June 1, 2009

Focus on Organics in Steam

David G. Daniels

Organic compounds can enter the steam cycle from a number of sources, including water treatment chemicals, or as part of a manufacturing process. Regardless of the source of the organics, their effects range from fouling polisher resins to causing significant steam turbine damage. Conventional water pretreatment systems are available to remove organics from water, but removing organic compounds at their source is the best place to start addressing the problem.

The presence of organic chemicals in feedwater and steam has been a power industry issue for a number of years and has been the subject of previous POWER articles (see "Organics in the Boiler and Steam: Good or Bad?" September 2006), as have the effects of organic compounds on the cation conductivity of feedwater and steam ("Cation Conductivity Monitoring: A Reality Check" May 2008).

Given the pervasiveness of these concerns, it pays to be aware of the latest understanding of how organics affect a power plant’s steam cycle and how their effects can be mitigated. That was the focus of the third conference on the interaction of organic compounds with water and steam, held in November last year in Lucerne, Switzerland. The conference was attended by90 participants from 19 countries and included 30 presentations over two and a half days on various aspects of the effect of organic compounds on water and steam to generate power. This article presents some conference highlights and current thinking about managing organics in the steam cycle.

Organics 101

Three major sources of organic chemicals in utility steam generating systems are:

• Natural organic matter (NOM), which consists of organic chemicals that pass through the pretreatment equipment and wind up in the boiler feedwater and steam.

• Treatment chemicals added to the feedwater that volatilize into the steam.

• Contaminants contributed by equipment such as condensate polishers.

Organic compounds, such as lubricating oils, can also contaminate the boiler, but these instances are rare. Each of the major sources poses a different risk for plant equipment.

The most common problem with organic compounds in the steam cycle is their detrimental effect on cation conductivity. In essence, heat and temperature in the steam cycle break down large and sometimes non-ionic organic compounds into shorter carbon molecules, and often carboxylic acids, which are ionic. Organic chemicals that create anionic species (acetate, formate, and dissolved carbon dioxide) in the steam or condensate will contribute to cation conductivity. Their presence makes it difficult for the operator to know if contaminants such as chloride or sulfate are in the steam, and therefore pose a risk to the turbine. It’s like driving in a rainstorm with bad windshield wipers — poor visibility prevents you from identifying a hazard early enough to respond.

Improvements in demineralized water purity, and the ability to improve the detection of contamination, have allowed turbine original equipment manufacturers (OEMs) to define a new "normal" for a steam cycle operating with no contamination from chloride and sulfate, thus lowering the acceptable cation conductivity. These lower limits leave little if any room for the contributions from carbon dioxide or other organic breakdown products.

For this reason, owners may void their warranty with the turbine manufacturer if their plant operates above this limit unless they can prove that the cation conductivity is not coming from chloride or sulfate.

Limiting organics can be a particular problem during commissioning, when there may be traces of contamination in the piping that can take a long time to remove.

Complicating matters further, industry studies have examined steam turbines that operate with cation conductivity that is consistently higher than EPRI and turbine manufacturers’ recommendations. The results showed that these turbines did not have a higher rate of turbine-related failures than their counterparts that operate at levels of cation conductivity that are within the current guidelines.