Handbook of Reliability, Availability, Maintainability and Safety in Engineering Design - Part 40 docx

4.2 Theoretical Overview of Availability and Maintainability in Engineering Design 373
i) Establishing Maintenance Strategies for Engineering Design
From the three fundamental principles of a maintenance strategy, it is evident that
all required maintenance work is made up of one or more types of maintenance that
accomplish specific technical benefits. As stated previously,it is the combination of
these different types of maintenance that constitutes a maintenance strategy.
From an engineering design perspective, a maintenance strategy is the establish-
ment of the most effective combination of the different types of maintenance to
be carried out on specific equipment in order to achieve the most desired technical
benefit from that equipment. This is determ ined through designing for reliability,
availability, maintainability and safety (i.e. designing for engineering integrity—
where in this case, the concept of safety is considered as part of designing for re-
liability). On the other hand, the most effective com bination of the different types
of maintenance for completed engin eered installations (i.e. a maintenance strategy
for operational systems and equipment) is established through a RAMS (reliability,
availability, maintainability and safety) program (DoD 5000.2-R. 1997). The deliv-
erable results are the establishment of operations and maintenance procedures and
work instructions in which the different types of maintenance are effectively com-
bined into maintenance strategies for specific equipment. The established mainte-
nance strategies for the effective care of the condition of engineering equipment are
takenupinaRAMS program.
The RAMS program The goal of the RAMS program is to establish policies and
strategies for effective care of the condition of engin eering systems and eq uipment
through the implementation of various RAMS methods and techniques. The objec-
tives of the RAMS program are to:
• Ensure effective care of equipment condition.
• Optimise the technical benefits derived from equipment reliability, availability,
maintainability and safety.
• Establish priorities for achieving targeted quality and safety.
• Establish maintenance strategies for carrying out the most applicable and effec-
tive types of maintenance and use of appropriate maintenance procedures and

work instructions.
• Ensure a correct balance of costs against desired technical benefits.
The immediate benefits of the RAMS program are in the establishment of mainte-
nance policies and strategies through an analysis and understanding of the follow-
ing:
• The systems process, equipment functions, failure modes, failure effects, failure
causes and failure consequences, and the criticality of equipment failures result-
ing in safety h azards, d owntime, and consequential damage,
• Identifying equipment conditions and failure characteristics and establishing ef-
fective m aintenance through the correct combination of the different types o f
maintenance by prioritising the related technical ben efits to be achieved,
374 4 Availability and Maintainability in Engineering Design
• Avoiding consequential damage and establishing the necessary maintenance pro-
cedures, work instructions and logistic support for equipment care and product
quality,
• Comparing design integrity as a benchmark against measures of operational in-
tegrity.
The benefits achieved through the establishment of maintenance policies and strate-
gies can be summarised in three fundamental principles of a RAMS program, each
relating targeted results and design requirements (in seque ntial order) of safety, re-
liability, availability and maintainability to the desired techn ical benefits, perfor-
mance measures, consequential effects on the designed equipment, and the required
types of maintenance.
Principles of a RAMS program in maintenance strategy The first RAM princi-
ple in a maintenance strategy is the following logical sequence:
Targeted result:
SAFETY
|
Technical benefit:
RELIABILITY

|
Performance measure:
MTBF
|
Effect on equipment:
PHYSICAL CONDITION
|
Type of maintenance:
PREVENTIVE MAINTENANCE
The second RAM principle in a maintenance strategy is the following logical se-
quence:
Targeted result:
UTILISATION
|
Technical benefit:
AVAILABILITY
|
Performance measure:
POTENTIAL USAGE
|
4.2 Theoretical Overview of Availability and Maintainability in Engineering Design 375
Effect on equipment:
OPERATIONAL CONDITION
|
Type of maintenance:
ROUTINE MAINTENANCE
The third RAMprinciple in a maintenancestrategy is the followinglogical sequence:
Targeted result:
QUALITY
|

Technical benefit:
MAINTAINABILITY
|
Performance measure:
MTTR
|
Effect on equipment:
REPAIRABLE CONDITION
|
Type of maintenance:
DEFECT MAINTENANCE
j) Maintenance Cost Optim isation Modelling
Returning to the definition of the goal of maintenance as “that maintenance ac-
tion necessary to achieve the correct balance between the costs of input resources
and the benefits derived from the performance of effective maintenance a ction”, an
additional principle in the understanding of the goal of maintenance, and of mainte-
nance as a whole, is the concept of “the correct balance between the costs of input
resources and the benefits ”.
In a developed maintenance strategy for engineering design, there are two basic
types of maintenance costs that relate to the required input resources for effective
maintenance:
• Costs arising from corrective maintenance action.
• Costs arising from preventive maintenance action.
Costs arising from corrective maintenance action are the costs of rectifying defects
and fixing or repairing equipment. They increase exponentially according to the
extent of usage that the equipment will be subject to, and according to the extent of
failures resulting in downtime.
376 4 Availability and Maintainability in Engineering Design
The manpower costs of corrective maintenance action are partly due to the time
taken to restore the equipment to its expected op erational condition within a min-

imum period of time or disruption to the overall operational process through the
application of defect maintenance. Corrective maintenance costs are thus dependent
upon the extent of defect maintenance, the effect of which is determined by MTTR,
the performance measure of the equipment’smaintainability.As noted before, main-
tainability is primarily a design parameter, and designing for maintainability defines
how long equipment is expected to be down after failure, which has a direct impact
upon corrective maintenance costs.
Costs arising from preventive maintenance action are the costs of detecting po-
tential failures and avoiding functional failures. They increase linearly according to
the age of the equipment and according to the extent of the maintenance schedules
resulting in downtime.
The manpower costs of preventive maintenance action, which comprises both
scheduled routine maintenance procedures, and scheduled preventive maintenance
procedures incur a cost in direct proportion to the amount of routine maintenance
being carried out, and to the amount of preventive maintenance being scheduled.
Preventive maintenance costs are thus dependent upon the extent of routine and
preventive maintenance, the effect of which is d etermined by potential usage and
MTBF respectfully, which are the measures of performance of equipment avail-
ability and reliability. The inherent availability of equipment is its potential usage
with respect to the operable time established from designing for availability, and the
inherent reliability of equipment is initially established by its physical design and
quality of manufacture estab lished from designing for reliability.
By far the largest portion of preventive maintenance costs is associated with
scheduled shutdowns and overhauls. Shutdowns and overhauls are scheduled ac-
cording to the expected life of the major critical components in process engineering
systems and equipment. In certain types of industries, particularly in refineries, sev-
eral different types of shutdowns can be scheduled. They are:
• Interim shutdowns for vessel inspections.
• Open and clean shutdowns.
• Annual shutdowns for replacement of worn components.

• General overhauls for plant and equipment refurbishment.
The scheduled frequency and duration of interim shutdowns for vessel inspections,
and of open and clean shutdowns can be determined according to a maintenance
strategy in which the most suitable scope of preventive maintenance work is already
established during the engineering design stage. The extent and duration of annual
shutdowns for replacement of worn components can also be determined during the
engineering design stage, and depends not only upon the expected useful life of the
critical components of the process engineering design (i.e. failure characteristics)
but also on the complexity of integrated systems, the level of equipment and/or
component redundancy (i.e. process characteristics), as well as their relevant extent
of usage.
4.2 Theoretical Overview of Availability and Maintainability in Engineering Design 377
General overhauls for plant and equipment refurbishment or rebuild are predom-
inantly scheduled on the basis of results obtained, firstly, from condition monitoring
carried out either periodically or continually and, secondly, from condition mea-
surement carried out during interim shutdowns for vessel inspections and open and
clean shutdowns. In principle, however, it is obvious that the costs of corrective
maintenance action as well as the costs of preventive maintenance action can be ra-
tionalised or, in fact, reduced according to the balance o f defect maintenance with
routine maintenance and scheduled preventive maintenance, based upon a particular
maintenance strategy. Such a strategy has its developed beginnings during the engi-
neering design stage, and is progressively modified and improved during the life of
the plant.
Mathematical model of preventive maintenance replacement costs The opti-
mum operational period b etween annual shutdowns for replacement of worn com-
ponents can be determined under a maintenance strategy of periodic replacement,
irrespective of the age condition of the equipment’s components. According to
this strategy, components are replaced at predetermined intervals, CL, typically the
length of the preventive maintenance cycle. If a component fails within this preven-
tive maintenance cycle, it is minimally r epaired to last for the remaining time of the

cycle. Such a minimal repair job, with relatively negligible repair time, implies that
the component’s failure rate
λ
(x), corresponding to its failure probability density
function f (x) at the time of failure x (i.e. the instantaneous failure rate), remains the
same as it was before the failure (Kececioglu 1995).
The cost function for the model is expressed as
C
pm
=
C
pr
+C
mr
E[
α
(T
p
)]
T
p
(4.106)
where:
C
pm
= preventive maintenance cycle costs
C
pr
= the cost of preventive replacement
C

mr
= the cost of minimal repair
E[
α
(T
p
)] = the expected number of failures in interval T
p
and
E[
α
(T
p
)] =
T
p

0
λ
(x)dx (4.107)
where
λ
(x)= f(x)/R(x) (4.108)
and:
λ
(x)=the equipment time dependent failure rate
f(x)=the equipment failure probability density function
R(x)=the equipment reliability function.
378 4 Availability and Maintainability in Engineering Design
Substituting Eq. (4.107) into Eq. (4.106) gives the following result

C
pm
=
C
pr
+C
mr

T
p
0
λ
(x)dx
T
p
(4.109)
In the case where
λ
(x), the equipment time-dependent failure rate, has an exponen-
tial failure probability density function, i.e.
λ
(x)=
f(x)
R(x)
=
λ
e
−
λ
x

e
−
λ
x
=
λ
differentiating with respect to T
p
and setting the resultant equal to zero gives the
following:
dC
pm
dT
p
=
−C
pr
+C
mr
λ
T
p
T
2
p
= 0 .
The optimum operational period between annual shutdowns for preventive replace-
ment is then
T
p

= C
pr
/C
mr
1/
λ
. (4.110)
Optimal preventive replacement age of components subject to functional fail-
ure In many cases, systems and equipment are subject to functional failure, where-
by the equipment or a component of the equipment has to be replaced. Where such
functional failure is unexpected, it is not unreasonable to assume that a failure re-
placement is more costly than a preventive replacement. For example, a preventive
replacement is planned, and arrangements are made for it to be conducted with-
out unnecessary delays, o r the unexpected failure may have caused consequential
damage to other components. In order to reduce the number of failures, preventive
replacements are made. However, a balan ce is required b etween the amount spent
on preventive replacements, and the resulting benefits, i.e. reduced failure replace-
ments.
Such a preventive replacement policy, or preventive maintenance strategy, is one
where preventive r eplacements are mad e according to the ‘right’ age of the compo-
nent, and failure replacements are done only when n ecessary, to minimise the total
expected cost of replacing the component over a period of time. In this optimisa-
tion approach, when functional failures occur in equipment, failure replacements
are made. The time at which preventive replacements are made d epends upon the
age of the component.The problem is to balance the cost of preventivereplacements
against their benefits of reduced failure replacements, which is done by determining
the optimal preventive replacement age for the component so that the total expected
costs are minimised over a period of time.
This is achieved with preventivereplacement m odelling with the following prop-
erties (Vajda 1974):

• C
p
is the cost of preventive replacement.
• C
f
is the cost of failure replacement.
• C
c
is the total expected replacement cost per cycle.
4.2 Theoretical Overview of Availability and Maintainability in Engineering Design 379
• T
c
is the expected cycle length.
• f(t) is the probability density function o f failures of the component.
The replacement policy is to perform preventive replacement once the component
has reached a specified age, plus failure replacements when necessary, where the
specified age is represented by t
p
. The objective is to determine the optimal replace-
ment age of the component to minimise the total expected replacement cost over
a period of time.
In this problem, there are two possible cycles of operation: one cycle is deter-
mined by the component reaching its planned replacement age, t
p
, and the other
cycle is determined b y the component ceasing to operate due to functional failure
occurring before the planned replacement time. The total expectedreplacementcost,
C(t
p
), over a period of time t

p
is given by
C(t
p
)=
Total expected replacement cost per cycle
Expected cycle length
(4.111)
C(t
p
)=C
c
/T
c
where the total expected replacement cost per cycleC
c
is given as (the cost of a pre-
ventive replacement cycle multiplied by the p robability of a preventive replace-
ment) + (the cost of a failure replacement cycle multiplied by the probability of
a failure replacement)
C
c
= C
p
R(t
p
)+C
f
[1−R(t
p

)] (4.112)
where R(t
p
) is the reliability of the component succeeding to last over the period of
the preventive replacement cycle t
p
. R (t
p
) is the probability of no failure occur ring
in the time period t
p
, and the expression [1 −R(t
p
)] is the probability of failure
occurring in the time perio d t
p
, which is the failure density function .
Thus:
C
c
=[C
p
×Reliability]+[C
f
×Failure density] .
The expected cycle length T
c
is given as (the length of the preventive replacement
cycle multiplied by the probability of a preventive replacement) + (the expected
length of a failure replacement multiplied by the probability of a failure replace-

ment)
T
c
= t
p
R(t
p
)+t
f
[1−R(t
p
)] . (4.113)
In this case, t
f
is the mean time to fail (MTTF) of the component. Here, it is im-
portant to take note of the description of MTTF, compared to MTBF, the mean time
between failures. The differencebetween MTTF and MTBF is in their usage. MTTF
is applied to items that are not repaired but replaced, such as components, whereas
MTBF is applied to items that are repaired. Therefore:
T
c
=[Replacement age·Reliability]+[MTTF·Failure density]
380 4 Availability and Maintainability in Engineering Design
The replacement model relates replacement age t
p
to the total expected replacement
cost over a period of time, where
C(t
p
)=

C
p
R(t
p
)+C
f
[1−R(t
p
)]
t
p
R(t
p
)+t
f
[1−R(t
p
)]
(4.114)
C(t
p
)=
[C
p
·Reliability]+[C
f
·Failure density]
[Age·Reliability]+[MTTF·Failure density]
.
Thus, the essential integrity measures for determining the total expected replace-

ment cost, C(t
p
), over a period of time t
p
, in addition to the cost of preventive re-
placement C
p
and the cost of failure replacement C
f
are the component (or equip-
ment) reliability and failure density. Values for the specific costs of C
p
and C
f
as
well as component reliability and the failure density (or 1–reliability), and MTTF
must be evaluated in order to determine the minimum total expected replacement
cost C(t
p
) over the period of time t
p
. Preventive replacement age is where C(t
p
) is
minimum.
Cost of input resource of spares A significant portion of preventive maintenance
costs, during ramp-up and the specified warranty period, as well as the remaining
life-cycle stages of an engineered installation, is the input resource of spares. Spares
for engineered installations can be grouped according to two categories:
• Contract spares

• Maintenance spares.
Contract spares are normally part of the initial procurem ent of systems and equip-
ment, and are determined by available reliability data from the manufacturer or ven-
dor. The main concern with contract spares is not so much the quantity,or individual
cost, but rather their identification. Determination of maintenance spares is achieved
through the method of maintenance spares requirements planning (SRP).
SRP can be defined as “a strategy involvingthe purchasing, supply, identification,
storage and issue of spare parts which improves system maintenance and results in
an increase of plant availability”.
SRP is different from inventory control. SRP is better suited to maintenance
spares that have a high-risk component failure and estimated equipment failure rate.
Inventory control is better suited to maintenance spares with low-risk component
failure and estimated stock levels. With SRP, the required spares are calculated ac-
cording to the estimated failure rate of the relevant equipment, and according to the
criticality of the equipment with regard to downtime costs.
Inventory control is a resource management system that makes use of calculated
order-points, reorder quantities, and forecasts of the stock level at which stock must
be replenished as well as the quantity to be ordered. It is evident that SRP considers
single items of spare parts for equipment when they are needed, whereas inventory
control considers many items to be placed into stock until they are needed.
SRP deter mines the efficiency level of the availability of spar es for maintenance,
and thus minimises downtime as well as avoids holding unnecessary spare parts in
stock. Inventory control determines the service level of the stores in not being out of
4.2 Theoretical Overview of Availability and Maintainability in Engineering Design 381
stock with spare parts, and thus also optimises on spares stock levels (Orlicky et al.
1970).
Both SRP as well as inventory control are important to managing spare parts for
maintenance, but it is essential to understand that each of these methods are ap-
plied to specific types of spares. The types of maintenance spares that are m anaged
through SRP and inventory control are determined from the demand for these spares

by the type of maintenance action. There are two types of d emand for maintenance
spares:
• Dependent demand
• Independent demand.
Dependent demand for maintenance spares relates to the need for the replacement
of other components of which the maintenance spare is a part. Dependent demand
is based on the systems h ierarchy structure of the process or equipment that forms
the b asis of a bill of spares for a spares requirements planning system. Independent
demand for maintenance spares relates to the demand of the maintenance spare on
its own, and is not subject to the need for other components or parts. Independent
demand is based on forecast usage of the spares that forms the basis of order-points
and reorder quantities for an inventory con trol system. It is evident fro m these de-
scriptions that different categories of spares can be grouped under the two types of
demand. There are several general categories of maintenance spares:
• Consumable materials (materials that are used up through the maintenance ac-
tion, such as oils, greases, waste cloth, etc.).
• Consumable spares (spares that are used up in the operation of the equipment or
process, such as filters, pump impellors, turbine blades, tube bundles in coolers,
etc.).
• Replacement spares (parts that become worn through excessive usage or insuf-
ficient routine maintenance, or that need to be replaced due to defects, damage
or failure. These spares are mostly the p arts of components such as bearings,
sleeves, liners, etc.).
• Repairable spares (assembled units that are repaired or overhauled through the
replacement of parts and then returned to stores (RTS) for later re-issue, such as
electric motors, valves, pumps, etc.).
• Critical spares (spares that are kept in stores for insurance against hazardous fail-
ures of critical equipment, such as special high-pressure or acid resistant valves,
high-voltage electrical parts, etc.).
• Strategic spares (spares that are kept in stores for insurance against high down-

time costs due to long ordering lead times, such as special alloy parts, specialised
engineered parts, etc.).
There is a further category that is called capital spare s, which are not really main-
tenance spares and consist of assembled units that are very expensive and are usu-
ally categorised by very high capital equipment industries such as power generation
plants. Most stores in industry make use of an ABC classification system to cate-
gorise the types of stock being held but, in many cases, this ABC classification has
proved to be inadequate to support effective maintenance strategies.
382 4 Availability and Maintainability in Engineering Design
Dependent demand maintenance spares usually consist of some replacement
spares, repairable spares, critical spares and strategic spares that are stocked be-
cause of the risk or frequency of failure of the relevant equipment. These spares
are controlled through a spares requirements planning (SRP) system. Preventive
maintenance makes use of dependent demand maintenance spares, and is therefore
associated with SRP.
Independent demand maintenance spares usually consist of consumable mate-
rials, consumable spares and some replacement spares that need to be stocked ir-
respective of the frequency of component replacement. These spares are controlled
through an order-pointand reorder quantityinventory control system. Routine main-
tenance makes use of independent demand maintenance spares, and is thus associ-
ated with inventory control.
Because the sort of maintenance spares that are controlled through an SRP sys-
tem are typically the logistic support spares required for shutdowns and general
overhauls (i.e. some replacement spares, repairable spares, critical spares and strate-
gic spares that are stocked because of the risk or frequency of failure of the relevant
equipment), SRP is extremely important for the effective application of preventive
maintenance, and also for the effective use of contracted maintenance crews during
shutdowns and overhauls (Hillestad 1982).
Mathematical modelling of spares requirement Most spares requirements opti-
misation models assume the constant failure rate to be a good approximation for

a constant demand rate, even if components have non-constant failure rate distribu-
tions. Such a failure rate is fundamentally a measure of the intrinsic failure charac-
teristics of a componentbrought about by usage stress and load over time. However,
it is not quite correct to express the demand rate for a spare simply by the intrinsic
failure characteristic of a component.
In most cases, the demand for a given spare is the result of a number of fac-
tors. Firstly, there may be several different items of equipment that require the same
spare. Secondly, there could be several similar parts in each component. Thirdly,
there are usually a large number of similar components within each system. Clearly,
it is cumbersome to derive the exact spares demand based o n the component fail-
ure rate. Furthermore, it is somewhat unrealistic to assume a specific failure rate
of a component within a complex integration of systems with complex failure pro-
cesses. At best, the intrinsic failure characteristics of components are determined
from quantitative probability distributions of failure data obtained in a somewhat
clinical environment under certain operating conditions. As indicated before, the
true failure process depends upon many other factors, including, for example, rou-
tine and preventive maintenance. It is generally accepted that preventive mainte-
nance affects the failure properties of components, although it is debatable whether
the end result is positive or negative from the point of view of equipment residual
life.
When modelling spares requirements, the foremost criterion to take cognisance
of is that the need for spares is determined by a spares demand. This demand is
formed by and dependent upon several factors, such as (Alfredsson et al. 1999):