are often the basis for significantly improving the design and operation of chemical and petro- chemical plants.
1 1-1 Review of Probability Theory
Equipment failures or faults in a process occur as a result of a complex interaction of the indi- vidual components. The overall probability of a failure in a process depends highly on the na- ture of this interaction. In this section we define the various types of interactions and describe how to perform failure probability computations. Data are collected on the failure rate of a particular hardware component. With ade- quate data it can be shown that, on average, the component fails after a certain period of time. This is called the average failure rate and is represented by p with units of faultsltime. The probability that the component will not fail during the time interval (0, t) is given by a Poisson distribution1 :
where R is the reliability. Equation 11-1 assumes a constant failure rate p. As t + oo, the reli- ability goes to 0. The speed at which this occurs depends on the value of the failure rate p. The higher the failure rate, the faster the reliability decreases. Other and more complex distribu- tions are available. This simple exponential distribution is the one that is used most commonly because it requires only a single parameter, p. The complement of the reliability is called the failure probability (or sometimes the unreliability), P, and it is given by
The failure density function is defined as the derivative of the failure probability:
The area under the complete failure density function is 1. The failure density function is used to determine the probability P of at least one failure in the time period to to tl:
f