Day-by-day On Line Wash

I

Thread Starter

ibr2016

Our gas turbine 9e GE gas turbine.

I want to know the advantages and disadvantages of making a day-by-day on line wash?

And is this correct: Doing on line wash every two days?
 
Compressor washing frequency, on-line or off-line, is entirely dependent on the types of ambient contaminants present in the air ingested into the axial compressor inlet. Some site have very little dust and airborne contaminants and low humidity; these sites generally require very infrequent compressor washing, and on-line washing is almost useless in this type of environment.

Some sites are located near refineries where the atmosphere is full of hydrocarbon-based vapours which are regularly ingested into the axial compressor inlet. The location may also have high humidity and lots of airborne dust. These sites benefit from frequent axial compressor washing, both on-line and off-line.

Some site have very dusty roads which are heavily traveled by automobiles and lorries (trucks) which emit large amounts of exhaust, with a lot of diesel soot. These sites also benefit from more frequent axial compressor washing.

Some sites are located on or very near sea shores, where high ambient humidity with high salt content is present. These sites can benefit from more frequent axial compressor washing.

Also, the type and filtration efficiency/micron rating of the turbine inlet air filters, and pre-filters if so equipped, also can impact the frequency of axial compressor washing.

Now to the efficiency of the two types of axial compressor washing: off-line and on-line. Off-line axial compressor washing--done properly--is always more effective than on-line washing. Soaking with detergent, and rinsing wash the entire axial compressor and generally remove more dirt and contaminants. But, done improperly--especially when rinses are not done properly--off-line washing can actually be very ineffective and wasteful.

To perform off-line washing it's necessary to shut the turbine down, and cool it--either naturally or by crank-cooling. This means the unit is not producing power, and if the gas turbine exhausts into a HRSG (Heat Recovery Steam Generator--a boiler) to produce steam for a process or a combined cycle application--there is lost steam production which can in some cases be more problematic than lost electrical production and may require auxiliary boilers to be started and stopped. Additionally, off-line washing also involves a "soak" period after detergent and water are admitted into the axial compressor to allow the detergent to do its magic on the dirt and contaminants on both the axial compress stator and -rotor. Then, the rinsing has to begin--and depending on how much detergent was used rinsing can consume a LOT of water, as well as a LOT of time. (Failure to remove all detergent can result in loss of washing efficiency as well as a coating on the compressor internals that actually attracts and collects some dirt and contaminants after re-starting and during operation.

So, the biggest knock against off-line axial compressor washing is the time and lost production required to cool the axial compressor and turbine to be able to admit water and detergent, as well as the time and amount of water required for proper rinsing before the unit can be re-started. But, the long-term benefits of a proper off-line wash are far greater than any on-line wash.

On-line axial compressor washing, using detergent (and most GE-design heavy duty gas turbines with DLN combustion systems do not use detergent for on-line washing, or at least it's not generally recommended by the OEM), is really only effective for the first four or five stages of the axial compressor as the water /detergent is vapourized as it passes through the compressor.

Multiple sites around the world have trended the results of on-line compressor washing and have discovered that over the course of a four, or five or six on-line washes the power recovery is pretty much negligible. In other words, the first on-line wash is fairly effective, the second less so, the third even less, and so on, until the power recovery is really undetectable. At that point, it's prudent to shut down the unit and perform a proper off-line axial compressor wash, and when done the power recovery is significant. And, then they perform a series of on-line washes until the power recovery is negligible, at which time the unit is shut down and another off-line wash is performed.

The primary benefit of on-line washing is it negates the need for shutting the unit down and cooling it--maximizing on-line generation and production as long as possible. However, at some point, the effectiveness of on-line washing is so reduced it is necessary to shut the unit down, cool it and perform a proper off-line wash to restore compressor effectiveness.

When deciding when to perform any axial compressor wash it's necessary to monitor compressor "efficiency" and this is really only done best when the unit is running at Base Load (CPD-, or CPR-, biased exhaust temperature control active AND the IGVs fully open), when power output and axial compressor discharge pressure can best be compared against other day's values at Base Load with the IGVs full open. It's relatively difficult, without pretty costly software, to monitor and compare axial compressor efficiency at various part load conditions to determine when it's best to perform an axial compressor wash (on- or off-line).

Making comparisons at Base Load is relatively easy--record the same data points at roughly the same time each day (say dawn, or mid-day) and plot them or analyze them over time (a few days, a couple of weeks, a month or two, etc.). The turbine manufacturer usually provides a correction curve which can also be used to minimize the impact of large swings in ambient temperature over the course of a year which may or may not be necessary at some sites. And, then immediately after an axial compressor water wash is performed and the unit has reached Base Load and the unit temperatures (exhaust and wheelspace) have stabilized take another set of data and compare it to the last set of data to determine the effectiveness of of the wash. This data can also be compared to previous post-wash data to observe the overall condition of the compressor when clean; usually over time and between Major maintenance inspections the compressor efficiency will deteriorate slowly as clearances degrade and axial compressor components wear (normally) and this is to be expected. But, accelerated performance degradation can also be indicative of more serious problems or ineffective washing procedures.

Unless the site ambient conditions and airborne contaminants (dust from nearby roads or process plants (like cement plants, or construction, etc.) it's not usually prudent to wash frequently. BUT, the decision of when to wash should always be made with hard, actionable data--achieved by recording and monitoring power output and axial compressor discharge pressure over time at the same operating point (Base Load with IGVs fully open), and making any corrections using the OEMs temperature correction curves for the most detailed comparisons if necessary (large ambient swings over the course of the year). Just washing every day, or every other day, without monitoring the effects on unit operation is probably not very effective. Again, on-line washing is really only effective for the first few stages of the axial compressor, and has been proven to be effective only for a period of time after which it's necessary to shut down and perform a proper off-line axial compressor wash.

So, without knowing a LOT more about the site conditions than you have told us it's not possible to say anything more about your particular situation and circumstances. The above should help with your understanding of conditions and the benefits of both types of axial compressor washes. But, data, recorded and analyzed over time, is the best way to determine when each type of wash should be performed and the effectiveness of each type of wash.

Hope this helps!

 
we have on line wash past day and i have record data before and after before (mw=105, cpd=10.4, compressor discharge temp=369).
at online wash time (mw=107, cpd=10.5, compressor discharge temp=356).
after (mw=105, cpd=10.4,compressor discharge temp=362).
this online wash my be the tenth one after off line wash.
 
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