Encyclopedia of Environmental Science and Engineering, Volume I and II

ELECTROSTATIC PRECIPITATION 289

at Column QQ36 at the +126−2 elevation. Detail B in

Figure 11 shows the ball bearing type that was selected. The

bearings (50 per assembly) were^7 / 8  in diameter, and were

made of M-50 steel as were the top and bottom assembly

plates. The ball bearing retainer was made of stainless steel.

The reaction of the G4 girder at Column QQ36 was 640 kips,

which ruled out a ball bearing design as the size that would be

required was too large for the space available. It was decided

to hang the huge girders in a pendulum fashion from a bracket

above, and allow them to expand by swinging. The connection

finally used is shown in Detail A in Figure 11. For the hanger

rod material, a high temperature service chrome-moly steel

was selected (ASTM 193-Grade B7). The bar material was

heat treated and stress relieved. Mill tests on the material used

indicated yield points of 98 KSI and higher. The total required

length of only 4¼ diameter rod was 19−10

Horizontal bracing has to be added to the 36 Column

Line to carry interior lateral loads out to the vertically

braced RR and MM column lines. This was accomplished

by adding two horizontal trusses, one at elevation 38-11

and the other at elevation 114−11 (see Section 2–2 in

Figure 11). It was possible to place the lower truss just

below the expansion connections for the precipitator sup-

port steel at elevation +42−8 so the “vertical” members

of the horizontal truss could also act as supports for the ball

bearing assemblies required for the lower units. This was

not the case for the upper units as the existence of other

steel (including the G4 girders) made it impossible to install

the upper truss any closer to the +126−2 elevation. New

support brackets were added to the 36 line columns to sup-

port the assemblies for the upper precipitator units.

The last major design obstacle to be overcome was the

exterior column movement that would result from the 2½

outward expansion of the framing at elevations +42−8 and

+126−2. It was accomplished by reinforcing the column

base billets (and in a few cases by extending the footings as

well) to transfer the eccentric column loads which would

be carried down predominantly by the exterior flanges.

Section 3–3 in Figure 11 indicates the type movement

expected. It was also necessary to cut free the support steel

of some platforms at elevation +25−2 so as not to restrain

the exterior columns close to their bases which would

induce high moment forces in the columns. This platform

steel was resupported on lubrite plate bearings before it was

cut free to expand. The column splice plates were found to

be flexible enough to carry the moments through the joints

at the expanding levels.

Construction Stage

Once the necessary alterations were designed and approved,

the difficult task of implementing the changes in the field

still remained to be done. One major limitation in this phase

of the job was that the precipitator erector was not anxious

to have any building connections cut free until his erection

work was essentially completed. He had placed a crawler

crane on a runway structure atop the 126−2 support steel

to erect the top units, and he felt that the vibrations already

being experienced with crane movements were large enough

with the original fixed connections in place, without increas-

ing them by adding expansion joints. This meant that the

expansion connections would have to be effected while they

were carrying their full design dead loadings. This loading

amounted to approximately half the total design load with

the remaining half consisting primarily of fly ash loadings.

The erection work was done in three separate stages.

First, the column base plates were reinforced as this material

was easy to obtain and required little fabrication. Second,

all the remaining work except for cutting free the expan-

sion connections was erected. That is, the bracing changes

required to fix the lower support columns, the stiffening

of the upper centerline support girders, the two horizontal

trusses along the 36 Column Line, and the brackets to support

the ball bearing assemblies. During this stage also, the ball

bearing assemblies were jacked up under the support gird-

ers, and the hanger rod connections at column QQ36 were

installed. The assemblies were positioned by jacking up the

support brackets. The G4 girder connections required the

hanger rods to be lowered down through the upper bracket

into position. After the top and bottom bearing and rocker

plates were in place, the upper nuts were then turned until

a snug fit was obtained. The third and final stage included

the actual burning free of the original bolted connections in

a very careful manner. Less than a sixteenth of an inch drop

was observed in the G4 girder elevation after the rods were

completely loaded at this point. The entire cutting operation

at all elevations was done in less than four weeks.

Operating Experience

All precipitator units have been operating successfully.

The tie-in to the boilers being done in two stages. The two

southerly units being connected first and then the northerly

ones directly after. This was accomplished by taking out the

south and north boilers alternately, thus reducing the rate of

the unit during this period to 500 MW. The complete tie-in

period was just less than six months.

Position readings of several exterior columns were taken

before any precipitator units were energized. Readings were

taken at the +42 and +126 elevations of each column mea-

sured. These “cold” readings were taken to obtain base mea-

surements from which to determine the outward movements

of the columns once in operation. “Hot” readings were taken

last summer with all the units energized and showed that

the north columns had moved a maximum of 1½ to the

north, and the east columns had moved a maximum of 1^11 / 16 

to the east. Both north and east maximum movements were

recorded at the +126−2 elevation. Comparing this to the

design movement of 2½ it appears that the assumed steel

temperature rises were ample. The fact that the +126−2

framing is actually expanding more than the +42−8 fram-

ing is not surprising. It was expected that the upper reaches

of the building would be hotter by convection, and because

the elevation +126−2 framing has an operating unit both

above and below it. The lower elevation steel is more readily

cooled by outside air being pulled through louvers at grade

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