The valve is connected to the pipe using pipe threads. … Most valves employ the tapered thread type. The valve is welded directly to the pipe. This connection is usually used where perfect shutoff of leakage is crucial, Most common on urban water and sewerage and gas pipelines

Flanged connections are the most common method of connecting valves to piping systems regardless of pressure levels and valve size. They are made by using two flanges, which are attached using stud bolts and nuts. They are quick and easy to assemble and disassemble for repair or replacement purposes.