Help! Calculating Water Flow Rate in Pipe

[Sevy35]

Please help I'm going round in circles!

Water on tank 12m high, feeding pipe 200m long and 125mm diameter. Open end.

Given friction factor of 0.008 for the pipe and total of 3.2K for various fittings.

Asked to find water flow rate in Kg/s in the pipe.

Been looking at this for ages now and have got really confused!

So far :

As the tank and the outlet are open to the atmosphere we can use v= √2gh to find the velocity of the water?

V=√(2*9.81*12)

V=15.34m/s

Mass flow rate =pVA
Where p is the density of water 1000kg/m3
V is mean velocity 15.34m/s
A is cross sectional area normal to the direction of flow 0.125mXπ2
2
m = 1000 x 15.34 x (0.125xπ2)
2

 

[tiny-tim]

Hi Sevy35!

… Been looking at this for ages now and have got really confused!

Yes … since last December, I see! … https://www.physicsforums.com/showthread.php?t=278890

So far :

As the tank and the outlet are open to the atmosphere we can use v= √2gh to find the velocity of the water?

No, you can use ρgh, but only to find the contribution of the height to the Bernoulli's equation.

As LowlyPion said, you must subtract the loss through friction.

Remember the background: Bernoulli's equation is basically a conservation of energy equation (per unit mass, ie using density instead of mass), and if you have friction, you must subtract the energy lost from friction (equals the work done by friction).