The Essentials of Turbine Cleaning

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 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, Andrew F Bromley carried out a fouling cost prediction for three engines based on a reduction in energy output over a period of 8000 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, the four main cleaning options are shown in Table 1 along with their advantages and disadvantages. Abrasive cleaning is seldom carried out anymore but the other three types are performed across the world. 

Type	Advantages	Disadvantages
Abrasive	A high level of performance recovery at a low cost	Can clog cooling holes in blades increasing thermal fatigue and likely to cause erosion to surface coatings
Hand cleaning	Restores turbine to "as new" condition offering maximum performance recovery	Lengthy shutdown and a large amount of manhours required
Online	Performed in service, no need to reduce speed or lower output	Extra loading of the turbine and increased erosion risk from large droplets
Offline	Deep cleans turbine resulting in significant performance recovery without shutdown	Requires a reduction in operating speed and multiple rinse cycles

Table 1: Advantages and disadvantages of different cleaning regimes

You might be surprised at the difference cleaning your turbine can make. Figure 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. 

Figure 1: Effect of cleaning on net work - gas turbine performance optimization using compressor online water washing technique – Ezenwa Alfred Ogbonnaya

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? 

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 maximize efficiency returns, the right injection fluid must be selected for the foulants.

The three main categories of wash fluid are de-mineralized water, solvent, and surfactant-based fluids. Some are used as a mixture to widen their effective range of foulant removal. The advantages and disadvantages of these can be seen in Table 2.

Cleaner	Mixed	Effective against	Advantages	Disadvantages
De-min Water	No	Salt	Environmentally friendly	High surface tension, only effective against water soluble molecules
Solvent	Yes, with water or neat	Oils, hydrocarbons, salt if mixed	Improved cleaning ability	Not environmentally friendly and hazardous to work with, can harden seals/cause deterioration
Surfactant	Yes, with water or neat	Oils, hydrocarbons, salt if mixed	Bio-degradable, better surface wetting, non-hazardous	Not suitable for heavy fouling

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. 

In order 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.
In order 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 is shown in Figure 2, which shows conductivity and appearance changes from effluent taken after the wash and 1st/2nd rinse cycles. 

Figure 2: Effluent samples taken during an offline wash - Gas turbine performance deterioration and compressor washing – Cyrus Meher-homji, et al.

A significant difference is seen 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 cleaning hardware be retrofitted?

The benefits of turbine cleaning have been outlined, but online cleans require specialist hardware that may not 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.