Stress in a Boiler Shell

[fredrosse]

In the industrial power generation realm, watertube boiler drums generally require much thicker sections where the tubes pierce the drum. This is because the costs of materials justify making the wall thickness of the drum variable, and as small as practical. The scale of these boilers is far removed from our steamboats, as illustrated in a few photos that follow:

[fredrosse]

Another example, showing the size of a typical utility power generation boiler drum. These boilers typically operate above 2400 PSI (164 Atmospheres), and produce a few million pounds per hour steam flow. This photo is taken inside the drum of a large boiler. Note the personal safety equipment, steel toe sandals, clear plastic helmets, etc.

[fredrosse]

Delivery of a large boiler drum. The power output from this steam plant is roughly enough to run about 300,000 typical steam launches which require about 3 horsepower each.

[Kelly Anderson]

The stresses in the vicinity of the extra holes are higher, but still sufficiently low as to not require increased wall thickness.

True. In our small sizes any additional thickness needed is often covered by extra material provided by a corrosion allowance anyway.

Take my example of a Blackstaffe given above. For that steam drum, the i.d. is 5.046" and at 150, psi, the stress at an efficiency of 100% is only 1,466 psi. In mild steel, stress in the boiler shell up to 15,000 psi could be tolerated, which means a wall thickness of only .025 could by rights do the job. In the worst case with the holes drilled in the drum, the efficiency is 38%, meaning the minimum wall thickness needed rises to a whopping .066, leaving us .192 extra for corrosion. I think we can sleep at night.

[tandemcompound]

after welding a Vertical fire tube boiler, will it need to be stress relieved?

[Kelly Anderson]

When made of less than 3/4" thick steel, stress relief is not required, but isn't a bad idea.

[Lopez Mike]

The main shell of my locomotive boiler is just about right for a diver's air tank. It would easily take a few thousand pounds pressure. A589, 45,000 psi strength. 10" dia., .37" wall calculates out to 3330 psi. I think at 150 p.s.i. rating it will suffice.

I selected it for low expense and as much weight as possible as the tractive effort of a locomotive is proportional to it's weight. I cheat to add weight everywhere it doesn't show.

Mike