Typically a blowdown valve is small; not over about NPS 3. A typical opening or closing speed for a ball valve is one inch per second. A throw-down BDV opening speed requirement is 10-seconds. For high pressure valves (class 600) consider a choke downstream to control the blowdown rate.
good advice. We used to sixe RO's downstream of valve for 15,000 pounds/hr. Some pipeline companies install upto 8" wide open ball valves under the impression that it was a DOT mandate to empty the contents of the blowdown piping in under 15 secs or 30 or 45 seconds, THEY weren't sure.
They finally allowed me to put in RO's when the land owner threaten to sue them and have them remove the 16,000 HP station if they didn't stop spraying oil all over his land.
