(a) Explain the concept of a fail-safe and fail-set system on a ship, providing examples of each system. (6)
(b) Describe the advantages and disadvantages of both systems. (5)
(c) How does the design difference impact overall reliability and safety of the vessel? (5)
Structured for DG Shipping MEO Class II examination scoring criteria.
A fail-set system, on the other hand, locks the system in the position it was in at the time of the failure, maintaining the current state rather than moving to a safe end position. This allows the plant or equipment to remain stable and potentially continue operation or wait for a controlled shutdown. An example is the boiler water level control valve that remains in the position it was before the control air supply failed, giving time to normalize conditions or re-establish control air.
System | Advantages | Disadvantages |
Fail-safe | - Ensures system moves to safe condition automatically on failure. - Minimizes risk of damage or accident immediately. | - May cause abrupt shutdown or change that disrupts operation. - Could lead to loss of stability if moved suddenly in some systems. |
Fail-set | - Maintains stable operation or condition during failure. - Allows time for safe, controlled shutdown or rectification. | - If failure occurs in dangerous or unsafe position, risk can persist. - Does not automatically protect system from harm in all cases. |
Fail-safe systems generally improve safety by ensuring that any failure leads to a condition that minimizes harm or damage, increasing protection for machinery, environment, and personnel. However, their automatic action can sometimes lead to operational interruptions or require backup systems to deal with the consequences of the fail-safe position.
Fail-set systems prioritize operational reliability and stability during failure by holding the current state, thus avoiding abrupt changes that may cause further damage or unsafe situations. But they may not always prevent hazards if the position at failure is unsafe.