Page 607 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Implementation and Communication Best Practices Best Practice 1 2.1

The amount of time equipment operates in one year
Operating hours per year
Reliability (% =) 100
8760 hours

Fig 12.1.6 Reliability e critical equipment

Hours per year spared equipment operated as a percentage of the
hours it was required to operate
Fig 12.1.3 The life span of rotating equipment
Yearly hours in operation
Reliability (% =) 100
Reliability terms and definitions Yearly main unit forced outage hours
Before we can optimize reliability, certain terms and definitions Fig 12.1.7 Reliability e general purpose (spared) equipment
need to be presented. These terms are shown in Figure 12.1.4.

Availability
Reliability
Availability Availability considers preventive and predictive maintenance
Maintainability downtime as shown in Figure 12.1.8.
Cost of unavailability One measure of both reliability and availability is mean time
between failure (MTBF). See Figure 12.1.9.
Fig 12.1.4 Reliability terms

The amount of time equipment operates in one year as
Reliability
a percentage of the available hours per year
Reliability is the ability of the equipment unit to perform its
Yearlyoperatinghours
stated duty without a forced (unscheduled) outage in a given Reliability(% =) 8760 planneddowntime ( T & Isorturnarounds) 100
period of time (see Figure 12.1.5).
The definition of reliability for critical (un-spared) equip-
ment is presented in Figure 12.1.6. Fig 12.1.8 Availability
In the case of general purpose equipment (spared), reliability
is not usually calculated since a spare unit should be available for
operation if required. In the case of unreliable general purpose Total operating hours
units, reliability could be defined as shown in Figure 12.1.7. MTBF = Number of failures
Note in Figures 12.1.6 and 12.1.7 that reliability does not
account for planned downtime for preventive and/or predictive
maintenance. Fig 12.1.9 Mean time between failure

Maintainability
Simply stated, maintainability is the ability to perform all
maintenance activities; preventive, predictive and forced
outage in a minimum time that requires rotating equipment
unit shutdown. It is understood that the total maintenance
time required will restore the unit to its original ‘new’
condition.
One parameter that can be used to measure maintainability is
mean time to repair e MTTR as shown in Figure 12.1.10. The
lower the MTTR, the greater the maintainability.

Cost of unavailability
All terms discussed so far, reliability, availability and maintain-
ability directly affect the product revenue of the plant. Product
Fig 12.1.5 The rotating equipment unit revenue is the value obtained from one day’s production

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