The steam engine

This is the one place on this site where reading first is not optional advice. Read Steam: Its Generation and Use and the boiler-design sections of the engineering books in the library before cutting any plate.

Write down, for every boiler: its design working pressure, the plate thickness and material, the safety valve setting, and who inspected it and when. Boiler explosions killed people in large numbers throughout the nineteenth century, and the causes were known and preventable.

A Newcomen-type engine fills its cylinder with steam at little more than atmospheric pressure, condenses it with a spray of cold water to make a vacuum, and lets the atmosphere push the piston down.

It is enormously inefficient — around 1% of the fuel's energy becomes work — and almost impossible to explode, because the boiler barely rises above atmospheric pressure. That is exactly the right trade for a first engine, and it pumps mines, which is what steam engines were first built for.

This is the gate. An out-of-round or tapered cylinder leaks steam past the piston and the engine will not work. Historically, Watt's engine had to wait for Wilkinson's cannon-boring machine, which bored a cylinder true to about the thickness of a thin coin over its whole length.

Bore with a bar supported at both ends — see machine tools — and measure the bore at several depths and across two directions. Fit the piston with packing or rings that seal against the true bore.

In the atmospheric engine, cold water sprayed into the cylinder chills it every stroke, and the next charge of steam wastes itself reheating the metal. Watt's separate condenser condenses the steam in a separate vessel kept cold, with an air pump to clear it, while the cylinder stays hot.

This roughly tripled efficiency and is conceptually simple once seen. Lag the cylinder to keep it hot.

Before any boiler is fired under pressure:

• A safety valve set at or below the design working pressure, that cannot be tampered with — locked or sealed — tested by lifting.
• A pressure gauge and two independent water-level gauges.
• A fusible plug in the crown of the firebox that melts and releases steam onto the fire if the water level falls.
• A hydraulic test: fill the boiler completely with cold water and pump it to about one and a half times working pressure, inspecting for leaks and distortion.

Never test with air or steam — a failure then releases stored energy explosively; a water-filled failure only leaks.

The water level is watched continuously and kept within the marks. If it drops out of sight in the gauge, or plates show red:

1. Do not feed water. Cold water onto overheated plate makes steam faster than any valve can release, and the boiler fails catastrophically.
2. Pull or smother the fire, close the dampers.
3. Stop drawing steam gradually, and clear everyone from the boiler room.
4. Let it cool completely, then inspect before refilling.

Scale insulates the plate from the water, letting it overheat, weaken and fail. This is the most common route to boiler failure, and it is chemistry, not mechanics.

• Use the softest water available — rainwater is excellent; hard well water is poor.
• Settle and pre-boil hard water in a separate tank where scale can form harmlessly.
• Blow down — open the bottom valve briefly — daily, to flush sludge.
• Open and inspect the boiler internally at least yearly, measure the scale, and remove it.

An inspector independent of the owner examines every boiler on a schedule. Insured, independent boiler inspection, introduced in nineteenth-century Britain, sharply reduced explosions.

Keep a log for every engine: hours, pressure, water level checks, blowdowns, faults. Then put it to work: pumping mines, then mills and factories, then rail, then ships.