Asset management comprises of a range of techniques to ensure maximum production at a minimum cost.
This is achieved through an optimum combination of design configuration, maintenance strategy and operational procedures. RAM analysis can be used to assess each one of these elements empowering the analyst to predict the performance for a range of combinations.
PMRs fall into two categories:
- Intrusive – potentially causing production downtime; and
- Non-intrusive – causing no direct downtime, but may have an indirect effect by absorbing maintenance resources elsewhere to deal with more critical jobs.
On a typical plant the vast majority of PMRs are non-intrusive activities having regular frequencies. The importance of a PMR can be determined by its priority rating, similar to ad-hoc corrective maintenance jobs. Generally, intrusive PMRs have relatively higher priority ratings over non-intrusive ones, however, there are some exceptions, for example safety checks, certification requirements etc.
When analysing PMRs the main areas of concern are:
- Are the intrusive PMRs providing benefit or are they causing too much production downtime? Cost/benefit optimization required.
- Can the proposed PMR strategy be liquidated by the resources and support logistics in place? Is there a sensible balance between utilization of resources and slippage/backlogs? Which resources are causing bottlenecks, which ones are under-utilised? Integrated logistics support optimization required.
- Are the safety and certification activities being dealt with effectively?
Therefore, from a modelling point of view the key issues are:
- Impact of PMRs on overall production performance;
- Determine critical PMRs;
- Slippage and backlog assessments;
- Utilization of resources and maintenance delays; assess burden of PMRs over ad-hoc requirements and optimize logistics;
- OpEx profile assessments.
To exemplify how Preventive Maintenance Routines (PMRs) can be modeled, consider the following example. A valve actuator has a replacement time of 20 hours, however, an unscheduled failure results in a 120 hour system shutdown. The failure pattern can be described by a Weibull distribution with a Characteristic Life of 0.7 years and a shape factor of 2.5 – age dependent failure!
Schematic of a valve
The question is: will replacement at monthly, yearly or five yearly intervals provide optimum availability?
Looking at performance figures for each option:
|Frequency||Monthly||Every two months||Every 6 months||1 year||5 years|
This output clear says that replacing the actuator of the valve every 6 months improves the overall performance of the system.
This is a simple example for a single equipment item but this analysis can be extended to include all the PMRs in the plant. Intelligence is built over the different maintenance strategies and decision related to priority and criticality of maintenance activities.
Author: Victor Borges