One purpose of the small valve on top of the shell or the water column on most boilers is to:
a. Signal when the boiler is full of water r.
b. allow water into the boiler when the boiler is being drained or cooled preventing the formation of a vacuum.
c. allow water to escape before the steam stop valve is opened during the warm-up period.
d. allow air to escape from the boiler when the boiler is being filled with water.
e. allow air into the boiler when the boiler is being filled with water.
Answer: D