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in my plant 100% feedwater control v/v is air to close but
due to instrument air fail,the valve opens but instead of
drum level high its get tripped in low low drum level.the
urv of level tx is 0 and lrv is -800.pls tell me the reason.
for such behaviour.
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Answer Posted / 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
Measure First
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