I work in the oil and gas industry and frequently encounter an issue involving isolation valves that do not seal 100%, and that begs me to ask the following question... I often need to isolate particular sections of piping so that a segment can be opened/disassembled for maintenance. When I have isolated a particular section of pipe (by closing valves and diverting the flow) from an operating system, I then bleed off the pressure in the isolated section (volume of isolated pipe varies from 3 cubic meters up to about 18 cubic meters). I often bleed the isolated section's pressure down to about 10psi and company procedures dictate that I then I observe a digital pressure gauge (for a period of 5 minutes) to confirm that the closed valves are infact holding, and that mainline pressure has been 100% diverted. There is often upwards of 300-800 psi of crude oil still flowing in the system which I have diverted by the earlier closing of certain valves. Occasionally, the pressure in the isolated section climbs slowly, maybe 10 psi/minute (give or take). Sometimes the valves hold completely and no pressure rise occurs in the isolated section and then we're all happy. But when the pressure does climb, how can I get a "liters/minute" flow rate so that I know how much product will be leaking past the valves after I have drained the isolated section and I open that isolated section for maintenance? I need to know if the leakage will manageable or not, while the isolated section is disassembled. I assume the variable numbers you need to know are: isolated pipe volume: 5 cubic meters time: 60 seconds pressure rise: 10 psi mainline pressure: 500 psi incoming flow rate (leakage past closed valve): ?? Liters/minute Is there a formula for this that will work on various sizes of isolated pipe sections? Maybe mainline pressure is irrelavent? Do you need more info? Thanks!