Consider a plant supplying gas to offsite via a twin stream pressure control station. Each stream is regulated by two valves (top of
Figure 11.1). Each valve is under the control of its downstream pressure. Each valve is closed by the upstream gas pressure via its pilot valve, J, but only when its pilot valve, K1, is closed. Opening pilot valve K1 relieves the pressure on the diaphragm of valve V, allowing it to open. Assume that a HAZOP (HAZard and OPerability) study of this system establishes that downstream overpressure, whereby the valves fail to control the downstream pressure, is an event which could lead to one or more fatalities.
Since the risk is offsite, and a two-fatality scenario assumed, a target maximum tolerable risk of 10−5 per annum has been proposed.
Assume that a quantified risk assessment has predicted a probability of 20% that failure, involving overpressure, will lead to subsequent pipe rupture and ignition. Furthermore it is predicted that, due to the high population density, fatality is 50% likely.
Assume also that the plant offers approximately 10 risks in total to the same population.
It follows that the target failure rate for overpressure of the twin stream subsystem is
Assume, however, that field experience of a significant number of these twin stream systems shows that the frequency of overpressure is dominated by the pilots and is 2.5 × 10−3 pa.