Condition Monitoring

Condition Monitoring

Condition monitoring is the process of monitoring a parameter of condition in machinery, such that a significant change is indicative of a developing failure.

It is a major component of predictive maintenance. The use of conditional monitoring allows maintenance to be scheduled, or other actions to be taken to avoid the consequences of failure, before the failure occurs.

Nevertheless, a deviation from a reference value (e.g. temperature or vibration behavior) must occur to identify impeding damages

Predictive Maintenance does not predict failure.

Machines with defects are more at risk of failure than defect free machines. Once a defect has been identified, the failure process has already commenced and CM systems can only measure the deterioration of the condition.

Intervention in the early stages of deterioration is usually much more cost effective than allowing the machinery to fail. Condition monitoring has a unique benefit in that the actual load, and subsequent heat dissipation that represents normal service can be seen and conditions that would shorten normal lifespan can be addressed before repeated failures occur.

Serviceable machinery includes rotating equipment and stationary plant such as boilers and heat exchangers.

Methods Of Cm

1.     Screen monitoring records video or static images detailing the contents, or screen capture, of the entire [video display] or the content of the screen activity within a particular program or computer application. Monitoring tools may collect real time video, accelerated or [time-lapse] video or screen shots, or may take video or still image captures at regular intervals (e.g., once every 4 minutes). They may collect images constantly or only collect information while the user is interacting with the equipment (e.g., capturing screens when the mouse or keyboard is active).

2.     Data monitoring tracks the content of and changes to files stored on the local [hard drive] or in the user's "private" network share.

3.     Keystroke monitoring (e.g., number of keystrokes per minute) may track the performance of keyboard-intensive work such as word processing or data entry. Keystroke logging captures all keyboard input to enable the employer to monitor anything typed into the monitored machine.

4.     Idle time monitoring keeps track of time when the employee is away from the computer or the computer is not being actively used.

Benefits

o       Screen monitoring records video or static images detailing the contents, or screen capture, of the entire [video display] or the content of the screen activity within a particular program or computer application.

o       Monitoring tools may collect real time video, accelerated or [time-lapse] video or screen shots, or may take video or still image captures at regular intervals (e.g., once every 4 minutes).

o       They may collect images constantly or only collect information while the user is interacting with the equipment (e.g., capturing screens when the mouse or keyboard is active).

o       Data monitoring tracks the content of and changes to files stored on the local [hard drive] or in the user's "private" network share.

o       Keystroke monitoring (e.g., number of keystrokes per minute) may track the performance of keyboard-intensive work such as word processing or data entry. Keystroke logging captures all keyboard input to enable the employer to monitor anything typed into the monitored machine.

Idle time monitoring keeps track of time when the employee is away from the computer or the computer is not being actively used.

Load Testing

o       Load testing is the process of putting demand on a system or device and measuring its response.

o       Load testing is performed to determine a system’s behavior under both normal and anticipated peak load conditions.

o   It helps to identify the maximum operating capacity of an application as well as any bottlenecks and determine which element is causing degradation.

o       When the load placed on the system is raised beyond normal usage patterns, in order to test the system's response at unusually high or peak loads, it is known as stress testing.

o       The load is usually so great that error conditions are the expected result, although no clear boundary exists when an activity ceases to be a load test and becomes a stress test.

o  There is little agreement on what the specific goals of load testing are. o  The  term is  often  used  synonymously with concurrency testing, software performance testing, reliability testing, and  volume testing.

Load testing is a type of  non-functional testing.

Types Of Condition Monitoring Systems

Condition monitoring systems are of two types: periodic and permanent. In a periodic monitoring system (also called an off-line condition monitoring system), machinery vibration is measured (or recorded and later analyzed) at selected time intervals in the field; then an analysis is made either in the field or in the laboratory.

Advanced analysis techniques usually are required for fault diagnosis and trend  analysis. Intermittent monitoring provides information at a very early stage about  incipient failure and usually is used where (1) very early warning of faults is required, (2) advanced diagnostics are required, (3) measurements must be made at many locations on a machine, and (4) machines are complex.

In a permanent monitoring system (also called an on-line condition monitoring system), machinery vibration is measured continuously at selected points of the machine and is constantly compared with acceptable levels of vibration.The principal function of a permanent condition monitoring system is to protect one or more machines by providing a warning that the machine is operating improperly and/or to shut the machine down when a preset safety limit is exceeded, thereby avoiding catastrophic failure and destruction. The measurement system may be permanent (as in parallel acquisition systems where one transducer and one measurement chain are used for each measurement point), or it may be quasi-permanent (as in multiplexed systems where one transducer is used for each measurement point but the rest of the measurement chain is shared between a few points with a multiplexing interval of a few seconds).

In a permanent monitoring system, transducers are mounted permanently at each selected measurement point. For this reason, such a system can be very costly, so it is usually used only in critical applications where: (1) no personnel are available to perform measurements (offshore, remote pumping stations, etc.), (2) it is necessary to stop the machine before a breakdown occurs in order to avoid a catastrophic accident, (3) an instantaneous fault may occur that requires machine shutdown, and (4) the environment (explosive, toxic, or high-temperature) does not permit the human involvement required by intermittent measurements.

Before a permanent monitoring system is selected, preliminary measurements should be made periodically over a period of time to become acquainted with the vibration characteristics of the machine. This procedure will make it possible to select the most appropriate vibration measurement parameter, frequency range, and normal alarm and trip levels.

Establishing A Condition Monitoring Program

A condition monitoring program may be established to check the satisfactory operation of a single machine or, more usually, it is established to check the operation of  a number of machines, perhaps all the machines in an entire plant. The following steps are usually considered in the establishment of such a program, depending on the type of machine and impact of failure of operation machines might have.

Step 1. Determine the type of condition monitoring system, described in the preceding section, that best meets the needs of the plant.

Step 2. Make a list of all of the machines to be monitored (see, for example,Table 16.1), based on the importance of these machines in the production line.

Step 3. Tabulate the characteristics of the machines that are important in conducting vibration analyses of the machines of step 2.These characteristics are associated with machine construction such as the natural frequencies of shafts, casings, and pedestals, and operational and defect responses.A tabulation of machine frequencies is important because fault analysis is conducted (Table 16.2) by matching machine frequencies to measured frequencies appearing in a spectrum. The following machine characteristics provide the necessary information for fault analysis.

_ Shaft rotational speeds, bearing defect frequencies, number of teeth in gears, number of vanes and blades in pumps and fans, number of motor poles, and number of  stator slots and rotor bars.

Vibratory forces such as misalignment, mass unbalance, and reciprocating masses.

_ Vibration responses due to process changes, such as temperature and pressure.

_ Fault responses associated with specific machine types, such as motors, pumps, and fans.

_ Sensitivity to instability in components, such as fluid film bearings and seals due to wear and clearance.

_ Loads or changes in operating conditions.

_ Effects of mass unbalance, misalignment, distortion, and other malfunction/defect excitations on vibration response.

Condition Monitoring Of Machinery

TABLE 16.1 Machinery Classification for Monitoring

Machinery classification Result of failure

Critical Unexpected shutdown or failure causes significant production loss.

Interrupts production Unexpected shutdown or failure causes minor interruptions in production.

Causes inconvenience Inconvenience in operation, but no interruption in production. Noncritical Production is not affected by failure.

Step 4. Select the most appropriate vibration measurement parameter. When an accelerometer is employed as the sensing device in a condition monitoring system, the resulting acceleration signal can be electronically integrated to obtain velocity or  displacement, so any one of these three parameters may be used in measurements.

The appropriate parameter may be selected by application of the following simple rule: Use the parameter which provides the “flattest” spectrum. The flattest spectrum requires the least dynamic range from the instrumentation which follows the transducer.

For example,Fig. 16.1 shows a velocity spectrum and a displacement spectrum obtained under identical conditions. The dynamic range (i.e., the range from the highest to the lowest signal level) required to measure the displacement spectrum is much larger than the range for the velocity spectrum; it may even exceed the available dynamic range of the instrumentation.Therefore, according to this rule, velocity measurements should be selected.

The flattest spectrum rule applies only to the frequency range of interest. Therefore, the parameter selection, to some extent, depends on the type of machine and the type of faults considered.

Step 5. Select one of the following vibration pickups that will best meet the requirements of step 4.

Displacement Transducer. A displacement transducer is a transducer that converts an input mechanical displacement into an electrical output that is proportional to the input displacement. Displacement transducer of the eddy-current type (described in Chap. 12), which have noncontacting probes, are commonly used to measure the relative motion between a shaft and its bearings. This information can be related directly to physical values such as mechanical clearance or oil-film thickness, e.g., it can give an indication of incipient rubbing. Shaft vibration provides information about the current condition of a machine and is principally used in permanent monitoring systems, which immediately shut the machine down in the event of trouble. The use of displacement transducers is essential in machinery having journal bearings. However, proximity probe transducers (1) usually are difficult to calibrate absolutely, (2) have limited dynamic range because of the influence of electrical and mechanical runout on the shaft, and (3) have a limited high-frequency range.

Accelerometers and Velocity Pickup. Pickups of this type, described in Chap.

12, are usually lightweight and rugged. They are always used for detecting faults  which occur at high frequencies (say, above 1000 Hz), for example, to detect rolling element bearing deterioration or gearbox wear. Acceleration measurements of bearing vibration will provide very early warning of incipient faults in a machine.

Figure 16.1 Displacement and velocity spectra obtained under identical conditions. The velocity spectrum requires a smaller dynamic range of the equipment which follows the transducer.Therefore, it is preferable.

Step 6. Select the measurement locations. When a periodic (off-line) monitoring  system is employed, the number of points at which measurements are made is limited only by the requirement for keeping measurement time to a minimum. As a general rule, bearing vibration measurements are made in the radial direction on each accessible bearing, and in the axial direction on thrust bearings. It is not usually necessary to measure bearing vibration in both the horizontal and the vertical direction, since both measurements give the same information regarding the forces within the machine; this information is merely transmitted through two different transmission paths. This applies for detecting developing faults. It will later be seen, however, that in order subsequently to diagnose the origin of the impending fault, measurements in both the horizontal and the vertical direction may give valuable information.When measuring shaft vibrations with permanently mounted proximity transducers, it is convenient to use two probes on each bearing, located at 90 from each other, thereby providing an indication of the orbit of the shaft within the bearing.

Axial displacement transducers, programmed to shut the machine down on preset levels, are mounted where a thrust measurement will protect the machine rotating parts, such as blades, from rubbing the stationary casing due to faultinduced axial forces.

When a permanent (on-line) monitoring system is employed using a seismic pickup, the number of measurement points usually is minimized for reasons of  economy. Selection must be made following a study of the vibration spectra of different bearings in order to locate those points where all significant components related to the different expected faults are transmitted at measurable vibration levels if full spectrum comparison is performed. If only broadband measurements are monitored, then a further requirement is that all frequency components related to the expected faults must be of approximately the same level within the selected frequency range. Otherwise, measurements must be made in selected frequency bands.

Step 7. Select the time interval between measurements. The selection of the time interval between measurements requires knowledge of the specific machine. Some machines develop faults quickly, and others run trouble-free for years. A compromise must be found between the safety of the system and the time taken for measurements and analysis. Measurements should be made frequently in the initial stages of a condition monitoring program to ensure that the vibration levels measured are stable and that no fault is already developing.When a significant change is detected, the time interval between measurements should be reduced sufficiently so as not to risk a breakdown before the next measurement.The trend curve will help in determining when the next measurement should be performed.

Step 8. Establish an optimum sequence of data acquisition. The sequence in which data acquired in a condition monitoring program must be planned so that the data are acquired efficiently. For example, the data collection may be planned on the basis of plant layout, on the type of data required, or on the sequence of components in the machine train, from driver to driven components.

Principles And Methods

As a starting point for any discussion on condition monitoring it is useful to define what is meant by the term, and to describe how it relates to other techniques used in the operation and maintenance of machines, such as alarm and shut down systems or methods for failure and problem investigation.

The crudest method for operating machines is to run them until they fail, and then to try and repair them in order to make them fit for further service. This method of operation can be very expensive in terms of lost output and machine destruction, and in addition can involve hazards to personnel. It is now well recognised that, particularly in the case of large and expensive plant, it is more economical and operationally satisfactory to carry out regular maintenance. This involves the maintenance of the machine or its various components at regular intervals, to reduce the likelihood of failure during a time when the machine is required to be available for use.

The problem in planning this type of maintenance lies in the choice of an appropriate maintenance interval for the machine, because the actual running time before maintenance is really needed is not constant, but varies from one occasion to another, due to differences in the operation of the machine in the behaviour of its components.

Fig. 1 shows how the running time to failure of a typical machine would be likely to vary if no preventive maintenance were carried out. The vertical line in this diagram represents the safe time interval between preventive maintenance work which could catch all the failures before they occurred. If this safe overhaul interval is chosen, however, there will be many occasions when the machinery will be overhauled long before it is really necessary,such as in those cases at the right hand side of the curve where it could have run on for much longer without failing.

This situation wastes production time, and by increasing the frequency of maintenance operations increases the incidence of human errors on reassembly of the machine.

A more satisfactory compromise in terms of maintenance strategy is to carry out preventive maintenance at what may be irregular intervals, but to determine these intervals by the actual condition of the machine at the time. For such condition-based maintenance to be possible,it is essential to have knowledge of the machine condition and its rate of change with time. The main function of condition monitoring is to provide this knowledge.

There are two main methods used for condition monitoring, and these are trend monitoring and condition checking. Trend monitoring is the continuous or regular measurement and interpretation of data, collected during machine operation, to indicate variations in the condition of the machine or its components, in the interests of safe and economical operation.

This involves the selection of some suitable and measurable indication of machine or component deterioration, such as one of those listed in Fig.2, and the study of the trend in this measurement with running time to indicate when deterioration is exceeding a critical rate.

The principle involved is illustrated in Fig.3, which shows the way in which such trend monitoring can give a lead time before the deterioration reaches a level at which the machine would have to be shut down.

This lead time is one of the main advantages of using trend monitoring rather than simple alarms or automatic shut down devices.

Vibration Monitoring System (VMS) Specification

To identify potential turbine generator fault mechanisms and so enable informed operational decisions, sophisticated vibration data analysis is required. To do this a modern dedicated vibration monitoring system (VMS) is required. Vibration Diagnostics can provide vibration specification and commissioning services.