The drainage system (1) contains a group (2) of valves, comprising a first pressure regulating valve (70) that opens at a predetermined pressure and permits the passage of fluid, a second valve (71) that closes at a predetermined pressure above the first opening pressure, allowing fluid to drain through the second valve when the pressure is between the opening and closing pressures, and a third pressure regulating valve (120) fitted in parallel with the first two. The third valve is designed to open at a pressure at least 10 per cent above the closing pressure, and the second and third valves are arranged so that fluid can drain through the third valve when the second one is closed.
