What Is Small Bore Piping?
Quick Answer: Small bore piping (SBP) refers to piping NPS 2 (DN 50) and below, including NPS 1/2, 3/4, 1, 1-1/2, and 2. These lines are particularly vulnerable to vibration-induced fatigue failure, and most EPC projects apply specific design, support, and connection rules to mitigate this risk. The most common connection types for small bore piping are socket weld and threaded (screwed) joints.
Small Bore Piping Classification
| Size (NPS) | Size (DN) | Classification | Typical Connection |
|---|---|---|---|
| 1/2” | 15 | Small bore | Socket weld or threaded |
| 3/4” | 20 | Small bore | Socket weld or threaded |
| 1” | 25 | Small bore | Socket weld or threaded |
| 1-1/2” | 40 | Small bore | Socket weld or threaded |
| 2” | 50 | Small bore (boundary) | Socket weld, threaded, or butt weld |
| 2-1/2” and above | 65+ | Large bore | Butt weld |
The NPS 2 boundary is the most common cutoff, but some project specifications set the small bore limit at NPS 1-1/2 or extend it to NPS 3. The applicable limit must be defined in the project piping material specification.
Connection Types and Standards
| Connection | Standard | SIF | Advantages | Limitations |
|---|---|---|---|---|
| Socket weld | ASME B16.11 | 2.1 | Leak-tight; good for high pressure; self-aligning | Crevice at root; not suitable for crevice-corrosion-sensitive services (SS in seawater) |
| Threaded (NPT) | ASME B1.20.1, ASME B16.11 | 2.3 | No welding required; easy disassembly | Potential leak path; limited to low-vibration services; requires thread sealant |
| Butt weld | ASME B16.9, B16.25 | 1.0 | Best fatigue life; full-penetration joint; no crevice | Difficult fit-up for small sizes; requires skilled welders |
| Compression (tube) | ASME B16.4, proprietary | N/A | Easy assembly; no hot work | Limited to instrument tubing; not for process piping per B31.3 |
Key Design Rules for Small Bore Piping
| Rule | Requirement | Reason |
|---|---|---|
| Maximum unsupported span | 50% of the large bore span (typically 1.5-2.0 m for NPS 3/4 to 1”) | Small bore pipe has lower stiffness; susceptible to vibration |
| Branch connection | Use branch fittings (weldolet, sockolet, threadolet) or forged tee | Stub-in connections on large headers create high stress concentrations |
| Minimum wall thickness | SCH 80 or SCH 160 (socket weld) per most project specs | Provides corrosion allowance and mechanical strength for small OD pipes |
| Valve connections | Match valve end connection (socket weld or threaded) to pipe class | Mixing connection types creates fabrication errors |
| Dead leg limit | Maximum 1.5 x pipe OD (some specs require zero dead leg) | Dead legs trap fluid and accelerate corrosion |
| Support type | U-bolts, clamps, or channel-type supports welded to structure | Guide and restrain to prevent vibration excitation |
Vibration Risks
Small bore piping is the leading cause of piping fatigue failures in operating plants. The Energy Institute report (2008) found that over 60% of hydrocarbon releases from piping were from small bore connections. Key risk factors:
| Risk Factor | Mitigation |
|---|---|
| Connections to reciprocating equipment (pumps, compressors) | Brace within 2 pipe diameters of the branch; use bracing per EI guidelines |
| Long unsupported cantilever runs | Add intermediate supports; reduce span length |
| Heavy inline components (valves, instruments) | Support the valve body independently; do not rely on the pipe for support |
| Socket weld root gap | Maintain 1.6 mm (1/16 in.) gap per ASME B31.3 before welding; lack of gap causes root cracking |
| Threaded joints in vibration service | Avoid threaded connections; use socket weld or butt weld instead |
Small bore piping connection types, schedules, and support requirements are specified in the project pipe class specification.
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