Drain valves are important items in all steam-handling installations, since they enable the operation and economy of the process to be handled properly. In combined cycle power plants, however, their role is fundamental; they have to undergo severe partialization sequences during start up and commissioning, and then they are required to ensure a line tight shut off under full plant operating pressure.
Bonetti Y Pattern Valves for this application, which has performed very
well in this application and can be seen as a satisfactory solution to the energy loss problem.
Valve characterics are the following:
- ‘y’ style, a design which offers fluid jets an almost straight path, with a very smooth ‘s’ bend;
- reduced port construction, further reducing the bend in the direction of the flow;
- trim shaped for a two-stage operation (double step function) with a large stellited area:
- partialization step, total or partial opening at the same time of the pressure reducing trim and the main seating, the latter offering the flow a much wider passage than that allowed by the partialization trim;
- shut off step, where the main plug reaches its seat when the partialization plug has completely closed its passage.
During the partialization step, the process fluid laps the main seating surfaces at a very low velocity, since in this passage there is almost no pressure drop, such a drop being entirely generated in the pressure reducing trim.
With this design (double step design) the seating surfaces do not have to endure a drop in pressure at any stage of the operation under flow, apart from the negligible time in each start up sequence. In this way, no seat damage is caused, and continuous line shut off is guaranteed.