This study examines the transient behavior of a seal injection system, for four boiler circulating water pumps, in an effort to optimize seal flush rates under startup conditions. During startup, seal injection supply water experiences a large increase in pressure, going from 1.8–26.2 MPa. This large increase in supply pressure presents a challenge in maintaining the desired differential pressure across the seals, and hence the optimum seal flush rate. Overshoot of the control valve position can result in starving the seals of seal water. Delayed responses expose the seals to excessively large differential pressures.
The seal injection system was modeled using PIPENET™ Vision. The model consists of a detailed replica of the seal injection system pipe network. Initial and boundary conditions were obtained from plant DCS data and pump OEM specifications. A baseline model was developed and validated using actual system response data. Extended models considered two types of control systems, manual and differential pressure-control; as well as, control valves with various flow characteristics: linear and equal percentage. Additionally, a diffuser breakdown assembly and startup control valve were also introduced as control components into the model.
Results show that the implantation of a diffuser breakdown assembly in series with the primary control valve (modified linear), in conjunction with automated controls produced a differential pressure of 436 kPa which was within the OEM specified range of differential pressures (345–690 kPa). A startup control valve used in series with the primary control valve also produced acceptable results (388 kPa). The proper design and operation of seal injection systems is vital to extending time between overhauls, thereby reducing maintenance costs. The use of the aforementioned control components in series with control valves is common for boiler feedwater regulation systems during startup; however, this is the first application known to the authors for pump seal injection systems in fossil plants. The results of the hydraulic simulation outlined in this study show this application is viable.