DESIGN CONSIDERATIONS

When specifying a new HRSG unit, owners and/or engineering procurement contractors (EPCs) prepare detailed technical specifications to which original equipment manufacturers (OEMs) are expected to bid. Without specific requirements, bidders will always choose the least expensive capital cost approach. Some decisions can be made by the customer at this stage that can provide substantial payback in the unit's useful life and reliability, even though initially appearing slightly more expensive. A few of these design considerations are listed below.

HRSG CHEMICAL TREATMENTS

A number of possible chemical treatment regimes are discussed in this document but no single set of chemistry parameters can be correct for all HRSGs. This document is intended to provide sufficient information to allow HRSG owners or their consultants to select chemistry control limits suited to the particular configuration of their own plant. Since some of these parameters can be determined only after commissioning, a set of conservative chemistry limits using general assumptions is also suggested for use during commissioning until more refined limits can be established.

During start-up and shutdown, chemistry may drift outside of the established limits and it will be important to define the allowable deviations. Particularly during start-up, grab sample testing and on-line analysis should be used as soon as possible to determine the actual chemistry of the water and steam in the unit. Prompt action can then be taken to reach normal operating limits as soon as possible.

Considering the complex nature of HRSG piping combined with the cycling service under which many units operate, serious consideration should be given to the potential hazards of the misapplication of high concentrations of phosphates, chelants, polymers, or complex organic treatments.

If contaminants that require precipitation, complexation, or dispersion regularly enter the HRSG, all possible steps should be taken to improve the condensate return and feedwater quality instead of attempting to remedy the problem internally after the water has entered the HRSG.

However, for HRSGs that use softened water make up, a variety of traditional chemistry programs may be used.

Copper materials in the feedwater and steam systems limit the feedwater pH. The lower pH required makes the HRSG more susceptible to flow accelerated corrosion (FAC). It is strongly recommended that copper materials not be used in combined cycle plants. Aluminum alloys should never be used ANYWHERE in the HRSG water and steam cycle including the makeup water system.

 
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