CleaverBrooks FLX Operation, Service And Parts Manual page 59

Pressure Vessel Care
control system, in order to lower the concentration of solids
in the water.
Solids are introduced to the boiler with the feedwater, even
though this water may be treated prior to use. These solids
become less soluble when the water is heated and evaporated,
and tend to accumulate on heating surfaces.
Periodic blowdown and chemical treatment are necessary to
prevent concentration of solids in the boiler water, and
attachment of these solids to waterside heating surfaces
(scaling).
Scale has a low heat transfer value and acts as an insulating
barrier on heating surfaces. A buildup of scale will result in
lower operating efficiency and, consequently, higher fuel
consumption. More importantly, scale buildup can result in
overheating of boiler metal. This can result in tube failures or
other pressure vessel damage.
! DANGER
Boiler and water level control blowdown
must be performed on a regular basis to en-
sure that concentrated solids are removed
from the boiler and in order to avoid damage
to the equipment.
Water column and gauge glass blowdown valves are located
on the water column assembly. The boiler blowdown
tapping(s) can be found at the bottom of the lower drum.
Most blowdown lines are provided with two valves. These
are generally a quick-opening valve nearest the boiler and a
slow-opening globe-type valve downstream. Valves will vary
depending upon pressure involved and the make or
manufacturer.
Blowdown Procedure
Blowdown is most effective when the boiler water is hot and
the burner is being fired at the lowest rate. This ensures that
the water in the boiler is being circulated, and that the solids
in the water are in suspension.
WARNING
! DANGER
Be sure that the blowdown piping is in good
condition, the discharge vents are clear of
obstruction, and that the waste is piped to a
safe point of discharge, in order to avoid
serious personal injury or death.
If a quick-opening valve and globe-type or slow-opening
valve are installed, the quick-opening valve is normally
opened first and closed last. Control of the water released
from the boiler is accomplished with the slow-opening valve.
750-177
CAUTION
! DANGER
When initially opening the blowdown valve,
open the valve slowly to heat the discharge
piping. Failure to follow this procedure
could result in rapid expansion and damage
to the piping.
The drop of the water level in the gauge glass can be used in
determining the length of time that the blowdown valve is left
open. This is to be used as a reference only, as proper water
analysis on a regular basis will serve as an indicator of the
effectiveness of the blowdown procedures used.
! DANGER
Do not pump the lever action valve open
and closed when draining water during
blowdown. The hydraulic forces resulting
from this pumping action could break the
valve bodies or pipe fittings in the blow-
down lines.
Blowdown valves should be closed in a specific order after
draining water for blowdown. Close the downstream (slow
opening) valve first, followed by the quick-opening valve
next to the boiler. Open the downstream valve slightly to
release the water trapped between the valves, then close the
valve again.
The water column and gauge glass should be blown down by
draining until the water in the gauge glass is clear. Open and
close the water column and gauge glass blowdown valves
slowly, allowing the water in the gauge glass to rise to a
normal level before repeating the process.
Under no circumstances should a blowdown valve be left
open and unattended during the blowdown operation.
Frequency of Blowdown
In practice, the boiler blowdown valve(s) should be opened
periodically in accordance with a set operating schedule.
Frequency and duration of the blowdown are to be
determined by chemical analysis of boiler water and
waterside boiler condition, as observed during regular
inspections.
From an economy standpoint, frequent short blowdown is
preferred to irregularly scheduled, lengthy blowdown. This is
particularly true when the suspended solids content of the
water is high.
Chapter 3
CAUTION
CAUTION
3-5