# PressureDrop/Pressure/Volumetric flow rate calculation (in a Pipe)

• I
Hello,

If I have the following known variables:

- P1, Pressure of fluid entering the pipe = 50 psi
- L, Length of pipe = 10 in
- D, Diameter of pipe = 0.5 in

I want to learn how to calculate:
- P2, Pressure of fluid at the end of pipe.
- Q1, Volumetric flow rate of fluid entering pipe.
- Q2, Volumetric flow rate of fluid at the end of the pipe.

Fluid is water.

How can I calculate the pressure and volumetric flow rate at the end of the pipe? What assumptions can I use to get a basic value? The pipe diameter does not change. If I differentiate the volume of pipe over one second does this mean that my Q2 will be 1.96 in^3/s ?

Thanks

You need more information. Either the flow rate, or the conditions at the end of the pipe.

Does the pipe dead-end at a plug? Does it go into a tank at some known pressure? Does it just open-end to atmosphere?

Look up bernoulli equation.

https://en.wikipedia.org/wiki/Bernoulli's_principle

russ_watters
Mentor
If the pipe is open to atmosphere, the pressure at the end is atmospheric.

In that case, for such a short length of pipe, I'd probably assume the pipe is a low efficiency nozzle (maybe 25%?) and use Bernoulli's equation to calculate the exit velocity.

Thank you for the replies.

The pipe is connected to another restriction at the end and I do know the restriction size but I do not know what the pressure or flow rate is at the end of the pipe. The purpose is to create a mathematical computer model, so what if I assumed the initial pressure at the end was atmospheric for the first iteration? Can I then use bernoulli's equation to compute the velocity at the end of the pipe and then feed that back into the Bernoulli equation assuming no height change therefore pgh =0?

Edit: I have just realised that I still do not know the initial flow rate Q1

russ_watters
Mentor
The pipe is connected to another restriction at the end and I do know the restriction size but I do not know what the pressure or flow rate is at the end of the pipe.
"Another restriction"? Besides just the pipe or is there more you haven't told us? Can you draw us a picture? Knowing what the entire system looks like is pretty critical for finding the flow through it!
The purpose is to create a mathematical computer model, so what if I assumed the initial pressure at the end was atmospheric for the first iteration?
After the restriction or before it? Is there anything else after the restriction you haven't told us about?
- Q1, Volumetric flow rate of fluid entering pipe.
- Q2, Volumetric flow rate of fluid at the end of the pipe.
[separate post]
Edit: I have just realised that I still do not know the initial flow rate Q1
Unless there is a receptacle or additional opening somewhere in your system, conservation of mass demands the flow in and flow out be the same.

Chestermiller
Mentor
Is the fluid compressible or incompressible?