Boiler Shell, Drums, Furnace and Tubes: The Pressure Parts
The pressure parts are the parts that hold steam and water under pressure: the shell or drums, the furnace, the tubes and the plates that join them. They are what the regulations inspect, what a hydraulic test proves, and what fails if the boiler is mistreated.
You'll learn
- Name the pressure parts and what each does
- Explain why wall thickness depends on diameter and pressure
- Understand why furnaces are corrugated and tubes are small
- Recognize the ways pressure parts fail
Before this: Fire tube, water tube and smoke-cum-water tube
The parts that hold the pressure
- Shell: the large cylinder of a fire tube boiler, holding the water, the steam space and the tubes.
- Drums: the steam drum (and sometimes a mud drum) of a water tube boiler, where steam separates from water and sludge collects.
- Furnace: where the fuel burns. In a shell boiler it is a large tube inside the shell; in a water tube boiler its walls are rows of tubes.
- Tubes: fire tubes carry hot gas through the water; water tubes carry water through the hot gas.
- Tube plates and headers: the thick plates or pipes the tubes are fixed into.
Everything else, from fans to controls, can be replaced on the run. The pressure parts are the boiler, in the eyes of the law: they carry the registration number, they are what an inspector examines, and they are what a hydraulic test pressurizes.
Why thickness grows with diameter
Pressure inside a cylinder tries to split it along its length. The stress in the wall, called hoop stress, grows with pressure and with diameter, and falls as the wall gets thicker:
| Symbol | Meaning | Unit |
|---|---|---|
| Internal pressure | MPa | |
| Diameter | mm | |
| Wall thickness | mm | |
| Allowable stress of the steel at its temperature | MPa |
- Design pressure 10.54 kg/cm²(g) = 1.034 MPa. Allowable stress 120 MPa (illustrative).
- A 2,000 mm shell:
- A 63.5 mm tube: 0.27 mm.
Furnaces, tubes and how pressure parts fail
A shell boiler’s furnace tube is pressed from outside by the water around it, which tries to collapse it rather than burst it. A plain tube under external pressure buckles at a fraction of the load it could take from inside, so large furnaces are corrugated: the ridges stiffen the tube against collapse without making it thicker, and also let it expand along its length as it heats.
Pressure parts are cooled by the water on their other side, and almost every failure starts when that cooling is lost or weakened:
- Overheating from low water: a furnace crown or tube uncovered by water heats far past its design temperature and bulges or ruptures.
- Overheating from scale: a deposit on the water side insulates the metal, which then runs hotter than designed even with the water level correct.
- Corrosion: oxygen pitting on the water side, acid attack on the gas side where gas cools below its dew point.
- Fatigue and cracking: repeated heating and cooling, especially fast starts, cracks tube plates and ligaments between tube holes.
- Leaking tube joints: tubes expanded into plates can loosen from thermal cycling.
Notice that the metal’s own temperature limit, not the steam’s, governs. Steel that is perfectly safe at 200 °C with water behind it can reach 500 °C or more under a flame without it, where its strength is a fraction of design. Module 12 covers tube failures in more detail.
Key takeaways
- Pressure parts: shell or drums, furnace, tubes, tube plates and headers.
- Hoop stress grows with pressure × diameter, so large shells need thick walls; small tubes do not.
- Furnace tubes are corrugated to resist collapse from outside pressure.
- Most failures start when the water side stops cooling the metal: low water, scale or corrosion.