The Effect and Remedy of Nozzle Loads on Boiler Feed Pumps

Many frequently encountered start-up and operational problems in current design combined cycle power plants can be traced to pipe strain and nozzle loads placed upon pumps. This is most dramatic when the boiler feed pump is affected. Many of the symptoms are significant and can include misalignment, dynamic (changing) alignment, pump or motor vibration, bearing problems, mechanical seal failures and seizure of equipment on start-up and shutdown. While these are all nuisance items that can plague plant shake-down and commissioning, some can generate huge costs and plant unscheduled outages. More profoundly, these symptoms are often all present, making accurate diagnosis of the true cause very difficult. The real cost of these problems can be seen in plants missing commercial operation dates. Some of the piping issues that can cause these symptoms will be discussed, items including hydraulic aspects of the piping design, straight runs, horizontal runs, venting, location of minimum flow valve, and pipe hanger location. Also, the use of pre-fabricated pipe and spool pieces will be discussed. A short discussion will also be presented on how these piping issues impact various designs of pumps, such as barrel pumps, horizontal split case and ring section type pumps. This will also include some comments on pump mounting issues such as base-plate installation, the use of pin & key blocks and pedestal design. All of the discussions will be summarized and then presented with several recommendations for piping repair, operational changes, and material reinforcement. Optional pump features will be presented, indicating what items can be used to improve operation and reliability when abnormal nozzle loads are expected, including comments on internal clearances, wear part metallurgy and bearing upgrades. These recommendations will be compared against several field experiences for confirmation, with some focus on nozzle load data vs. design, and operation prior to and after strain removal. This combination of field results and engineering analysis of this topic should prove quite useful to the engineer attempting to diagnose any symptoms found in the field. Often times several symptoms may be present, making diagnosis difficult and it is only the methodical steps of symptom elimination that will get the new power plant on the way to commercial operation.