A relief valve is normally closed, senses its inlet and opens to cap maximum system pressure. A reducing valve is normally open, senses its outlet and closes progressively to maintain a lower downstream pressure. One limits the system; the other regulates a branch, and confusing them is the usual error.

Supplying a branch at lower pressure than the main system - pilot circuits, clamps that must not crush delicate work, lubrication feeds, and any sub-circuit whose components are rated below system pressure. It lets one power unit serve functions with quite different pressure requirements.

Because the spring chamber must be referenced to tank. Any back pressure there adds to the spring force and raises the effective setting, so a valve draining internally into a pressurised return line will hold a higher outlet pressure than it is set to, and the error varies with return line conditions.

One that will also relieve downstream pressure back to tank if it rises above the setting from an external source. A plain reducing valve only throttles the supply, so a load pushing back on a cylinder can raise branch pressure above the set value with nothing to release it.

Wherever downstream pressure can be raised by something other than the supply - a load acting on a cylinder, thermal expansion in a closed branch, or an intensifying effect. If that is possible, a plain reducing valve leaves the branch unprotected above its setting.

No - it throttles, and the pressure difference becomes heat. It is a control component rather than an efficiency one. Where a large flow is being reduced substantially, a separate supply or a variable pump arrangement is the efficient answer and the reducing valve is the convenient one.

By adjusting to the required downstream pressure with the branch in its normal working condition, reading a gauge downstream rather than relying on the adjustment scale. Setting it unloaded gives a figure that will not hold once the branch is working, which is a common source of confusion.