring needs a supervisor), tags for HMI and historian.
• Site. Hazardous area classification and panel location against the approvals in the module technical data.
• Lifecycle. Status of every new catalogue number on the day of the order.
• Testing and handover. Simulate each channel from the sensor end, prove every relay at the shutdown input, record as-left setpoints, and back up the monitor configuration with the controller backups (AssetCentre automatic backup). Decide who reads the spectra after handover.

See also breakdown support and maintenance and the preventive maintenance checklist.