What is water hammer? Water hammer (or hydraulic shock) is themomentary increase in pressure, which occursin a water system when there is a suddenchange of direction or velocity of the water.When a rapidly closed valve suddenly stopswater flowing in a pipeline, pressure energy istransferred to the valve and pipe wall.ater hammer (or hydraulic shock) is themomentary increase in pressure, which occursin a water system when there is a suddenchange of direction or velocity of the water.When a rapidly closed valve suddenly stopswater flowing in a pipeline, pressure energy istransferred to the valve and pipe wall.