Answer / carl_ellis

Air-to-close means that the valve fails open.

A boiler feedwater valve should 'fail open' so that the drum
level never drops too low, which can be an unsafe condition
and/or a damaging condition.

For that same reason, a boiler's feedwater valve should be
driven open on a low-low drum water alarm to prevent the
drum level from dropping too low and creating an unsafe
condition and/or a damaging condition.

Carl Ellis
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