Quick answer: flanged ends let you remove the valve for maintenance or replacement, at the cost of two gasketed joints that can leak. Butt-weld ends become part of the pipeline, removing potential leak paths, but the valve can only be removed by cutting the pipe.
Comparison
| Point | Flanged ends | Butt-weld ends |
|---|---|---|
| Maintenance access | Easy — unbolt and remove | Requires cutting and re-welding |
| Leak paths | Two flange joints and gaskets | None beyond the weld itself |
| Installation | Fast, no hot work in the line if pre-assembled | Requires qualified welding and NDT where specified |
| Weight | Higher because of flanges and bolting | Lower |
| Typical use | Process plants, utility lines, valves likely to be serviced | Pipelines, high pressure service, buried lines, critical leak-tightness |
Other end connections
- Socket-weld: small sizes, simpler alignment, suitable for lower pressure classes
- Threaded: small sizes and utility lines where welding is impractical
- Wafer and lug: butterfly valves clamped between flanges
- Hub and clamp connectors: where a mechanical joint with high integrity is required
How to decide
- Will the valve need to be removed during the life of the plant? If yes, flange it
- Is the line buried, or is leak-tightness critical? If yes, weld it
- Does the project specification fix the end connection? Follow it
- Check gasket type, bolting material and flange facing against the line specification