Trade Fundamentals: Pressure, Flow and Drainage Principles
What this domain tests
Trade Fundamentals is the foundation domain: the exam checks whether you understand what plumbing is for, how water behaves in pipes, and the vocabulary and units used in the trade. Expect questions on pressure, head, flow, gradient, fixture units and the difference between supply and drainage.
Core rules and concepts
- Plumbing has two systems: supply (potable water under pressure) and drainage (soil and waste carried by gravity).
- Pressure at a point depends on the height of water above it: p = ρgh. A 1 m head of water is about 9.81 kPa, or 0.098 bar.
- Flow rate Q is volume per time, in L/s or L/min. Velocity must stay within accepted limits to control noise and pipe erosion.
- Drainage relies on gradient, not pressure; pipes must fall continuously toward the drain.
- Fixture units (FU) let different fixtures be added into one design load.
- Keep units consistent: metres, litres, seconds and pascals.
Worked example
A tap is 7 m below the water level in an overhead tank. Ignoring friction, static pressure = ρgh = 1000 × 9.81 × 7 = 68,670 Pa = 68.7 kPa. Since 1 bar = 100,000 Pa, that is about 0.69 bar. When the tap is opened, the pressure at the tap is lower because of friction losses in the pipe and fittings. The reasoning: identify the head, convert it to pressure, then state that friction reduces the available pressure.
Traps that cost marks
- Saying pressure doubles when pipe diameter doubles; pressure depends mainly on head, not diameter.
- Using 1 bar = 10 m exactly; it is about 10.2 m of water.
- Treating drainage as a pressure system; drains need fall and venting.
- Mixing L/min and L/s without converting.
- Forgetting that fixture units are not litres per minute.