SPED Fluid Dynamics & Flow Analysis 3 โ Questions and Answers
Question 1: In gas pipeline design, the Weymouth equation is most accurate for:
- Low-pressure liquid systems
- High-pressure, large-diameter, long gas transmission lines (Correct answer)
- Steam distribution networks
- Two-phase slug flow regimes
Correct answer: High-pressure, large-diameter, long gas transmission lines
The Weymouth equation is empirically developed for high-pressure gas transmission pipelines and assumes fully turbulent flow.
Question 2: Choked flow in a gas line occurs when the gas velocity at the restriction reaches:
- The Mach number of 0.5
- The local speed of sound (Mach 1) (Correct answer)
- Twice the upstream velocity
- The critical Reynolds number
Correct answer: The local speed of sound (Mach 1)
Choked (critical) flow occurs when gas velocity reaches Mach 1 at the minimum cross-section; further reducing downstream pressure cannot increase mass flow.
Question 3: The equivalent length method for minor losses converts a fitting's resistance into:
- An equivalent pipe diameter
- A length of straight pipe with the same frictional pressure drop (Correct answer)
- An equivalent flow area
- A velocity head correction factor
Correct answer: A length of straight pipe with the same frictional pressure drop
The equivalent length Le expresses a fitting's pressure loss as the length of straight pipe that would produce identical friction loss, so Le = KยทD/f.
Question 4: Flashing in a liquid piping system occurs when:
- Flow velocity exceeds the speed of sound
- Static pressure drops below the fluid's vapor pressure (Correct answer)
- The Reynolds number exceeds 10โท
- The pipe wall temperature exceeds the fluid boiling point
Correct answer: Static pressure drops below the fluid's vapor pressure
Flashing occurs when local static pressure falls below the saturation (vapor) pressure, causing partial vaporization of the liquid.
Question 5: For a gas flowing through a pipe, the Panhandle A equation accounts for which factor that Weymouth does not explicitly include?
- Pipe roughness effects at lower Reynolds numbers (Correct answer)
- Liquid holdup in two-phase flow
- Soil thermal conductivity
- Valve loss coefficients
Correct answer: Pipe roughness effects at lower Reynolds numbers
The Panhandle A equation was developed for partially turbulent flow regimes and includes an efficiency factor that implicitly accounts for friction variation with Reynolds number.
Question 6: The velocity of a pressure wave traveling through a liquid-filled pipe (wave speed 'a') is most significantly reduced by:
- Higher fluid density
- Pipe wall elasticity and larger diameter-to-thickness ratio (Correct answer)
- Higher fluid viscosity
- Rougher pipe interior surface
Correct answer: Pipe wall elasticity and larger diameter-to-thickness ratio
Wave speed a = โ(K/ฯ ยท 1/(1 + KD/Ee)), so higher pipe compliance (larger D/t or lower elastic modulus E) significantly reduces the wave propagation speed.
Question 7: The Strouhal number in vortex-induced vibration (VIV) of piping relates:
- Pressure drop to velocity head
- Vortex shedding frequency, flow velocity, and pipe diameter (Correct answer)
- Fluid density to viscosity
- Wave speed to pipe length
Correct answer: Vortex shedding frequency, flow velocity, and pipe diameter
Strouhal number St = fD/V relates vortex shedding frequency f, pipe diameter D, and flow velocity V; for circular cylinders St โ 0.2.
SPED Fluid Dynamics & Flow Analysis 3
In gas pipeline design, the Weymouth equation is most accurate for: