TABLE OF CONTENTS Overt and covert failures
Failure direction
Directed failure states
What instrument failures indicate
In the previous chapter we talked about failures in general. In this chapter we will discuss several ways of classifying failures: overt and covert, unpredictable and directed, and several types of directed failures, in which the instrument itself detects the failure and directs it toward a particular end state.
Failures can be overt, which means they are self-revealing: they announce themselves as a failure to perform a function that is monitored by another device or by plant personnel. An example of this might be a level-control valve installed on the inlet of a tank that is designed to shut when it fails. If the level decreases, an operator or low-level alarm detects the failure. Many instruments have directed failure modes that make failures more obvious, such as fail-closed or fail-open. In continuous control systems such as basic process control systems (BPCS), many failures are self-revealing because they are continuously monitored by operators or alarm systems.
In demand systems, such as safety systems, failures are not always so obvious. These systems only operate when requested or "demanded." In these systems, and occasionally in continuously operated systems, failures can "lie in wait" and fail at what seem the most inopportune times. These are called hidden, covert, or latent failures. Such failures often appear after troubleshooting another failure, after a demand is placed on the system, or during routine testing. Testing is the most...
