ELECTROSTATIC PRECIPITATION 287
Initially, there was no reason to believe that these units
would create an excessive expansion problem for the sup-
porting steel, even with the high operating temperature
expected. Calculations indicated that only a 20° temperature
rise would result overall within the enclosed building with
the possibility of a few local hot spots developing similar to
what you would normally find in the boiler house.
Boiler 30 started out in construction as an oil fired unit
with provisions included to convert to coal fired at some
future date. It was decided during the initial construction to
convert immediately even if it would not be ready to burn
coal until two years after its initial operation burning oil.
Due to the close erection schedule which the coal con-
version work on the unit was to follow, it was necessary to
start the equipment foundations and supporting steel in the
field as quickly as possible. Thus the steel was designed
before the equipment design was completed.The four units were designed to be installed on two eleva-
tions in a double deck fashion as shown in Figure 9. The lower
units were supported from steel erected on the 42 8 eleva-
tion (Grade elevation is 15 0 ) which allowed for a con-
venient column arrangement below. The upper units, however,
were to be supported from the 126 2 level and presented
a more difficult column design. Since the interior columns
that supported the lower units could not be carried up through
the equipment to the upper supports, deep girders had to be
utilized to span over the lower units to provide the required
support. The maximum span required was 90−0 which
resulted in a girder depth of 91½ for a total girder weight
of approximately 110 tons. A high yield strength steel was uti-
lized for these girders (ASTM A440 − f y = 46 KSI). Bracing
the building was also a challenge because of the equipment
space requirements, and could only be provided around the
periphery of the units except below the 42 8 elevation.
When the hopper detail drawings were received for the
precipitator sections, it was noted that the upper hopper
plate stiffeners were located very close to the steel support
girders (see Section 1−1 in Figure 11). It was obvious that
the hopper insulation above these upper stiffeners would be
almost impossible to filed install with the stiffeners in place.
But more important, the supporting steel member was now in
a “heat pocket” which would not afford much air movement
for cooling. Since the supporting members were designed
for the same top elevations in both directions, this greatly
reduced the possibility of air movement longitudinally along
the interior members. This condition existed at all hopper
locations at both levels. The possibility of over-heating the
supporting steel resulting in an excessive outward movement
of the support columns now had to be reckoned with for the
safety of the structure.
Schemes were immediately proposed to provide some
type of forced ventilation or cooling system which were
dismissed because of lack of space as well as for economy
reasons. Conditions in the field at the time were such that
erection of the lower steel supports was nearing completion,
and the lower precipitator shell plates were being delivered
to the job site. A solution had to be found which would not
delay the precipitator erection, and yet result in a stable
structure under significant expansion movement.
The answer was to reduce the relative expansion of sup-
port framing in any one direction by providing expansion
points at certain key connections. The centerlines for the north
and south units were located on the 390 and 320 Column
Lines respectively (see Figure 11). It was felt that the build-
ing should move north and south of these lines symmetrically
as the building heated up during operation. This could be
done by stiffening the support steel on these centerlines and
by cutting loose the connections on the 36 Line and replace
the fixed connections with movable ones. This meant that the
maximum expansions would take place by moving the 29 and
430 Column Lines outward and at the same time allowing the
center of the building (36 Line) to absorb the inward expan-
sions on sliding connections. This approach had the advan-
tages of having to cut loose only one column line instead of
several, and also it reduced individual relative expansions toCOLD (^) PR
ECIP
ITAT
OR
(^99). (^5) %
(^99). (^75) %
99.5 %
HOT PRECIPITATOR 99.75 %
COLLECTION EFFICIENCY
COLLECTING AREA - SULPHUR
300°F AND 600°F
PERCENT SULPHUR IN COAL
(^00)
100
200
300
400
500
700
600
800
900
1000
1100
1200
1300
1400
1500
1600
1700
1800
1900
2000
2100
2200
2300
2400
2500
0.5 1.0 1.5 2.0 2.5
COLLECTION AREA PER MW
SQUARE FEET/
MW
FIGURE 10
C005_005_r03.indd 287C005_005_r03.indd 287 11/18/2005 10:22:02 AM11/18/2005 10:22:02 AM