Offline Water Wash System for F5 GTG

[norzul]

Hi,

Anyone got experience with regards to frequency and duration for offline water wash for frame 5 gas turbine?

norzul

 

[gtsim]

Gas turbine compressor wash frequency depends on a numbers of factors such as rate of fouling, production, down time for wash, fuel cost etc. Generally, if you are running on an engine limit such as the EGT, speed or power limit then the wash frequency must be increased as lost production due to compressor fouling will be significant and may amount to about £300,000 to £500,000 a year for a 25MW engine. In this event wash frequency of 4 to 6 weeks seems about optimum. However, if you are not operating on an engine limit then no loss is production would occur due to fouling and wash frequency may be significantly reduced, particularly if fuel cost is minimal or not significant as we may find in exploration and production.

 

[norzul]

Dear gtsim,

In our case, we have 4 GTGs (F5 size) + 1 STG (similar size to GTG) and plan to operate at about 60% load for all of the generators. Each GTG will be operated at about 14.4 MW each (rated load about 23-25 MW). Based on your explanation, I would consider the frequency of offline water wash is not critical to us.

Normally based on your actual experience doing 4-6 weeks offline water wash, what will be the increase of heatrate percentage at the end of the 6th week?

In addition, what is your frequency of online water wash?

Thanks

norzul

 

[gtsim]

Norzul,

Yes, that’s correct. In your case you should consider reducing your wash frequency.

The heat rate would increase by about 1% to 1.5% due to fouling after 6 weeks. It does not increase very much after that. In fact we are monitoring a 40MW gas turbine offshore (exploration and production) running at about 60% load. We wash about one a year. Fuel cost unimportant in this application (at least for now).

There is another smaller unit (7MW) we monitor, which has on-line wash facilities. On-line wash frequency is set by the OEM and is about once every 3 days or so. Although there is a benefit it is not as effective as off-line wash on this unit. IN general, there are other issues with on-line washing where compressor damage has occurred due to compressor blade erosion. Also please see thread666-147405 for some useful information on washing.

Regard,
gtsim

 

[toothless]

Calculate the isentropic efficiency of the turbine to determine how dirty the compressor is. We on-line wash every day (water only) and off-line wash about twice per year.

 

[gtsim]

Hi toothless,

I respectfully disagree that using the isentropic efficiency of the compressor alone would be sufficient to optimise the compressor wash.

Knowing only the isentropic efficiency is not sufficient as this efficiency can decrease if the load conditions are such that the compressor operates at a low efficiency part of the characteristic. We need to compare it with the expected isentropic efficiency at any operating condition.

The loss in isentropic efficiency of the compressor is quite small due to fouling. It is the loss in flow capacity that is significant. For example, after about 500 hours of operation after a compressor wash, the isentropic efficiency decreases by about 1% where as flow capacity decreases by about 3%. Furthermore, we need the change in both flow and efficiency characteristic due to fouling before we can determine the loss in power and increased heat rate due to compressor flowing before we can properly optimise the wash frequency. Other engine faults such as hot end damage will also affect the wash frequency. Also, as stated above, we need to know the downtime for wash, unit price (i.e. revenue), fuel cost, engine load, any standby availability to determine the wash frequency so that the lost revenue due to washing is minimised.

Your comments on this will be very much appreciated thanks.

Regards,
gtsim

 

[norzul]

Hi,

Does OEM suppose to normally recommend the frequency of online and offline washing? I've checked the manual. Nothing mentioned about the frequency.

Thanks

norzul

 

[TurbineDoctor]

The Off-Line compressor wash gives you the most "bang for the buck" vis-a-vis performance improvement. In addition to monitoring unit performance, my 36 years of experience with GE gas turbines has shown that a borescope inspection or at least a manual visual inspection through the inlet bell mouth will tell you what type of fouling you are dealing with.
I have found that manual cleaning of the IGV's and the bell mouth area is very effective in terms of the quality of the wash. Use the same detergent as you will use for the off-line wash cycle. Make sure that your people are properly clothed with rubber gloves and a face shield. Use sponges or SOFT brushes (NEVER metal brushes!) on the IGV's.
When you do the off-line wash, after you've injected the detergent mixture, and the unit coasts down to a rest, let the unit soak for at least 30 minutes. This soak period enhances the effectiveness of the detergent.
The rinse cycle is also crucial. Rinse until the drains run clear. It is also good practice to run a conductivity check of the rinse water.