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Friday, March 21, 2014

Spring Cleaning the Forest

Our home-owner's insurance policy was recently canceled because "you no longer meet our underwriting requirements, due to being located in a wildfire area".  Well duh.  I thought they understood that when they first issued the policy.

Because the insurer cancelled my home-owner's policy, I also cancelled the RV, Motorbikes, and cars.  They don't get to pick and choose what property of mine they get to insure.  They also saved me some money, because I found cheaper rates with a different insurance company (Geico).

Anyway... this is the second time that we have owned property.  The first property we owned was 2-1/2 acres in the Mojave Desert.  That property didn't require any maintenance.  Even tumbleweeds would blow right across the land because it was not fenced on the downwind side. 

Now that we are living in the forest, things require a lot more upkeep.  The insurance cancellation brought the realization that we have been here for two years now, and I haven't yet created a satisfactory fire-safe zone around the house and shop.  We have been busy getting the interior of the house set up, and ignored the grounds.

There is a lot of hard work involved with creating a fire-safe zone.  The  recommended minimum distance between a structure and any tree is 25 feet, with the forest thinned (and undergrowth removed) to a distance of 50 feet, with all tree branches removed to a level of 6 feet.

Personally I think 50 feet is a good radius for no trees.  I don't want the possibility a burning tree just 25 feet from my house.  Actually I don't want any tree close enough to fall onto the house.  Although two sides of our house have excellent clearance, I still have quite a bit of removal and thinning to do on the remaining sides.

This is an excellent time of year to clean up, because the forest is very wet, so it's safe to burn.  It's quite difficult to get a pile of twigs, branches and logs to catch fire.  Both times I burned, it took a full gallon of gasoline (and some diesel) to get the pile to stay lit.  This means that the forest won't catch fire whenever a spark flies off the fire.  The only bad thing is that downed trees located in the shade are often still frozen to the ground :)

This past week I got started on the process of making the house fire-safe by removing several years worth of fallen dead trees.  I have owned a 18" McCullouch MacCat chainsaw for the past 20 years, and it has served pretty well for odd pruning jobs and taking down small trees.  The first time I used it to gather soft wood for the fireplace, it proved not to be up to the task.  It doesn't have the power or the cutting ability to quickly get through a bigger log.  This past week I damaged the blade by forcing the saw into a log, and I realized that with this level of work, I needed a bigger, professional saw.

So... This year I got an early birthday present:  A Stihl MS 362.  This sucker comes with a 25" bar, but it is powerful enough to run a 36" bar.  It is amazing how quickly and easily it cuts compared to the MacCat.  This thing goes through logs like they aren't there.  Instead of spending minutes getting a log cut, it's over with in seconds.  It definitely makes the job easier. 

I also bought a skip-tooth chain to try out aggressive cuts, but haven't tried it out yet.  A skip-tooth chain makes kickback more severe, so I want to get used to the saw before putting on that chain.  Kickback is where the blade pops up after grabbing and releasing in a cut, or getting caught in wiry ground branches, especially at lower engine speeds. 

With the new saw, I can round up a lot of trash wood and a few chunks of firewood in short order.  Note:  This is not a lifted SoCal poser truck with chrome and aluminum hardware underneath.  I also don't cry like a girl when it gets a dent or scratch. 

Lodgepole pine - crabgrass tree of the forest.  Not even worth cutting up for firewood.

Saturday, March 01, 2014

Compressor Destruction :(

I have always been amazed by how quickly and destructively large powerful machines can fail.  I happened upon some pictures recently of an Industrial Combustion Turbine failure, brought on by icing.  The other type of combustion turbine is aero, like you might find on an aircraft.  Industrial (or "Frame") turbines are quite heavy and cannot be flown.

A gas turbine consists of three sections:  Compressor, Combustor, and Turbine.  At the inlet of the compressor, air pressure is reduced because the compressor is sucking air in.  This inlet pressure drop also causes a slight temperature drop, which is how this turbine's problem began.

Take a look at this video of an aircraft testing an aero turbofan engine.  You can see water vapor entering the engine.   This water vapor appears because the drop in pressure (a.k.a. suction) at the compressor inlet is also causing air temperature to drop.  The temperature is falling below the dewpoint - that is, moisure present in the air is condensing suddenly due to the drop in temperature.  Those swirls are condensing water vapor entering the engine.

Certain weather conditions, however, will cause moisture in the air to change from harmless vapor to ice.  Relative humidity must be high, ambient temperature has to be near the dew point, and temperature has to be near or below freezing.  Remember that air temperature drops as it enters the compressor, so the temperature doesn't have to be 32 degrees.  Inlet icing can occur with ambient temperatures as high as 38 degrees.

Icing is only an issue for the first row of compressor blades, because they are the coldest ones.  Water vapor can form, and freeze to the blades, if they are cold enough.   Icing is not a problem further inside the compressor, because as the air pressure increases, the air temperature also increases.   In fact, at the discharge of the compressor of a stationary gas turbine, the air temperature is about 700 degrees F. 

If necessary, a portion of this 700 degree air can be "bled off", and brought forward to the inlet.  This raises the inlet temperature just enough to prevent ice from forming. This is called the Inlet Bleed Heat system.  You take a small hit on engine output, but it definitley beats scrapping the engine.

Gas Turbine compressors are designed using complex fluid dynamics calculations to maximize airflow.  Maximizing airflow requires minimizing vortex formation, and minimizing interstage air leakage along the shaft and casing.  Thus everything inside is machined with very fine clearances for high efficiency.  For these reasons the blades are machined with pretty tight tolerances, and normally the airflow in the machine is in line with what the software calculated.

Inlet icing throws all that fluid modeling out the window, because if the blades are covered in ice, airflow will be erratic.   

This machine may have experienced ice buildup on the stationary compressor blades in the first row. The ice caused the air to flow unevenly onto the the rotating blades.  Each time a rotating blade passed a stationary blade with ice on it, turbulent/uneven airflow would cause a rocking motion as the rotating blade was buffeted by varying amounts of air.  This would actually be a very high speed vibration, since the rotor spins at 60 times per second.  The vibration created metal fatigue at the base of the blade (like bending a coat hangar over and over until it snaps in two) and eventually the blade came loose from the rotor. 

With the rotor spinning at 60 revolutions per second, the blade had plenty of centrifugal (centripetal) stress to shear it off, once the base had gathered enough cyclical fatigue to create a crack.

What followed after the compressor blade released compounded the failure.  It was pulled through the machine by the air stream and impacted most (perhaps all) of the other blades.  What a mess.  Moral of the story:  Don't allow the inlet to ice up.

Below: A new, clean compressor.  Inlet to the left side, discharge is near the circular flange to the right.

Below:  A severely damaged compressor.  It appears that all of this damage was caused by a single blade passing through the compressor.  (Inlet is to the left side, so the loose blade came from that row).  None of the blades on this rotor escaped harm.  It is not clear if the combustor section or turbine section were also damaged.

With the rotor removed, you can see that most of the stationary blades are also damaged.  The case will undoubtedly need some repairs as well.  Sad sad stuff here.