# Pressure change with a pipe diameter change

I have a question regarding a water line from the street to a house. What happens to the pressure in a water line if it goes from a 1" diameter pipe (at street) and increases to a 1.5" diameter pipe for 300 feet from the street to the house and then back down to a 1" pipe inside the house?

• Look for Venturi Effect – Anubhav Goel Aug 3 '16 at 7:11

If the flow rate is zero then the pressure is the same everywhere in the pipe.

If there is some flow rate then the pressure drop per unit length of pipe is given by the Hagen-Poiseuille equation if the flow is laminar, or the Darcy-Weisbach equation if the flow is turbulent. The pressure gradient along the pipe is going to look something like:

so the pressure will fall more slowly along the bigger pipe. However to make this quantitative i.e. to calculate the gradients of the $P:L$ lines you will need one of the two equations mentioned above.

• I don't understand why this question keeps "popping up". John Rennie gave a very good answer to a somewhat ambiguous question. – David White Sep 4 '16 at 14:39

The pressure is clearly going to drop. Water would not flow without a pressure drop.

Going from 1" to 1.5" alone does not change the pressure. The linear flow rate drops by 1 / (1.5)^2.

Then when it goes back to 1" the the linear flow rate increases. At the front of the water line the end of the line you have the same volume and linear flow.

Along the 1" pipe you will have a linear pressure drop. Sudden expansion will have a pressure drop. The 1.5" pipe will have a pressure drop that will be less per linear foot than the 1" due primarily to a lower linear velocity. Then sudden contraction will have pressure drop. Then along the final 1" pipe you will have a pressure drop. If it is a short 1.5" section is may not make up for the pressure drop associated with the contraction and expansion. Yes both have a pressure drop as it is a disruption in flow. PIPE EXPANSIONS AND CONTRACTIONS

It depends on the flow rate and type of pipe but for equal length of pipe the 1.5" could have 1/10 the pressure drop of the 1".

I think as the radius increases, the velocity decreases(flow rate is constant) and pressure gets increased (fom street to house) and then due to sudden decresase in radius of pipe to 1'' at house pressure gets decreased and vel increases..

• The pressure is not going to increase in the direction of flow in a horizontal pipe – paparazzo Jul 29 '15 at 18:15