Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ASME B16.14-2018 — Ferrous Pipe Plugs, Bushings, and Locknuts With Pipe Threads
🔌 ASME B16.14-2018 establishes dimensions, tolerances, materials, pressure-temperature ratings, marking, and testing for ferrous pipe plugs, bushings, and locknuts with pipe threads (NPT). These small parts are among the most common “supporting actors” in piping systems — sealing spare connections, reducing line sizes, or locking threaded joints — and are used in enormous quantities in petrochemical, utility, fire-protection, and plumbing lines. Covered types include square-head plugs, bar or slotted-head plugs, countersunk plugs, outside bushings, inside (face) bushings, and locknuts of various sizes.
💡 Though “small,” plugs, bushings, and locknuts are a critical link in the sealing chain: thread accuracy and taper determine seal reliability, and selecting the wrong class can cause leaks or insufficient pressure capability.
1. Scope and Pressure-Temperature Ratings 📌
Section 3 provides pressure-temperature ratings for each product type: the rating of plugs and bushings varies with material, size, and temperature and is typically derived from the allowable stress of carbon/alloy steel, linking with the class system of ASME B16.5/B16.11; locknuts primarily carry axial locking loads and must be strong enough for the threaded joint they secure. Nominal size follows the NPS of the connected pipe; reducing bushings are identified by both large-end and small-end sizes.
| Type | Threaded End | Typical Materials | Typical Use |
|---|---|---|---|
| Square-head plug | Single NPT end | A105 carbon steel, A182 stainless | Sealing spare tee/coupling outlets, equipment drains |
| Bar / slotted-head plug | Single NPT end | A105 and other ferrous | Manual sealing in tight spaces, wrench or screwdriver |
| Countersunk plug | Single NPT end | A105 and other ferrous | Flush/hidden sealing in counterbored seats |
| Outside bushing | External thread (large), internal (small) | A105 and other ferrous | Size reduction and reducing adapters |
| Inside / face bushing | Internal and external at same end | A105 and other ferrous | Reducing threaded openings |
| Locknut | Internal thread, hex or round | Ferrous materials | Locking threaded connections against loosening |
⚠️ When a plug seals a pressurized line, its rating must cover the system design pressure and temperature. Never substitute drainage-grade cast iron plugs for pressure-rated plugs — materials and thread strengths differ, and mixing risks blowout.
2. Material and Threading Requirements 🔧
Products are made of ferrous (carbon or alloy steel) forged/machined material, commonly ASTM A105 (carbon steel) and A182 (stainless and alloy steel). Threads are cut to ASME B1.20.1 taper pipe threads (NPT); sealing relies on the wedging action of the taper — the thread tightens progressively as it is screwed in, combined with sealant tape or compound, to provide a pressure-tight seal. The Standard also specifies pattern taper requirements so the transition between threaded portion and body has adequate strength without stress concentration.
💡 NPT sealing works by metal-to-metal wedging plus filler in the thread clearance: use sufficient sealant tape/compound and tighten to the specified turns. Too few or too many turns can cause leakage or cracking of the mating part.
3. Dimensions, Tolerances, and Marking 📏
Section 7 tabulates key dimensions for each type: head width and height for square-head plugs, slot width and diameter for bar/slotted plugs, countersink depth and angle, outside/inside diameters and lengths for bushings, and thickness and outside diameter for locknuts, with tolerances. Marking requires the maker’s name or trademark, material identification, and size for traceability and installation checks.
| Inspection Item | Typical Requirement | Engineering Significance |
|---|---|---|
| Thread accuracy | NPT per ASME B1.20.1 | Reliable sealing, interchangeability |
| Head type dimensions | Square/bar/countersink specified | Proper wrench or screwdriver engagement |
| Body length and wall | Per tabulated dimensions and tolerances | Full thread engagement turns |
| Taper transition | Pattern taper per Standard | Avoids thread-root stress concentration |
| Marking | Maker/material/size | Traceability |
🚨 When sealing openings in high-pressure systems (Class 600 and above), verify thread engagement turns and material grade against the Standard; always depressurize before removing a plug to prevent high-pressure fluid from jetting out.
4. Engineering Practice Essentials 🛠️
- 🔍 Selection workflow: determine the pipe size, pressure, and temperature of the opening → choose type (plug/bushing/locknut) → select material grade → verify NPT and dimensions → match sealant.
- 🔧 Installation: clean threads, wrap sealant tape in the direction of the thread (3–4 turns), tighten progressively; use an adjustable wrench for square heads, screwdriver or bar for slotted heads.
- 📊 Pressure testing: after sealing, perform a hydrostatic test to verify tightness at a pressure not exceeding the rating at the test temperature.
- 📋 Removal safety: depressurize before removal; inspect thread wear on reused plugs and discard badly worn parts.
5. Frequently Asked Questions ❓
❓ Q: What is the difference between B16.14 plugs and B16.11 threaded plugs?
A: B16.14 is a standalone standard for plugs, bushings, and locknuts with more plug styles (square, bar, countersunk); B16.11 is the forged socket-welding and threaded fitting standard whose threaded plugs are built to its class system. Both share compatible thread and sizing conventions; choose per the governing document.
A: B16.14 is a standalone standard for plugs, bushings, and locknuts with more plug styles (square, bar, countersunk); B16.11 is the forged socket-welding and threaded fitting standard whose threaded plugs are built to its class system. Both share compatible thread and sizing conventions; choose per the governing document.
❓ Q: Which pipe sizes do the two ends of a bushing fit?
A: The external (large) end screws into the larger opening and the internal (small) end receives the smaller pipe; sizes are stated large × small, e.g., 2×1 means the 2-in. end fits a 2-in. opening and the 1-in. end fits a 1-in. pipe.
A: The external (large) end screws into the larger opening and the internal (small) end receives the smaller pipe; sizes are stated large × small, e.g., 2×1 means the 2-in. end fits a 2-in. opening and the 1-in. end fits a 1-in. pipe.
❓ Q: What is the function of a locknut?
A: A locknut screws onto a threaded nipple or stem and, by jamming (backing up) against the joint, prevents loosening from vibration and thermal cycling and can set or fix axial position; select a locknut matching the thread size.
A: A locknut screws onto a threaded nipple or stem and, by jamming (backing up) against the joint, prevents loosening from vibration and thermal cycling and can set or fix axial position; select a locknut matching the thread size.
🔍 Q: Why do sealed plugs occasionally leak?
A: Common causes are insufficient or wrongly wound sealant tape, worn/damaged threads, too few turns, mismatched taper, or too low a material grade. Inspect thread condition, install per the Standard, and prove tightness by hydrostatic test before service.
A: Common causes are insufficient or wrongly wound sealant tape, worn/damaged threads, too few turns, mismatched taper, or too low a material grade. Inspect thread condition, install per the Standard, and prove tightness by hydrostatic test before service.
