Hi, yesterday my dad and I were debating whether flow rate stays constant throughout a system. I told him we learned this in physics and he refused to believe it. He's a plumber so he may have thought his reasoning was more justified than mine. He finally said, "I can disprove it now" and opened the tap a little and then a lot. Obviously when it was open a little the pressure was not greater than when it was opened a lot. So opening the tap must change the flow rate, which would mean the conserved flow rate would only exist within the system. So if I were to open or close a valve in a pipe in the system would flow rate still remain constant on both sides of the valve and, if not, why would it differ? I'm guessing if you close the valve all the way then open it you're changing the flow rate because there's no water in front of that water to restrict it. But if the valve is left quarter inch open for a while the water pressure would be greater than if we left the valve an inch open for a while, but they both would have the same flow rate so long as the opening on the tap is constant.