Discussion:
Below is a so called "piping and instrumentation diagram (P&ID) for a batch chemical process. Assume that high pressure steam, a hydrocarbon mixture, and a catalyst are fed into a tank, and that a single operation of the system takes ten hours. If the pressure in the tank exceeds some level, a weld will burst, destroying the tank. The pressure is regulated by a computer-operated control circuit, designed to maintain the tank pressure between specified minimum and maximum values. If the computer fails, a light comes on in a control room to signal an operator to regulate the pressure manually. Assume that the steam supply exceeds normal pressure 0.1% of the time, that the computer controls overpressures 98.5% of the time, and that the operator is available to see and react to the light 75% of the time. The tank is also equipped with a primary pressure relief valve (PRV 1) that is expected to open successfully 99.9% of the time. The primary pressure relief valve is backed by a secondary pressure relief valve (PRV 2) that is expected to open successfully 95% of the time that it is needed.
Below is the fault Tree for this example.
1. Calculate the probabilities of tank failure (unsafe operation) by inserting numbers into the fault tree diagram. You may write in the values, scan the sheet, and submit it as part of your assignment. You may also set up a fault tree diagram on an Excel spreadsheet and do the calculations that way.
Be sure to show all your calculations, so that you may receive part credit if the answer is only partially correct.
2. What measures could be taken to reduce the probability of unsafe operation?