POWER September 2014 - 41

WATER
1. To clean or not to clean? For cleaning with the bead blast
method, this chart shows the recommended parameters for immediate
and near-term cleaning. Source: EPRI
60
55
50
45
40
35
30
25
20
15
10
5
Cleaning recommended this outage
Committed to
moving your
performance
forward
Plan cleaning for next outage
No cleaning needed
1,500 1,700 1,800 1,900 2,100 2,300 2,500 2,700 2,900 3,100 3,300 3,500
psig
The middle area between the green and red line indicates that deposits
are beginning to accumulate to the point where cleaning should be
considered and probably budgeted for the next major outage or within
the next two years. If that is the case, grab another tube sample close
to the next outage and see if the DWD has increased and is close to or
in the " Cleaning recommended " area. If not, you may be able to get
by for another year or two.
Although the heat flux in a heat-recovery steam generator (HRSG)
C
M
is far lower, the circulation issues can be far greater due to the multiple
assemblies and configuration with the drum. So the industry has
applied close to the same DWD criteria for cleaning for an HRSG as
for a conventional fossil-fired unit.
As a general rule, the DWD criteria for HRSG tubes is about 20%
higher than for a conventional boiler.
Occasionally, a utility will want to take multiple tube samples and
Y
CM
MY
CY
CMY
have them analyzed. In these cases, the DWD result that should be
used to determine the need to chemically clean is the tube that is the
most heavily deposited. Remember, what you are trying to determine
is if there is sufficient deposit anywhere in the boiler to cause underdeposit
corrosion or overheating. You are not trying to determine the
average amount of deposit in the whole boiler or even the average in
the high-heat area. One isolated area of hydrogen damage or overheated
tubes is enough to cause a number of forced outages or extend
a planned outage, and certainly reason enough to chemically clean.
If the DWD results indicate the need to chemically clean, now is
K
the time to do it. Procrastination with cleaning is detrimental on a
number of levels.
First and foremost is the damage done to the tubes. Under-deposit
corrosion rates and long-term overheat damage are exponential, not
linear. A delay of one or two years on a dirty boiler can result in major
tube damage.
Second, you don't save as much as you think. Cleaning a very dirty
boiler is significantly more expensive than cleaning a boiler that has just
crossed into that " Cleaning recommended " region. The additional costs
in solvent (see sidebar), time to get the tubes clean, multiple cleaning
steps, startup delays, and dealing with excessive amounts of cleaning
wastes are all consequences of postponing a needed cleaning.
So you need to clean; now what?
Planning for a Cleaning
The first decision is what solvent to clean with. There are five commonly
used cleaning solvents. Each has advantages and disadvantages.
If you have been operating a plant for awhile, you may simply go
with the solvent and procedure you used last time. But it is worth a
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September 2014 | POWER
www.powermag.com
41
Total project supply - boiler
chemical cleaning advisory and
project management services. We do it
so you won't have to! Our service call in
# 877-311-0101. Learn more at
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POWER September 2014

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