Читать книгу System Reliability Theory - Marvin Rausand - Страница 151

A heavy (i.e. heavier than air) and dangerous gas is used in a chemical process. If a gas leakage occurs, it is important to raise an alarm and shut down the process as fast as possible. For this purpose, a safety‐instrumented system (SIS) is installed, with one or more gas detectors. The SIS has three main parts (i) gas detectors, (ii) a logic solver that receives, interprets, and transmits signals, and (iii) a set of actuating items (e.g. alarms, shutdown valves, door closing mechanisms). The purpose of the SIS is to give an automatic and rapid response to a gas leakage. Many more details about SIS may be found in Chapter 13.

Assume that a gas leak has occurred without any response from the SIS. Possible causes of the failure may include the following:

 A primary (i.e. random hardware) failure of the SIS.

 The installed gas detectors are not sensitive to this particular type of gas, or have been mis‐calibrated.

 The gas detectors have been installed high up on walls or in the ceiling (remember, the gas is heavier than air.)

 The gas detectors have been installed close to a fan (no gas will reach them.)

 The gas detectors have been inhibited during maintenance (and the inhibits have not been removed.)

 The gas detector does not raise alarm due to a software bug. (Most modern gas detectors have software‐based self‐testing features.)

 The gas detector is damaged by, for example, sand‐blasting. (Has happened several times in the offshore oil and gas industry.)