How to get the best from online cleaning and fired washing

In Rochem's experience the effectiveness of the performance of on-line chemical washing is dependent on:

a. The right injection system - The chemical injection system must be able to deliver the cleaning fluid into the airflow in such a way that comprehensive wetting of the compressor is always achieved.

It is a totally pointless exercise to inject any cleaning solution into the operating compressor if it does not actually come into direct contact with the fouled compressor airfoils. 

b. The right chemical - The selected chemical must be able to actually break down and remove the fouling.  There is little point in injecting any cleaning chemical into any compressor if it is incapable of effectively breaking down the type of foulant present in that compressor. For example, simple water based detergents cannot be as effective as solvents against the hydrocarbon based oily/carbonaceous deposits which form the basis of most industrial compressor fouling.

c. The right duration of wash - Since a good proportion of the injected cleaning solution will pass directly into the combustion system without making contact with the compressor blades, the actual period of injection of the cleaning fluid must be sufficiently long to provide enough continuous blade wetting and contact time to break down the base foulant.

An injection of cleaning solution into the operating compressor which lasts only a few minutes has little chance of successfully removing the fouling. Rochem's experience is that on-line wash times of 10 to 20 minutes are needed to produce the best results.

d. The right washing frequency - For consistently good on-line cleaning results it is vital to maintain a washing frequency which matches the rate of fouling of the compressor. 

The objective of on-line washing is to prevent or control the rate of compressor fouling to prevent or reduce the rate of performance loss. Therefore it is pointless to wait until the compressor has built up sufficient fouling to cause a significant performance loss before carrying out on-line cleaning.

How to get the best from offline cleaning, on-crank washing and crank washing

Getting the best from off-line cleaning processes requires all the following elements, particularly if no on-line washing is performed:

The correct:

1. frequency of washing

2. method of injection

3. choice of chemical

4. preparation of chemical

5. pre-heating of the chemical

6. soak time

7. post rinse procedures

a. Washing frequency - If no on-line cleaning is carried out the only choice is to shut down as regularly as possible for off-line cleaning. Even if a typical industrial gas turbine is off-line cleaned once per month, it will still probably suffer an average annual output loss of 3 to 5% and an increased fuel burn of 1 to 3% purely as the result of compressor fouling.

b. Method of injection - Even though the cleaning solution is normally injected at a relatively low cranking speed it is still vitally important to ensure that the solution comes into contact with all the compressor airfoil surfaces. Correct nozzle spray patterns to give complete facial coverage of the compressor are therefore vital.

High flow deluge wash systems using relatively crude spray nozzles are no guarantee of achieving this.  In fact, particularly in the case of large capacity gas turbines, the fluid from such systems does not give comprehensive face wetting of the compressor even though large volumes are being injected. Also, the fluid droplets can be so large that the minimal mass air flow and low air speed while under crank cannot support them so the majority of the injected fluid gets not much further than the 3rd or 4th stage before draining back to the plenum and doing no cleaning work at all. 

c. Choice of chemical - As with on-line cleaning there is little point in using any chemical for off-line cleaning which does not have the power to remove the deposits from the compressor blades.

If, for example, the foulant is known to be oily/carbonaceous material a water-   based detergent will never be as effective as a solvent-based cleaner.  And, as in the case of Rochem's F3 formula, there are natural solvent products available which have excellent levels of biodegradability which are ideal for this type of application

d. Preparation of chemical - The typical way of preparing a chemical for off-line injection is to first mix it into a solution with water (usually a 4:1 water/chemical mix) before heating it to 50 to 60 Deg C then injecting it while the compressor is being turned at cranking speed.

This can be reasonably effective so long as the correct chemical is selected and the compressor is not too heavily fouled.

e. Correct pre-heating - When using a water/chemical solution it is virtually essential to pre-heat the solution before injection in order to enhance the chemical cleaning reaction which can be increased by a factor of 2 or 3 just by heating it to 50 to 60 Deg C. 

e.g. consider the difference in using cold water and detergent as opposed to hot water and detergent to wash dishes.  Attempting to wash a compressor with cold cleaning solution as opposed to warm is no different. 

When injecting concentrated chemical the need for pre-heating is less essential but it is still good practice to heat it to 30 to 40 Deg C to maximize its chemical cleaning reaction. 

However, in hot climates when ambient temperatures are in the order of 30 Deg C or more there is no need to heat the concentrate further since the compressor surfaces are at the same temperature even after total cool down and thus have no cooling effect on the chemical.

f. Soak time - Since there is no mechanical cleaning action or agitation involved once the chemical has been injected it is vital to allow sufficient soak time for the chemical to thoroughly penetrate right through the surface deposits. If it is prematurely rinsed off, deposits can easily be left behind which the post water rinsing is unable to remove.

In the case of a water/chemical solution rinse time should be a minimum of 30 to 60 minutes even for lightly fouled compressors and up to several hours for more heavily fouled compressors.  When using concentrated chemicals a good rule of thumb is about half the normal soak time.

g. Post rinse procedures - Thorough post rinsing is essential to obtain a good cleaning result and to ensure that no potentially harmful loosened deposits are left anywhere in the gas turbine.

When the compressor is crank-washed a lot of contaminated fluid is drained from the compressor casing into inaccessible places such as bearing labyrinths, instrument bleeds, cooling ducts etc. and unless this fluid and the deposits it is carrying are completely flushed out it is quite common for corrosion and potentially catastrophic blockage to take place after the gas turbine is back in service.

Rochem recommends that at least two complete successive warm water rinses and one final cold water rinse be carried out after each off-line wash. The reason for the final cold water rinse is to quickly break down any remaining foamy chemical residue and flush it away. And the reason it is important to flush out foamy residues is simply that it can hold a lot of deposit in suspension.

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