Flanged connections
Flanged end connections use a raised or flat face on the valve end, mated to a corresponding flange on the pipe and secured with bolts through matching bolt holes. Flanged valves are the standard in large-bore process pipework because they can be removed and reinstalled without cutting the pipe — the valve drops out once the bolts are removed and the pipe flanges are jacked apart by the flange gap.
In Europe, the standard is EN 1092-1, which defines flange dimensions (OD, bolt circle, bolt count and bore) for each combination of DN and PN rating. In North America and for equipment to ASME standards, the standard is ASME B16.5. The two are not interchangeable — the bolt circles and flange ODs differ even at notionally equivalent sizes and pressure ratings.
EN 1092-1 defines several face types: raised face (RF), flat face (FF), and tongue-and-groove (TG). Raised face is standard for most industrial service. Flat face is used when mating to cast iron flanges that might crack under the point loading of a raised face gasket. Always specify the face type on flanged valve orders.
Threaded connections
Threaded end connections suit smaller bores (typically DN15 to DN50) where flanging is impractical or disproportionately expensive. The two main thread standards are BSP (British Standard Pipe, used throughout Europe and most of the world) and NPT (National Pipe Thread, used in North America and common in oil and gas globally).
BSP threads use a parallel profile (BSPP) for fittings that seal on a face or O-ring, or a tapered profile (BSPT) for thread-sealing connections. NPT threads are always tapered and seal on the thread. The two are not interchangeable — an NPT fitting will not seal correctly in a BSP port, even if it partially engages.
Threaded valves are suitable for low and medium pressure applications. At higher pressures, the thread provides both the mechanical joint and the seal — thread quality, sealing compound and assembly torque all affect the result. For critical or high-pressure applications, socket-weld or butt-weld connections are preferred.
Wafer and lug wafer connections
Wafer-body valves (principally butterfly valves and some check valves) fit between two pipe flanges and are clamped in position by the flange bolts passing around the outside of the valve body. They have no integral flanges, which minimises face-to-face length and weight. Wafer valves are low cost and suit installations where a flanged bypass or isolation valve is available for maintenance access.
The key limitation of wafer bodies is that they cannot be removed from the pipeline without removing the pipe flanges on both sides — both the upstream and downstream flanges must be unbolted and the pipe jacked apart to extract the valve. This is acceptable in most applications but impractical in buried or confined installations, or where regular maintenance access is required.
Lug wafer (or lug type) bodies have tapped lugs around the perimeter that accept individual bolts from each pipe flange independently. This means the downstream pipe can be disconnected without disturbing the upstream connection — the valve remains bolted to the upstream flange and acts as a line-blind. Lug bodies are specified wherever end-of-line service is possible, or wherever one side of the pipeline may need to be removed independently.
Butt-weld and socket-weld connections
Butt-weld end connections are welded directly to the pipe with a full-penetration weld, producing a joint with no mechanical connection, no gasket and no bolt. The result is the strongest, most leak-proof connection available, and is standard for high-pressure, high-temperature, cryogenic and hazardous media pipework where a flanged joint's potential leak path is unacceptable.
The trade-off is that a butt-welded valve cannot be removed without cutting the welds — either by cutting the pipe or by grinding out the weld bead. In practice, butt-welded valves are specified where reliability and leak integrity are paramount and valve replacement is expected to be rare.
Socket-weld connections insert the pipe end into a socket in the valve body and fillet-weld around the outside. They are used in smaller bores (typically DN50 and below) and offer similar integrity to butt-weld at lower cost, with slightly less strength at the pipe–socket interface.