Preface:
There have been a
number of questions
in the WTUI Forum
sections recently
about different
aspects of
waterwashing. I
thought perhaps I
could help out by
writing down a few
thoughts and
observations and
posting them here
for everyone to
read. I hope this
helps out.
Overview:
Gas turbine
compressors have
been cleaned in one
form or fashion
since they came into
existence. The
movement of vast
volumes of air
through the machine
almost guarantees a
deposition of
indigenous dirt over
time, no matter how
efficient the
applied inlet filter
design might be.
Many cleaning media
have been used over
the years, from
plain water to nut
shells to carbon
cinders. Each of
these have
advantages and
drawbacks. The
current
"state-of-the-art"
methodology utilizes
water and detergent
solutions so, for
the purposes of this
article, that is
where we will focus.
Off-line
Waterwashing:
Off-line
waterwashing refers
to washing the unit
while it is in an
unfired state. If
the unit has just
been brought
offline, the engine
manufacturer will
usually have a
sustained
temperature
specification that
the engine must be
cooled to prior to
washing. Once the
engine is properly
cooled, wash
preparations may
begin.
Most engines have
instrumentation that
is water-sensitive.
Pressure tap lines
that can be flooded
and retain water,
optical flame
scanners that can
become clouded with
residue, etc., all
must be addressed.
Usually these can be
protected fairly
easily by removal or
disconnection.
Waterwash solution
must be prepared for
injection. The OEM’s
usually provide some
kind of injection
ring or manifold at
the front of the
engine for this
purpose. Although
this is a relatively
effective means of
injecting the
solution, it may be
necessary to augment
the system if there
is too much solution
lost out the front
of the bellmouth.
This may be an
especially
irritating problem
on twin spool
machines (LM-5000,
LM-6000), where the
LP spool is not
spinning fast enough
to ingest all of the
solution and
significant amounts
are wasted on the
inlet plenum floor.
Several users have
added direct
injection ports,
utilizing existing
compressor casing
bosses, with good
success. Such an
addition should be
undertaken only
after consulting
qualified technical
personnel and should
be implemented with
hardware consistent
with the
application. It
should be noted that
OEM’s are usually
against tinkering
around like this, so
be ready for some
mild negative
feedback.
There are a number
of quality waterwash
detergents being
marketed in the
community. They come
in two basic
formulas:
water-based and
solvent-based. Both
of these formulas
have three building
blocks: a detergent
(cleaner) package, a
surfactant package
and a carrier
medium. The cleaners
are there for
obvious reasons. The
surfactants are
there to provide a
residual coating on
the airfoils to
retard the
redeposition of
dirt. The medium, or
carrier, is there to
aid the movement of
the cleaners and
surfactants to the
cleaning site and to
insure homogenous
blending with the
dilution water.
Almost every
detergent
manufacturer I am
aware of recommends
heating the
detergent/water
solution to 140F -
180F prior to
injection. This
helps the solution
break down the dirt.
If possible, this
should always be
done. It is
especially true on
dirt that contains a
large percentage of
unburnt
hydrocarbons.
Which detergent you
choose is up to you.
There are all kinds
of differentiators
that can be applied
to the
decision-making
process (price being
the most prevalent),
but I have found
that the best one
is: "How well does
it clean?". Each
installation has a
different dirt
constituency and the
various cleaners
react to them
differently. Also,
some detergents may
be more effective or
less effective at a
dilution ratio other
than that which is
recommended by the
manufacturer. Don’t
be afraid to
experiment. Not
enough detergent and
the airfoils aren’t
clean. Too much
detergent and you
will see a greasy
black deposit on the
suction (back) side
of the airfoils.
This is excess
dehydrated
surfactant and can
accelerate the
waterwash cycle.
This is a
trial-and-error
process that may
take as long as a
year to work
through, but in the
end you will be
rewarded with faster
and more effective
washes.
As important as the
detergent is, so is
the water with which
you are diluting it.
You should always
strive to use the
best quality water
obtainable for your
washes. DI or RO
water is preferred.
Most engine
manufacturers have a
"must-meet"
specification for
water, but the
cleaner it is, the
better it is. Also,
any handling
equipment (buckets,
funnels, hoses,
etc..) should be
rinsed and as clean
as possible.
The number of wash
and rinse cycles is
another
"up-for-grabs"
issue. In my
experience, once I
have settled on a
detergent and a
dilution ratio I
monitor three or
four washes with a
borescope to see how
many wash cycles are
needed to get the
compressor
acceptably clean.
From then on, I
stick with that
number unless I have
mitigating factors
such as a huge dust
storm or an extended
run. Rinse cycles
are equally as
important. The best
way to monitor the
effectiveness of the
rinse is by
comparing the rinse
effluent to the wash
influent. In most
cases, you can do
this with a simple
conductivity meter.
If the influent is
10 micromho’s, a
target for the
effluent must be
obtained. I have
found that 10 cycles
is about right. So,
in our example, 100
micromho’s at the
exhaust drain would
be what we’re
looking for. Barring
the availability of
a conductivity
meter, a clear
discharge for two
rinse cycles is
usually OK.
Once all of the
washing and rinsing
is done, the unit
should be reinstated
and fired as quickly
as possible. This is
to insure that any
waterwash residue is
either burnt up or
expelled from the
engine and no
cooling passages or
other sensitive
areas are
contaminated. If the
engine is slated to
remain offline, a
ten minute runup to
idle will accomplish
this. A washed
engine should never
be left offline
without being run.
Online Waterwashing:
In the past ten
years it has become
fashionable to
perform online
washes. These washes
are performed while
the engine is being
fired and usually
utilize a different
injection system.
Care should be taken
to monitor the
engine closely while
it is being washed
to insure smooth and
safe operation. In
most baseloaded
cases, it is
advisable to back
the engine off it’s
max power setting
for this procedure
to allow the fuel
control system some
room to operate
should there be a
problem.
Although several
detergent
manufacturers have
touted their wares
as being effective
in the online
setting, I have
found that you can
achieve the same (if
not better) results
using plain water. I
think that most
experienced users
feel the same way.
What you’re trying
to do is get rid of
some of the dirt at
the front of the
compressor before it
has a chance to bake
on. Plain water can
do this and will not
disturb the
surfactant left by
the offline wash.
Detergent solutions
will reduce the
dirt, but also seem
to impact the
effectiveness of the
surfactant coating.
A plain water online
wash every two to
three days seems to
be about the most
optimal program.
Final Thoughts:
This should give you
some things to
consider about your
waterwashing
program. I would
recommend that you
sit down with your
field personnel and
insure that they are
in alignment with
you on the purpose
of the exercise. It
is not to dribble
water and detergent
around the site and
drag some hoses so
you can write in the
log book "Waterwashed
Unit #2". It’s to
get the engine
clean. Once
agreement is reached
here, the learning
and perfecting
process can begin.
When a new or newly
overhauled engine
arrives on your
site, you should
always program an
offline waterwash
into your
installation plans.
That way, you are
launching a nice,
clean engine off
into a service run
with a good, strong
surfactant residue
on the air foils.
This should be the
start of an optimal
production run and
prevent a rapid
initial buildup of
dirt.
Always take the
opportunity to
perform a visual
"shake-down" of your
gas turbine and the
package interior
while an offline
waterwash is being
performed. Many "gotchas"
have been eliminated
by observant
operators while
using this time to
check things out.
Always make the best
use of any time
spent with your
engine down.
Once you have
established "the
drill", stick to it.
Make sure
everyone sticks
to it. Having a
repetitive process
makes
troubleshooting much
easier. Good luck!