Key Requirements and Target Indicators for Boiler Feed Water

Boiler feed water is the treated water entering a boiler system, typically comprising makeup water and returned condensate. Its quality directly determines boiler safety, thermal efficiency, and service life. Poor feed water quality leads to scaling, corrosion, carryover, and even catastrophic tube failures. 

Therefore, strict chemical control standards have been established by organizations such as the ASME, as well as national standards including China‘s GB/T 1576 and European EN 12952-12.

Hardness – Zero Tolerance for Scaling:

Hardness, primarily calcium and magnesium ions, is the leading cause of scale formation inside boiler tubes. Scale acts as an insulating layer, reducing heat transfer efficiency and potentially causing tube overheating and rupture. For low‑pressure boilers, total hardness should be kept below 0.3 ppm, while high‑pressure systems require values below 0.1 ppm. Zero hardness is the ideal target for all industrial boilers to prevent scale deposition.

pH Value – Maintaining a Protective Alkaline Environment:

The pH of boiler water must be controlled within an alkaline range to minimize corrosion of carbon steel components. For low‑pressure steam boilers (below 300 psi), a pH of 10.5–11.5 is recommended; for high‑pressure steam boilers (above 300 psi), the target is 9.0–9.6; for hot water boilers, 8.5–9.5 is typical. Excessive acidity accelerates metal dissolution, while overly high alkalinity may cause caustic embrittlement.

Dissolved Oxygen – Preventing Pitting Corrosion:

Oxygen is a primary corrosive agent in boiler systems, causing pitting corrosion on tube and drum surfaces. Carbon steel components are particularly vulnerable when dissolved oxygen exceeds permissible limits. For low‑pressure boilers, dissolved oxygen should be kept below 0.007 mg/L (7 ppb), while high‑pressure systems require less than 0.005 mg/L (5 ppb). Mechanical deaeration combined with chemical oxygen scavengers such as sodium sulfite is commonly employed to achieve these stringent targets.

Suspended Solids and Total Dissolved Solids (TDS) – Avoiding Carryover and Deposits:

Suspended solids in feed water concentrate in the boiler drum as steam is generated, leading to sludge accumulation, foaming, and carryover of water droplets into the steam line. 

TDS limits are pressure‑dependent: for boilers operating below 50 psi, maximum TDS is approximately 2500 ppm; at 50–300 psi, the limit rises to 3500 ppm; above 750 psi, TDS must be reduced to 750 ppm or lower. Solids cause deposit formation on heat transfer surfaces, reduce efficiency, and increase blowdown frequency.