When is a steam blanket lay-up considered lost?

A steam blanket lay-up is considered lost when the steam pressure falls to atmospheric pressure. This is because a steam blanket creates a protective layer of steam above the water, preventing heat loss and maintaining a certain temperature within the system. When the pressure drops to atmospheric levels, the steam blanket is no longer effective, leading to potential heat loss and a change in the water chemistry that could lead to corrosion or damage. The integrity of the lay-up is compromised, necessitating immediate attention to maintain system safety and equipment longevity.