Failures hidden modes

The main purpose of identifying failure detection methods is to determine whether the failure mode is "hidden," i.e., not detectable for some period of time. If there is no means to detect failure, "none" should be entered into the worksheet. 

The FHA should identify all functions associated with the system under smdy. The equipment s functionality, capability, and limitations must be deliberately incorporated (i.e. there must be no hidden functionality, as a function not assessed might have undesired failure modes). 

A preventive maintenance policy has an important role to achieve higher levels of availability in any industrial system. However, in many industrial plants, there are safety systems whose components are in standby mode. In standby mode, there is no way to know a priori if the system s components are in operational condition or not. For that, periodic tests should be scheduled so that hidden failures can be revealed and corrective maintenance actions be taken. 

The new approach is applicable for low demand SIFs, which are the most common SIFs in the od and gas industry. A low demand SIF is passive during normal operation and is intended to respond if a specific hazardous event occurs. In passive mode, limited information is available about the system state. Regular functional testing is therefore important to reveal failures that otherwise would be hidden until the next demand. The approach presented in this paper suggests criteria for how to adjust the functional test intervals, based on operational experience. 

Abstraction is a view of an object that focuses on the information relevant to a particular purpose and ignores the remainder of the information [12], So, to define hierarchical abstraction from the failure realization to the specification, the difference between their purposes has to be determined, which is only the hierarchy level. This means that the specification has to abstract from the same information as the realization, plus from the inner details of the component that are shown in the realization. For the hierarchical abstraction of the functional specification from the functional realization this means that the subcomponents and their collaboration are hidden, as well as that the functionality of the component is specified only in terms of the component itself The failure realization must have the same level of hierarchical abstraction as the fiinctional realization and the failure specification the same level as the functional specification. This requirement is true for SpeedControl. The hierarchy level of the failure realization in Figure 4a is consistent with its functional reaUzation in Figure la, because it only shows failure modes of the functional in- and outputs as well as failure modes of the subcomponents that are shown in the functional reaUzation. The hierarchy level of the corresponding failure specification in Figure 4b is consistent with the functional specification in Figure lb, because it hides all details of the subcomponents and only shows the failure modes of SpeedControl (S). 

An essential requirement of the diagnostic tests for the PLC is that they should cover a high percentage of potential faults. Diagnostics linked to appropriate control actions convert potentially dangerous hidden faults into safe mode failures of the PLC. If a substantial number of potential faults remain that cannot be detected by diagnostics then much of the benefit is lost. 

---