In This Article
- 1.Wafer vs Lug Butterfly Valve — Dead-End Capability
- 2.API 6D Bi-Directional Seating
- 3.Other Valve Types — Dead-End Ratings
End-of-line (dead-end) service means the valve is the last valve on a pipeline — the downstream side is either blind-flanged, open to atmosphere, or has a removable spool. This creates a unique mechanical challenge: the valve must retain the upstream pressure even when the downstream piping is completely removed. Not all valve types can do this — selecting the wrong type leads to catastrophic blowout when the downstream flange is unbolted with the line pressurised.
Wafer vs Lug Butterfly Valve — Dead-End Capability
Wafer butterfly valves are held between pipe flanges by through-bolts passing through both flanges. There is no load path from the valve body to the individual flanges — the valve is simply squeezed in place. If the downstream flange is removed, the valve body is free to be ejected by upstream pressure. Wafer butterfly valves CANNOT be used for dead-end service.
Lug-type butterfly valves have threaded inserts (lugs) in the valve body that accept stud bolts from each flange independently. This creates a direct mechanical connection between the upstream flange and the valve body — the valve can retain pressure with the downstream flange completely removed. Lug butterfly valves ARE suitable for dead-end (end-of-line) service.
API 6D Bi-Directional Seating
API 6D requires bi-directional seating for pipeline ball and gate valves — the valve must seal equally well with pressure from either the upstream or downstream side. This is the pipeline standard approach to dead-end service: the valve provides full rated pressure retention regardless of which side is pressurised. The test in API 6D Section 9 requires a high-pressure body test with the body cavity depressurised (simulating dead-end) — the seat must hold without leakage.
Other Valve Types — Dead-End Ratings
| Valve Type | Dead-End Capable? | Notes |
|---|---|---|
| Wafer butterfly valve | No | No mechanical attachment to either flange independently |
| Lug butterfly valve | Yes (with limitations) | See manufacturer's dead-end pressure rating — may be derated |
| Double-flanged butterfly valve | Yes | Full-face flanged; full pressure rated dead-end |
| Ball valve (trunnion, API 6D) | Yes | Bi-directional by design; full pressure both directions |
| Gate valve (API 600) | Yes (body) | Gate holds both ways; verify seat rating at dead-end |
| Globe valve | Yes (flow direction matters) | Normally installed with flow under the plug; reverse possible |
| Wafer check valve (API 594) | No | Not designed for dead-end; use flanged check valves |
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