Page 28 - Boiler plant and distribution system optimization manual

Boiler Plant Orientation 13

THE CAUSES OF PRIMINg
Water Surging
TDS TOO HIgH
• Sudden fluctuations in steam
• Corrosive to boiler metal
pressure [demand]
• Causes foaming and carryover
• Water level too high
• Alters boiling patterns in tubes
• Operating above boiler rating
leading to deposits.
• Damaged steam separating equip-
ment

solids at a level less than 8 percent of the TDS. water controls or incorrect blowdown procedures.
Antifoam agents are added to boiler water to help Figure 1.15 illustrates how priming occurs.
control foaming.

START-UP
PRIMING CARRYOVER
As steam pressure is increased, the water
Priming carryover is caused by liquid surges level should be carefully controlled within nor-
in the steam drum that throw water into the steam mal limits. Prior to picking up load, it is desirable
space where it is carried into the steam header. to keep the water level near the lowest safe lev-
Priming is caused by a mechanical problems or el to allow for thermal expansion of the water as
mechanical properties such as oversensitive feed- the steam generation rate increases. When raising

Figure 1.15—violent ebullition caus-
es changes in drum water level and
unacceptable discharge of water in
the steam. The upper sketches show
two conditions registering normal
water level. In the right drum the
true water level is higher because it
is being forced up by violent steam
action of steam in the water causing
an undetected water level offset. In
the left sketch a low steam gener-
ation rate does not offset the water
level from the observed level. In the
lower sketch, longitudinal mound-
ing near the steam nozzle is caused
by violent steam formation aided
by low steam pressure in the steam
system piping. Also, dynamic forces
may set up a sloshing action causing
localized high water and wave like
motion contributing to priming.