Selection and Operation of Variable Speed Boiler Feed Pumps in a Triple Pressure Combined Cycle Plant

This paper discusses various issues that pertain to the proper selection and operation of variable speed boiler feed pumps in a combined cycle plant equipped with a triple pressure boiler. There are numerous selection criteria to be met in order to achieve the best operational efficiency with variable speed pumps. In addition, it is imperative that all cases of operation be analyzed to ensure proper pump selection. The triple pressure Heat Recovery Steam Generator (HRSG) is typically found in the combined cycle plant. Unlike its fossil fired predecessor, it requires a high component flow through each section of the HRSG. By far the most difficult choice when selecting the boiler feed pump for the boiler arrangement is how to balance the pressure and flow requirements of each section of the boiler with the pressure requirements that result from varying loads on the plant. The primary benefit of the variable speed pump is economy of operation at reduced load, yet this is also the very issue that makes selection difficult. Each pressure component of the HRSG has varying requirements that are non-linear with plant demand. A secondary but less tangible benefit from variable speed operation is improved life of system components, with the most significant benefit relating to the main feedwater control valve. However, coincident with the benefits of the variable speed pump are several operational and selection complications that make proper design challenging. Some of the issues to consider in the initial design include: control scheme, thermal plant design, and potential operational modes, all of which contribute to the challenges in designing the pump intermediate and high pressure sections for variable speed operation. These issues are analyzed and discussed. In addition, various details critical to pump design and selection that have been acquired through recent experience are incorporated in the various sections. If successfully selected and properly planned to work in concert with the remaining plant hardware, the benefits of a variable speed pump can be significant. The reduced power input to the pump can be a significant financial savings along with several less quantifiable but equally valuable benefits like increased control valve life, increased pump life and better resistance to system upsets.