Boiler Feedwater and Condensate

Feedwater is the water that is pumped into a boiler, made up of treated make-up water and condensate returned from the steam users. Condensate is steam that has given up its heat and turned back into water. Together these two streams form a loop whose condition decides how much corrosion product, oxygen and hardness reaches the boiler.

How does the feedwater and condensate system work?

Steam leaves the boiler, does its work in heaters, process equipment or turbines, and condenses. Condensate drains through traps and return lines to a collection tank, where it meets make-up water. A deaerator or feedwater tank removes dissolved gases, and the feed pump sends the mixture to the boiler. The pipework is usually carbon steel, sometimes with copper alloy components in heaters and condensers.

What are the water-chemistry risks?

• Oxygen corrosion: oxygen that survives deaeration, or enters at a tank vent or through a leak, pits feed lines, economisers and boiler surfaces.
• Carbon dioxide attack: carbon dioxide released from alkalinity in the boiler dissolves in condensate as carbonic acid, which lowers pH and thins pipe walls, often as a groove along the bottom of the line.
• Iron and copper transport: corrosion products carry back to the boiler and add to deposits.
• Contamination: leaks in heat exchangers or condensers can bring process fluid or cooling water into the condensate.
• Idle corrosion: wet, stagnant lines suffer attack during shutdown.

How are feedwater and condensate monitored?

Monitoring tracks feedwater and condensate pH, dissolved oxygen where measured, the residual of the oxygen scavenger, conductivity of the condensate, which reveals contamination, and iron and copper in samples as evidence of corrosion. Inspection of traps, tank vents and line sections shows where attack is happening. Trends matter more than a single reading.

Which chemical categories are relevant?

Our datasheets describe oxygen scavengers in two forms, one DEHA based and one sodium bisulfite based, a neutralizing amine for steam and condensate, and a multipurpose boiler product. Treatment rates are set by our technical team from the water analysis and the operating conditions, fed with a chemical metering pump.

Points to take into account when treating feedwater and condensate. These are qualitative guidance and not a recommendation for a specific plant.

• The choice between oxygen scavenger types, which depends on the boiler, the feedwater system and where the steam goes.
• Where the neutralizing amine is fed so that it reaches the whole condensate network.
• Whether steam contacts product, which limits the chemicals that may be considered.
• The metals in the system, particularly copper alloys, which react differently to amines and to pH.
• Dissolved oxygen entering at tanks, pump seals and vents, which treatment alone cannot offset.
• Idle periods and lay-up practice.
• Condensate return share and the reliability of traps and lines.

Our technical team reviews the water analysis and system description before any program is proposed.