Backpressure is nothing more than the resistance to flow at any point in a system, expressed in terms of pressure. Usually thought of (somewhat erroneously) in the context of what resistance a particular downstream flow element will impart to a piping system at, or near to the flow element's upstream connection point, in order to drive flow through it. Often thought of without any limitation of how much flow may be under consideration, or the fact that backpressure is likely to change considerably at different flow rates.
"The filter has a backpressure of 75 psig", implies that 75 psig, just upstream of the filter, will be sufficient, or the minimum pressure needed, to pass either a certain, or perhaps totally unspecified, flowrate through the filter. If the flowrate is unspecified, the engineer is expected to assume whatever flowrate is required to make the statement true. If handed to you across the counter, it is likely to be without any consideration of the system's outlet pressure at the time when it is installed, or operating. If viewing within a system, or nearly installed in a system, the backpressure value probably already has the system's outlet pressure already added.
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
Pressure existing at the outlet of a pressure relief valve is defined as back pressure.
there are two types of back pressure:
Superimposed:which is pressure at the outlet of a pressure relief valve when it is required to operate.
Built-up:back pressure which develops in the discharge system after relief valve open.
