Calculate flow rate in parallel pipes

In summary: The current through an electric circuit is determined by the voltage and the current through a parallel circuit is determined by the flow rate. If the flow rate is the same in each pipe, the pressure drop thru each pipe will also be the same. If the flow rate is different in each pipe, the pressure drop thru each pipe will be different. I have a 30" pipe that splits into 3 parallel paths, each 24" in dia. The pipes converge into a 30" outlet and then flows at about 40,000 gallons per minute at full head pressure. Assuming the same head pressure and one path closed off (i.e. now only flowing through 2 parallel paths), how can I determine my new flow
  • #1
steves1080
65
1
So I have a 30" pipe that splits into 3 parallel paths, each 24" in dia. The pipes converge into a 30" outlet and then flows at about 40,000 gallons per minute at full head pressure. Assuming the same head pressure and one path closed off (i.e. now only flowing through 2 parallel paths), how can I determine my new flow rate from the outlet? Testing this would be a real hassle. I'm just looking for a good estimate, not necessarily right on the money. Thanks
 
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  • #2
Hint: Ohm's Law.

You want to know the current for two identical resistors in parallel given the current for three and the same voltage across them.
 
  • #3
Good analogy. But do I know the current? I know the "current" for 3 open parallel paths, but if I eliminate one path, I can't exactly assume it's the same flow rate as before - right?
 
  • #4
You know the total flow into the parallel pipes must equal the total flow out. This is the continuity relation. If you block one pipe but don't change the flow rate into the network, the the flow velocity must increase.

You must also have the same pressure drop (or resistance to flow, to continue the electrical analogy) from the entrance of the parallel pipes to their exit, but having all of these pipes the same diameter and length makes it easy to satisfy this requirement. If the flow velocity is the same in each pipe, then the pressure drop thru each pipe should also be the same, as long as the pipes are identical.
 
  • #5
steves1080 said:
Good analogy. But do I know the current? I know the "current" for 3 open parallel paths, but if I eliminate one path, I can't exactly assume it's the same flow rate as before - right?
That is correct - the same pressure difference can produce a different flow rate like the same voltage can produce different currents. Your main advantage is that you know the pipes are identical so the resistance to flow will be identical and that the pressure difference (voltage) is the same.

Write out the equations - use dummy variables for everything you don't know.
Just pretend it's an electric circuit.
 

Related to Calculate flow rate in parallel pipes

1. What is the formula for calculating the flow rate in parallel pipes?

The formula for calculating the flow rate in parallel pipes is Q = V1 + V2 + V3 + ... + Vn, where Q is the total flow rate and V1, V2, V3, etc. are the individual flow rates in each pipe. This formula assumes that the pipes are all of equal diameter and length.

2. How do you determine the individual flow rates in parallel pipes?

To determine the individual flow rates in parallel pipes, you need to know the diameter, length, and pressure difference of each pipe. Then, you can use the Darcy-Weisbach equation (Q = (π/4)*D^2*√(2ghf)) to calculate the flow rate in each pipe. Once you have the flow rate for each pipe, you can use the formula mentioned in the first question to calculate the total flow rate.

3. Can parallel pipes have different diameters and still be used to calculate flow rate?

Yes, parallel pipes can have different diameters and still be used to calculate flow rate. However, the formula for calculating flow rate will be slightly different. It will be Q = (V1/A1) + (V2/A2) + (V3/A3) + ... + (Vn/An), where A1, A2, A3, etc. are the cross-sectional areas of each pipe. This formula takes into account the different diameters of the pipes.

4. Is there a maximum number of parallel pipes that can be used in flow rate calculations?

No, there is no maximum number of parallel pipes that can be used in flow rate calculations. However, as the number of pipes increases, the complexity of the calculations also increases. It is important to ensure that the pipes are properly sized and that the pressure difference is evenly distributed among them in order to get an accurate calculation of flow rate.

5. How can you increase the flow rate in parallel pipes?

There are a few ways to increase the flow rate in parallel pipes. One way is to increase the pressure difference between the inlet and outlet of the pipes. Another way is to decrease the length of the pipes or increase the diameter of the pipes. Additionally, using pipes with a smoother surface can reduce friction and increase flow rate. It is important to note that these changes should be made carefully and with consideration of the system as a whole to avoid any negative impacts.

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