Relationship between water flow and pressure drop

KE = (1 - V2^2/2g)(1 - V1^2/2g)In summary, the conversation discusses the relationship between flow and pressure drop in a building's water pipe inlet. The Bernoulli equation is mentioned as a means to predict the residual pressure at different flow rates, taking into account the inner diameter of the pipe and ensuring that units are consistent. The possibility of kinetic energy correction factor is also mentioned, although it is considered negligible in this particular scenario.
  • #1
TSN79
424
0
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?
 
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  • #2
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:
  • #3
Lombasto said:
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

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
lingesh said:
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..

That's because his original answer is incorrect. If you want to use Bernoulli's equation, you will need additional terms to account for the head loss in the pipes and fittings.
 
  • #5
The valve position is going to induce a pressure drop with a velocity increase in the fluid.

Piping losses would be negligible for this purpose. If it was dealing with a section of pipe, then you would consider it. The only thing that isn't on there that could possibly effect the answer would be the kinetic energy correction factor.
 

1. What is the relationship between water flow and pressure drop?

The relationship between water flow and pressure drop is inverse. This means that as the water flow increases, the pressure drop decreases, and vice versa. This is due to the principle of fluid dynamics, where an increase in flow rate leads to a decrease in static pressure.

2. How does the diameter of a pipe affect the pressure drop?

The diameter of a pipe has a significant impact on the pressure drop. As the diameter decreases, the pressure drop increases. This is because a smaller diameter pipe has less surface area for the water to flow through, resulting in higher velocity and therefore, higher pressure drop.

3. Does the viscosity of water affect the pressure drop?

Yes, the viscosity of water does affect the pressure drop. Higher viscosity means that the water is thicker and more resistant to flow, leading to a higher pressure drop. This is why fluids with higher viscosity, such as oil, have a higher pressure drop compared to water.

4. Can the length of a pipe impact the pressure drop?

Yes, the length of a pipe can also affect the pressure drop. The longer the pipe, the higher the pressure drop, as the water has to travel a greater distance and encounters more resistance. This is why it is important to consider the length of pipes when designing a system to minimize pressure drop.

5. How does the roughness of a pipe's surface affect the pressure drop?

The roughness of a pipe's surface can also impact the pressure drop. A rougher surface creates more friction, resulting in a higher pressure drop. This is why smooth pipes are preferred in systems where pressure drop needs to be minimized. However, some roughness is necessary for turbulent flow, which can help in reducing pressure drop.

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