Module 05 · Boiler Water4 min read
Priming, Foaming and Carryover: When Water Leaves With the Steam
A boiler should send out steam and keep its water. Carryover is when boiler water, with everything dissolved in it, leaves with the steam instead. It wets the steam, contaminates the process, deposits solids in valves and superheaters, and can hammer the mains.
You'll learn
- Distinguish priming, foaming and carryover
- Know the causes of each
- Recognize the symptoms downstream
- Prevent and correct carryover
Before this: TDS control and blowdown, Wet, dry and superheated steam
BeginnerThe idea in plain words, and where you meet it. No maths.
Water where steam should be
- Priming: slugs of boiler water thrown into the steam outlet, usually by a sudden surge in steam demand or a water level that is too high.
- Foaming: a stable froth on the water surface, caused by dissolved solids, alkalinity, oil or organic contamination, that fills the steam space and is drawn out with the steam.
- Carryover: the general term for any boiler water leaving with the steam, from fine mist to slugs.
Carried-over water is not just water: it is concentrated boiler water, carrying dissolved solids and treatment chemicals into the steam system.
IntermediateThe formula, its units, and a worked example on real plant numbers.
Causes
| Cause | Why |
|---|---|
| High boiler water TDS or alkalinity | Makes the water foam; the froth fills the steam space |
| Oil or organic contamination | Often from condensate; a powerful foaming agent |
| Water level too high | Less steam space for separation; spray reaches the outlet |
| Sudden large steam demand | Pressure drops, water swells and surges toward the outlet |
| Running far above rating | Steam velocity through the water surface is too high for separation |
| Low steam pressure for the load | Steam occupies more volume and rushes out faster |
Several of these act together. A boiler running at low pressure, high load and high TDS, with a sudden process start-up, is the classic case.
ExpertThe fine print: where the simple model breaks, and what it costs.
Symptoms and prevention
- Gauge glass level unstable or jumping.
- Water hammer in the steam mains, especially after a load surge.
- Steam traps and control valves fouled with deposits; process heaters losing output.
- High conductivity in condensate that has not been contaminated by the process — the dissolved solids came from the boiler.
- Superheater and turbine deposits, where fitted: the most expensive consequence.
- Hold TDS and alkalinity within limits with correct blowdown.
- Keep the water level at its normal working level, not above.
- Keep oil and process contamination out: monitor condensate and dump it when contaminated.
- Avoid sudden large steam demands; open process valves gradually and keep the boiler within its rating.
- Do not run at much lower pressure than designed: steam volume and outlet velocity rise.
- Fit a steam separator after the boiler where clean, dry steam is critical.
Key takeaways
- Priming throws slugs of water; foaming fills the steam space with froth; both cause carryover.
- Causes: high TDS or alkalinity, contamination, high level, sudden demand, overload, low pressure.
- Carryover brings boiler solids into valves, traps, superheaters and the process.
- Prevent with blowdown, level control, clean condensate, gradual loading and a separator.