Pipefitter Pipe Systems and Fluid Mechanics 5 — Questions and Answers
Question 1: A pipefitter needs to calculate the equivalent length of a 4-inch standard 90° elbow for pressure drop calculations. What method is used?
- Measure the centerline radius of the elbow
- Use the manufacturer's K-factor or equivalent pipe length tables (Correct answer)
- Multiply pipe diameter by 10
- Use the Darcy-Weisbach equation directly on the elbow geometry
Correct answer: Use the manufacturer's K-factor or equivalent pipe length tables
K-factors and equivalent pipe length tables published by manufacturers or engineering references are used to convert fittings into an equivalent straight pipe length for pressure drop calculations.
Question 2: In a closed-loop hydronic heating system, what is the role of the expansion tank?
- To store excess water for makeup
- To accommodate volume changes as water expands and contracts with temperature (Correct answer)
- To increase system pressure during peak demand
- To separate air from the water loop
Correct answer: To accommodate volume changes as water expands and contracts with temperature
The expansion tank provides a cushion to absorb the increased water volume as it heats up, preventing dangerous pressure buildup in the closed system.
Question 3: Which pipe sizing principle requires that velocity be limited to prevent erosion in copper tubing carrying hot water?
- Keep velocity below 2 ft/s regardless of temperature
- Limit velocity to 4–8 ft/s for hot water in copper to prevent erosion-corrosion (Correct answer)
- Velocity has no effect on copper corrosion
- Keep velocity above 10 ft/s to prevent scaling
Correct answer: Limit velocity to 4–8 ft/s for hot water in copper to prevent erosion-corrosion
Hot water in copper piping should generally be kept between 4–8 ft/s; higher velocities cause erosion-corrosion that degrades the pipe wall over time.
Question 4: What phenomenon occurs when a fluid's absolute pressure drops to its vapor pressure within a flowing system?
- Water hammer
- Cavitation
- Flashing (Correct answer)
- Surging
Correct answer: Flashing
Flashing occurs when liquid pressure drops to its vapor pressure, causing some of the liquid to instantaneously vaporize into steam or vapor within the pipe.
Question 5: In a parallel pipe system, which characteristic is the same for all parallel branches?
- Velocity
- Flow rate
- Pressure drop across each branch (Correct answer)
- Pipe diameter
Correct answer: Pressure drop across each branch
In parallel piping, the pressure drop across each branch between the two common junction points must be equal, regardless of individual flow rates or pipe sizes.
Question 6: A system curve plotted on a pump performance graph represents what relationship?
- The pump's head output versus flow rate
- The required system head versus flow rate for the piping network (Correct answer)
- The efficiency of the pump at various speeds
- The NPSH required at different capacities
Correct answer: The required system head versus flow rate for the piping network
The system curve shows the total head required to overcome friction and static elevation in the piping network at various flow rates, intersecting the pump curve at the operating point.
Question 7: What is the function of a balancing valve in a hydronic piping system?
- To prevent backflow in the system
- To throttle flow to achieve design flow distribution among parallel circuits (Correct answer)
- To relieve excess pressure automatically
- To control fluid temperature by mixing hot and cold streams
Correct answer: To throttle flow to achieve design flow distribution among parallel circuits
Balancing valves are manually adjusted to restrict flow in circuits that would otherwise receive excess flow, ensuring each branch receives its design flow rate.
Pipefitter Pipe Systems and Fluid Mechanics 5
A pipefitter needs to calculate the equivalent length of a 4-inch standard 90° elbow for pressure drop calculations.
What method is used?