What is a blocking valve? 

A dual-functional device, featuring interchangeable blind plates and flow sleeves, suitable for both hydrostatic testing and pipeline use.

The hydraulic test plug valve is used in the main steam pipeline of thermal power generating units, serving as a sealing device for the pressure test medium in the pipeline during the overall hydraulic test of the unit. It is installed on the steam pipelines at the outlet of the boiler superheater and the inlet and outlet of the reheater, functioning as an isolation device for static hydraulic tests. After the hydraulic test, internal components such as the internal plug plate and support plate are removed, and a flow sleeve is installed to put the pipeline into normal use. When a hydraulic test is needed again in the future, the flow sleeve is removed, and internal components such as the plug plate and support plate are reinstalled.

Source: Baidu Encyclopedia