On structured fault tree construction by modularizing control loops

The objectives of this article are: (1) to develop the fault trees for control loops in a way that they all appear in a proper form and as modules in the fault tree for the whole system; and also (2) to modularize each control loop in a system properly to establish its unit model. These two methods are essentially equivalent. Either of them can be applied to facilitate constructing fault trees for the whole system. To accomplish such equivalent or parallel objectives, we will first take the feedback and feedforward control loops in the heat exchanger system as examples and then: (1) construct the fault trees of deviations in controlled variable for the two control loops in the way that all of their non-basic terminal events should be deviations in variables on those streams that connect to the loop from their outside in the system; and (2) establish the unit model for each of such two loops. One of the purposes of (2), which will not be addressed here, is to regard each control loop as a dummy unit and hence reduce the system to facilitate system input and fault tree construction eventually for complete automation purpose.