11.2 High-Pressure Steam Generation and Boiler Auxiliaries
Key Takeaways
- A water-tube boiler carries water inside the tubes with hot gas outside, and it is the design used for high-pressure and high-capacity service.
- Under ASME Section I, a boiler with more than 500 square feet of heating surface requires two or more safety valves.
- A deaerator removes dissolved oxygen and carbon dioxide from feedwater, which is the primary defense against feedwater system corrosion.
- Continuous surface blowdown controls dissolved solids in the steam drum, while bottom or mud drum blowdown removes settled sludge.
- An economizer recovers heat from flue gas into the feedwater, and an air preheater recovers it into the combustion air.
Two boiler architectures
| Fire-tube boiler | Water-tube boiler | |
|---|---|---|
| Arrangement | Hot combustion gas passes inside tubes surrounded by water | Water passes inside tubes with hot gas outside |
| Pressure capability | Lower; the whole shell is a pressure vessel | High; only the tubes and drums hold pressure |
| Capacity | Lower | High |
| Water volume | Large, so it responds slowly and stores energy | Smaller, so it responds quickly |
| Failure consequence | Large stored energy makes a shell failure severe | Tube failure is more contained |
| Typical use | Building heat, small process | Utility and large industrial plants |
For the high-pressure work this module covers, the water-tube boiler is the reference design.
Pressure parts and circulation
| Component | Function |
|---|---|
| Steam drum | Separates steam from water; houses separators and dryers; the chemical feed and continuous blowdown connections are here |
| Mud (lower) drum | Collects sludge and provides a header for the tubes; the bottom blowdown connection |
| Downcomers | Carry cooler water from the steam drum down to the mud drum, usually outside the gas path |
| Risers / generating tubes | Heated tubes in which steam bubbles form; the density difference drives natural circulation |
| Water walls | Tube panels forming the furnace enclosure, absorbing radiant heat |
| Superheater | Raises steam above saturation temperature after it leaves the drum |
| Desuperheater / attemperator | Injects water to control final steam temperature |
| Economizer | Recovers flue gas heat into incoming feedwater |
| Air preheater | Recovers flue gas heat into the combustion air |
Natural circulation works because the heated riser tubes contain a steam-water mixture that is less dense than the solid water column in the downcomers. Anything that blocks or overheats a riser — scale, a plugged tube, flame impingement — disrupts that circulation and leads to tube overheating.
Safety valves and water level
Safety valves are the last line of defense against overpressure, and their requirements come straight from ASME Section I:
- Every boiler must have at least one safety valve.
- A boiler with more than 500 square feet of heating surface must have two or more safety valves.
- Safety valves are set and sealed by a certified organization; field adjustment is prohibited.
- Superheater outlets carry their own safety valve, set to open before the drum valves, so steam flow through the superheater is maintained during a relief event and the tubes are not starved and overheated.
- Relief piping must be supported independently and drained, so its weight and condensate do not load the valve.
Water level is the other life-safety item. Low water uncovers heated tubes, which then overheat and rupture.
| Device | Function |
|---|---|
| Gauge glass | Direct visual indication; blow down and verify it on every shift |
| Water column | The chamber the gauge glass and level switches connect to |
| Try cocks | Manual valves to confirm level when the glass is in doubt |
| Low-water fuel cutoff | Shuts off the burner before the level falls below safe |
| Feedwater regulator | Modulates feed to hold level against changing steam demand |
A gauge glass showing a rock-steady, unmoving level should be treated as suspect — in service the level normally shows slight movement. A blocked connection can leave a false level in the glass.
Feedwater treatment and deaeration
Boiler water chemistry failures cause more damage than mechanical failures.
- Dissolved oxygen causes rapid pitting corrosion of the feedwater system and boiler. The deaerator heats feedwater with steam and vents the liberated oxygen and carbon dioxide to atmosphere; an oxygen scavenger chemical removes the last traces.
- Hardness (calcium and magnesium) forms scale on heat transfer surfaces. Scale is an excellent insulator, so it drives tube metal temperature up and efficiency down. Softening or demineralization removes it upstream, and internal treatment keeps the residue as a non-adherent sludge.
- Carbon dioxide dissolved in condensate forms carbonic acid, which attacks condensate return lines. Neutralizing and filming amines address it.
- Dissolved solids concentrate in the drum as pure steam leaves, and above a limit they cause carryover — foaming and priming that sends water into the steam line.
Blowdown controls solids:
| Type | Location | Purpose |
|---|---|---|
| Continuous (surface) blowdown | Steam drum, just below the water line | Controls dissolved solids concentration |
| Intermittent (bottom or mud drum) blowdown | Lowest point | Removes settled sludge in short, sharp blows |
Bottom blowdown is done in short bursts so the drum level is not lost and so the flow scours sludge rather than just drawing clean water.
Steam turbines
Many industrial pumps and compressors are turbine driven. Two broad designs:
- Impulse turbine — steam expands in fixed nozzles and the resulting high-velocity jets strike moving blades. Pressure drop happens in the nozzles.
- Reaction turbine — steam expands as it passes through the moving blades themselves, which act as nozzles, and the reaction force turns the rotor.
Common features a mechanic works on: the trip and throttle valve, the governor that holds speed, the overspeed trip, gland (labyrinth) seals with a gland condenser, thrust and journal bearings, and the steam chest and nozzle block. Turbines are warmed slowly and drained thoroughly before rolling, because water entering a turbine at speed destroys blading immediately.
A power boiler has 700 square feet of heating surface. Under ASME Section I, how many safety valves are required?
What is the primary function of a deaerator in a boiler feedwater system?
Why is bottom blowdown performed in short bursts rather than as a slow continuous flow?