Relief valves are available in two types, spring or weighted type.
Spring type valves are typically the most popular and least expensive for a given flow and pressure. A spring is used to provide a force against a plate that prevents the air or gas from escaping as long as it operates below the set point of the valve. If the air or gas pressure on the plate exceeds the force exerted by the spring, the valve opens allowing air to bleed off and preventing over-pressurization of the blower.
Weighted relief valves are available for pressure systems only. They must be mounted in a vertical upright orientation to function properly. A cap rides over a vented piston that forces the cap up as pressure builds. Many customers prefer these weighted relief valves due to their ease of operation and simple maintenance.