EPRI Cycle Chemist Guidelines

Dissolved Oxygen

Monitoring of oxygen is necessary for the following reasons: • Copper alloy corrosion in condensate and feedwater systems is controlled by ORP. The valueof ORP is influenced by the levels of oxygen and reducing agent. • Oxygen reacts synergistically with chloride, resulting in hydrogen damage boiler tube failures.

• In all-ferrous feed-water systems, very low levels of oxygen (<1 ppb) in conjunction with high levels of reducing agent (hydrazine or alternative) result in very strong reducing conditions. Under such conditions, the feed-water reacts with the ferrous alloys in the feedwater system to generate corrosion products. These corrosion products are subsequently transported to the boiler, which leads to a) increased chemical cleaning requirements, and b)increased risk of boiler water-wall tube failures by under-deposit corrosion. • Flow-accelerated corrosion of carbon steel components is accelerated by over-feeding reducing agents and stopped by eliminating them and/or adding oxygen at concentrations above 20 ppb in high purity feed-water. Overfeeding of reducing agents is, therefore, not recommended and oxygen in high purity water is beneficial in reducing generation of irono-xides. • Many plants with all-ferrous feed-water systems operate without the use of reducing agents,and have significantly reduced iron transport. Dissolved Oxygen Target Value in Economizer Inlet
This dissolved oxygen target provides an indication of the oxygen control for reducing, oxidizing and oxygenated feed-water, and is thus important for controlling feed-water material corrosion in all-ferrous and mixed-metallurgy feed-water systems.

Dissolved Oxygen Target Value in Drum Boiler Water This dissolved oxygen target is provided as an indication of potential boiler water-wall corrosion.Its level is determined by the oxygen at the economizer inlet and the boiler recirculation ratio.

Dissolved Oxygen Target Value at De-aerator Outlet monitoring of oxygen at the de-aerator outlet serves as a check for the effectiveness of de-aeration.

Dissolved Oxygen Target Value at Condensate Pump Discharge. This new dissolved oxygen target value (10 ppb) is provided to minimize total corrosion in mixed-metallurgy low pressure feed-water heaters and to provide an indication of air in-leakage into the cycle for all feed-water systems.


Silica
Monitoring of silica is necessary because of the following reasons:
• precipitation of silica forms silicate deposits on the turbine that are not soluble in water and are very difficult to remove;
• silicate deposits cause losses in turbine capacity and efficiency; and
• significant amounts of silica can enter the condensate/feedwater undetected in the nonreactive colloidal form through the makeup, causing boiler water and steam target values to be exceeded in spite of apparently good-quality makeup.

Silica Target Value in Steam Based on the solubility of silica in superheated steam, a limit of 10 ppb was chosen for units with reheat. Compliance with this target value should limit deposition of silica in the turbine

Silica Target Value in Boiler Water Vaporous carryover is the major contributor to silica in steam as illustrated by the partitioning predictions in Figure 3-13. Also Figure 3-3 shows the distribution ratio, or concentration in thesteam to concentration in the boiler water, of silica as a function of pressure.

High boiler waterpH reduces silica carryover. This effect is insignificant for the pH range (9.0–9.6) used in high-pressure boilers as shown in Figure 3-13 and, for that reason, is not considered in theseguidelines.

Operation at higher boiler water pH values will cause these guidelines to be slightlyconservative. The limits for silica are based on the need to limit the amount of silica carried overinto the steam and is based on partitioning data and the thermodynamic code.3-35


Silica Target Value in Condensate and Makeup Water
The target value of 10 ppb silica in condensate is necessary for these reasons:
•
to achieve recommended steam purity under normal operation without relying on boilerblowdown, leaving the full blowdown capability available to control transients and maintainthe chemistry within guidelines at all times, and
•
to allow maximum flexibility during excursions.The target value for makeup water is the same as that for the condensate: however, this targetvalue should be modified if necessary to be consistent with the capabilities of the makeuptreating equipment design