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.