PRV

A pressure regulating valve or PRV placed in a piping system regulates the pressure in the system downstream of the PRV to a set pressure. When fluid pressure on the inlet side of the PRV is set greater than the PRV's set pressure, the valve opens to allow fluid to pass through the valve until the downstream set pressure is achieved.

Here is an example of a pressure regulating valve.With the PRV installed, the pressure in the low pressure side of the system will not go above the set pressure of 70 psi regardless of the downstream loads.  

Notice the PRV is set to 67% open.

Here are two examples when the demand flow rates are halved and when they are doubled.

Notice the valve position of the control valve changes to allow more or less fluid through the pipeline to maintain the downstream pressure to the set value.  

BPV

A backpressure valve or BPV placed in a piping system regulates the system pressure upstream of the BPV to a set pressure.  When the fluid pressure on the inlet side of the valve is greater than the BPV's set pressure, the valve opens to allow fluid through the valve.  As more fluid is allowed through the valve the inlet pressure decreases.  This continues until the valves inlet pressure is equal to the BPV's set pressure.

Here is an example of a system with a back pressure valve.  With the BPV installed, the pressure in the high pressure side of the system will not go below the set pressure of 90 psi regardless of the downstream loads. 

Once again, here are two examples, one with the flow demand values halved and the other with them doubled.

Notice the valve position of the control valve changes to allow more or less fluid through the pipeline to maintain the upstream pressure to the set value.