Pipe Fabrication and Joining Flashcards

When performing a socket weld on a 2-inch Schedule 80 carbon steel pipe, what is the correct gap that must be maintained between the pipe end and the socket bottom before welding?

Answer: 1/16 inch (approximately 1.6 mm)

ASME B31.1 and B31.3 require a 1/16-inch gap between the pipe end and the socket bottom in socket weld fittings. This gap accommodates thermal expansion during the welding process. Without it, expansion can cause stress cracking or socket failure. Bottoming out is a common field error that invalidates the joint.

A pipefitter is joining dissimilar metals — carbon steel (P-1) to austenitic stainless steel (P-8) — using a butt weld. Which filler metal classification is most appropriate to prevent carbide precipitation and maintain corrosion resistance at the weld interface?

Answer: ERNiCrFe-6 (nickel-based alloy)

Nickel-based filler metals (such as ERNiCrFe-6 or Inconel-type fillers) are the industry-standard choice for dissimilar metal welds between carbon steel and austenitic stainless steel. They provide a metallurgically compatible buffer that resists carbide precipitation, accommodates the different coefficients of thermal expansion, and maintains corrosion resistance at the fusion boundary. ER308L would work in low-temp service but is prone to dilution cracking at the carbon steel side.

During hydrostatic testing of a newly fabricated carbon steel piping system, a small persistent seep is found at a threaded NPT joint that was sealed with PTFE tape. The system is at 1.5× the design pressure. What is the correct immediate action per standard practice?

Answer: Depressurize the system fully before attempting any repair or re-make of the joint

Any leak found during hydrostatic testing requires full depressurization before repairs are made. ASME B31.3 Clause 345.9.1 prohibits tightening fittings or making repairs while the system is pressurized above atmospheric. A threaded joint under hydrostatic pressure can suddenly fail, creating a projectile hazard. Adding sealant to a live pressurized joint is also prohibited.

A pipefitter must field-fabricate a 45° lateral branch connection on a 10-inch nominal diameter run pipe without using a prefabricated fitting. Which layout method produces the most accurate saddle cut on the branch pipe?

Answer: Computer-generated paper template (pipe wrap) calculated from actual OD measurements

A computer-generated or mathematically derived paper template (pipe wrap template) based on the actual measured outside diameter of both the run and branch pipes is the most accurate method for a non-standard lateral saddle cut. Unlike the wrap-around rule method (which approximates), the template accounts for the true intersection geometry of two cylinders at a compound angle. Trigonometric layout is accurate but extremely time-consuming for compound angles; the template approach is both accurate and efficient.

When making a roll groove on a ductile iron pipe for a mechanical grooved coupling, the groove depth is found to be 0.008 inches shallower than the minimum specification. What is the correct disposition of this pipe section?

Answer: Cut off the grooved end and re-groove further back on the pipe

A groove that is too shallow cannot be corrected by re-rolling in place — re-grooving a shallow groove on the same end will not reliably achieve the correct depth because the pipe wall has already been work-hardened by the initial roll and the groove geometry is compromised. The correct procedure per AWWA C606 and manufacturer specifications (Victaulic, Gruvlok, etc.) is to cut off the non-conforming end and re-groove on fresh pipe material. Applying sealant is never an acceptable substitute for dimensional compliance.

A brazed joint on 2-inch copper refrigerant piping (ACR tubing) shows a pinhole void visible on the outside of the joint after brazing. The pipe will carry R-410A at 600 psig. Which corrective action is technically correct?

Answer: Cut out and replace the entire fitting and pipe section with a new properly brazed joint

Any visible porosity or void in a brazed refrigerant joint is cause for rejection and replacement. Re-heating an already-made joint risks oxidizing the base metal, vaporizing residual flux (producing toxic fumes), and creating further voids. Applying filler to the exterior of a void creates a cosmetic patch, not a metallurgical bond — the void remains internally. For high-pressure refrigerant systems, ASHRAE and refrigerant system piping codes require the joint to be cut out and remade. There is no repair-in-place allowance for voids in brazed joints.