Sunday, January 25, 2015

A simple prop made from PVC water pipe fittings

Sometimes a simple, inexpensive model is just the right prop to  illustrate a point in your presentation.

Several years ago I gave a speech to twenty members of my Toastmasters club about a corrosion problem that I’d seen two decades earlier. It involved special bolts used at ground level in a flange joint on a fire hydrant, instead of regular hex-head bolts as shown above.

When a hydrant is located near a road, it can be hit by a wayward vehicle. It is desirable to have the flange joint break just above ground before the hydrant body does. That special feature is the difference between a regular hydrant and a “traffic hydrant.”

One way to do this is to have the cast iron bottom flange made in pieces (or with grooves) so that it breaks before the body of the hydrant does.

Another way is to neck down the cast-iron bolts at the joint, so these special “breakable bolts” (as shown above) fail first. This inexpensive fix once was used by some manufacturers. The bolt head even could be shaped like a T (to indicate traffic).

The problem with breakable bolts is that the smaller diameter section is hidden away. It is a good place to trap water and road salt. If you forget to undo and inspect some of those bolts periodically, they will corrode away without being noticed. Then when the valve on a hydrant is opened all the bolts can break, accidentally launch the hydrant skyward like a rocket, and severely injure someone. This is why some municipalities forbid the use of breakable bolts. 

I wanted to make a larger-than-life (~3X) model to show the head and neck from a breakable bolt to my audience. (There already were PowerPoint slides of regular hydrants). So, I headed to my friendly, local plumbing supply store and bought a two-foot length of one-inch PVC pipe, two caps, a tee, two couplings, two reducers, and a two-foot length of half-inch pipe. I cut short lengths of pipe, and slipped all the pieces together. If I had a larger audience, then I could have instead used two-inch PVC pipe.

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