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Criticality

Asset Criticality Studies

Criticality Assessment
A Criticality Assessment plays a vital role in all Best Practice Maintenance Programs.

Assets that have an immediate and critical effect on the reliability of the production process will be highlighted by the assessment. The highest level of protection is then targeted at these assets.

Typically this would mean some kind of condition based maintenance regime with multiple technologies being deployed.

Case study
A recent Criticality study carried out by MMS for one of our clients revealed that 80% of maintenance spare parts held were not needed but other very important assets had no spares available. A condition monitoring program was immediately started to protect these assets.

MMS uses six Criticality criteria to assess how assets should be monitored for maximum reliability.

Principles of Criticality
The study uses six criteria to rate each element of the production equipment, three related to consequences of plant failure on production, and three to the probability of failure. These criteria are given a simple 0 – 4 rating, with a 8 rating for stop of production.

Consequence Criteria
Production: gives a measure of the effect that failure would have on production.

0 No effect, as there is either full redundancy with duty and standby machinery or it is a machine in a non critical application
2 Some effect depending upon demand, but there is some spare capacity to accommodate loss of service of one item
4 Noticeable effect on production, either in loss of quality or reduced speed of production
8 Immediate stop of production, with the need to undertake immediate repair to restore production

Maintenance: indicates the measure of the likely difficulty of maintaining an item or component, with regard to access, time required and cost of spares.

1 Low cost (spares readily available, low time involvement), readily accessible
2 Medium/low cost, some preparation required for access (removal of lagging etc.)
3 High/medium cost, restricted access and labour intensive
4 High cost, restricted access and labour intensive

Damage: gauges the results of component failure and subsequent damage to other components, or the impact on safety of personnel.

0 No secondary damage effects or risk to personnel
2 Minor damage to other areas, no risk to personnel e.g. bearing failure of pump drive motor
3 Moderate damage to other components or machines, potential risk to personnel e.g. failure of gearbox bearing or bottom roller bearing in paper mill
4 Serious/catastrophic failure – seizure of cylinder bearing leading to shearing of spindle and collapse
1 Failure interval 4 years or greater
2 Failure interval circa 2 years
3 Failure interval 1 – 2 years
4 Failure interval less than 1 year
1 Continuous operation, 30 – 95% of rated duty
2 Intermittent operation, 30 – 95% of rated duty
3 Continuous operation at either 100% or greater duty, or less than 30% of related duty
4 Intermittent duty, either at 100% or less than 30%
1 Clean product, i.e. fresh water, or light load i.e. paper roller
2 Medium loading, i.e. some solids present or under tension i.e. Wire drive motor
3 Solids present and/or heavy loading i.e. press rollers
4 Solids and heavy loading present, with harsh regime i.e. high temperatures

The results of the first three factors, when added together give the consequence factor (CF) and the last three the probability factor (PF). When the two resultants are multiplied together the criticality rating (CR) is achieved.

Obviously any plant or component with a Production Effect of 8 should be monitored, irrespective of the resulting CR number.

Hydrapulpers 2 4 0 6 2 3 3 8 48
Disch. pumps 4 2 0 6 2 4 3 9 54
Water pumps 2 3 0 5 2 4 1 7 35
Fire Pumps 0 3 4 7 2 4 1 7 49
Holding Tk Agitators 2 2 0 4 2 1 3 6 24
Holding Tk Pumps 0 2 0 2 4 4 3 11 22

This document is Copyright Machine Monitoring Systems Ltd (MMS) 1999 and must not be reproduced or otherwise used without the prior written consent of MMS Ltd.

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