Slam a valve shut on a moving column of liquid and the kinetic energy has nowhere to go but into pressure: a surge wave races along the pipeline at the pressure-wave celerity a, set by the Korteweg relation from the fluid’s bulk modulus and how much the pipe wall stretches. The upper bound on the spike is the Joukowsky equation, dP = ρ·a·Δv (or dH = a·Δv/g as head), and it applies whenever the closure beats the relief wave home — that is, whenever the valve shuts within the critical pipe period 2L/a.
The numbers escalate quickly. A 1000 m steel line, 0.5 m bore, carrying water at 2 m/s has a wave speed of 1191 m/s and a critical period of 1.68 s: any closure faster than that produces the full Joukowsky head of 243 m — roughly 23.8 bar on top of the operating pressure. Stretching the same closure to 10 s invokes the Michaud reduction dH = 2·L·v/(g·tc) and drops the surge to 40.8 m, about a sixth of the instantaneous case.
Every mitigation attacks one of the two factors in ρ·a·Δv: slow the velocity change (closure times beyond 2L/a, gentler pump ramps), or slow the wave itself — flexible materials like PVC and HDPE have far lower elastic moduli than steel, so the celerity and the surge both shrink. Surge tanks, air vessels, and relief or air-release valves absorb what remains.