Plumbing Science
This domain tests the physics you apply every day - pressure, flow, energy and heat - and whether you can use the numbers to choose pipe sizes and spot an installation that will not work.
Pressure and head
Pressure is force per unit area (1 bar = 100 kPa). Head is the equivalent column of water: 1 bar ≈ 10.2 m, and 1 m of water ≈ 9.81 kPa. Static pressure at a tap is the head above it; dynamic pressure falls once water flows because of friction.
Flow, velocity and friction
Flow rate Q = velocity × area. Friction loss rises with velocity squared, so a small increase in speed costs a lot of pressure. Domestic velocities are normally kept below about 1.5 m/s for cold and 1.0 m/s for hot to control noise and erosion-corrosion.
Heat and energy
Q = m × c × ΔT, with c = 4.186 kJ/kg·K for water. Latent heat of vaporisation is about 2257 kJ/kg. Water expands when heated; a sealed system needs an expansion vessel, and a blocked cold feed can cause a cylinder to collapse.
Other science you will be asked about
- Siphonage and trap seal loss.
- Air locks, water hammer and shock arrestors.
- Thermal expansion of plastics versus copper.
- Gas laws applied to compression and air testing.
Worked example
0.3 L/s through 15 mm copper. Internal area = π × (0.0075)² = 1.77 × 10⁻⁴ m². Velocity = 0.0003 ÷ 1.77 × 10⁻⁴ = 1.7 m/s. That exceeds 1.5 m/s, so the pipe would be noisy and erosion-corrosion is likely: upsize to 22 mm, which gives about 0.8 m/s.
Traps that cost marks
Confusing bar with metres of head; ignoring friction when predicting tap pressure; assuming flow alone governs noise when velocity does; mixing kJ with kWh.