Introduction

In this column I will discuss condition monitoring for large rotating machinery that use journal bearings, such as turbines and compressors. We will also review industry specific terminology and related standards.

1. Condition Monitoring of Large Rotating Machinery and API Standards

Photograph 1. FK Series Non-contact displacement / vibration transducer

The measurement of shaft vibration is an essential part of condition monitoring and vibration diagnosis for large rotating machinery. Large rotating machinery supported by journal bearings, such as steam turbines and centrifugal compressors, are two examples where non-contact vibration sensors are used to measure rotating shaft behavior.

The performance of eddy-current sensors is not affected in the presence of machine lubricants; whereas, optical or capacitance type non-contact sensors may be affected. As a result, we recommend using an API 670 compliant eddy-current displacement sensor to measure shaft vibration.

The American Petroleum Institute standard 670, commonly referred to as API 670, provides detailed requirements for equipment that may be used to monitor and protect critical rotating machinery. This standard is used by customers operating oil refineries, petrochemical plants and other industrial operations. In its fifth and most recent edition, the standard has made considerable changes. The table below outlines the previous four editions.

As table shows, the standard was originally for non-contact systems to monitor shaft vibration and axial position, but later the second and third edition added monitoring for casing vibration and bearing temperature. In its fourth edition, rotational speed and over-speed detection were added, and the title was changed to Machine Protection Systems. The new title reflected a broader coverage of standards, which now included piston rod drop for reciprocating compressors.

While API 670 is an American standard, it is commonly used in petroleum plants around the world due to the lack of a comparable alternative. Interestingly, it is not intended for the commercial power generation turbine industry. However, since shaft vibration and axial position are common measurement parameters to the petroleum and power generation industry, sensors and monitors that confirm to API 670 are used for Turbine Supervisory Instruments (TSI). In the next article, I will explain the principles of an API 670 compliant eddy-current displacement sensor.