End connections affect installation, leakage paths, maintainability and inspection. They also determine which dimensions must be frozen before fabrication. A photograph rarely carries enough information to order a replacement valve.
01 / Routes
Each connection transfers a different installation problem.
Flanged valves can be removed without cutting the line, but the joint adds gaskets, bolting, alignment and flange-face control. Threaded valves are compact and familiar at smaller sizes, but thread standard, sealant, engagement and piping loads matter. Welded ends remove a detachable joint and transfer control to weld preparation, procedure, heat input, examination and future removal planning.
The right choice follows the piping class, service risk, maintenance strategy and construction practice—not a general ranking of “better” and “worse”.
| Connection | Useful when | Questions to close |
|---|---|---|
| Flanged | Removal, inspection and standardized mating joints are priorities. | Standard, class/PN, facing, finish, face-to-face, gasket and bolting. |
| Threaded | Compact small-bore installation is permitted by the piping class. | NPT/BSPT/BSPP or other form, gender, seal method and engagement. |
| Socket weld | Small-bore welded construction is specified. | Socket dimensions, gap practice, WPS, material and examination. |
| Butt weld | Permanent welded construction and smooth bore continuity are required. | End preparation, wall schedule/bore, transition, WPS/PQR and NDE. |
02 / Flanges
Freeze the mating-joint definition before approving a drawing.
ASME B16.5 covers pressure-temperature ratings, materials, dimensions, tolerances, marking and testing for defined pipe flanges and flanged fittings. The valve design route may additionally involve ASME B16.34 and a face-to-face standard. Those documents answer different questions.
Record raised-face, flat-face, ring-type-joint or other facing; surface finish where specified; bolt-hole orientation; gasket selection; and the mating flange standard. For wafer or lug valves, include the available bolt pattern and clearance around the body.
03 / Threads
A nominal thread description must identify the thread system.
NPT, BSPT and BSPP are not interchangeable labels. Tapered and parallel thread forms rely on different sealing arrangements, and pitch or profile differences can allow partial engagement that looks plausible but is unsafe.
State the standard, nominal size, internal or external thread, allowable sealant and any cleanliness requirement. Confirm that the piping design allows threaded construction for the service and size.
04 / Welding
A weld-end valve needs fabrication information, not just “SW” or “BW”.
For butt-weld ends, match the end preparation and inside diameter to the adjoining pipe. Heavy-wall differences may require a controlled transition. For socket welds, confirm the socket dimensions and the project’s fit-up practice.
The purchase package should address material identification, weldability, preheat/postweld heat treatment where applicable, weld procedure qualification, welder qualification and examination. If the valve contains temperature-sensitive seats or seals, agree how welding heat will be managed.
End preparation, bore, wall transition and face/end-to-end dimension.
Approved WPS/PQR and essential variables for the materials.
Cleanliness, heat control, orientation and disassembly instructions.
Weld map, welder identity, examination and repair history.
05 / Replacement
Measure the installation; do not order from the old nameplate alone.
Capture face-to-face or end-to-end length, pipe centerline, flange details, accessible bolt removal, insulation, actuator envelope and maintenance clearance. Check whether field piping has been pulled into alignment around the old valve.
A replacement that matches nominal size and class can still require spool modification if its dimensional series differs. Put the approved general arrangement drawing into the order deliverables.
Ends shall be [type] to [standard and edition], [class/PN], [facing/thread/end preparation]. Supplier shall confirm mating dimensions, face-to-face or end-to-end dimension, bore transition, gasket/bolting interface and installation limitations on the approved drawing.
Sources / editorial control
Source basis
- ASME B16.34-2025. Official scope for flanged, threaded, welding-end and flangeless valves.
- ASME B16.5-2025. Official flange-standard scope page.
Review limit: Connection dimensions and allowable construction depend on the governing piping code, piping class and licensed standard editions. No dimensional table is reproduced here.