Pipe Fabrication Methods Flashcards

When fabricating a piping spool with a DN200 (8-inch) carbon steel pipe using SMAW, the preheat temperature requirement per ASME B31.3 is primarily driven by which combination of factors?

Answer: Wall thickness and carbon equivalent (CE) of the base metal

Per ASME B31.3, preheat requirements for carbon steel are governed by the carbon equivalent (CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15) and the section thickness. Higher CE values and thicker walls increase susceptibility to hydrogen-induced cracking, mandating higher preheat temperatures. Ambient temperature can modify the requirement but is not the primary driver.

A fabricator is producing a mitered elbow from 12-inch Schedule 40 pipe using three cuts to achieve a 90° change in direction. What is the correct miter angle at each cut?

Answer: 30°

A 3-piece mitered 90° elbow requires 3 cuts, but it actually consists of 2 end pieces and 1 middle piece — effectively 2 weld joints producing the full turn. Each weld joint deflects 45° (90° ÷ 2 joints), so each cut is made at 22.5° from square, but the miter angle — measured from the pipe centerline — at each cut face is 30°. Using the formula: miter angle = total angle ÷ (2 × number of welds) = 90° ÷ (2 × 1.5 welds for 3-piece) = 30° per cut face. Each of the 3 cuts is made at 30° to produce the 3-piece 90° miter.

During hydrostatic testing of a fabricated spool per ASME B31.3, a small through-wall leak is discovered at a socket weld fitting. After draining and repairing by re-welding, what is the correct retesting procedure?

Answer: The spool must be retested at the full specified hydrostatic test pressure for the minimum required hold time

ASME B31.3 requires that any repair to a joint that failed hydrostatic testing must be followed by a complete retest at the full specified test pressure for the full required hold time. There are no provisions to reduce hold time or substitute a lower-pressure pneumatic test after a discovered leak — the entire test must be repeated to demonstrate the integrity of all joints, including the repair.

A fabrication specification calls for a 2-inch Class 3000 half-coupling socket welded to a 2-inch Schedule 160 header using a full-penetration fillet weld with a 1/16-inch gap at the bottom of the socket. Why is this gap intentional?

Answer: To prevent stress concentration from differential thermal expansion during welding and service

The 1/16-inch (approximately 1.6 mm) gap at the bottom of a socket weld is intentionally specified to prevent the pipe end from bottoming out hard against the socket shoulder. If there is no gap, differential thermal expansion during welding (and service cycling) creates compressive stress at the pipe end that can crack the fitting or the weld. The gap allows free axial movement and reduces the risk of crevice corrosion-induced stress cracking at the socket root.

When performing PWHT (Post Weld Heat Treatment) on a P-No. 4 (Cr-Mo) alloy steel spool per ASME Section I requirements, the cooling rate from the hold temperature must be controlled below 800°F (427°C). What is the primary metallurgical reason for this restriction?

Answer: To prevent re-hardening of the heat-affected zone through martensitic transformation during rapid cooling

P-No. 4 Cr-Mo steels (e.g., 1¼Cr-½Mo, 2¼Cr-1Mo) have sufficient hardenability that rapid cooling from PWHT temperatures can re-austenitize portions of the HAZ if overheated, or cause re-hardening through martensitic transformation if cooled too quickly through the critical temperature range. Controlled slow cooling below 800°F prevents this re-hardening, maintains the tempered microstructure achieved during PWHT, and reduces residual thermal stresses that could initiate cracking in service.

A fabricator is tasked with making an NPS 6 branch connection into an NPS 10 header using a weldolet in a high-pressure Category M fluid service. Which condition would require the use of a reinforcing pad (repad) in addition to the weldolet?

Answer: Neither a repad nor any additional reinforcement is needed — the weldolet is a self-reinforcing fitting by design

Weldolets are integrally reinforced branch connection fittings manufactured to MSS SP-97 and are designed to provide all required reinforcement for the branch opening per ASME B31.3 calculations. Unlike a plain branch connection cut into the header, the weldolet's integral boss supplies the metal that replaces material removed from the header wall. Adding a repad over a weldolet is not only unnecessary but is prohibited in many specifications because it can trap moisture and cause underbead corrosion. The other options describe situations where additional checks may be needed but do not override the self-reinforcing nature of a properly selected weldolet.