condition monitoring Archives - Machinery Monitoring Solutions

Do You Make These Mistakes in Condition Monitoring?

The Problems with Condition Monitoring

For condition monitoring, to be a ‘success’ as a maintenance strategy, assumes that the equipment is monitored on a frequent and regular basis; typically once-per-month. However, the reality is often that it is considered less important than other tasks assigned to maintenance staff.

In the case of the offshore Oil & Gas industry, bed space, helicopter seat allocation and conflicting priorities, conspire against the CM work being carried out regularly. It is not seen as a positive and beneficial activity, rather it is viewed as a problem, which will be fitted in where and when possible, given the above restrictions. Management commitment and support however, is often the main missing element for CM to work; they perceive it as a cost, rather than a benefit.

The other element which is often overlooked is that CM is only appropriate for certain types of equipment and failure modes, in which a reasonable time exists between detection of the failure and its ultimate failure. This is illustrated below in the “P-F Curves”. This P_F interval is the time from the point (P) – the potential failure point, to the functional failure point (F).

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Condition Monitoring_Potential Failure Curve 1

P-F Curve

 

Condition Monitoring_Potential Failure Curve 2

P-F Curve…Detailed Analysis

 

Condition Monitoring_Potential Failure Curve 3

P-F Curve…More Detailed Analysis!

An example of a functional failure (F) is when a car engine seizes due to a lack of oil, and the car stops. The potential failure (P) is when, say, a pump impellor is worn, but still pumps the fluid, but not operating at the required performance (i.e., flow rate) level required. Note that ‘failure’ in this context does not necessarily mean the equipment is physically damaged, but rather it may only mean that it is not performing or functioning to the required or specified level.

Therefore, if the P-F interval is short and the CM measurement interval is relatively long, there is a high probability of a failure occurring. Similarly, if the P-F interval for a particular failure mode is say 6 weeks, and the CM measurement task is only implemented either (a) infrequently or (b) every two months for example, then again, an undetected failure is likely to occur.

 The Benefits of Condition Monitoring

Condition Monitoring, if correctly applied, and given the essential management support and commitment, can save money. Most company accountants regard the maintenance budgets as another overhead on the operating cost. Certainly, CM equipment, software, computers and training all cost money in order to implement CM, but the benefits far outweigh the costs.

Savings which can arise from the implementation of CM:

  1. A reduction in spares holding – only order spare parts when required
  2. A reduction in breakdown failures
  3. A time interval to plan a shutdown for repairs on equipment
  4. A reduced cost for warehousing spare parts inventory
  5. Only replacing parts when their condition dictates this should be done
  6. Because this method employs a range of non-invasive techniques, and no invasive actions are required, there is also a reduction in human intervention factors, which contribute to increased costs [Recall the old saying: “If it isn’t broke, don’t fix it”]

Machinery failures do not follow any specific and predictable failure pattern unfortunately, but are subject to:

  • how well maintained the equipment is
  • Is the equipment being operated within its design parameters
  • Is the equipment being subjected to high and cyclic stresses
  • Is the machine being operated in a hostile environment

All of the above factors can affect the life of a machine and its components, making time-based or preventive maintenance strategy risky, as well as costly. The above points are therefore, perhaps, the most compelling argument in favour of employing CM, where appropriate, as a part of the maintenance strategy in a plant.

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 Maintenance Strategies

There are three maintenance strategies commonly employed, and these are:

  • Breakdown (or Run-to-Failure)
  • Time-based (or Preventive)
  • Condition Monitoring (or Predictive)

The Breakdown maintenance strategy is a traditional method where machines are simply run until they fail in service. This in principle gives the longest time between shutdowns, but failure, when it does occur, can be catastrophic and result in severe consequential damage – which is often referred to as secondary damage. This can therefore lead to an increase in repair times and may eventually impact on production. There is still a case for breakdown maintenance, in situations where there are large numbers of small machines, where the loss of one machine has no impact on safety and production. Failure can also include economic failure or performance degradation to a point considered to be where the equipment is no longer providing an acceptable level of performance for its task.

Preventive maintenance is where the plant management decides, and takes some actions, based on a fixed time or operating hour interval, to carry out maintenance tasks, which are deemed to be shorter than the expected time between failures. However, It does mean that the vast majority of machines will run longer by a factor of two or three. The advantage of this method is that most maintenance can be planned well in advance and that catastrophic failure is greatly reduced. The disadvantages, in addition to the fact that a small number of unforeseen failures ca still occur, are that too much maintenance is carried out and an excessive amount of perfectly usable parts are replaced. There is also the possibility of introducing faults, which would otherwise not happen, due to wrong part replacements, incorrect installation, poor workmanship etc.

This method is still widely applied, particularly where statutory regulations require inspections on a regular basis – e.g., pressure vessels, lifeboats etc.

Condition based or Predictive maintenance is a diagnostic testing method, whereby the potential for breakdown of a machine is predicted through the monitoring and trending of a parameter or parameters, to enable maintenance to be carried out at the optimum time, as a result of regular measurements or assessment of plant condition.

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Exactly What is Condition Monitoring?

Put more succinctly, Condition Monitoring (or “CM”) is the process of monitoring a parameter (or parameters) which reflect the condition or performance of a piece of equipment, in order to identify any significant degradation, which is indicative of a developing fault, or an unacceptable drop in equipment performance.

The key word is ‘monitoring’ in the context of condition monitoring. This conditional monitoring task requires regular checks of the selected key parameters, which have been selected, to be in a position to identify the onset of a failure or drop in performance. Then subsequently schedule the appropriate maintenance intervention, in a timely manner, to prevent failure and avoid its consequences. CM, which is a non-invasive technique, which is usually employed on rotating equipment such as: pumps, compressors, fans, turbines and electric motors etc. The ultimate aim of condition monitoring is to only perform maintenance work only when necessary.

The most common technique used in condition monitoring is vibration analysis, where measurements are taken on machine bearing housings with transducers – which are normally accelerometers, in triaxial directions, as shown below. This system employs portable battery-powered instruments called data collectors/analysers; this methodology is referred to as an “offline” system, using instruments such as the Adash A4900 VA4 Pro 4-channel unit below.

Condition Monitoring_Vibration Analysis 1

Accelerometer

Condition Monitoring_Vibration Analysis 2

Adash A4900 VA4 Pro 4-channel unit

However, on more critical machines, eddy-current displacement transducers are used, which directly observe the rotating shafts to measure the radial and axial displacement of the shaft as shown below. These displacement transducers are permanently mounted on the machine housings, and monitor the condition of the machine on 24/7 basis, and is known as an “On-Line” system.

Condition Monitoring_Eddy Current Displacement Transducer

Eddy Current Displacement Transducer

 

Condition Monitoring_On-Line System

Machine Shaft

The signals from these eddy current ‘prox probes’ are fed back to a permanently installed rack system, such as the Sensonics G3 system below, which is designed to protect these high capital investment and production critical machines from failure.

Condition Monitoring_Permantly Installed Rack System

Sensonics G3 System

 

Other Condition Monitoring Techniques

There are other condition monitoring techniques to consider, and machines may have one or more of these applied, depending on its criticality and likely modes of degradation and the cost of failure. These other CM techniques (in addition to vibration analysis) include:

  • Spectrographic Lube Oil Analysis
  • Ferrography
  • Thermography
  • Acoustics
  • Performance analysis

Lube oil analysis is perhaps the next most widely employed CM technique after vibration analysis, and perhaps, more increasingly, thermography and performance analysis.

When you call it a day on the plant, is your mind more at ease thanks to condition monitoring?

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