Parallel Configuration in the Sense of Reliability

We consider the system of Fig. 9.15. The fluid can reach the outlet in sufflcient quantity through any one of the two valves. Hence, the undesired event interruption of flow can only occur if both valves fail adopting their closed position ( AND gate, logical intersection). 

For this case we have the following structure function [30] 

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Fig. 9.15 Flow sheet and fault tree for a parallel configuration in the sense of reliability...

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