Effects of a sudden change in pipe diameter

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Discussion Overview

The discussion revolves around the effects of a sudden change in pipe diameter on fluid flow and potential structural issues in a vertical pipe system. Participants explore the implications of replacing a larger diameter pipe with a smaller one at a coupling point, considering both flow dynamics and structural integrity.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about the consequences of replacing a 150mm diameter pipe with a 100mm diameter pipe at a coupling, questioning the effects on fluid flow and structural integrity.
  • Another participant introduces the concept of "sudden contraction," noting it leads to an irreversible pressure drop similar to that of a long pipe, suggesting that calculations can be made to determine the pressure drop.
  • A follow-up question addresses whether the sudden contraction would affect the coupling, particularly in the context of the pipe being used for emergency drainage when the main pipe fails.
  • One participant discusses potential stresses due to internal pressure and mentions that if the pipe and coupling are rated for the pressure, they should not pose a structural issue. They highlight the importance of considering reaction loads at the outlet due to high-velocity fluid expulsion.
  • Another participant acknowledges the outlet's straight configuration, indicating an understanding of the previous points raised.

Areas of Agreement / Disagreement

Participants express varying levels of concern regarding the structural implications of the sudden diameter change, with some focusing on fluid dynamics and others on structural integrity. No consensus is reached on the overall impact of the change.

Contextual Notes

Participants discuss the potential for pressure drops and structural stresses but do not resolve the complexities involved in the calculations or the specific conditions under which the system operates.

bellshom
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Hey,

I was wondering what were would happen to the fluid flow or if this would cause any structural problems to the following case.

A vertical pipe of 150mm diameter and roughly 3 metres in length is at the bottom of a large tank. At the bottom of the pipe is a coupling which leads to another pipe. What would happen if the bottom pipe was removed and replaced with a pipe of around 100 mm diameter.

Where the sudden change would appear at the coupling, where the diameter would change instanously from 150 mm to 100mm. Would this set up cause any problems and what affect would it have on the flow.

Thanks
 
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Hi bellshom, welcome. What you're talking about is called a "http://www.google.com/#sclient=psy&...&aql=&oq=&gs_rfai=&pbx=1&fp=cc7ad1a43d378bba"", and it looks like this:
[PLAIN]http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/FLUID-MECHANICS/lecture-14/images/fig14.3.gif
It creates an irreversible pressure drop similar to a length of pipe but one that's relatively large. You can calculate the drop in pressure as shown in one of the following:
http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/FLUID-MECHANICS/lecture-14/14-7_losses_sudden_contract.htm
http://www.codecogs.com/reference/engineering/fluid_mechanics/pipes/the_flow_through_pipes.php
 
Last edited by a moderator:
Cheers,

So would this sudden contraction have any effect on the coupling. The pipe is pretty much used to dump the tank if the other main pipe fails. Just wondering due to it being vertical when the tank gets dumped would it do anything to the coupling. If that makes sense

Thanks
 
There are stresses due to the internal pressure, sure. But if the pipe or coupling is rated for the pressure, there's not an issue with those parts. The only structural issue might be a reaction load at the outlet - the fluid coming out is analogous to a jet exhaust, so it pushes on the end of the pipe. There have been cases where relief valves for example have elbows on the outlet that instead of pointing down, they turn sideways. When the relief valve lifts and fluid is expelled, it's been known to cause a jet powerful enough to unscrew the relief valve. Similarly, pipes venting to atmosphere have to be anchored near the outlet to prevent them from whipping around when a high velocity fluid is expelled. You'll often find a T on the end of a vent line to help balance the effects of the fluid jet.
 
ah okay, the outlet is straight for the set up. Thanks for the information.
 

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