# Pressure discontinuity In piping sections of constant diameter the [[Choked flow|choke]] point—if present—will occur will occur at the choking velocity (Mach number = 1) and at the downstream end. A pressure discontinuity—a sudden drop in pressure—will occur immediately downstream of the choke point. Where the pipe diameter differs it is possible for several choke points to develop in a system; these occur where the area for flow is a local minimum. The choke point furthest upstream determines the overall mass flow, with the other choke points determining the rest of the system's pressure profile. In a lot of relief systems (not all) the flow is choked across the relief valve orifice, meaning the system flow will be independent of backpressure (as long as the downstream pressure is below that dictated by the [[Critical pressure ratio|critical pressure ratio]]). ## Example pressure profile The pressure downstream of a choke point cannot be determined from the upstream condition. It is best to work backwards from a known discharge pressure. The following is a typical pressure profile for a pressure relief system with critical flow. ![[Discontinuity_pressure_profile.png]]