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Failure Modes

Last spring I was doing some work to my gutters and downspouts.  I was running a new downspout extension out past the front of my porch to make it easier to maintain.  So I bought a piece of downspout material at my local hardware store and I was running it along the wall of my front porch.

One of my neighbors was walking by at the time and stopped to see what I was doing.  And he asked me why I was installing the downspout in that orientation.

First let’s back up a little bit and look at how a metal downspout is made.  You start with a long flat piece of metal, as wide as you’ll want the circumference of the downspout to be.

Then you fold up one side.

Then you fold it again.

Then you fold the other side.

And then you fold it again.  Now you have a nice rectangular tube, with a seam running along its length.

So you’ll seal up that seam.  Maybe you’ll crimp it, or weld it, or use rivets.  But you’ll seal it up.

Now you’re ready to sell this to some homeowner, like me, to install.  And how should I install this?  Well, the seam is kind of ugly if you look really closely at it, and so you might be tempted to say that I should install this with the seam facing the house in order to hide it.  And in fact that’s what my neighbor was suggesting.

But I was installing it with the seam facing out, for all the world to see.  And here’s why:  If this downspout fails for any reason, where’s it most likely to fail?  At the seam – that’s the weak spot.  And if it does fail it’s likely to spew water out, maybe at a high velocity if there’s a heavy rain.  And I don’t want water being sprayed against my house like that.  I want water spraying away from my house.

This is the idea of a failure mode.  It’s a matter of understanding how something might fail and designing it or installing it to minimize any damage if it does fail.  Failure mode analysis can be used when designing or installing any object, and it can be used when designing a process.  How is the process most likely to break down and fail, and how can we design the process so that if there’s a failure it will cause the least damage?

If you want to learn more about this idea you can search online for “fail safe mode” or “failure mode and effects analysis.”

I’ve seen several cases of downspouts leaking from their seams and allowing water to spray against the house and cause damage.  Somebody tried to hide the seam rather than use good failure mode analysis and face the weak spot away from the house.  So when you’re installing or even maintaining something in your home, spend a couple of minutes and think about how it might fail, and think about what you can do to minimize the consequences in case of failure.

Grease traps

Many of the older cities in the Chicagoland area have combined storm and sanitary sewers, meaning that the rain water drainage is collected through the same pipes as the sewage from houses.  This type of system is almost never used anymore when constructing a new system, but many of the older cities still operate this way.

In these older cities with combined sewers it’s very common for houses to have a grease trap built in.  Because the sewer system is combined, and because back when the system was built it couldn’t handle grease very well, grease traps were a way to prevent grease from entering the sewer system.  Here’s a brief description of a grease trap.

 

 

 

A grease trap is a pit buried underground where water from the kitchen drain runs to.  The exit pipe to the sewer is elevated a little bit from the bottom so that there’s always some water in the pit.  There’s a trap built over the sewer pipe, so that grease floating on the surface of the water is held back, but the water is able to run under the trap and out the sewer pipe to the sewer main.  Any solid debris will settle to the bottom.

This way the grease is held back out of the sewer system.  Note that only water from the kitchen flows into the grease trap.  There’s no water from any other sink and certainly no water from any toilet coming into the grease trap.  But there might be water from your gutters flowing into the grease trap.  Where your gutter downspouts go underground they generally empty into the grease trap.

With this arrangement you’ll need to clean out the grease every so often.  This used to be a much bigger issue back when we used a lot more grease in our cooking.  Now you should just throw away your grease rather than putting down your kitchen sink drain.  So how often you need to clean out the grease trap depends on a lot of factors, but it’s likely to be many years (probably a decade or more) before anything will need to be cleaned out.  You can hire somebody to do this for you, or you can just lower a bucket on a rope to scoop out the grease and discard it.

In well over half of the grease traps I see now the trap has completely deteriorated, usually to the point where it’s completely gone.  This isn’t a problem anymore and I don’t recommend that you do anything about it.  There’s just no reason to.  And some grease traps have been bypassed, so that the drainage from the kitchen goes directly to the sewer and doesn’t go into the grease trap at all.

These days the biggest issue with a grease trap is the condition of the ring and the lid.  If the top concrete ring is damaged or the metal lid is broken then somebody could fall through.  But as long as the ring and lid are in good condition then you have very little to worry about with a grease trap.