All-volatile treatment is only applicable to units using demineralized water (< 0.1 pS/cm specific conductivity) makeup, excellent control over feedwater purity, and continuous monitoring to ensure that feedwater and steam purity is maintained.
If the unit suffers even minor contamination while using AVT, corrosion of the HRSG tubes is likely. Also, the use of AVT presumes that tube surfaces are clean. Changing from another treatment to AVT or using AVT on an HRSG that was not chemically cleaned prior to commissioning is inadvisable.
All-volatile treatment (AVT) has a long history of successful use in conventional fossil-fired watertube boilers and is applicable to HRSGs where feedwater purity is consistent with Table 9a. In conventional boilers the treatment regime consists of the use of ammonia or volatile amine and a volatile oxygen scavenger such as hydrazine. In HRSGs, concerns about FAC typically preclude the use of oxygen scavengers during continuous operation for units with all-ferrous materials. As long as the HRSG contains only ferrous materials, the pH should be raised sufficiently to maintain a pH greater than 9.4 at 25oC in the LP drum to minimize FAC. The LP feedwater prior to the LP drum should always be maintained at a pH greater than 9.6 at 25oC.
Ammonia is the simplest volatile compound for raising the pH of the feedwater in the economizers and evaporators. Neutralizing amines have also been used but there are some disadvantages. All neutralizing amines contain carbon and, depending on system temperature and pressure, may eventually break down into ammonia, carbon dioxide, and organic acids such as acetic acid. All of these, except ammonia, will increase the cation conductivity of the feedwater and steam. The cation conductivity limits in steam were established assuming only a small contribution from organic acids or carbon dioxide. Plants that use amines may find that they have difficultly determining whether an elevated cation conductivity of the steam results from organic acids or to the presence of chloride and sulfate in the steam.
Many of the commonly used neutralizing amines have a higher preference for the liquid phase than ammonia and will condense preferentially from the steam into the first condensate, increasing the pH to protect the turbine or condenser materials. Often, a combination of two or more amines is used to cover a variety of conditions in the steam/wa- ter cycle. Proper selection and application of volatile alkalizing agents is important to reduce the rate of FAC in HRSGs.
Ammonia effectiveness at pH elevation is reduced at high temperatures because the ammonia is primarily present in molecular form, and therefore may not protect the HRSG materials. Any acid forming species, i.e. chloride, can cause corrosion in the tubing even when the sample pH at 25oC indicates the drum water is alkaline if the rise in pH is strictly from ammonia. Therefore, it is strongly recommended that before choosing AVT treatment, steps be taken to ensure the consistent purity of plant feedwater. Such steps may include use of a titanium- tubed condenser and/or full-flow condensate polishing.
High pH in the feedwater from ammonia will cause ion exchange polishers in the system, including cation conductivity resin columns, to exhaust more quickly since non-ammoniated resin removes the feedwater ammonia. Resin exhaustion should be considered when setting the upper limit for feedwater pH and selecting the operating form of the resin in the condensate polisher.