Condition Monitoring based on Acoustic Emission

Condition Monitoring Based on Acoustic Emission

Trevor Holroyd

When we talk about machinery condition monitoring, we’re talking about condition in the sense of a machines ability to continue to perform its intended function in an efficient manner. Unfortunately there is no way to directly measure machine condition and so it is necessary to infer it from indirect measurements. For Condition Monitoring (CM) purposes a range of technologies are available, each having its own strengths and weaknesses, and it is usual to consider each of them as a tool in the CM toolkit. The Acoustic Emission (AE) technique has a 40 year history of use for machinery condition monitoring and although it got off to a slow start, in recent years it has gained very widespread acceptance across industry. The particular strength of Acoustic Emission is its ability to directly detect the processes associated with wear and degradation (including friction, impacts, crushing, cracking, turbulence, etc,). It does this by detecting the surface component of stress waves that these processes invariably generate. These stress waves travel all over the machines surface, which means that sensor positioning is not critical.

Another convenient feature of Acoustic Emission signals is that they generally have a high Signal to Noise Ratio (SNR), which means that the signals related to machine condition can be clearly seen and are not buried in other inconsequential signals. This is a direct consequence of the high frequency and resonant nature of an Acoustic Emission sensors response. An example of an application that makes full use of this high SNR is the monitoring of blowers in a blower house. This application environment involves very high noise and vibration levels with adjacent identical blowers cutting in and out in response to process demands without warning. Despite this, clear trends of processed Acoustic Emission signals have successfully identified a number of faults including defective motor coupling (see Figure below showing good and bad examples) and build up of lime on the rotors enabling wash initiation and avoidance of rotor collision. The high and variable background noise together with the problems of cross-talk and sympathetic resonance from adjacent blowers prevents the successful use of Vibration techniques in this application.

Acoustic Emission sensors, signals and technology have little in common with those of the Vibration technique and this has many positive consequences in practical terms. For example, with Acoustic Emission it isn’t a pre-requisite to perform a frequency analysis (or FFT) and interpret the signal levels at all possible defect frequencies before determining whether or not there is a fault. In fact, with Acoustic Emission the usual measurement sequence is the logical and time saving one of: (a) instantly alert to the presence of a fault on a machine without knowledge of machine details (such as bearing type or number),(b) only look at the trend of readings to reveal the rate of degradation on those machines with a fault flagged,(c) only carry out an FFT if diagnostic information is required of a fault condition.

Of course, such a radical departure from the norm was initially met with a fair degree of scepticism along the lines of ‘It can’t be that easy otherwise why would people use Vibration?’ A fair point in its time! The only answer was to relentlessly demonstrate instant fault detection on the shop floors of those who were interested but doubtful. A not untypical example of readings from such a demonstration is shown below for one-off measurements taken with a portable Acoustic Emission based CM instrument (MHC type) on the various white metal bearings of a generating set in a power station. (Note: for this instrument the common interpretation for all types of machines, bearings and rotational speeds down to 35 rpm is that an item is suspect if Distress® is 10 or more.)

Prior to these measurements being taken, the client believed that the main exciter was as good as new! However upon seeing the readings the client explained that the exciter had previously shown problems of arcing in the bearings and the resulting surface damage had been blended out prior to the bearing being returned to service. The high SNR which Acoustic Emission signals enjoy has other consequence too, as will now be described:

Measurement Point	Distress®
HP Rotor DE	03
HP Rotor NDE	04
IP Rotor DE	05
IP Rotor NDE	03
LP1 Rotor DE	04
LP1 Rotor NDE	03
LP2 Rotor DE	03
LP2 Rotor NDE	03
Generator Rotor DE	04
Generator Rotor NDE	03
Exciter DE	19
Exciter NDE	16
Pilot Exciter	07

Very Slowly Rotating Machinery

At slow rotational speeds the rate at which Acoustic Emission signals are generated reduces, even becoming infrequent at the lowest speeds. However, this does not necessarily pose a problem provided signals are processed accordingly since Acoustic Emission sensors are insensitive to the low frequency sounds and vibrations which are ever present on the shop floor. Commercially available AE instrumentation is able to easily monitor down to 0.25 rpm without requiring special expertise.Non-repetitive or random fault types Because Acoustic Emission signals can be simply analysed directly in the time domain, without recourse to frequency analysis, they are equally sensitive to non-repetitive signals. One example of this has been the detection of carbonisation of oil in a high temperature white metal bearing where the small particles of carbon were crushed in the bearing as they randomly formed. Another example has been the detection of sporadic air bubbles in the oil flowing through a plain bearing.

Reciprocating Machinery

It is normal to get load reversals, valve openings/closures, sliding actions and gas transfers during the operation of reciprocating machinery and each of these has an associated Acoustic Emission signature. A full interpretation of the Acoustic Emission signal requires knowledge of the timing sequence of all these actions but it can also be useful to simply observe occasional or persistent variations from the norm. This is illustrated in the Figure below which shows the maximum, minimum and mean values of the Acoustic Emission envelope signal as a function of time within the cycle of a diesel engine over 12 successive engine cycles. Comparison of the minimum and mean waveforms clearly indicates an occasional drop out of part of the operating process (thought to be due to a sticking valve) which is only detectable because of the high SNR of Acoustic Emission signals – a low SNR would have required averaging to reveal the waveform in the first place thus masking exceptional occurrences.

Intermittent Actions

Similar comments to those for reciprocating machinery apply to intermittent actions. For a single action the high SNR of Acoustic Emission signals is particularly relevant in allowing the signals to be directly analysed without the need for averaging of multiple actions. The high SNR and the resulting suitability of time domain signal processing has had a positive impact on simplifying Acoustic Emission instrumentation. In particular for rotating machinery it has allowed generic time domain signal processing algorithms to be developed which are quick and easy to use. Initially this led to the creation of very simple yet effective portable instruments but, more recently, these algorithms have been incorporated into the AE sensor itself (see example shown below). The importance of this development is that it eases the way for the utilisation of Acoustic Emission sensors with other technology sensors (such as rms vibration and temperature) in shop floor installations as it removes the need for a separate Acoustic Emission instrument. With increased understanding of the fundamentals underpinning the use of AE for CM and the overwhelming evidence of its benefits to industry, it is not surprising that AE has gradually moved centre stage over the years. In addition to having its own standard ISO 22096 (Condition monitoring and diagnostics of machines – Acoustic Emission) it is specifically listed in the CM standard ISO 18436 (Condition monitoring and diagnostics of machines – Requirements for qualification and assessment of personnel) as one of the four main CM techniques, together with Infra-red Thermography, Lubrication Management & Analysis and Vibration Analysis. Alongside these developments the British Institute of Non-Destructive Testing as a UKAS accredited, third party certifying body has recently created a certification programme of training and examinations that conforms to the ISO 18436 international standard.

Principles of Insulation Testing

Probably 80% of all testing. performed in electrical power systems is related to the verification of insulation quality. This Cadick Corporation Technical Bulletin briefly describes the fundamental concepts of insulation testing including – insulation behavior, types of tests, and some test procedures. For more detailed information, refer to the bibliography at the end of the paper.

Top Tips for Selecting Pressure Measurement Transmitters

When selecting pressure measurement transmitters, the first stage is whether to opt for a transducer or a transmitter. Although the terms are often confused, there are several differences between transducer and transmitter devices. A transducer creates a low-level electronic signal in response to changes in applied or differential pressure. As with transmitters, transducers feature an internal sensor that converts the applied force into an electrical signal, from which the measurement is derived.

Gearbox Diagnostics Fault Detection

The paper deals with the method of gearbox diagnostics fault detection, and shows that using: design, production technology, operational, change of condition (DPTOCC) factors analysis leads to the inference of gearbox diagnostic information. In the paper is a review of the current possibilities for using mathematical modeling and computer simulation for investigating the dynamic properties of gearbox systems. Computer simulation of dynamic behavior of gearboxes is a powerful tool for supporting gearbox diagnostic inference.

What is Vibration?

In its simplest form, vibration can be considered to be the oscillation or repetitive motion of an object around an equilibrium position. The equilibrium position is the position the object will attain when the force acting on it is zero. This type of vibration is called "whole body motion", meaning that all parts of the body are moving together in the same direction at any point in time.

Why Record? Infrared Video

As the infrared industry continues to develop, documentation will become more important and required. Infrared video recording and data logging creates a database, making the thermographer and the end user accountable for all items surveyed, thereby reducing the liability and improving the quality for both the end user and infrared thermographer.

What You Should Know Before you Buy – A Guide to Buying an Infrared Camera

During Academy of Infrared Training courses, we see many students who have purchased an infrared camera that is not suited for their intended use. These students have put their trust in a camera salesperson, and, many times, that salesperson was solely interested making a sale. Later, these students are disappointed to find out that the camera is not truly suited for their application. This article aims to help the camera buyer understand what basic camera specifications mean, and also help them determine what type of camera and options are suitable for their application. If you note your answers to questions throughout this article, by the end you should have a custom list of applicable specifications and options.

Vibration & Ultrasound Technologies: A Possible Integrated Inspection Tool?

The purpose of this paper is to introduce condition monitoring and reliability engineers to the principles of using ultrasound for the assessment of machine condition. Ultrasound can be a complimentary technology to vibration, thermography and lubrication monitoring. It must be emphasized that it is rarely successful as a stand alone technology for effective machine condition assessment and subsequent required maintenance planning. This paper concentrates on the use of airborne ultrasound as a complementary technique particularly for machinery that may be inaccessible due to guards or hazardous locations.

Condition Monitoring based on Acoustic Emission