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Gas Turbine Online Water Wash Requirements
Online water wash for frame 6 machines and the requirements to run on full load

Hi,

we have 6 nos Frame6 GE- GTG machines in our plant. We do have a water wash skid (GE Fanuc) installed from day one. However, it was not commissioned for online washer wash operation.

After discussion with GE Engineer, he explained the limitation is due to the requirement of the "Running the machine on a Full load, i.e. 26 MW". Our machines runs on not more than 20 MW on a load sharing/load shading modes.

Is there anything we can do to make this skid works on the normal load mode (i.e. 20MW)?

Hi, Nassor,

So, what kind of combustion systems do these six GE-design Frame 6 heavy duty gas turbines have?

Are you confident the Water Wash skid can be used (in automatic mode) for on-line water washing?

Do the gas turbines have on-line water wash spray nozzles?

DLN combustors do not like any on-line water wash detergent with any kind of flammable solvent content; it will flash and block the flame detectors and can result in a trip of the turbine. So, only water-based detergents can be used for on-line water washing of units equipped with DLN combustors.

One reason the OEM likes the units to be at higher loads when off-line water washing is that the IGVs are usually more open at higher loads which improves the cleaning effect of the detergent on the IGVs (or so I'm told, anyway). Also, if the units have DLN combustors I believe the OEM recommends on-line water washing only be done in Premix Steady State combustion mode, which is about 70-80% of rated load when Inlet Bleed Heat is not being used.

At higher loads, the Premix flame is more stable--the fuel/air mixture is a little richer (as rich as it's likely to get, anyway). And, flame stability is important when on-line washing a unit with DLN combustors.

And, that's another thing--if you're washing a DLN combustor-equipped unit with IBH while on-line, it should be off.

For units with conventional, diffusion flame combustors, it's more desirable to be at higher loads with the IGVs more open when on-line washing.

Most GE field service personnel don't have a lot of experience with on-line water washing (they're long gone before the unit is washed on-line!). It's only when problems come up that they get experience with the equipment, and there are usually problems with the water wash skid or the motor-operated valves near the bellmouth (when GE provides the skid and the isolation valves).

Me, I'm not generally a fan of on-line water washing for a couple of reasons. First, there is a diminishing return on each successive on-line wash; the performance never gets back to where it was before the previous wash, and eventually it doesn't restore much of any performance at all--which is the indication you really need to shut down and do a PROPER off-line water wash.

And, that's the second reason I'm not a big proponent of on-line water washing. As the water travels through the compressor it flashes to steam as it's heated, and then it doesn't really do too much. In fact, some machines have had a LOT of deposits on downstream compressor blading which was attributed to on-line water washing and detergent.

A properly performed off-line water wash WILL remove MOST of the deposits normally occurring in a unit, and will remove most of any deposits that occurred as a result of on-line washing. BUT, an improperly performed off-line water wash (with too much detergent and an insufficient rinse for the amount of detergent being used) will also cause deposits on the compressor blading. Using the proper amount of detergent (which is usually LESS than the detergent manufacturer's recommendation (because they want users to use AS MUCH detergent as possible, so they have to buy more!), with a proper soak period, followed by a proper rinse, and a manual inspection of the IGVs and compressor to assess the effects of the wash, is crucial to restoring as much performance as possible and for as long as possible. (What's the good in only washing 40-50% of the dirt off, and then re-starting the machine? It's only going to foul quicker because it wasn't clean when it was re-started, AND the performance gain will be lower than it would otherwise have been had a proper washing been performed, including the inspection (and subsequent washing, if necessary).

It does make economic sense for some applications to do on-line water washing. The cost of shutting down (lost generation), and cooling the unit to do an off-line water wash, and then washing and rinsing, can be expensive. And, sometimes the performance gains with initial on-line washing can be good. But, if you're only running the units at 20 MW or less, you can't really measure the performance increase (unless you have some pretty sophisticated fuel flow and performance monitoring equipment). GT performance (heat rate) is usually only guaranteed at Base Load when the IGVs are fully open.

Now, if you are determined to try on-line water washing, and the units do not have DLN combustors I would say you should try it--at a time when, should the unit trip, it won't cause a huge upset for the plant and any user it is supplying. If necessary you could force the permissive that would block it from running until it was at a higher load.

But, that entails risk. Everything entails some risk though. How much is it worth to find out if you can safely perform on-line water washing, and how much it might save over shutting down to do an off-line wash? (Again, I think that determination is going to be difficult if the units don't run much at Base Load. It will be a pretty subjective determination, at best.)

Hope this helps!

Please write back to let us know how you fare if you decide to try.

A COUPLE MORE THINGS TO CONSIDER...

I believe the spray nozzles for on-line water washing (if your unit(s) are equipped with on-line water washing spray nozzles--which are DIFFERENT than off-line water wash nozzles!) are sized for higher air flow-rates and fuel flow-rates. And, to my knowledge they aren't adjustable or variable, either automatically or manually. This is probably another reason why on-line water washing is "limited" to higher power outputs.

Sorry I forgot to mention those two things!

MANY years ago I had occasion to help rebuild a turbine at a site that tried running off-line water wash (using only water) for power augmentation for about four to six hours per day. Needless to say, a less-than-one year old turbine's compressor was reduced to nothing in about two months. Too much water flow for the axial compressor at rated speed....

Very expensive trial. And the Customer tried to say the Mark* was at fault because it didn't prevent them from forcing the logic to run the off-line water wash system when the unit was running. The controls are ALWAYS the problem.!.!.! NOT!!!!

By João Paulo on 10 August, 2019 - 10:19 am

Hi all,

I work in a power plant with two 9FB machines with the DLN 2.6+ combustion system. I have some questions about the water wash system operation.

This units are equipped with IBH. But we don’t have any restriction related to the on-line water wash and the operation of the IBH system, that's normal?

Another question that I have is: what is the correct amount of detergent to use in the off-line water wash? We use CLEANBLADE GTC1000 detergent, and the flow of detergent to the venturi is arround 10gpm. This amount of detergent is correct?

Thanks

Hello,

I would say, in my personal experience, that the situation you describe with no permissives or requirements is unusual, but with GE Belfort sequencing anything is possible. (They are extremely inconsistent. Consistently inconsistent, actually.) Actually, if the water wash system was NOT supplied by the turbine packager (GE; BHEL; etc.) and is not really "connected" to the Mark* turbine control (that is, the skid and pumps and such are not controlled by the Mark* turbine control except possibly for a START/STOP) then what you describe is entirely likely if the turbine packager was unaware of the provision of the water wash skid by a third party (or the EPC, Engineering/Procurement Contractor)--which is often done to "save" money.

Water wash detergent manufacturers will ALWAYS recommend the maximum allowable detergent--so that you will buy as much detergent as possible. Only the detergent manufacturer can say how much of their detergent is required for proper cleaning--but I can tell you honestly, that their typical written recommendation, again, is almost always excessive and could possibly lead to undesirable deposits in the engine.

How will you know this? When you open up the unit, especially the compressor casing, for a maintenance inspection you will see deposits on the last stages of rotating and stationary compressor blading, and possibly even on the combustion liners if the amount of detergent usage is excessive.

And that's the only way you will be able to determine for certain if the recommended usage is excessive or not. And, if the deposits are found during a maintenance outage, you should immediately contact the detergent supplier and manufacturer and have them come to site for a visual inspection of their own. Samples of the deposits should be gathered and sent to a lab for analysis.

Hope this helps!