| INTRODUCTION
In the late 70s performance loss as the result of
compressor fouling was generally regarded as a necessary evil inherent in the
operation of gas turbines. Little was done about it apart from shutting the
machine down from time to time to carry out a time consuming and laborious
crank-soak compressor washing or other cleaning procedures (see Fig 1) to
restore lost performance.
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METHODS OF CLEANING FOULED
GAS TURBINE COMPRESSOR
OFF-LINE CLEANING METHODS
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Crank-soak chemical washing using
in-built chemical injection water rinsing systems.
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Crank-soak chemical cleaning
using hand held hose or lance.
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Partial hand cleaning (e.g. Struts.
IGV's. 1st stage rotor & stator blading) using chemicals, rags, brushes
and water rinse.
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Full hand cleaning with compressor
covers removed using chemicals, various types of abrasives or even light
shotblasting techniques
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Steam cleaning.
ONLINE CLEANING METHODS
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Injection of abrasives (e.g.
crushed nutshells) into the compressor airstream to displace blade deposits
by high velocity impingement.
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Injection of plain water to remove
water soluble deposits.
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Injection of special chemical
solutions (solvent 8 aqueous based) to chemically dissolve and remove surface
deposits from the blades.
Figure 1. |
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However, with the popular resurgence of
the gas turbine as an industrial prime mover over the past decades, serious
interest in the problem of lost performance and increased fuel consumption
caused by fouling has led to the development of so-called 'on-line' or
'fired wash' compressor cleaning systems. The objective of these systems
being to chemically clean the compressor while the engine remains in operation
at up to full speed and load in order to extend the output for longer and
avoid increase in heat rate and subsequent increases in fuel consumption.
In reality, the number of companies and
individuals that have been seriously involved in the development of fired
wash systems over the years are few and far between. However,
since the process has, of recent, gained the official blessing of some
major gas turbine manufacturers, there has been a sudden proliferation
of system suppliers and even more running wash chemical suppliers - who,
in many cases, may have scant knowledge or experience of the fired wash
process and the gas turbines to which it is being applied.
If done properly, fired washing can be a
very safe and successful method of keeping gas turbines running more efficiently
and the process is being constantly improved and perfected and is, without
doubt, here to stay as more and more gas turbine manufacturers offer running
wash systems as a standard fit or recommended option.
However, it can also be a dangerous process
if injection systems or chemicals are incorrectly designed, fitted or used
and operators should be cautious when selecting any on-line cleaning system
(see Fig. 2 & 3)
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QUESTIONS TO ASK WHEN
SELECTING AN ON-LINE CLEANING SYSTEM
ABOUT THE INJECTION SYSTEM
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How long has vendor been in the
business.
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An installation reference list.
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Is the system known to engine
manufacturers Do they approve it or have no objection to its installation
and use.
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Is the system a recommended option
or installed as standard in new gas turbines by any manufacturers/packagers.
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Does the vendor design, manufacture,
install. service and guarantee the system himself (If not, why not).
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Are the materials of the system
of good quality
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How long does the vendor say
it should take to install the system. Some can take a few hours to install,
others can take weeks to install if it involves drilling thick casting
etc.
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Is the design safe. Could
it possibly damage the engine or injure those using it.
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Does the vendor have sufficient
liability insurance.
Figure 2. |
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Since on-line washing has rather suddenly
come into vogue even though deep rooted knowledge and experience of the
process is known to relatively few system supplier and operators it is
hoped that this paper will be of sound practical help to those operators
who would like to adopt online chemical washing procedures but who are
unfamiliar with the concept. |
QUESTIONS TO ASK WHEN
SELECTING AN ONLINE CLEANING SYSTEM
ABOUT THE CLEANING CHEMICAL(S)
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Does the vendor also manufacture
special cleaning chemicals for use with the system and are they tried,
tested and approved
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was the chemical on offer solely
developed for on-line cleaning or was it originally developed for some
other application not connected with gas turbines.
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Does the vendor offer a choice
of chemicals (i.e. solvent based and water based) to suit particular fouling
and/or environmental requirements.
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Is the chemical supplied as a
concentrate to save storage and transportation costs. Paying for water
in ready-to-use chemical solutions can be very expensive and unnecessary.
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Can the chemical offered also
be used safely and effectively for off-line compressor washing if need
be.
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Does the vendor offer ex-warehouse
availability of chemical.
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If the vendor only supplies chemicals
are you sure it is safe to use them in your injection system.
Recommendation: Be very wary
of using any chemical especially for on-line. fired washing, unless it
has been property tested and approved and has a good long term safety record
behind it.
Figure 3. |
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CAN ON-LINE WASHING REALLY BE COST EFFECTIVE
&
IS IT REALLY NECESSARY - OR JUST ANOTHER
FAD?
Due to the current Middle East crisis the
cost of fuel has increased by roughly 70% over the past 3 months. e.g.
No2 Diesel was, as of October 1, 1990, being quoted on Rotterdam market
at $300/tonne whereas 3 months prior it was priced in the region of $180/tonne.
Even if fuel drops back to pre crisis levels
in due time
fuel costs for operating any gas turbine
are still substantial (see Fig 4) even when the machine is kept in perfect
operating condition and at peak efficiency. A fouled compressor can
easily increase fuel consumption by 5% or more and, in real terms, a 5%
increase for the operator of a typical 25mW heavy industrial unit running
base load for say 8,000 hours/year
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