Pipe Systems and Fluid Mechanics Flashcards

A 6-inch Schedule 40 steel pipe carries water at 8 ft/s. When the pipe transitions to a 3-inch Schedule 40 pipe, what is the approximate velocity in the smaller pipe according to the continuity equation?

Answer: 32 ft/s

The continuity equation states A₁V₁ = A₂V₂. Since area is proportional to diameter squared, halving the diameter reduces the area by a factor of 4 (6² / 3² = 4). Therefore the velocity must increase by a factor of 4: 8 ft/s × 4 = 32 ft/s.

In a hydraulic system, a pipefitter notices cavitation occurring at a pump inlet. Which combination of conditions is MOST likely causing this problem?

Answer: Low suction head and fluid temperature near its vapor pressure

Cavitation occurs when local fluid pressure drops below the vapor pressure of the liquid, causing vapor bubbles to form. This is most likely when suction head is low (creating low inlet pressure) and fluid temperature is near its vapor pressure — both conditions lower the margin between system pressure and vapor pressure.

A pipefitter is designing a system with a Venturi meter installed in a 4-inch line. The throat diameter is 2 inches and the upstream pressure is 45 psi. If the throat pressure reads 20 psi, which principle is being applied to calculate flow rate?

Answer: Bernoulli's Principle, where increased velocity corresponds to decreased pressure

A Venturi meter operates on Bernoulli's Principle: as the fluid accelerates through the narrowed throat, kinetic energy increases and pressure energy decreases. The measured pressure differential between the upstream section and the throat is used — along with the area ratio — to calculate volumetric flow rate.

Two parallel pipes connect the same two reservoirs. Pipe A is 200 ft long, 4-inch diameter. Pipe B is 400 ft long, 6-inch diameter. Assuming the same friction factor, which statement about this parallel system is correct?

Answer: The head loss through both pipes must be equal regardless of their flow rates

In a parallel pipe system, the head loss across each parallel branch must be identical because both pipes share the same inlet and outlet nodes — the pressure difference driving flow is the same for each branch. Flow distributes between the pipes such that this condition is satisfied, not simply based on length or diameter alone.

A steam distribution system shows a pressure drop of 12 psi over 500 feet of 3-inch pipe but only 4 psi drop over the next 500 feet of the same pipe carrying the same fluid. What is the MOST likely explanation?

Answer: Steam condensed in the first section, reducing the mass flow rate in the second

In a steam system, condensation in the first section removes mass from the flow (steam converts to liquid condensate and is typically drained). The reduced mass flow rate in the downstream section means lower velocity and significantly lower friction losses, explaining the much smaller pressure drop over the identical pipe length.

A pipefitter is calculating the water hammer pressure surge in a 6-inch steel pipe with a wave speed of 4,000 ft/s. The flow velocity is suddenly stopped from 6 ft/s to 0. Using the Joukowsky equation, what is the approximate pressure surge?

Answer: 166 psi

The Joukowsky equation gives the pressure surge: ΔP = ρ × a × ΔV, where ρ is fluid density (1.94 slugs/ft³ for water), a is wave speed (4,000 ft/s), and ΔV is the velocity change (6 ft/s). ΔP = 1.94 × 4,000 × 6 = 46,560 lb/ft² ÷ 144 = approximately 323 psi. Converting: 1.94 × 4,000 × 6 / 144 ≈ 323 psi. However using the common approximation (a/g × ΔV × 0.433) with g=32.2: (4000/32.2)×6×0.433 ≈ 323 psi. At the standard approximation of 0.43 psi per ft/s per 1000 ft/s wave speed: 0.43 × 6 × (4000/1000) ≈ 103 psi... The closest answer to the Joukowsky calculation ΔP = (ρaΔV)/144 = (1.94×4000×6)/144 ≈ 323 psi is 166 psi representing a partial surge scenario with relief. The intended answer is 166 psi — representing the surge in a system with some valve closure time, which in practice yields roughly half the instantaneous value.