Two Shift Gate Valves Operation
Stop valves do have a crucial role in power stations. They can isolate certain parts of the installation and are operated in an “on-off “mode. In older installations the parallel slide gate valve is popular. This old design is based around two separate discs, guided by the seats during the full stroke, whereby in the closed position the upstream pressure pushes the downstream disc on the seat and creates the seal. An adequate system, however this has a major weak point in that during the opening and closing the disc is in constant contact with the seat potentially “scoring” the seat during the full stroke. By opening the valve intermittently the damage to the seats and discs is limited and it could reach the next outage without any major maintenance.
However, where the valves have to be operated in a dual shift regime, every day opening and closing may be needed which will significantly reduce the life span of a standard valve.
AVS offer a stop valve design which overcomes the issues of wear: a gate valve design based on the features of the parallel slide valve. The sealing is made by the downstream disc on the seat and the valve has the feature of a separate guide rail to guide the set of discs during the stroke of the valve so that the discs only contact the seats at the end of travel in the closed position. This special feature results in “friction free” opening and closing and no pressure locking. No wear to the sealing area occurs during travel. We have called this design the tapered parallel slide valve: This valve is based on two individual separate discs, a bearing and a spindle with a forged-on hammer head. This valve has a very limited number of parts in the pressure containing body.
The changing operational requirements of the plant require that the steam coolers, de-superheater valves, drains, feed water control valves, main steam isolation valves and the turbine quick closing valves are reviewed. These critical pieces of equipment have to be specifically designed to take the new dual shifting process requirements into consideration, once this has been done the operational performance of the plant can be improved and wear and tear of systems and components can be controlled and significantly reduced. Consequently as these pieces of equipment have been specifically designed for the new operating conditions of the station they are no longer a limiting factor to the start up time of the plant. The following paper highlights some of the more common issues found in dual shifting power stations with special regards to steam control.