# Effect of increase pipe diameter on fluid

## Main Question or Discussion Point

Trying to understand what will happen to the fluid in the following scenario.

Lets say there is a length of pipe with an OD of 4", what would happen if halfway along the pipe the pipe OD increased to 6" (gradual increase).

I believe the pressure of the fluid will decrease, would the flow rate also decrease.

If anyone could explain what actually happens that would be great.

Cheers

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So the flow rate remains the same,

What about the pressure and velocity? I am assuming a decrease in velocity.

Simon Bridge
Homework Helper
You should be able to work everything out from the flow rate being the same and that one follows from incompressibility :)

Of course, not all fluids are incompressible, but we are taking about flow through a pipe.
If all else fails - read the instructions (hint: that link I gave you.)

Gold Member
This depends on what you mean by flow rate. You have to be more specific than that. The mass flow rate will remain the same. For an incompressible fluid like water, that means the volumetric flow rate will stay the same. However, that means that the velocity of the flow changes. In your example, the velocity decreases when the diameter increases.

Well here is the situation, I can only change pipe diameter size, looking at changing a section of the 4" pipe to 6". Will more water be avaiable at the outlets with the increase in pipe size, will the pressure increase or decrease.

Gold Member
Unless you think water will magically be created in the pipe, then no more water will be available if you increase the diameter. You will have the same mass flow rate as before. Static pressure would rise.

Okay cheers,

Also would the pressure losses decrease with a larger pipe diameter.

If you increase the pipe diameter, you increase the amount of water a given section of the pipe will hold.If a house is 200 yards uphill from the water meter,you have to lay a inch and a half sized pipe,instead of the normal 3/4 inch pipe,to receive adequeate volume and pressure.It's like having a water tank to draw from instead of a straw.If your hot water is on a circulating system,you can design it the same as the cold.If not ,you have to go with a manifold system.Which is also cheaper and more effective for the cold as well.

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If you need easy to use formulas,pick a plumbing code book.

Alright cheers everyone, thanks for the help