Effect of pressure due to change in diameter of pipe

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Discussion Overview

The discussion revolves around the effects of pipe diameter changes on pressure in a fluid system, specifically comparing a 4-inch pipe to a 2-inch pipe connected to the same pump. Participants explore the implications of flow rate and pressure at the end of the pipes under different configurations, considering factors such as head loss and system demands.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that the pressure at the end of the 2-inch pipe will be different from that at the end of the 4-inch pipe, requiring different pump pressures to achieve the same flow rate.
  • Others question whether the flow rate or pressure can be the same in both systems, suggesting that the conditions at the end of the pipe (e.g., atmospheric pressure) are relevant to the discussion.
  • One participant mentions that the pressure difference may be attributed to contraction and head loss in the smaller diameter pipe compared to the larger one.
  • Another participant emphasizes that each piping system has its own pressure versus discharge relationship, which is not independently adjustable and involves kinetic head considerations.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether the flow rate and pressure can be the same in both systems, and there are multiple competing views regarding the effects of pipe diameter on pressure and flow characteristics.

Contextual Notes

There are unresolved assumptions regarding the specific conditions at the end of the pipes and the nature of the pump being used, which may influence the pressure and flow rate outcomes.

Nitheeswar
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The pump has an outlet size of 4 inch
1) In case 1, 4 inch pipe is connected at length of 20 m
2) In case 2, 2 inch pipe is connected using reducer
In both cases the pump outlet conditions are same( having same flow rate at a given pressure)
What happens to pressure at the end of the pipe in case 2?
 
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The pump has an outlet size of 4 inch
1) In case 1, 4 inch pipe is connected at length of 20 m
2) In case 2, 2 inch pipe is connected using reducer
In both cases the pump outlet conditions are same( having same flow rate at a given pressure)
What happens to pressure at the end of the pipe in case 2?
 
At which end of the pipe?
 
Nitheeswar said:
In both cases the pump outlet conditions are same( having same flow rate at a given pressure)
False.

All other things the same between the two systems:

If system2 is 20 ft. of 2 inch pipe (horizontal) and you want flow ##Q## out of it, it requires a certain pressure at the pump discharge ##P_2##.

If system1 is 20 ft. of 4 inch pipe (horizontal) and you want flow ##Q## out of it, it requires a certain pressure at the pump discharge ##P_1##, where ##P_1< P_2##.

In other words you need an effectively different pumps to achieve the flow rate in each system. Meaning impeller changes, speed changes, etc...

If you are running a positive displacement pump( aka constant volume pump) the flow rate will be equal, but not at the same pressure. The pressure adjust to whatever the systems demands at a flow with those pumps. Their pump curve is a vertical line at a fixed speed.
 
Last edited:
Nitheeswar said:
The pump has an outlet size of 4 inch
1) In case 1, 4 inch pipe is connected at length of 20 m
2) In case 2, 2 inch pipe is connected using reducer
In both cases the pump outlet conditions are same( having same flow rate at a given pressure)
What happens to pressure at the end of the pipe in case 2?
Isn't it one out of two: either the flow rate is the same OR the pressure is the same ?

And: doesn't it matter what's at the end of the pipe ? Atmospheric pressure, something else ?

##\ ##
 
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Is this pressure difference only due to contraction and head loss in 2 inch (compared to 3 inch)?
 
This is a duplicated thread.
 
By mistake, it is uploaded 2 times.please delete it
 
Is this pressure difference only due to contraction and head loss in 2 inch (compared to 4 inch)?
 
  • #10
Nitheeswar said:
Is this pressure difference only due to contraction and head loss in 2 inch (compared to 4 inch)?
Each piping system has its own pressure vs discharge relationship. They are not independently adjustable. It would also have to do with kinetic head consideration, not just head loss.
 

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