API 670 Turbine Vibration Protection System: What It Includes

An API 670 vibration protection system continuously monitors radial shaft vibration, casing vibration, rotor axial position, speed, overspeed, phase reference and critical temperatures. It consists of proximity and seismic transducers, a monitor rack with Alert and Danger setpoints, relay modules with voting logic that issue the trip command, and redundant power.

What API 670 is and where our data comes from

API 670 "Machinery Protection Systems" is an American Petroleum Institute standard. The current edition is the 5th edition of 2014 (256 pages), reaffirmed in 2022: API STD 670:2014(R2022). The standard is paid, so we do not quote it. Everything below is based on the public abstract of the standard and on Bently Nevada documentation, which states which API 670 requirements the equipment meets.

According to the abstract, API 670 describes the minimum requirements for a machinery protection system measuring:

Parameter	Typically measured with
Radial shaft vibration	Proximity probes
Casing vibration	Accelerometers, velocity sensors
Shaft axial position	Proximity probes
Shaft rotational speed	Proximity probes, magnetic pickups
Piston rod drop (reciprocating compressors)	Not covered in this article
Phase reference (Keyphasor)	Proximity probe
Overspeed	A separate system with redundancy
Critical temperatures (bearing metal, motor windings)	RTDs, thermocouples

The "Typically measured with" column is compiled from Bently Nevada transducer and monitor documentation, not quoted from the standard.

Typical system architecture

1. Transducers on the machine. Proximity probes for radial vibration and axial position, a phase reference probe, bearing temperature sensors and, where needed, accelerometers or velocity sensors on the casing.
2. Monitor rack. Modules compare measured values with Alert and Danger setpoints and check the health of every transducer and its wiring.
3. Relay modules. Relays actuate by configured logic: for example, a trip only on Danger from two channels at once.
4. Trip output. Relay contacts are wired into the machine's emergency shutdown circuit.
5. Annunciation and diagnostics. Analog and digital outputs go to the DCS for display and trending, data goes to the diagnostic system.

How many probes per bearing

For radial vibration, each bearing usually has two proximity probes at 90° (an X/Y pair). Fabrico describes this arrangement as typical. 3500 monitors are programmed in channel pairs: channels 1 and 2 perform one function, channels 3 and 4 the same or another.

What the Bently Nevada 3500 does

According to the Bently Nevada datasheet, the 3500 System provides continuous online monitoring suitable for machinery protection and is designed to meet API 670 requirements. It is intended for auto-shutdown protection applications that require extremely high reliability and availability. Listed applications include industrial gas and steam turbines, aeroderivative gas turbines, hydro turbines, compressors, turbo expanders, electric motors and generators, gearboxes, pumps and fans.

Rack components

Component	Required	Function
3500/05 rack	Yes	19" rack: 14 module slots plus power supply and TDI positions. The 12" mini rack has 7 slots
3500/15 power supply	Yes, one or two	Each supply can power the whole rack. With two, the lower is primary and the upper is backup, taking over without interruption
3500/22M TDI	Yes	Interface to configuration software and System 1. Not part of the critical protection path
3500/XX monitors	Yes, one or more	Measurement, Alert and Danger setpoints, transducer checks
3500/32M (4 relays) or 3500/33 (16 relays)	No, but strongly recommended	Trip relays with voting logic
3500/25 Keyphasor	No, one or two	Phase reference signal for the monitors
3500/92 communication gateway	No	Link to the DCS over industrial protocols
I/O modules, configuration software	Yes	Field wiring connection, setup

Four-channel monitors allow a full 14-slot rack to hold up to 56 channels.

Which modules cover the API 670 items

API 670 parameter	3500 module	Per the datasheet
Radial vibration, casing vibration	3500/42M	4 channels; proximity and seismic inputs; radial vibration, thrust position, eccentricity, acceleration, velocity, Shaft Absolute and more
Axial position, differential expansion	3500/45	4 channels; thrust position, several differential expansion schemes, case expansion, valve position
Phase reference	3500/25	2 channels per module; up to 4 Keyphasor signals per rack (up to 8 paired); monitors use it for speed, 1X amplitude and phase
Speed	3500/50M	2 channels; speed, rotor acceleration, zero speed, reverse rotation
Bearing temperatures	3500/60	6 channels; RTDs and thermocouples
Trip output	3500/32M	4 relays; AND/OR logic on Alert, Danger and Not OK statuses of any rack channels

Overspeed protection is a separate task

The 3500/50M tachometer module measures speed, but Bently Nevada warns plainly that it must not be used on its own or as part of a speed control or overspeed protection system, because it lacks the required redundancy and response speed. Speed setpoints on the 3500/50M are for annunciation only. For overspeed protection the manufacturer points to the 3701/55 Emergency Shutdown system.

Setpoints and alarm delays

Using the 3500/42M datasheet as an example:

Parameter	Value
Setpoints	Alert for each measured value, Danger for any two of them; 0 to 100% of full scale
Setpoint accuracy	within 0.13% of the desired value
Alert delay	1 to 60 s in 1 s steps
Danger delay	0.1 s or 1 to 60 s in 0.5 s steps

The delay prevents a short signal spike from tripping the machine. Its value is chosen for the specific machine.

What not to do

1. Wire the trip through 4–20 mA or Modbus. According to the 3500 System datasheet, these outputs are for annunciation and trending only: they lack the fault tolerance and integrity required. Automatic shutdown uses the 3500/32M or 3500/33 relay modules.
2. Use the Rack OK relay for shutdown. Dozens of different rack events drive it to NOT OK, so Bently Nevada recommends the Rack OK relay for general annunciation only.
3. Protect against overspeed with the 3500/50M. See above.
4. Fit proximity probes on too small a shaft. For the 3300 XL 8 mm, the minimum shaft diameter is 50.8 mm and the recommended minimum is 76.2 mm; on shafts smaller than 76.2 mm the scale factor generally changes.

API 670 transducers

According to the Bently Nevada datasheet, the standard 5-metre 3300 XL 8 mm system fully complies with API 670 for mechanical configuration, linear range, accuracy and temperature stability. Its sensitivity is 7.87 V/mm (200 mV/mil) and the linear range is 2 mm. 3300 XL 8 mm components are interchangeable without matching or bench calibration. An application on large fans is described in 3300 XL proximity probes on large fans.

For auxiliary equipment (pumps, draft fans, mills), a separate rack with 3500/42M monitors is often enough. How it differs from main turbine-generator protection is covered in CHP auxiliary equipment vibration monitoring: the 3500/42M module.

What to choose for a turbine-generator

• Rack and power — 3500/05 and two 3500/15 supplies for redundancy.
• Radial vibration — 3500/42M monitors and 3300 XL probes.
• Axial position and differential expansion — 3500/45.
• Phase reference — 3500/25.
• Bearing temperatures — 3500/60.
• Trip relays — 3500/32M.

KEG TRK supplies Bently Nevada equipment in Kazakhstan. Price on request.

Sources

• American Petroleum Institute. API 670:2014 Machinery Protection Systems: abstract, edition, status (standards shop page).
• Bently Nevada. 3500 System Datasheet 162096.
• Bently Nevada. 3500/42M Proximitor Seismic Monitor Datasheet 143694.
• Bently Nevada. 3500/45 Position Monitor Datasheet 141537.
• Bently Nevada. 3500/25 Enhanced Keyphasor Module Datasheet 141532.
• Bently Nevada. 3500/32M 4-Channel Relay Module Datasheet 141533.
• Bently Nevada. 3500/50M Tachometer Module Datasheet 141538.
• Bently Nevada. 3500/60 and 3500/61 Temperature Modules Datasheet 141540.
• Bently Nevada. 3300 XL 8 mm Proximity Transducer System Datasheet 141194.
• Fabrico. API 670 machinery protection: typical X/Y probe arrangement.