Pumps, Valves, and Hydraulics
Know the difference between centrifugal and positive-displacement pumps. Centrifugal pumps follow a pump curve: head falls as flow rises, and the operating point is where the system curve crosses the pump curve. Understand NPSH and cavitation (noise, vibration, pitting, lost capacity), affinity laws (flow ∝ speed; head ∝ speed²; power ∝ speed³), and specific speed as a pump-shape indicator. Positive-displacement pumps deliver nearly constant flow against varying pressure and need a relief valve; they must never be run against a closed discharge.
Hydraulics: static head, friction head, velocity head, minor losses, and the idea that friction rises roughly with the square of flow. Know valve types-gate (isolation), globe (throttling), butterfly (isolation/limited throttling), check (backflow prevention), pressure-reducing, and air/vacuum and surge relief. Traps: using a gate valve for throttling; forgetting the check valve or relief valve; mixing up head (feet) with pressure (psi)-1 psi ≈ 2.31 ft of water; and ignoring that adding a parallel pump increases flow without doubling head.
Rule of thumb: psi × 2.31 = feet of head; feet ÷ 2.31 = psi.
Worked example: A pump discharges at 60 psi. Head = 60 × 2.31 ≈ 139 ft. If a system curve shows 139 ft of head at 500 gpm, that is the operating point. Doubling speed would roughly double flow and quadruple head-check motor power, which rises about eightfold.