# How to find velocity of water flow and $Q$ if I know static pressure at the end of the pipe of known diameter?

I have a pipe. At its end I have mounted pressure meter. Of course there is no circulation. Water pressure meter is a kind of stopper for water. I know diameter of the pipe, and I know its pressure in static. How can I find velocity of water and its quantity $Q$ in that particular place (where pressure meter is) if I remove pressure meter and continue existing pipe with pipe of the same diameter?

As I understand $$Q = v(velocity) * S.$$ Then... I need velocity to count Q.

• Hi John Smith, 3 comments: 1) What is S? Is that the pipe area? 2) You say that 'pressure meter is kind of a stopper for water'. Does that imply that the water is not flowing, i.e. this is a static situation? 3) Don't forget, as I mentioned in this answer to your other question, that Bernoulli's equation is not valid for pipe flow (you applied the tag for bernoulli equation). – Time4Tea Aug 28 '18 at 1:38
• I think the answer to this is basically the same as the answer I gave to that other question. – Time4Tea Aug 28 '18 at 1:55
• There isn't enough information to answer the question. What's the source of the water? Is a pump providing the water flow? If so, what kind of pump (e.g., centrifugal, positive displacement, etc.). If a centrifugal pump, what does the pump curve look like? How long is the pipe, what diameter, and what relative roughness? Are there any elevation changes, bends, valves, "T"'s, or changes in diameter in the pipe? – David White Nov 26 '20 at 15:28

• Hi MJCROMER, per the Wikipedia page for the Darcy-Weisbach equation, that formula appears to only be for laminar flow. It may be ok for a viscous liquid, but I'm not so sure about water flowing in a long pipe. – Time4Tea Aug 28 '18 at 1:53
• It’s for laminar flow. You really don’t want turbulent flow as it is an inefficient way to move fluids. The onset of turbulence can be predicted by using Reynolds number, but that’s not needed to answer the original question. – MJCROMER Aug 28 '18 at 4:26
• en.m.wikipedia.org/wiki/Hagen–Poiseuille_equation – MJCROMER Aug 28 '18 at 4:30
• Sure, you don't want it to be turbulent, but for any meaningful flow of water through a pipe, you're most likely going to have it, whether you like or or not. – Time4Tea Aug 28 '18 at 10:08