POWER July 2023 - 29

MARNIE SURFACEBLOW
1. Marnie and Maya study plant drawings to get a better understanding of system designs.
Source: POWER
have you got that pump apart now? ...
You do? ... OK, great! Later this morning
I'm bringing some overpriced consultants
down to pester you. " Mary smiled
at Marnie. Marnie grinned back and said,
" It's alright Mary. I recognize with your
advanced age you likely need to hire people
like me to give you a hand. Are we all
hitching a ride on your mobility scooter? "
Mary cursed, and muttered, " I'm only
two months older than you! "
see is you are drinking diet soda! "
Marnie took a sip and shrugged, " It's
stronger than the coffee. "
Somewhat disbelieving of her senses,
Maya gave her boss the side-eye while
she reported on what she had found via
her simulations. Marnie nodded approvingly
and said, " Well done. Let's go take
this can of Diet ... whatever this is ... on
a tour of the plant. "
A scrubber recycle pump on the half-shell
lay exposed on a sturdy workbench. A frustrated-looking
maintenance technician was
chiseling what appeared to be semi-friable
concrete from the pump inlet and outlet
manifolds.
For the next hour Marnie asked for
various design drawings of the air quality
control systems, tracing through the
flow path with Mary. Meanwhile, Maya
input the scrubber process flow diagrams
(PFDs) and design data into one
of her custom-built simulators, occasionally
viewing the drawings with Marnie
(Figure 1) to check her data, then ran
some simulations of the system.
When it was time to walk to the maintenance
bay, Marnie walked over to
Maya and glanced at her spreadsheet of
simulation results. " Do you see what I
see, my faithful apprentice? "
Maya began to answer, then suddenly
her eyes widened and she said in a nervous
whisper, " Ma'am! One thing I DO
July 2023 | POWER
'Clotted' Pumps and Pipes
As Marnie and Maya waited for an elevator
that moved at glacial speed to take
them to the maintenance bay, Maya
asked, " Ma'am, I am certain I know what
one of the problems is, but there are actually
two problems, yes? "
" Indeed, " replied Marnie. " You could
call them twin problems-not identical,
but more fraternal. "
Maya sighed, " Each year I understand
more of your metaphors, but today I
have failed. "
Marnie laughed and patted her assistant
on the shoulder. " Don't worry,
Maya. Sometimes I don't understand
them either. "
A scrubber recycle pump on the halfwww.powermag.com
shell
lay exposed on a sturdy workbench.
A frustrated-looking maintenance technician
was chiseling what appeared to be
semi-friable concrete from the pump inlet
and outlet manifolds. Taking a short
break, the maintenance tech introduced
herself as Jessica and explained the situation,
adding, " It's not so bad this time.
The impeller seems good, just a little
grinding and balancing and it should be
ready by tomorrow. I'd like to change the
bearing just in case, and we don't know
how much of this rock is in the other
pipes, so if a piece breaks off and hits
the pump ... "
" It would be a metaphor for a venous
blood clot, just like I said! " Marnie
chimed, while Maya shook her head.
" You should also purge the entire scrubber
vessel tank, then send new slurry in
from the limestone mills at maximum
pump flow. In fact, I recommend strongly
you operate the limestone feed system
in a surge mode at low unit loads.
It'll throw your scrubber chemistry off a
little bit, but then most OEMs (original
equipment manufacturers) don't really
have recommended chemical balances
at very low loads. Few provide a PFD at
operation less than 50% of MCR, and for
this plant 60% is the lowest operating
PFD I could find, " Marnie said.
" I have conducted many chemical
equilibrium simulations, and I believe I
can optimize your limestone use to reduce
the required solids flow. Particle
drop-out will still vex you, it will simply be
less common, " added Maya. " Your piping
is sized for greater volume flows than you
utilize at low loads. The reduced sulfur
content from your current coal reduces
the volume flow further. The limestone
drops out at your very low flow rate. You
can witness this in your DCS (distributed
control system) historian, where there
are occasional pump pressure surges
as the obstruction grows, then a release
of pressure when it breaks free. It's not
practical to replace all your piping with
smaller pipe, as that would limit your fullload
capability, so I suggest you change
the pump flow and pressure profile. "
Upgrades Could Pay Off
Jessica and Mary looked down at the
pump parts. " That makes a lot of sense
to me. What if we installed a VFD on the
pump, so we could cycle the speed regularly
during operation? " Jessica asked.
Maya and Marnie nodded in approval.
" Indeed, and at the very low flow rates
you wish to operate at, the use of a VFD
may save you some energy as well. "
29
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POWER July 2023

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