Pipefitter Pipe Fabrication and Joining 4 — Questions and Answers
Question 1: When performing a socket weld on a 2-inch pipe, what gap should be left between the pipe end and the socket shoulder before welding?
- No gap; pipe must be fully seated
- 1/16 inch gap to allow for thermal expansion (Correct answer)
- 1/4 inch gap to allow filler metal flow
- 1/8 inch gap to allow for inspection
Correct answer: 1/16 inch gap to allow for thermal expansion
A 1/16-inch gap is maintained before socket welding to allow for thermal expansion and prevent distortion during the welding process.
Question 2: Which pipe end preparation is required for a Class 6000 threaded fitting on a high-pressure steam line?
- Standard NPT taper thread with sealant (Correct answer)
- Straight pipe thread with O-ring seal
- Butt weld end preparation
- Flanged connection with ring gasket
Correct answer: Standard NPT taper thread with sealant
Class 6000 threaded fittings use NPT (National Pipe Taper) threads with an appropriate high-temperature thread sealant for high-pressure steam service.
Question 3: A pipefitter needs to cut stainless steel pipe using a portable band saw. What blade characteristic is most important for this material?
- High carbon steel blade with coarse teeth
- Bi-metal blade with 14-18 TPI (Correct answer)
- Carbide-tipped blade with 4-6 TPI
- High-speed steel blade with 24-32 TPI
Correct answer: Bi-metal blade with 14-18 TPI
A bi-metal blade with 14-18 teeth per inch provides the right balance of heat resistance and cutting speed for stainless steel without work-hardening the material.
Question 4: During a hydrostatic pressure test, a threaded joint is found to be weeping. After depressurizing, what is the correct corrective action?
- Apply additional thread sealant over the outside of the joint
- Disassemble, clean, re-apply sealant, and reassemble (Correct answer)
- Tighten the fitting an additional half turn while pressurized
- Wrap the joint with self-fusing silicone tape and retest
Correct answer: Disassemble, clean, re-apply sealant, and reassemble
The proper fix is to fully depressurize, disassemble the joint, clean the threads, apply fresh sealant, and reassemble to the correct torque.
Question 5: What is the primary purpose of a purge dam when welding stainless steel pipe?
- To prevent the welder from being exposed to UV radiation
- To maintain inert gas backing on the weld root and prevent oxidation (Correct answer)
- To control the rate of heat input into the base metal
- To hold pipe sections in alignment during the root pass
Correct answer: To maintain inert gas backing on the weld root and prevent oxidation
Purge dams are used to isolate the weld zone inside the pipe so an inert backing gas (typically argon) can prevent oxidation of the root bead on stainless steel.
Question 6: A groove weld on schedule 80 carbon steel pipe requires a root opening of 3/32 inch. Which root opening tool is used to verify this dimension?
- A combination square
- A feeler gauge set to 3/32 inch
- A pi tape
- A wire gauge or spacer rod of the correct diameter (Correct answer)
Correct answer: A wire gauge or spacer rod of the correct diameter
A wire gauge or spacer rod of 3/32-inch diameter is inserted into the root opening to verify consistent spacing around the joint before tacking.
Question 7: When cutting carbon steel pipe with an oxy-fuel torch, the cutting oxygen lever should be opened at which point in the process?
- Immediately after lighting the torch
- After the base metal has reached bright cherry-red or orange heat (Correct answer)
- After establishing a carburizing flame
- While moving the torch along the cut line from the start
Correct answer: After the base metal has reached bright cherry-red or orange heat
The cutting oxygen jet is opened only after the steel reaches the ignition temperature (bright cherry-red to orange) so the exothermic oxidation reaction can sustain the cut.
Pipefitter Pipe Fabrication and Joining 4
When performing a socket weld on a 2-inch pipe, what gap should be left between the pipe end and the socket shoulder before welding?