We have 4 GT's with same Frame. Rated output 35.5 MW. Recently we observed that unit 1 and unit 3 GT's output reaches 29MW (at base load with 32degC ambient) just after a rainfall. Each time we notice AFQ drastically comes down, and consequently increases exhaust temperature (around 572 degC). After a offline water wash MW again increases along with AFQ. Matter to be noticed that change in AFDP is not too much after a rainfall.

Please give me a suggestion what necessary to do to avoid MW decreasing issue in such case?

 

You say you have recently noted this problem with two of the units. Have you checked the differential pressures across the turbine inlet air filters of all four units before and after the rain? How do they compare? (You want to do this when the units are running at the same loads.)

The kiel probes used as part of the Performance Monitor Package to measure air flow in the inlet have very small holes in the ends of the tubes--AND the holes in the tubes are supposed to be pointed <b>UP</b> into the air flow (which is usually coming down into the bellmouth). It is necessary to do regular maintenance on the keil probes; many sites use white vinegar or some similar liquid to soak the kiel probes in for two- to four hours to loosen the grime and dirt, and then use high pressure air and even small wires to clean out the orifice/hole, before rinsing them clean with warm water and drying them with clean dry instrument air and then re-installing them (again--with the openings in the ends of the tubes point INTO the direction of air flow (NOT with the direction of air flow).

There have also been lots of problems with leaking tubing fittings not properly tightened, and also with improper sloping of the sensing lines, which can collect moisture and cause erroneous readings.

My guess is that the time required to cool the unit(s) and perform a water wash is sufficient to allow the turbine inlet air filters to dry (they are probably ingesting too much rain/moisture). Rain hoods on the filters can help with this, especially if the units are situated such that the inlets of these two units face into the prevailing winds and rain (another thing to check--are these two units facing in a different direction than the other two units?).

It's also possible the turbine inlet air filters on these two units are different than those on the other two units, and may have elements that swell more when exposed to high humidity and/or rain.

Hope this helps!