Large rotating machines such as steam turbines, gas turbines, and centrifugal compressors are used as critical equipment in various plants. These machines are equipped with vibration sensors and condition monitoring equipment, also known as “monitors” or “monitor systems”, to monitor their condition during operation and to protect the machines when abnormal vibrations occur. In addition, a permanent or temporary vibration analysis system is used to investigate the cause of abnormalities.

　This column provides an overview of systems applied to condition monitoring, vibration analysis, and diagnosis of rotating machinery.

1. Outline of Condition Monitoring System for Large Rotating Machinery

　Non-contact displacement transducers (proximity transducers) are typically installed on large rotating machines to detect variations in dynamic displacement due to shaft vibration, which is the swing of the rotating shaft itself, and the transducer’s output signals (waveform signals) are converted into vibration amplitude by the vibration monitors. If the vibration amplitude exceeds the alarm threshold, the alarm relay is activated. The monitor is equipped with two types of alarm relay: one for Alarm (Alert) and another for Shutdown (Danger). Up to here, the scope is the displacement transducers and monitors shown in the lower part of Figure 2-1. Furthermore, the vibration analysis and diagnostic system can be constructed by taking the buffered outputs of the vibration monitors into the external analysis data acquisition units or by using the vibration monitors with built-in analysis boards. The buffer output is an unaltered analog replica of the transducer input signal that outputs from the rear terminal block and front BNC connector of the monitor. It is designed to prevent a short circuit of this output to monitor system ground from affecting the operation of the system.

　In some existing plants, the permanent vibration analysis and diagnosis system is not installed, but only the transducers and the vibration monitors are installed. In such cases, the portable vibration analysis system is employed to collect transient data during machine startup and shutdown, and to perform analysis and diagnosis of abnormal vibrations.

2. Condition Monitoring of Large Rotating Machinery and API Standards

　Shaft vibration measurement is an essential item of condition monitoring and vibration diagnosis for large rotating machines supported by journal bearings, such as steam turbines and centrifugal compressors. In such applications, non-contact displacement sensors (proximity sensors) are employed to detect and measure the behavior of the rotating shaft, as previously described. Non-contact displacement sensors include optical and capacitance sensors. However, these cannot be used in environments where machine oil splashes.

　Eddy current type displacement sensors, which are unaffected by such environments, are used instead. Figure 2-2 shows examples of the eddy current type displacement transducers. Displacement transducers for shaft vibration measurements generally use the eddy current type sensors that comply with API standard 670 ⁽¹⁾. These sensors are mounted to the bearing using mounting brackets or to the machine casing using sleeves. This allows for the measurement of relative vibration between the shaft and bearing or casing. The sensor setting gap is typically 1.2 mm or so.

　The API 670 is an American Petroleum Institute standard that provides detailed requirements for monitoring and protection equipment for critical rotating machinery in petroleum refining and petrochemical plants. The most recent version of this standard is currently the 5th edition, published in November 2014. As indicated by the title of Table 2-1, this standard was originally intended for non-contact shaft radial vibration and shaft axial position monitoring systems. Since the 2nd edition, casing vibration and bearing temperatures have also been included. The 4th edition incorporated overspeed detection and reciprocating compressor piston rod drop monitoring as critical machine protection parameters, prompting a title change to “Machinery Protection Systems.”

Table 2-1: Changes in the title of API Standard 670

Edition	Title	Publication
1st	Non-contacting Vibration and Axial Position Monitoring System	1976/06
2nd	Vibration, Axial-Position, and Bearing-Temperature Monitoring Systems	1986/06
3rd		1993/11
4th	Machinery Protection Systems	2000/12
5th		2014/11
6th	Under development as of 2025/03/31

　API 670 specifies detailed requirements for shaft vibration and axial position monitoring systems, which are critical monitoring parameters for large rotating machinery. This standard is widely applied for oil-related plants around the world. Additionally, sensors and monitors that are compliant with API 670 are utilized as turbine supervisory instruments (TSI) for power generation turbines in power plants.

3. Sensors for Vibration Measurements

　When measuring the vibration of rotating machinery, the measurement parameters are vibration displacement, velocity and acceleration, and one of these parameters is applied depending on the application, machine type, and the measuring point. For high speed rotating machines operating above the first critical speed and classified as flexible rotors supported by journal bearings, it is important to understand the dynamic behavior of the rotating shaft, both for the usual condition monitoring and for vibration analysis, therefore, as mentioned in the previous chapter, eddy current non-contact displacement sensors are commonly used to measure shaft vibration in displacement.

　Since eddy current displacement transducers respond from a stationary distance (DC component) to high frequencies, they are used not only for shaft vibration measurement, but also for axial position measurement, rotational speed measurement and phase reference detection.

　Table 2-2 lists the major parts of the requirements for non-contact displacement transducers described in API 670. In this standard, the sensor portion of a non-contact displacement transducer is referred to as a “proximity probe” and the transducer or driver is referred to as an “oscillator-demodulator”. Table 2-2 also uses this terminology.

Table 2-2: API 670 Requirements for Non-contact Displacement Transducer

Item	Requirements described in API 670 5th edition
Incremental Scale Factor	7.87 mV/μm ±5% at testing temperature
	7.87 mV/μm ±10% at operating temperature
Deviation from Straight Line	Within ±25.4 μm at testing temperature
	Within ±76 μm at operating temperature
Linear range	2 mm
Operating temperature	Proximity probe	: -35℃ to +120℃
	Extension cable	: -35℃ to +65℃
	Oscillator-demodulator	: -35℃ to +65℃
Humidity	100% RH (non-submerged, with protection of connectors)
Standard target material	AISI 4140 steel (JIS SCM440)
Probe tip diameter	Standard	: 7.6 mm to 8.3 mm
	Option	: 4.8 mm to 5.3 mm
Thread of probe body	Standard	: 3/8-24UNF Reverse mount
	Option	: 3/8-24UNF, 1/4-28UNF, M10×1, M8×1
Probe cable length	Standard	: 0.5 m (0.5 to 1 m)
	Option	: 1 m (0.8 to 1.3 m)
Extension cable length	Standard	: 4.5 m (min. 4.1 m)
	Option	: 4 m (min. 3.6 m)
Power supply	-24VDC

　The incremental scale factor (ISF) error indicates the sensitivity error for each 250 µm gap change relative to the standard sensitivity of 7.87 mV/µm and is related especially to the radial shaft vibration measurement error.
　In other words, [vibration measurement error] = [measurement value] × [ISF error].
For example, if the ISF error is ±5%, the error for a vibration measurement value of 100 µm p-p is ±5 µm p-p, and the error for a vibration measurement value of 40 µm p-p is ±2 µm p-p.

　Many commercially available non-contact displacement transducers that meet the requirements of Table 2-1 and conform to the API 670 have a wider operating temperature range than the standard requires. In addition, the standard specifies only a 5-meter system for the total system cable length (probe cable length + extension cable length), but commercially available products include not only 5-meter systems but also 9-meter systems to expand application.

4. Installation of Shaft Vibration Sensor

　Both the API 670 and ISO 20816-1 ⁽²⁾ standards provide the criteria for the installation of shaft vibration sensors using non-contact displacement transducers on machines. Figure 2-3 shows an image of the installation of the shaft vibration sensors based on the requirements of the API 670, which is more detailed. The shaft vibration sensor is installed close to the radial bearing, and normally two sensors are installed at each measurement position, forming angles of 90 degrees with each other in a plane perpendicular to the rotor axial direction. However, as shown in the Note in Table 2-3, the ISO 20816-1 standard states that “A single probe may be used at each measurement plane if it is known to provide adequate information about the shaft vibration”. However, in actual shaft vibration, the “orbit” which is the dynamic locus of the center of the rotating shaft, does not always show a perfect circle, and depending on the anisotropy of the bearing rigidity, the operating state, or abnormal state, it may show an ellipse or even a more complex locus. Therefore, when using “single-sensor measurement”, it is not possible to detect the maximum amplitude of the actual vibration, and there is a possibility that the vibration amplitude will be underestimated. Installing sensors in two directions at a 90-degree angle not only avoids the risk of underestimating the vibration amplitude value, but is also useful for vibration analysis, which will be discussed later, and is usually recommended.

Table 2-3: Installation rules for shaft vibration probes

Item	API 670	ISO 20816-1
Number of probes at each measurement plane	Two probes	Two probes (See Note)
Angle between paired probes	90°±5°	90°±5°
Perpendicularity to shaft axis	±5°	±5°
Mounting angle from vertical center	45°±5°	Not specified
Distance from bearing	Within 75 mm	Should be located at, or adjacent to, each bearing
Note: A single probe may be used at each measurement plane if it is known to provide adequate information about the shaft vibration.

In the subsequent column, an overview of the “Monitor System” and the “Vibration Analysis and Diagnosis System” will be provided.

Bibliography
(1) API Standard 670, Fifth edition, Machinery Protection Systems, (2014)
(2) ISO 20816-1:2016, Mechanical vibration — Measurement and evaluation of machine vibration — Part 1: General guidelines