ABSTRACT

This exercise is based on a real scenario. Spaces have been left for the reader to attempt the calculations. The answers are provided in Appendix 5.

11.1 The unprotected system

Consider a plant supplying a 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 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.