Final answer:
The lifting of a safety valve on a hot water boiler is usually caused by excessive pressure within the system, often due to an overfilled expansion tank without proper air cushion or a malfunction in the pressure-regulating components.
Step-by-step explanation:
When operating a hot water boiler, the safety valve may lift due to several reasons, one of which could be excessive pressure within the system. The safety valve is a critical component designed to release pressure and prevent potential damage or explosion if the pressure exceeds safe limits. In a hot water boiler system, excessive pressure can build up if the water temperature is too high, if there is a failure in pressure-regulating systems, or if there is an overfilling of the expansion tank. In particular, the expansion tank's function is to absorb excess pressure due to the thermal expansion of water as it heats. If the tank is filled with water rather than having the required air cushion, it cannot absorb this increased volume, resulting in increased pressure, which could trip the safety valve.
Other possible causes for a safety valve to lift can include a malfunctioning pressure relief valve or a blocked outlet which does not allow the steam or hot water to flow as intended, thereby increasing the pressure to dangerous levels. It's essential that safety mechanisms such as the safety valve are regularly maintained to ensure the safety and proper operation of the boiler system.
It is important to note that contrary to what might be assumed, safety valve lifting would not be caused by water being too cold or not having enough pressure. Instead, it typically occurs due to too much pressure, which might be erroneously assumed to be due to high efficiency or other factors leading to unsafe operating conditions.