Holes in Swiss Cheese.

by John


I decided that the time was right for me to fire up the traction engine.  I had seen the maker do it once, and another local expert do it again.  And I thought that I had taken in the essential steps and safety features.

So I wheeled the engine out of the shed onto a piece of thick plywood.  Filled all of the bearings with oil, pumped water into the boiler, opened the cylinder cocks, and lit the fire with kerosene soaked dry wood.   Then shovelled in some Welsh steaming coal.  The fire started up well, and within 15 minutes the pressure started to rise.

At 40psi I removed the fan on the funnel, and the pressure continued to rise.

At 60 psi I started the engine.  It turned over very nicely, and continued to run.  All good.

The pressure continued to rise, 80-90-100.   At 100 psi the safety valves started to vent, as expected, but the pressure continued to rise.   110-120 psi.

120 psi is well above expected pressure.  The safety valves continued to vent, but not quickly enough.  I decided that the fire was too hot.  What I did not realise, was that the cam which drives the water pump was slipping on its shaft.  The water pump was not functioning.   I could see that the boiler water level needed topping up, so I turned on the 160psi electric pump.  It did not seem to be working.  In retrospect, the nominal 160psi electric water pump spec is optimistic.  I used the hand pump.  There was some resistance which is good, but I could not see the water level rising.

To cut a long story short, I dropped the fire.  It was all very scary.  “Dropping the fire” involves pulling out 3 long metal pins which hold the fire pan in place.  The pan drops to the ground, spilling the burning coal beneath the traction engine.

Of course the pins, and the pan are hot hot.  And the burning coals are even hotter.

Then I could smell burning rubber.  Oh shit!  A piece of coal against one of the solid rubber tyres.  Panic pushed the engine a few feet away from the pile of burning coals, trying not to stand in them.   Stamped out the bit of plywood which had caught fire.  (I had rolled the engine onto a piece of thick plywood because it is difficult to roll it on the thick gravel which surrounds my shed.)

Steam engines are not for the faint hearted it seems.  Nor for the ignorant amateur.  I have a learning curve looming.

I had noted that quite a few nuts and other fasteners were not very tight.  I can only speculate about the reason for that, but the water pump failure due to a loose connection was a bit concerning.  So I have decided to disassemble the entire engine, check everything and reassemble it.  Should be interesting.

Considering the causes for this near disaster, I list the following in no particular order….

Shaft driven water pump failure due to an unexpected loose connection

Electric water pump not working at specification (to be confirmed)

Operator inexperience (unsure about hand pump pressures and valve positions, no experience in dropping the fire, furnace fire possibly too fierce, insuffient practice in emergency steps.  In retrospect I should have closed the fire damper, opened the fire door, and stopped the engine turning, and maybe used a fire extinguisher).

In medicine, disastrous outcomes are usually caused by multiple small mistakes, rather than a single big mistake.  “Holes lining up in the Swiss Cheese”  theory.

It seems that Swiss Cheese also occurs in steam engines.

p.s.  Note added 8 Nov 2017,  6 weeks later.  See my blog “The Boiler Inspector”.  It seems likely that the safety valves were not up to the job of venting adequate steam with a vigorous fire.  Another hole in the Swiss Cheese lined up.