Water Chemistry in HRSGs
The environment on the inside of HRSG tubing is an important factor in maintaining tube integrity. The chemical environment in the cooling water and steam is generally termed the ‘Water Chemistry’ of the unit. This book is not intended to be a reference for the complex subject of power plant water chemistry. This white paper provides a brief summary and an outline of the special features of HRSGs from a Water Chemistry perspective.
Raw surface or city water cannot be used directly in most HRSGs as impurities and other materials would damage the HRSG and downstream equipment including steam turbines and piping. The high pressures and temperatures in HRSGs result in chemical conditions damaging to power plant equipment from water that would be perfectly acceptable for drinking.
The water chemistry program at any power plant must accomplish the following:
- Control corrosion
- Control carryover of contaminants into steam
- Accommodate upsets in water quality (Buffering Capacity)
- Regulate vaporization of silica and other materials to steam
- Control depositsforming on boiler tube walls
- Control the adherence of sludge and other solids to piping and tubing
- Ensure adequate protective oxide formation on tube walls.
Key measures of water quality are:
Figure 1: Typical Feedwater pH variation
Raw water is treated to reduce suspended and dissolved solids, dissolved gasses (Oxygen and Carbon Dioxide), and organic materials before use in the HRSG. Typically as a final step in the processing, the water is passed through an ion exchanger. The resulting purified water is termed demineralized water. For high-pressure systems, it is recommended that demineralized water purity be equal to or better than the required steam purity. This is because at high temperatures and pressures, many contaminants such as silica will vaporize directly into the steam, and then condense out onto turbine blades and components causing damage.
The demineralized water must be further treated when used as HRSG feedwater. To control corrosion of the steels used in HRSG tubes, the dissolved oxygen content must be controlled to a defined band (2 ppb < DO < 7 ppb) and the pH kept relatively high (9.3 to 10.6 for all ferrous pipe and tubes, 8.8 to 9.2 for systems with copper alloy tubes in feedwater heaters). Ammonia (NH3) or other amines are typically used to control the condensate/feedwater pH. Oxygen scavengers such as Hydrazine (N2H4) are used to combine with dissolved oxygen to control oxygen levels. Other combinations of scavengers (and amines) can be used to provide protection of condensate or better stability.
The areas of the HRSG where boiling occurs are known as boilers, evaporators, or vaporizing sections. Chemical control is crucial in these areas. This is due to the fact that boiling allows impurities to concentrate, much as scale which forms on the bottom of a kitchen teapot. These concentrated impurities can form surface deposits or local acid forming salts, caustic forming salts, or sodium hydroxide. All these can cause tube corrosion, overheating and failure. Dissolved Oxygen (DO) can also be highly corrosive, especially in combination with chlorides or free chelant.
Boiler water treatment is designed to minimize the amount of impurities (by feedwater treatment and blowdown) and by buffering against the formation of acids or bases. However, most boiler treatment chemicals can also be corrosive or aid corrosion if not kept in the proper concentrations. Boiler water treatment programs in HRSGs include:
- Coordinated Phosphate
- Equilibrium Phosphate
- All Volatile Treatment(AVT) using Ammonia, Hydrazine or other amines (Cyclohexylamine, Morpholine, DEHA, etc.)
- Oxygen Treatment
All of these methods are used in HRSGs, and often more than one in a single HRSG. This is due to the fact that HRSGs often have more than one boiler. These operate at different pressures and temperatures, which affect the choice of chemical treatment methods.