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Pressure and vacuum relief valves are relieving devices which automatically relief excess pressure when abnormal operating conditions occurs. Such valves may be actuated directly by the system pressure upstream the valve or be piloted, for instance, by a magnet. Valves can modulate open and closed over the entire or substantial portion of the lift, or modulate open over only a small portion of the lift and then open suddenly to the fully open position. Accordingly, pressure relief valves may generally be divided into modulating and full-lift valves.