During the HAZOP analysis, the shutdown of the plant is not always considered as a possible consequence; so what are the consequences of a shortage of instrument air?

When the instrument air is cut off, the consequences vary depending on each device, and a specific analysis is required.

In the event of a loss of instrument air, the storage tank can provide supply for a certain period of time. For small installations, operating the control valves through bypass lines can allow continued operation. However, for most installations, it takes time to achieve complete and stable control, which requires human intervention; as a result, some units may cycle repeatedly with some leakage occurring, but generally there is no need to shut down the operation – making adjustments on-site will suffice to restore stability. A detailed plan is required for shutting down the instrument air; a detailed list should be prepared based on the on/off status of the air supply, and the sequence of adjustment for each unit must be determined. The possibility of a loss of instrument air is very low, unless there is a power outage across the entire plant; it is important to ensure that the gas storage tanks are properly maintained as backups.

The iron law of HAZOP is also the biggest problem in the industry today. When assessing the consequences, be sure not to take any protective measures into account; otherwise, you will not obtain the truly worst outcome.