# Basic questions about flowing water in a pipe

Hi, I have no background in physics or engineering, I am a computer guy who is tasked with building a toy simulator for piping diagrams. Hoping I can get some help in this forum. Basically I am given a piping diagram with a bunch of widgets (pumps, heat exchanger, radiators, etc) that are connect via pipes and I need to show an animation with water moving around, with the ability to click on each pipe to display its pressure, flow and temperature. I have a few qns (might be dumb, if so pls enlighten):

- When water is flowing in a pipe full of water, are the pressures at both ends equal? How do I compute the two pressures given the flow velocity, length and diameter of the pipe (no other variables given, I can make assumptions)?

- Given a pump that maintains a pressure difference of P, and a circular pipe that connects one end to the other: how fast is the water flowing? What is the pressure in the middle of the pipe? What is the pressure at the input end of the pump, is it negative? Suppose I take the same pump and hook it up to a heater. What is the pressure at the heater?

- Given the same pressure difference, does water in a longer or wider pipe flow faster/slower?

- What happens when the pipe is split into two via a splitter?

- What is a "vessel"? I know it's a container that fills up with water -- will water flow out even when its not full? If so how fast? If it's full it works like a pipe?

- Is 'flow' the speed of water moving in a pipe? If so how come I need two connections for a flow sensor but only one connection for a pressure sensor? I know in electrical engr potential difference requires two points and current only requires one.

- In a piping diagram there are gas pipes. Are these important? Can I calculate the flow, pressure and temperature of gas the same way as water (since they are compressible)? I presume pumps, vessels, etc made for water do not also work for gas.

Thanks in advance! Pls help if you can answer any of these questions. This is a very "ideal-world" simulator and ignores almost all variables other than pressure, flow and dimensions, so no friction or turbulence. Readings do not need to be correct, just believable.

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256bits
Gold Member
Quite a tall order to try to explain a textbook's worth of questions on fluid flow in a few paragraphs.

try looking at this site as a start:
http://en.wikipedia.org/wiki/Bernoulli's_principle

And you might want to forget about compressible flow with gases, until you can master incompressible flow.

I've been there but I don't understand a word of what it's saying. I have no background on this subject and that page seems to be trying to explain everything from first principles.

I would really appreciate if I can get some quick answers here as I really don't have the time or ability to read an entire textbook in a topic completely outside my field. Thank you!

256bits
Gold Member
Well then, to do it simple,

Pick a pressure that the pump will output and a flow rate.
Say 40psi and 10 gallons/minute.

Then have the circuit as follows:
Tank ( vessel ) , pump , pipe 1 , heat exchanger, pipe 2, split into pipe 3a and pipe 4a, radiator 3 and radiator 4, pipe 3b and 4b, merge into pipe 5 , and then back to the tank.

Make a chart like the following but add in length and area columns
Item ...... Pressure drop(gain).... pressure temp Q
Tank.............. 0 psi ................ 0 .......... 70 deg
pump............. 40 .................... 40 .......... 70 ......... 10 gpm
Pipe 1 ........... -2 .................... 38 ........... 70
Heat ex ......... -10 ................... 28 ........... 120
pipe 2 ........... -2 ................... 26 ........... 120
Pipe 3a.......... -4 ................... 22 ........... 120 ........ 6 gpm
radiator3........ -12 .................. 10 ........... 70
Pipe 3b.......... -4 ................... 6 ........... 70
Pipe 5 .......... -6 ................... 0 ............ 70 ......... 10 gpm

Also do the same for pipes 4a, radiator 4, and pipe 4b, splitting from pipe 2 and merging into pipe 5. flow for 4a, rad 4 and 4b would be 10-6 = 4gpm

Keeping it simple: you can use these rules
If the pipe is twice as long it has twice the pressure drop.
If the area of the pipe is halved then the flow rate is halved. ( not exactly true but OK here for a simulation )
So pipe 3a can be twice as long as 2 or half as big in area ( ie A = pi r^2 ) or a combination of both.

Assume that the pipes are horizontal at the same elevation.

Does that give you a start.

Last edited:
This definitely helps, thank you very much.

Just to clarify:
- Pressure decrease in one pipe = total pressure drop x (length of pipe / total length)?
- Flow rate and pressure are independent? (it'll work for me even if its a simplification)
- What if the lengths of pipes 3 and 4 are different? Is it like circuits where you use inverse mean?
- What happens if I replace the radiator at 3 with another pump? How does this change flow and pressure?
If any of these scenarios are too complicated then it's fine I just won't handle it.

Once again, thank you.

256bits
Gold Member
- Pressure decrease in one pipe = total pressure drop x (length of pipe / total length)?
If the pipe is all the same size then that would apply.

- Flow rate and pressure are independent? (it'll work for me even if its a simplification)
No. An increase in pressure increases the flow rate, but not on a one to one basis.
The whole pipe circuit determines the output pressure from the pump, so simply if you add more pipe or radiators the pressure output of the pump goes up to keep the same flow rate.
Note that through all your circuit what flow goes in must come out, and whatever pressure the pump outputs must return back to zero (0) around the circuit ( or all circuits )to the pump inlet.

- What if the lengths of pipes 3 and 4 are different? Is it like circuits where you use inverse mean?
For your simulation use pressure drop is proportional to the length and inversely proportional to the cross sectional area of the pipe. pressure drop =~ L/A
increase the length, you increase the pressure drop in the pipe length
decrease the area, you increase the pressure drop

Also use flow rate is inversely proportional to the cross sectional area and length.
Flow =~ 1/( A L )
Increase the length and the flow rate decreases through the pipe
decrease the area and the flow rate decreases

Inverse mean - similar to resitive circuits in series and parallel.

- What happens if I replace the radiator at 3 with another pump? How does this change flow and pressure?
The pressure after the pump would increase depending on how much you want it to in your simulation. Remember when 2 pipes split, a that point the pressure is the same; and when 2 pipes meet the pressure at that point is the same. If you add a second pump, make sure the pressure drops come out OK from the split to the merge - add extra pipe if you have to or another radiator in one of the splits.

Use the chart thing and you can play around with lengths, areas of pipes, add radiators or such to make everything come out alright.

Thank you very much, this helps tremendously. I can now get started and if I need more realism in the future, I will be better equipped to google. Thank you once again.