My fluid handling system requires a valve that will open at low pressures (2-6 PSI) and close at high pressures (8 PSI). Is there any commercially available valve that is suitable for this application?
Search back-pressure regulators. Basic idea is they have a dome with a membrane diaphragm pressurized to the control pressure. If the incoming pressure is higher than the dome it pushes the membrane up and shuts the flow.
This is a link to one. back pressure regulator