Blowdown of steam boilers is very often a highly neglected
or abused aspect of routine boiler room maintenance. The purpose of boiler
blowdown is to control solids in the boiler water. Blowdown protects boiler
surfaces from severe scaling or corrosion problems that can result otherwise.
There are two types of boiler blowdowns - continuous and manual. A continuous
blowdown utilizes a calibrated valve and a blowdown tap near the boiler water
surface. As the name implies, it continuously takes water from the top of the
boiler at a predetermined rate
A
continuous blowdown is an optional feature and may not be
included on your steam boiler. However, all steam boilers should include a
means for manual blowdown as standard equipment.
Manual blowdowns are
accomplished through tapings at the bottom of the boiler. These openings allow
for the removal of solids that settle at the bottom of the boiler. Manual
blowdown is also used to keep water level control devices and cutoffs clean of
any solids that would interfere with their operation. All steam boilers require
manual blowdown whether or not they are supplied with continuous blowdowns.
Blowdown should be done with the boiler under a light load.
Open the blowdown valve nearest the boiler first. This should be a quick
opening valve. Crack open the downstream valve until the line is warm. Then
open the valve at a steady rate to drop the water level in the sight glass ½
inch. Then close it quickly being sure that the hand wheel is backed off
slightly from full close to relieve strain on the valve packing. Close the
valve nearest the boiler.
Repeat the above steps if the boiler has a second blowdown
tapping. Water columns should be blown down at least once a shift to keep the
bowls clean. Care should be taken to prevent low water shutdown if this will
affect process load.
Please keep in mind that all blowdown piping should be
checked once a year for obstructions.
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