Backpressure regulates the higher inlet (upstream) pressure by opening up only as much as necessary to hold back the desired pressure at the inlet (upstream).
Back pressure regulators are often used for:
control of pressure in recirculating loops (i.e. pressure bypass regulators).
relief of excess system pressure.
The total backpressure is generated from two components; 1). superimposed backpressure and 2). the built-up backpressure.
Superimposed back pressure: the static pressure that exists on the outlet side of a closed valve.
Built-up back pressure: the additional pressure generated on the outlet side when the valve is discharging.
In a conventional design (no bellows), the superimposed backpressure will affect the opening characteristic and set value, but the combined backpressure will alter the closing (blowdown) and re-seat value