BGE Material Selection & Properties 3 — Questions and Answers
Question 1: Which material property determines how much a metal expands when heated, and why is it critical for pipeline design?
- Tensile strength
- Coefficient of thermal expansion (Correct answer)
- Brinell hardness number
- Yield elongation factor
Correct answer: Coefficient of thermal expansion
The coefficient of thermal expansion dictates how much a pipe grows or contracts with temperature changes; expansion loops and offsets must accommodate this movement to prevent stress failures.
Question 2: A BGE technician is selecting grounding electrodes for a new substation. Which material offers the best long-term corrosion resistance for buried ground rods in Maryland's soil?
- Bare copper-clad steel (Correct answer)
- Galvanized iron
- Stainless steel 316
- Aluminum alloy rod
Correct answer: Bare copper-clad steel
Copper-clad steel rods combine steel's mechanical strength with copper's excellent corrosion resistance in soil, making them the industry-standard choice for grounding electrodes.
Question 3: What is the maximum allowable operating pressure (MAOP) classification for PE 4710 gas distribution pipe in a typical residential distribution system?
- 60 psi
- 125 psi
- Up to 100 psi for medium-density PE in distribution class (Correct answer)
- 500 psi
Correct answer: Up to 100 psi for medium-density PE in distribution class
PE 4710 pipe used in distribution-class systems is generally rated up to 100 psi, sufficient for residential and light commercial natural gas service.
Question 4: Which transformer insulation system uses mineral oil as both a coolant and a dielectric medium?
- Dry-type cast resin (AN)
- Liquid-immersed oil-filled transformer (Correct answer)
- SF6 gas-insulated transformer
- Vacuum pressure impregnated (VPI)
Correct answer: Liquid-immersed oil-filled transformer
Liquid-immersed (oil-filled) transformers circulate mineral oil to dissipate heat and provide dielectric insulation between windings and the tank.
Question 5: A gas line crossing a river requires a casing pipe. What material is specified in DOT regulations for the carrier pipe inside the casing?
- PVC schedule 40
- Standard-weight carbon steel with external coating and cathodic protection (Correct answer)
- Copper type K
- Corrugated stainless steel tubing (CSST)
Correct answer: Standard-weight carbon steel with external coating and cathodic protection
DOT 49 CFR Part 192 requires steel carrier pipe with external corrosion coating and cathodic protection for crossings through casings under water bodies.
Question 6: What is the purpose of a zinc anode attached to a steel gas main?
- To increase pipe wall thickness
- To provide cathodic protection by sacrificing the zinc to prevent steel corrosion (Correct answer)
- To act as a pressure relief device
- To identify the pipe location with a tracer signal
Correct answer: To provide cathodic protection by sacrificing the zinc to prevent steel corrosion
Zinc is more electronegative than steel; it corrodes preferentially (sacrificially), protecting the steel pipe from electrochemical corrosion in the soil.
Question 7: Which conductor property is expressed in circular mils (kcmil) on utility material lists?
- Conductor insulation voltage class
- Cross-sectional area of the conductor (Correct answer)
- Conductor tensile breaking strength
- Conductor resistance per 1000 feet
Correct answer: Cross-sectional area of the conductor
Kcmil (thousand circular mils) describes the conductor's cross-sectional area, which determines its current-carrying capacity (ampacity).
BGE Material Selection & Properties 3
Which material property determines how much a metal expands when heated, and why is it critical for pipeline design?