# Relationship between water flow and pressure drop

1. Nov 2, 2015

### TSN79

A building has a water pipe inlet. I have found that at if I open the valve at this location fully, then I get 180 liters per minute at a residual pressure of 0,2 bars (static pressure is 5,4 bars). Is there a fixed relationship between the flow and pressure drop so that I can easily predict what the residual pressure will be at say 50 liters per minute?

2. Nov 2, 2015

### Lombasto

Yes, you want the Bernoulli equation for steady, incompressible flow.

P1/ρ + V12/2 + gz1 = P2/ρ + V22/2 + gz2

For your situation, gz1 = gz2 and can be ignored.

So:

(P2-P1)/ρ = (V22-V12)/2

thus ΔP = ρ(V22-V12)/2

where Vx = V°/A1 and A1 = πr2

In your case A1 = A2

So you will also need to know the inner diameter of your pipe.

ΔP = ρ/2((V°2/A)2-(V°1/A)2)

edit:
I don't know what your background is so if you use this equation you have to make sure your units work out.

If in doubt put pressure in Pascals (not kPa), volumetric flow rate in m3/sec, area in m2, density in kg/m3, answer will be in Pa

Last edited: Nov 2, 2015
3. Nov 2, 2015

### lingesh

My opinion is,
as the Area is uniform, acc to continuity eqn Q=A.V,i.e velocity at both ends is same,which makes RHS ZERO..

4. Nov 2, 2015