Generally, a pipeline’s reliability includes consideration of all leak/rupture scenarios PLUS all other scenarios that could prevent the system from performing its intended function. With the normal ‘intended function’ being something like: deliver product to point B from point A at specified flow rates, pressures, temperatures, and product quality, a reliability failure occurs when any of the delivery parameters is not met. So, blockages, contaminations, and others can cause reliability excursions. However, excursions do not necessarily impact a customer. If the customer is not impacted, perhaps there are no significant consequences. For a reliability assessment, it may be appropriate to only consider customer impact events as ‘failures’.
Generally, excursions from intended delivery parameters that affect a customer are included as ‘failures’ when the risk assessment is expanded beyond leak/rupture type failures. This makes the risk assessment more complicated but also more useful in certain applications. See detailed discussion in textbook.