Martin Howarth reveals what you need to know about cleaning gas turbines
The purpose of running a gas turbine is to harness work. Some of this goes back into compressing the fluid and continuing the process but everything else is your product. This may generate electricity or drive other machinery, but the net profit relies on gas turbine efficiency.
A fouled turbine affects output and throughput, so it is essential to remedy any fouling to maintain a productive, profitable turbine.
What is gas turbine fouling?
Fouling consists of deposited airborne particles ingested from the environment. These 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, affecting the aerodynamic performance and in turn compressor performance.
These 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 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 insignificant in comparison and you are likely to see a return on investment within the first year.
A further issue of fouling is the blocking or partial blockages of cooling passages in hot section stators and blades. This 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 offline – 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 difference 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. This data indicates an increase of 35% net work is seen when a combination of both on and offline plus hand wash is used. A significant 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.
This supports the recommendation of using both an online and offline wash system which can be done during operation, requiring no shutdown, unlike hand cleaning.
Water or chemical cleaners for gas turbine cleaning?
This 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 effective maintenance schedules but, to maximise efficiency returns, the right injection fluid must be selected for the foulants.
The three main categories of wash fluid are de-mineralised water, solvent and surfactant-based fluids. Some are used as a mixture to widen their effective range of foulant removal.
Another consideration when selecting wash fluid is whether it foams and how long it takes to rinse out of the turbine. Many of the most popular fluids on the market are surfactant based ,which are well suited to capturing foulant and carrying it through the engine without redepositing on later stages.
To select a chemical cleaner, operators should perform an effectiveness test with a foulant sample.
What if the foulant redeposits in later stages?
Once foulant has been removed by the wash fluid it will pass further into the turbine compressor.
To ensure no significant build-up occurs in later stages, a regular offline wash should be performed with a suitable number of rinse cycles.
The importance of rinse cycles can be seen in conductivity and appearance changes from effluent taken after a wash and after rinse cycles. What’s usually observed is a significant difference from the first drain during the wash cycle to the last on the second rinse cycle. This suggests the offline wash itself only loosens the dirt and the rinse cycle is what removes it from the compressor. This highlights the importance of offline cleaning to remove foulant and any later stage deposits.
Can hardware be retrofitted for gas turbine cleaning?
The benefits of turbine cleaning have been outlined, but online cleans require specialist hardware that may not be have been installed when the machinery was manufactured. However, it is often possible to retrofit this hardware and add online cleans to the schedule.
The costs associated with installing new cleaning equipment are often returned many times over in fuel savings alone, not to mention sparing the expense of excess scheduled and unscheduled downtime.
Martin Howarth is managing director of Rochem Fyrewash.