ENGINE & TURBINE TECHNOLOGY
Martin Howarth reveals what you need to know about cleaning gas turbines
he purpose of running a gas turbine is to harness work. Some of this goes back into compressing the fl uid and continuing the process but everything
THE ESSENTIALS OFTURBINE CLEANING T
else is your product. T is may generate electricity or drive other machinery, but the net profi t relies on gas turbine effi ciency. A fouled turbine aff ects output and
throughput, so it is essential to remedy any fouling to maintain a productive, profi table turbine.
WHAT IS FOULING? Fouling consists of deposited airborne particles ingested from the environment. T ese can include salts, minerals or hydrocarbons, but also aggressive gases such as SOx, NOx or Cl2
. Once these
particles adhere to the compressor blading, an increase in surface roughness is seen, aff ecting the aerodynamic performance and in turn compressor performance. T ese chemicals also react with moisture
from the environment creating acidic compounds: causing corrosion; reducing the operational life of the machine; and increasing maintenance costs. In 2012, engineer Andrew F Bromley carried out a fouling cost prediction for
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three engines based on a reduction in energy output over a period of 8,000 hours. He discovered a huge potential monetary cost to running a fouled turbine, suggesting that the installation and running cost of a turbine wash system is relatively insignifi cant in comparison and you are likely to see a return on investment within the fi rst year. A further issue of fouling is the blocking
or partial blockages of cooling passages in hot section stators and blades. T is results in improper cooling and accelerated thermal fatigue on components.
WHAT ARE THE OPTIONS WHEN CLEANING A GAS TURBINE? Having decided it is important to combat foulants, there are four main cleaning options – abrasive; hand cleaning; online; and offl ine – each with its own advantages and disadvantages. Abrasive cleaning is seldom carried out any more but the other three types are performed across the world. You might be surprised at the diff erence
cleaning your turbine can make. Fig. 1 shows the net work produced before and after a wash on a 45MW turbine collected over 15 weeks at a gas turbine plant. T is
data indicates an increase of 35% net work is seen when a combination of both on and offl ine plus hand wash is used. A signifi cant increase is also seen when only online washing is carried out, albeit with a downward trend post-wash which could easily be controlled with a regular wash schedule. T is supports the recommendation of using both an online and offl ine wash system which can be done during operation, requiring no shutdown, unlike hand cleaning.
WATER OR CHEMICAL CLEANERS? T is should be considered on a case to case basis as it depends on the type of fouling seen. Some fouling can be mitigated with fuel selection and eff ective maintenance schedules but, to maximise effi ciency returns, the right injection fl uid must be selected for the foulants. T e three main categories of wash fl uid
are de-mineralised water, solvent and surfactant-based fl uids. Some are used as a mixture to widen their eff ective range of foulant removal.
Another consideration when selecting
wash fl uid is whether it foams and how long it takes to rinse out of the turbine. Many of the most popular fl uids on the market are surfactant based ,which are well
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