Why are direct acting relief valves not used on high pressure and high flow hydraulic systems?
As a direct relief valve opens, its spool or poppet pushes against a spring. As the spring compresses, the system pressure necessarily rises due to the spring constant. To handle large flows, you need a big valve. To minimize the pressure rise due to the spring constant you need a long spring. To minimize the pressure drop due to the valve body, you need a big valve body, which in turn needs a bigger spring. As the operating pressure goes up, you need stiffer (typically bigger)springs. Eventually the cost of the valve is prohibitive and a pilot operated valve is called for. There are benefits to using direct operated valves which may out weigh the costs, but most direct operated valves are limited to medium and low pressure settings.
