Boiler Water Carryover: Causes, Detection & Prevention

What is Boiler Water Carryover?

Carryover is the presence of boiler water (liquid) in the steam leaving the boiler. It occurs when water droplets are entrained in the steam flow and carried out of the boiler drum. The contaminated steam carries dissolved solids, treatment chemicals, and water into the steam distribution system, where it causes a cascade of problems.

The impact of carryover extends far beyond the boiler itself:

• Dissolved solids deposit on turbine blades, causing erosion and imbalance (for power boilers)
• Water droplets in steam reduce the heat transfer coefficient at heat exchangers — wet steam transfers heat less efficiently than dry steam
• Treatment chemicals (sodium sulfite, amines) contaminate process steam used in food, pharmaceutical, or textile production
• Water slugs cause water hammer in steam piping, potentially rupturing pipes and fittings
• Scale-forming minerals in carryover water deposit in superheaters, causing tube overheating

Types of Carryover

• Foaming (Bubble carryover): Excessive foam forms on the boiler water surface. When the foam level reaches the steam outlet, bubbles are carried with the steam. This is the most common type and is caused by high TDS, suspended solids, or contamination by oils and organics.
• Priming (Surging): Sudden surges of boiler water are physically thrown up into the steam outlet by rapid changes in steam demand or boiler load. This is a hydraulic/operational problem, not a water chemistry problem. Often occurs when load increases suddenly.
• Dripping (Atomised carryover): Fine water droplets are mechanically entrained in high-velocity steam leaving the water surface. This occurs even without foaming and is primarily caused by high steam velocity at the drum water surface — often due to an undersized boiler drum.

Causes of Carryover

Detecting Carryover

• Steam purity testing: Sample the steam at the boiler outlet and test for conductivity or sodium content. Any reading above the baseline indicates water in the steam. Target: <0.1 ppm sodium for process steam, <0.01 ppm for turbine steam.
• Steam gauge glass foaming: If the boiler water gauge glass shows foaming or fluctuating levels, carryover is likely. The water surface should be calm and clearly visible.
• Steam line temperature: Wet steam has a lower temperature than dry steam at the same pressure. If the steam temperature at the boiler outlet is below the saturation temperature for the operating pressure, water is present.
• Downstream deposits: Inspect steam traps, strainers, and heat exchangers for signs of scale or chemical deposits. White or brownish deposits on steam-side surfaces indicate carryover.
• Water hammer: Frequent water hammer in the steam header, particularly near the boiler, is a strong indicator of water entering the steam line.

Prevention

1. Control TDS: Maintain boiler water TDS below the limit for the operating pressure (typically 3,000–3,500 ppm for low-pressure, lower for high-pressure). Use continuous surface blowdown controlled by a conductivity sensor.
2. Maintain proper water level: Keep the boiler water level at the design centreline — never above the high-water mark. High water level reduces the disengagement space above the water surface.
3. Avoid sudden load changes: Where possible, ramp steam demand gradually. If sudden loads are unavoidable, consider installing a steam accumulator to buffer the demand.
4. Use antifoam chemicals: Polyamide or silicone-based antifoam agents can be dosed to reduce foaming. Use sparingly — overdosing can actually increase carryover.
5. Remove oil and organics: If process contamination is the source, identify and fix the leak. Install oil separators in the condensate return system.
6. Inspect steam separation internals: During annual shutdowns, inspect and clean the steam separation devices (baffles, demister pads, cyclone separators) in the boiler drum. Damaged internals are a leading cause of persistent carryover.
7. Control treatment chemical dosing: Excessive sulfite or amine dosing can contribute to foaming. Dose based on actual test results, not on a fixed rate.