SPED Pipe Support Design 2 — Questions and Answers
Question 1: Which load category per ASME B31.3 is used to evaluate sustained (primary) stresses in a piping system?
- Occasional loads — wind and seismic only
- Pressure, weight, and other sustained mechanical loads combined (Correct answer)
- Thermal expansion loads only
- Hydrostatic test pressure only
Correct answer: Pressure, weight, and other sustained mechanical loads combined
ASME B31.3 sustained load cases combine internal pressure, deadweight (pipe, fluid, insulation), and other continuously acting mechanical loads to check primary stress limits.
Question 2: What is 'cold spring' in pipe support and piping design?
- A cryogenic-rated spring hanger assembly
- An intentional cut-short or offset during installation to pre-compensate for thermal growth (Correct answer)
- A support used exclusively in cold-service (sub-zero) piping
- A spring constant measured at ambient temperature
Correct answer: An intentional cut-short or offset during installation to pre-compensate for thermal growth
Cold spring pre-stresses the system by installing the pipe in a slightly compressed or offset state so that thermal growth moves it toward its stress-free position, reducing hot loads on supports and equipment nozzles.
Question 3: When calculating allowable pipe support span, which single geometric factor most increases the permissible span length?
- Higher operating temperature
- Larger pipe diameter and schedule (greater section modulus) (Correct answer)
- Thicker insulation layer
- Higher fluid specific gravity
Correct answer: Larger pipe diameter and schedule (greater section modulus)
A larger pipe cross-section increases the moment of inertia and section modulus, directly raising the bending stiffness and allowable span before stress or deflection limits are reached.
Question 4: A constant-effort (constant load) hanger is preferred over a variable spring hanger when:
- The pipe operates below 200°F with negligible thermal movement
- Thermal movement is so large that a variable spring would have unacceptable load variation (>25%) (Correct answer)
- The pipe is subject to frequent external vibration
- The line is a small-bore drain with minimal load
Correct answer: Thermal movement is so large that a variable spring would have unacceptable load variation (>25%)
When vertical thermal displacement is large, a variable spring's load change (from cold to hot travel) can exceed 25%, making a constant-effort hanger necessary to maintain nearly uniform support load.
Question 5: What maximum mid-span deflection is most commonly specified in piping project standards to avoid flow and stress problems?
- 1 inch regardless of pipe size
- 1/2 inch (12.7 mm) or as defined by the applicable code and project specification (Correct answer)
- 10% of the pipe nominal diameter
- No explicit limit — stress alone governs
Correct answer: 1/2 inch (12.7 mm) or as defined by the applicable code and project specification
A 1/2-inch sag limit is the widely accepted industry default, preventing ponding in liquid lines, vapor locking, and appearance issues while keeping stresses within acceptable ranges.
Question 6: How are 'occasional loads' such as wind and seismic treated differently from sustained loads in ASME B31.3?
- They are ignored if the pipe is insulated
- They are evaluated with the same stress limits as sustained loads
- They are combined with sustained loads but checked against higher allowable stress limits (Correct answer)
- They only apply to outdoor piping above 50 ft elevation
Correct answer: They are combined with sustained loads but checked against higher allowable stress limits
ASME B31.3 allows a higher allowable stress (typically 1.33× the basic allowable) for occasional load combinations because such events occur infrequently and for short durations.
Question 7: What is the function of a 'limit stop' (travel stop) in a pipe support system?
- It restricts pipe displacement to a defined range, allowing normal thermal movement but preventing excessive travel (Correct answer)
- It sets the maximum allowable support span on engineering drawings
- It is a regulatory shutdown device on pressurized lines
- It permanently anchors the pipe in all directions
Correct answer: It restricts pipe displacement to a defined range, allowing normal thermal movement but preventing excessive travel
A limit stop acts as a mechanical boundary that permits controlled thermal displacement within a preset range while blocking abnormal or emergency movements beyond that range.
SPED Pipe Support Design 2
Which load category per ASME B31.3 is used to evaluate sustained (primary) stresses in a piping system?