Question 5 of 27

Question 1.5 (5 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

1.5.1 — water hammer. Water hammer is a transient, high-magnitude pressure surge (positive) or drop (negative, potentially cavitation-inducing) that propagates as a pressure wave through a pipeline when the flow velocity is changed suddenly — the kinetic energy of the moving water column is converted almost instantaneously into pressure energy, at a wave speed governed by pipe stiffness and fluid compressibility.

1.5.2 — major causes. Any pipeline fitting that can close or change state QUICKLY: fast-closing check (gate) valves (exactly the fitting referenced in Question 6.1.5), rapid pump shut-down or trip (power failure), and sudden pump start-up against a closed or nearly-closed valve are the principal causes; the faster the closure relative to the pipeline’s pressure-wave travel time, the more severe the surge.

1.5.3 — two major effects. (1) A large, short-duration POSITIVE pressure spike that can burst pipe, joints, or fittings, or damage the pump casing/seals. (2) A NEGATIVE pressure (vacuum) phase on the rebound that can cause column separation and cavitation, collapsing pipe (especially thin-wall HDPE) and causing violent secondary hammer when the vapour cavity re-collapses.

1.5.4 — mitigation. Slow-closing or spring-loaded/damped check valves (rather than a simple fast-slam swing check); surge tanks or air chambers/accumulators near the pump to absorb the pressure wave; controlled (ramped) valve operation and soft-start/soft-stop on pump motors; pressure-relief or surge-relief valves set to open on an over-pressure spike; and keeping pipeline runs as short and direct as practical to reduce wave travel time.