2.2 Maintenance
2.2.7 Reliability and Performance Metrics Measures
A KPI is defined by Bower (2003, 77) as “the measure of performance asso- ciated with an activity or process critical to the success of an organisation”. Levitt (2011, 218) also states that a measure can be regarded as any means which can be used to understand what is happening within a business unit. Furthermore, the author states that a KPI is a measure of success for a busi- ness function and that there are several types of indicators that are important to maintenance management which include quantitative and directional indi- cators, actionable indicators and financial indicators (Levitt, 2011, 218).
The Asset Performance and Health Monitoring AM subject is outlined in sectionA.1.6.5and defined byGFMAMas (GFMAM, 2014, 47) “the processes and measures used by an organisation to assess the performance and health of its assets using performance indicators”.
Importantly,KPIsneed to be used to monitor the assets and theAMS(IAM 2014a, 62; GFMAM 2014, 47).
Each element of a strategic plan can be allocated a specificKPIwhich can be broken down into various functions or levels of performance indicators. KPIs
can be used to understand performance trends related to business processes, departments and functions which need to be improved in order to achieve the organisational objectives (Parida and Kumar, 2006, 20). Furthermore, each organisation needs to monitor the performance to identify improvement areas (Parida and Kumar, 2006, 20).
Lagging performance measures take into account part performance of assets while leading performance measures are intended to predict the future per- formance. These two indicator types are generally combined to see how the processes are performing and which outcomes are being achieved (IAM 2014a, 62; GFMAM 2014, 47). Weber and Thomas (2005, 7) also state that organ- isations need to have leading KPIs (process based) and lagging KPIs (result based) measures (Weber and Thomas, 2005, 7). The authors present a study that identified leading and lagging KPIs for the organisational maintenance function and performance (Weber and Thomas, 2005, 7).
Performance measures or KPI targets need to align with organisational ob- jectives and stakeholder requirements which are expressed within theOSPand theSAMP. This approach assists in creating theLine-of-Sightand understand- ing the performance of physical assets and plays an important part in business and risk management (IAM 2014a, 62; GFMAM 2014, 47).
KPIsare important strategic indicators of performance and are an aggrega- tion of various performance indicators. The core purpose of aKPIis to under- stand where there is room for improvement (Kobbacy and Murthy, 2008, 461). AppropriateKPIsneed to be developed to evaluate actual performance against the targets. These KPIs set the continuous improvements process in motion by identifying gaps and opportunities which are addressed though selecting and implementing suitable strategies to harness the benefits (Andrawus, 2008, 43).
The discussed principles are depicted in figure 2.22.
Asset Management
System Assets
Management Review
Reactive
Monitoring MonitoringPro-Active
Past or Existing Non-Conformities E.g. Asset Condition E.g. Asset Failures or Incidents Asset Manage System Lagging KPI P No E .E.gg.. PProcess MMaturity Continuous Improvement Leading KPI
Figure 2.22: System reliability model based on component probability density functions (Adapted from IAM (2014a, 63))
According to Palmer (2006, 329) people can relate to overall plant avail- ability and Nilsson (2009, 6) notes that reliability can be measured using key performance indicators (KPIs) such availability, failure rate and repair time. The author also notes that availability is a fundamental measure of reliability. An availability KPI is expressed in equation2.2.4:
Availability = M T BF M T BF + M T T R (2.2.4) Mean Time Between Failures (MTBF) indicates the mean exposure time between failures. Failure rate (λ) indicates number of failures per time unit
Nilsson (2009, 6). When failures are exponentially distributed (λ) is constant over the time and according to equation 2.2.5 this gives:
λ = 1 M T BF (2.2.5) Repair time (r) is the mean time to replace or repair a failed component, also called Mean Time To Repair (MTTR). Availability can be calculated as 1 - Unavailability (U), which in its turn can be approximated as U = λ · r (Nilsson, 2009, 6).
In the case of maintenanceKPIsthere should be a direct correlation between the maintenance activities and the KPI. During the process of defining a KPI
for maintenance a good test will be to understand whether, if the measured function performs as it should, would there be a proportional change, or will theKPIbe affected through other external factors? (Weber and Thomas, 2005, 4). Maintenance performance indicators include planned coverage, proactive versus reactive WOs, reactive work hours, work type, availability forecast, schedule compliance, backlog WOs and WOs completed, total cost of main- tenance, total production cost, mean time to repair (MTTR), maintenance cost per hour and manpower utilisation (Levitt 2011, 238; Kister and Hawkins 2006, 172; Peters 2014, 68; Parida and Kumar 2009, 22; Palmer 2006, 329).
A KPI related to solar PV power plants is Performance Ratio (PR) that is measured for the degree of utilisation of an entire PV system. Essentially
PR, which is a dimensionless indicator, is widely used as a measure of the quality of thePVsystem. It describes the relationship between the actual and theoretical or reference energy output of thePVplant. The difference between 100% and the PR value aggregates all the possible energy losses including inverter efficiency, wire losses, panel degradation, mismatch, shading, dust, soiling, thermal inefficiencies (temperature effects) and system failures Reich et al. (2012).
According to the International Electrotechnical Commission (1998) PR is defined as the ratio of so-called finalPVsystem yield (Yf) to so-called reference yield (Yr) and denoted asPR in equation 2.2.6:
P R = Yf Yr