Water chemistry for today's combined cycles

May 29 2012 - TI Staff

Below are the extracts from a paper titled, ‘Considerations in Selection of the Appropriate Cycle Chemistry Program for Combined Cycle Power Plants’ by Colleen M. Layman, Bechtel Power Corporation, Frederick, MD USA, submitted at the Electric Power 2012 Conference Program.

Selection and proper application of an appropriate cycle chemistry program is at the heart of ensuring short-term availability and long-term reliability of a combined cycle power plant. The promotion of today’s ‘fast-start’ combined cycle designs rely heavily on a successful chemistry program as a prerequisite to implementation of the quick startup option for these units.

(A HRSG. The water chemistry program should be tailor made for fast cycling units)

Despite designers’ and equipment suppliers’ best efforts to standardize combined cycle system designs to streamline the engineering, construction and operations of these units, there is still no “one size fits all” chemistry option that works for every installation. Factors such as the extent of cycling operations, type of unit cooling, equipment metallurgy, operating pressure and temperature, steam export requirements, and OEM warranty limits need to be evaluated in the selection of the best cycle chemistry program. 

In selecting the best cycle chemistry treatment program and deriving operational chemistry limits specific to a given unit, one should begin by first compiling and analyzing the steam turbine, HRSG and combustion turbine original equipment manufacturers’ (OEM) specified chemistry limits. The easiest way to do this is to begin with the steam turbine OEM limits and work backwards through the cycle to calculate the chemistry required in the HRSG and condensate/feedwater to meet these limits, considering the manner in which the plant is designed to operate.

Design or operation limits
In some instances, however, the HRSG or combustion turbine components may have limits based on their design or operation that are more stringent than the chemistry requirements dictated by the steam turbine limits. One specific example of this situation is a combustion turbine design where IP steam is utilized for cooling. In this instance, the purity of the IP steam to meet the combustion turbine requirements is more stringent than the purity that would normally be required based on steam turbine OEM limits. This is why it is important to collect and review the chemistry guidelines supplied by the OEMs for major pieces of equipment. 

Use of high quality makeup and maintenance of condensate/feedwater purity are primary concerns for projects that utilize a “fast start” or “rapid response” design.   Such plants should ideally include permanent condensate polishers as part of their standard design to maintain condensate and feedwater purity and minimize chemistry holds. For the Benson once-through design that Siemens Energy provides, for instance, condensate polishers are necessitated as the HP portion of the HRSG is designed to operate on OT chemistry and condensate polishers are key under this chemistry regime to successful implementation of the treatment program. For other ‘fast start’ or ‘rapid response’ cycle designs, AVT chemistry programs coupled with a condensate polisher are the best choice to maintain a clean cycle and respond rapidly to changes.  

Miscellaneous considerations  
Plants that are designed to include an air-cooled condenser (ACC) for condensation of the steam turbine exhaust have special considerations that will impact the cycle chemistry treatment program and the associated operating limits. The ACC design consists of a large surface area for condensation of the exhaust steam. While this works well for heat transfer purposes, it can upset the steam-water cycle chemistry.

Newly erected ACCs are difficult to adequately clean and tend to contribute a substantial amount of contaminants to the cycle during initial start-up and even during unit restart if vacuum has been broken. The large surface area also increases the likelihood of iron transport in the system, particularly during initial startup and during unit restarts, and the potential for air in-leakage in the system. The inclusion of a condensate polisher as part of the plant design if utilizing an ACC should be considered. 

In order to minimize FAC in an ACC, the pH in the early condensate must be increased above that required for an equivalent water cooled condenser. It is recommended that the HP feedwater pH be maintained in the range of 9.6 to 9.8 to minimize FAC in the ACC. This may require supplemental chemical injection be provided for the HP steam drum or HP feedwater.  

Steam from auxiliary boilers is frequently utilized in combined cycle power plants for pegging the deaerator during startup, holding vacuum overnight, or hotwell sparging. The purity of the steam coming from the auxiliary boiler must be the same as the steam produced in the main cycle. Therefore, the chemical treatment program utilized for the auxiliary boiler must be compatible with the operating pressure and temperatures of the main cycle regardless of the fact that auxiliary boilers are designed to operate at much lower pressures and temperatures. 

Once the unit has been designed, constructed, and commissioned or – for an operating unit – once a chemistry program conversion has been completed, operators must be trained to understand and operate the plant using the selected chemistry control program. Methods of monitoring and analysis must be established to ensure that the chemistry program is maintained, and it should also have management buy in and support. The success of the program should also be continually verified and adjustments made, through the use of routine HRSG and turbine inspections, corrosion and deposition monitoring and testing, failure-rate analysis, and program audits.