TABLE OF CONTENTS In Chapter 9 block and age preventive replacement policies were discussed. There may be some situations, however, in which neither of these policies may be applied. For example, the age-based preventive replacement policy can not take advantage of periods when replacement is particularly cheap, and it may be difficult to schedule several replacements at the same time. Also, after an in-service failure, the block policy may result in the replacement of a nearly new component.
To overcome these disadvantages, Woodman [1] proposed a method by which the drawbacks of both policies can be avoided. In fact the policy that is developed includes the age and the block replacement policies as its special cases. The improvement is brought about by the simple expedient of replacing a component after a failure, but not always replacing it when other replacement opportunities occur. Such policies, where replacement is not obligatory at every opportunity, are called Optional Replacement Policies. There are many industrial situations in which optional policies are relevant. For example, a plant is frequently operated on a semi-continuous basis with regular shifts provided for maintenance; these present opportunities for preventively maintaining due components. An Optional Replacement Policy is required under these conditions since it would rarely be economical to replace all components at every maintenance shift, because not all ofthem will have reached and/or exceeded their prespecified replacement age.
General results about the best optional policies have been obtained for components...
