Heat Soaking

After a person has burned out a few superheater sections of a monotube boiler everyone, without exception, will stop by and say “all you needed to do was to let it heat soak; everyone knows that”.   What they do not say is that they learned the lesson by burning out their share of superheaters.  They also do not say why they did not mention this delicate subject to the new steam person before the superheaters were burned out.

The new steam person may not be aware of the melting point of steel.  All you need to know is that it is below the temperature of combustion of burning fuel.  Therefore any boiler has the potential for burning out, melting, developing holes, or generally failing.  What keeps the hot end of a monotube from getting hot and melting is steam flowing through it, hence cooling it off.

The problem has to do with getting one of these things started from cold.  In a Doble style of a boiler the burner is at the top and the water is at the bottom and the top half of the coils is empty of water.  So, when the burner comes on it has to push combustion gases past all of the cold steel tubing which has a lot of specific heat in it, or lack thereof, and so the gases are cooled off by the time they get to the part with water so the water does not get heated up very fast and so there is no steam being made to move through the pipe to cool it.

The solution is to turn the burner on for 10 seconds or so and then to turn if off for 20 seconds and then to repeat many times.  This gives the heat in the upper coils time to flow through the pipe and heat up the water.  After a while steam is made, pressure comes up on the pressure gauge, and then a valve can be opened to let some steam flow.  Usually some hot water spits out and whatnot until hot steam is getting made.  Then it is ready to go to the engine.  If the engine has a dog clutch then the engine is warmed up.  If not then one has to slowly roll the automobile around the yard rocking it back and forth to let the water out and warming up another few hundred pounds of iron.  One does not want to go out on the freeway until everything is warmed up or many embarrassing things will happen.

Another solution to heat soaking is to have an electrically powered small water pump all hooked up to pump water into the superheater so that as soon as it is hot the driver can turn on the pump, open up the valve, and get steam flowing through the superheater section cheerfully cooling it down so the burner can be left on continuously to heat up the rest of the coils and the water so contained.

The lesson to be learned is that if one understands the problem then one can figure out how to deal with it.  When working with steam no one tells you what the problems are going to be.

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