Questions about T-type diaphragm valves

Hydraulic test valves mainly come in two structural types: the plug-type and the flap-type; currently, most units in China use the plug-type. Figure 1 shows the pressure test valve of the plug-type design used on the outlet pipe of the reheater in a 300MW boiler; it is model SD61H-64DN400, with a maximum operating steam temperature of 350°C and an operating pressure of 6.6 MPa. It consists of a valve body, valve cover, sealing ring, four-way ring, test plug, support plate, pre-tightening screw, and guide tube, etc.; its specific structural components depend on its operating condition. Installed at the outlet of the 300MW boiler superheater, the inlet of the reheater, and on the main steam pipeline, as an isolation device for hydraulic testing. It features a pressure-self-tightening sealing structure with welded connections at both ends. It features a flat O-ring seal, with the valve seat made by stainless steel surfacing. The middle chamber sealing ring can adopt an internal pressure self-sealing structure made of reinforced flexible graphite or a metal sealing ring. During the overall hydraulic testing of the unit, a special wrench is required to adjust the setting screw in order to apply an initial preload to the valve disc; when the force of the medium acts on the valve disc, the sealing force increases, thereby achieving a reliable seal. The middle cavity section features a pressure self-sealing structure.   　　During pipeline testing, the support plate and the plug plate are installed inside the valve. The preload screws are tightened to create a certain initial sealing pressure between the sealing surfaces of the plug plate, thereby preventing the flow of the medium toward the outlet side. The pressure of the medium generates sufficient sealing pressure between the plug plate and the sealing surface of the valve body; thus, sealing is ensured regardless of the pressure of the testing medium in the pipeline, allowing for testing of the pipeline on the side of the support plate. Once the testing is complete, remove the plug, install the guide tube and valve cover, etc.; at this point the valve functions as a pipe, and the plug can be reinstalled during the next water pressure test.   　　When in normal use, a plug-type valve functions like a section of pipe; its flow resistance can be disregarded, and it has no impact on the thermal efficiency of the unit. The downside is that removing parts such as the valve cover and sealing rings during testing is time-consuming and labor-intensive; it can easily damage the sealing rings, especially the metal ones, so spare parts are necessary