Steam Generation, Steam Systems and Steam Traps
From water to steam
Heat added to saturated water first raises it to saturation temperature, then evaporates it at constant temperature (latent heat), then superheats the vapour. Saturated steam is in equilibrium with water; superheated steam has been heated above saturation and contains no moisture. Dryness fraction describes the proportion of vapour in wet steam - wet steam carries water droplets that reduce heat transfer and can erode turbines.
Steam systems
Steam leaves the boiler through the stop valve, travels in mains, and gives up latent heat in the load. Priming and foaming carry water over with steam; causes include high TDS, high level, sudden load change and oil contamination.
Steam traps - the examiner's favourite
The trap passes condensate and air but retains steam. Types:
- Mechanical: float-thermostatic, inverted bucket - operate on density difference.
- Thermodynamic: operate on velocity/pressure difference of flash steam.
- Thermostatic: operate on temperature difference (balanced pressure, bimetallic).
Symptoms: a failed-open trap blows live steam (energy loss, noise); a failed-closed trap causes waterlogging, water hammer and poor heating.
Traps in questions
Do not confuse superheat with latent heat, or priming with foaming. Remember traps discharge condensate, not steam, in normal service.
Worked rule of thumb
If a steam main is cold at the far end and hammers, suspect failed-closed traps or blocked strainers. If the trap discharges continuously with live steam, it has failed open.