Pressure Vessel Deflection 1

[smart2009]

Hi,

I have observed in many cases that horizontal pressure vessel with saddle support due to the internal pressure gets circumferentially compressed (vertical deflection).

Can anyone explain why it behaves so.

Regards,
smart009

 

[pennpiper]

Gravity!

prognosis: Lead or Lag

 

[smart2009]

How Gravity causes circumferential compression.

there are other loads acting like thermal and other nozzle loads, but why vessel shows compressive deflection vertically.

Let me know if my question is not clear.

 

[dhengr]

Smart2009:
Draw a cross section through the tank and the saddle, and a free body diagram of the various forces on this cross section. I’m not sure that I understand your question, but I think it is that there might be a slight ovaling or downward deflection of the top of the tank in the region of the saddle. Assuming a cylindrical tank with hemispherical ends, the internal pressures are self equilibrating as normal pressures and hoop stresses and tensile stresses along the length of the tank shell; except for localized secondary stresses. But, in addition, you now have the saddle reaction pushing upwards on the tank shell and you need an equal and opposite downward force system from the top of the tank for equilibrium. These two force systems are a concentration of half of the gravity loads on the tank in the immediate area above the saddle. Unlike the pressure forces, these gravity forces will tend to force/pull the top of the tank down a little bit. The saddle supports the bottom half of the tank and tends to maintain its shape, but the top half is free to move down. This can cause some concentrated secondary stresses in the tank shell right at the top of the saddle, caused by these two different force systems. I assume you’ve run your tank design through some computer software, and that software should show this action and these stresses added to the pressure stresses.

 

[JStephen]

I'm not sure I understand the question, either. Generally, gravity effects will make the vessel sag in various ways, although I wouldn't expect this deflection to be visible; if it is, the saddles may be underdesigned. Generally, internal pressure would tend to make the vessel round up and straighten out, and wouldn't be the cause of vertical deflection.

If you're not familiar with it, look through Zick's paper on the topic, or review the topic in various vessel design handbooks. Zick's paper:

http://lms.i-know.com/pluginfile.ph...resses in Large Horizontal Cylindrical PV.pdf

Note: this paper has been updated a time or two, and this may not be the most recent version.

 

[smart2009]

hi,

here is the image showing how it is behaving.

 

[GenB]

Unloaded it can show some deflexion but once pressurize it it will disapear as pressure will stiffen the plates.

 

[gr2vessels]

It seems the selection of words is a bit confusing;- The shell under pressure does not deform, it tends to 'round up' compared with depresurized shape. However, under very low pressure and internal hydrostatic head, the vessel might tend to take a deformed shape, most likely within the allowable stresses. Imagine smart009 an inflated long baloon and a deflated one;- that's your pressure vessel. Fill it half with water, then blow air in it and put it on the edge of two books. It will hang a bit over the 'saddles' and will take an oval shape, but will not burst. Put some more air in it, the baloon will stiffen up and rounding up in transversal shape.
Cheers,
gr2vessels

 

[smart2009]

Got your points guys. Initially it was difficult for me to find the reason why vessel deforms under internal pressure.

The region of the vessel between the two saddle deforms i.e it takes oval shape. to be more clear top side of vessel deforms more than the bottom side of the vessel.

Gravity, thermal loads & nozzle loads might play a role. Let me recollect all your points and think again.

thanks all, if you have any other points to add please, it will help me to understand better.

Thanks,

 

[pennpiper]

Smart2009,
You want an intelligent answer but you have not provided any details on which we can determine that answer.
Such as:
- The diameter of the vessel?
- The tangent to tangent length?
- The number of Saddles?
- The tangent to center of each saddle?
- The wall thickness of the vessel shell?
- The type (shape) of the Heads?
- The material of the vessel?
- The age of the vessel?
- The commodity in the vessel?
- The normal operating pressure of the vessel?


"Intelligent questions earn intelligent answers"

prognosis: Lead or Lag

 

[corus]

The vessel is basically a thin walled beam that is partially restrained radially by the spherical ends, and partially by the saddle supports if they are welded on, otherwise they provide simple supports. In bending under self weight the cylinder will remain circular at the spherical ends. If it's assumed the saddles just provide simple supports then bending will be at a maximum at the centre and the vessel shape will distort asymmetrically at the centre producing ovalisation of the shape. It's genergally called the Brazier effect. Internal pressure in the cylinder may offset this ovalisation to some extent. You can see the same ovalisation if you bend a pipe over your knee. The pipe will ovalise until buckling occurs on the compressive side.

 

[smart2009]

thanks a lot everyone