How to Drain the Steam Boiler

A ship's boilers must be drained for servicing.

When a ship's master sends the order "Finish with engines," that tells the ship engineer that it's time to take the ship's steam boilers off line. When the engineer is also told to "go to cold iron," that means it is time to shut down the boilers and, using the scum-blow valves on the bottom of the boiler's drum -- its body -- to drain the water from the boiler, for maintenance, repair or control replacement.

Instructions

1. Turn the burners down to their minimum level after you receive the order to “Finish with engines.” Allow the burners to drop to the low-fire level. Open and shut off the automatic feedwater-control valve, preparing the boiler to drain when the water-column water-level-glass gauge -- often called the boiler's "sight glass" -- empties.

2. Allow the steam pressure to drop naturally. The cooling rate should not exceed 150 degrees F per hour -- an average of 2 1/2 degrees per minute. If the temperature drops more quickly, close the boiler-vent valve until the boiler temperature is dropping between 2 and 2 1/2 degrees per minute, based on the boiler’s temperature gauge.

3. Open any unopened vents when the steam pressure drops to between 15 and 20 lbs. per square inch, according to the boiler’s steam-pressure gauge. Close the main steam-stop valve on the steam line. Open the steam drum's air-release vent valve.

4. Allow the boiler to cool to less than 200 degress F, according to the boiler’s temperature gauge. Allow the fan to operate for five minutes to prevent steam drum distortion.

5. Open the overboard discharge valve. Open the two boiler blow-down valves. Fully open the scum blow valve on the bottom of the boiler. Allow the water to drain. Close the overboard-discharge valve, the two boiler blow-down valves and the scum-blow valve after the flow of water from the overboard discharge ceases.

Tips & Warnings

• This process is both hazardous and complicated. It must be performed only by persons holding the necessary license or endorsements such as "Qualified Member of the Engine Department."

References

• "Operation and Maintenance of Machinery In Motorships"'; N.E. Chell; 1999