Compressor Stall on Industrial Gas Turbines - Inlet Filter Replacement

 "Think of the universal substance, of which thou hast a very small portion; and of universal time, of which a short and indivisible interval has been assigned to thee; and of that which is fixed by destiny, and how small a part of it thou art." - Marcus Aurelius

 

Not too long ago, I made a post about compressor stalling.  That post started off with aircraft engine stalls and their causes, and then concluded with industrial engine stalls.  At the time, the power plant where I work was dealing with the effects of a badly fouled Gas Turbine inlet air filter.  

There are seasonal effects that can worsen a fouled inlet filter.  When the filter gets wet - especially when it is fouled - it doesn't want to pass air.  When the wet filter freezes, it will pass even less air.  On a cold, wet morning, the combination of wet filter media, wet debris clogging the filter, and freezing combine to prevent air from passing.  

If the inlet duct pressure reaches a certain negative pressure, the Gas Turbine will automatically run back to a lower power level to quickly clear the condition.  It does this so that the engine requires less air, and this takes the engine out of a possible stall situation.  However, well before the turbine reaches the point of automatically running back, it is already taking a significant hit on power output, due to lack of air.    

Once the inlet filter has reached this point, it's necessary to take the power plant offline and replace it - which I helped to do recently.  It's an all-hands affair, and it's a pretty physical job.  There are about one thousand individual filter elements to dispose of, and one thousand to replace.  Access is limited, so the filters are handed up to the top of the filter house in a daisy-chain of workers.  The job isn't great ergonomically - there's a lot of overhead lifting involved with the filter elements.  After-care involves hot pads, hot tubs (if you have one) and OTC pain relievers.

Some pictures of the recent filter change out.  The road is barricaded, and individual filter elements are tossed out of the filter house to the ground below.  At first the filter elements land in the bins.  Once the bins fill up, the filter elements roll all over the road.  Afterwards, they are stacked out against the fence once the demo portion has been completed.  The demo alone takes about an hour and a half, with half a dozen guys throwing them down and then later stacking them out.

Below:  The filter house above, and the trash dumpsters where the filter elements sometimes land.

Below:  A view of the landing zone from inside the filter house above.

Below: Mounting posts for each filter element.  There are four levels like this.  Inside of the holes is the clean side.

 

 

 Below:  A look down along the catwalk from level four.

Each filter element consists of a conical piece that goes up against the wall, and a cylinder piece that slides on in front of the conical piece.  In the image below, some of the conical elements are in place.  The blue stuff is a treated pre-wrap filter that removes coarse material.  The pre-wrap is intended to extend the life of the main filter element, but in my opinion, it's pretty useless.  Wildfire smoke goes right through that stuff and fouls the actual filter.

One other maintenance item that is usually performed following the inlet filter replacement is an offline compressor water wash.  The Gas Turbine is spun at low speed ~ 850 RPM, and soapy water is injected into the inlet from a spray ring on the compressor bellmouth.  

Below:  Me about 15 years ago at the compressor bellmouth.  You can see three spray nozzles.  One is left of my shoulder, and another is above and right of my head on the center section.

These nozzles squirt soap into the spinning compressor, which then is allowed to coast down to zero speed, with soap spraying the entire time.  Then the compressor soaks for 30 minutes, while soap is purged from the spray system.  After the soak, the turbine is spun back up, and the rinsing begins.  Demineralized water is sprayed intermittently into the compressor inlet for the next 4-6 hours, depending on how long it takes to get the rinse water conductivity low enough - less than 10 micromho per square inch.  

Afterwards, the water wash set-up has to be switched and the associated turbine valves moved to their normal operating positions.  This is a pain in the ass, because the valves are inside the turbine compartment.  They are difficult to reach, and having been in a very hot environment (>200 degrees F) for months, they are very difficult to operate.