Effects of a sudden change in pipe diameter

[bellshom]

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

 

[Q_Goest]

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

 

[bellshom]

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

 

[Q_Goest]

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.

 

[bellshom]

ah okay, the outlet is straight for the set up. Thanks for the information.