POWER September 2023 - 36

MARNIE SURFACEBLOW
2. Marnie and Maya used laser levels to help identify the problem in the scraper conveyor
room. Source: POWER
Seeing puzzled looks from the plant
staff, Maya yelled, " Kindly turn on the
laser levels! Thank you! " The scraper
conveyor room suddenly was crisscrossed
with six fans of oscillating
green beams, immediately revealing
that the entire conveyor room floor was
bowed upward.
Preston gaped incredulously, while
Annik ran to read the digital measurements.
She rose and shook her head,
saying, " But ... how? The scraper conveyor
trough is almost 20 centimeters
higher at the center than at hatchway #1,
and about 10 centimeters higher than it
is at hatchway #5? Is this normal? "
" Nope, not by a long shot, " replied
Jeff. " We measure the sag in this room
twice a year, and it normally tends to dip
about five to 10 centimeters in the middle.
How on earth ... "
" ... do we get more coffee up here, "
interrupted Marnie. " That, my friends,
is a question unanswerable by mere
mortals, such as I. But, we can tell you
what's going on with the floor. "
Leveling the Playing Field
" I had one idea about the conveyor
problems from the steamboat anchor
chain story. In that case, a new cargo
master was atrociously loading the
hold, torquing the ship structure. This
bowed the deck upward, pinching the
anchor chain and eventually breaking
it. Then I recalled another story, " Marnie
explained. " A grain silo with 2,000
tonnes of oats slowly filled with 1,000
tonnes of water from a leaking sprinkler.
The added weight dragged the
silo down almost two meters before it
failed. No one was injured, but 3,000
tonnes of oatmeal makes the most
36
dreadful mess! "
Enjoying being the center of attention,
while Maya started already pricing parts,
Marnie continued. " It's a question of balance.
Jeff, you said the coal silos hold
about 16 hours at full output-I figured
that at about 600 tonnes each. But for
the pellet silos, adjusting for fuel density,
heat content, and plant efficiency differences,
a full pellet silo barely weighs half
what a coal silo does. So, with relatively
heavy full coal silos on either end of the
room, not only are they sagging, but
they're also bending the floor and fuel
trough upward in the middle. "
" This might not have been critical,
but there were complications, " added
Maya. " This partly charred wood pellet
fuel is much more plastic than torrefied
pellets, and it can build a layer between
the scraper blades and the trough, until
it forms a solid wedge. This presents
significant resistance to the scraper conveyor
movement, increasing the motor
amperage. And when you started wetting
the pellets to help them stick to the
main conveyor belts, this accelerated the
buildup and increased the resistance to
movement. We found many trends of
scraper conveyor motor amperage increases
corresponding with both your
water misting and the silo loading. "
" Your scraper conveyor is tough, I'll
grant you that, " said Marnie, " but it's not
made of adamantium. At some point this
biomass buildup, combined with traversing
the upward bowing of the floor, was
going to break the chain. "
There was general agreement and
nodding, and work teams broke into
groups to discuss potential hazards from
the new findings. Preston and Annik
stood with Marnie and Maya. " Let me
www.powermag.com
guess, " said Preston, " this is going to be
expensive, eh? "
" The answer is 'yes-with-an-if' or 'nowith-a-but,'
" Marnie said with a grin.
" First, eliminate the wetting mist and install
variable-speed drives on your main
feed conveyors. I think you'll find at half
the speed you'll actually end up with
about twice the capacity. Importantly,
less water means less buildup under the
scrapers. These lightly cooked pellets
usually rely on binders to reduce fines,
and it looks like the binders work well for
a little rain, but soon wash out and form
deposits in your conveyor track. "
" Next, since you mostly solved the
pellet grinding issues, must both coal
silos stay so full? " Marnie asked. " How
much load-shifting power reserve do you
need? I'll bet you can get by with eight
hours, so keep the coal silos half to twothirds
full. That should equalize out the
loading on the local superstructure and
keep the flooring much more level. "
Tapping away on her tablet to tabulate
costs, Maya added, " If you are willing
to invest in greater reliability, you
can
purchase
replacement
conveyor
belts with regular ridges on them to
assist in moving material. They can be
dear, especially on older power stations,
but they do indeed work. "
" Worst case, we would need to beef up
the coal silo and scraper conveyor structure,
if you have about $10 to $20 million
to spend. I'd get a bonus-and a BMW
M5 has caught my eye-however, " Marnie
sighed exaggeratedly, " even though
I'd achieve annual sales targets early, I
can't ethically recommend something
that major without trying some cheaper
and potentially better options first. "
Thrown by Marnie's deadpan humor,
there was a brief pause before both
Preston and Annik burst out laughing.
" Thank you, Ms. Surfaceblow, Ms. Sharma.
Let's talk about some of the cheaper
options first. " Following their hosts as
they returned to the meeting room, and
while riding the plant elevator to the first
floor, Marnie whispered to Maya, " Rats,
I really wanted that BMW. "
After the elevator stopped and while
continuing their walk to the plant offices,
Maya patted Marnie's shoulder and
said, " There, there, ma'am. Do not be
unhappy. I ordered you a set of green laser
levels as a gift, and yes, their housing
is green as well. " Marnie's joyous shout
echoed through the plant. ■
-Una Nowling, PE is an adjunct professor
of mechanical engineering at the
University of Missouri-Kansas City.
POWER | September 2023
http://www.powermag.com
POWER September 2023

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