Introduction

Machinery uptime is related to availability but not synonymous with it. Availability or service factor may be simply stated as:

where A = availability, MTBF = mean time between failures, MTTR = mean time to repair.

Availability in our context is recognizing the fact that many plant items can be repaired when they fail or their probability of failure can be reduced. This is illustrated in Figure 14-1. It shows that the securing of a required function of plant equipment is a combination of its built-in reliability and its maintainability.

Figure 14-1: Machinery availability components.

Reliability, shown as a component of quality in Figure 14-2, exists as a requirement of plant equipment from its beginning to the end of its working life. The creation of reliability lies essentially in the sphere of design.

Figure 14-2: Machinery reliability and quality.

We have seen that acceptable built-in equipment reliability is achieved by the evolution of design procedures, codes of practice such as API standards, or methods that have shown good results in the past. As a consequence, the hydrocarbon processing industry, for example, usually refers to equipment as "reliable" or "unreliable" in a qualitative sense.

In certain services, where high reliability is required, the industry has, often intuitively, made use of the redundancy principle. Examples are installed spare pumps, compressors and heat exchangers. However, in the recent past, many companies have provided non-spared installations in traditionally redundant services. It is here where built-in reliability is of utmost importance.

Maintainability is defined...