Basic Pipe Stress Question

[Jonnox]

I have a pipe welded to a large heavy base and the other side is capped off. If I neglect all other stresses other than the tensile stress in the cross sectional area of the pipe due to the pipe end being capped, what would it be???

I'm no expert by any means but just trying to get my head round a few things.

Example, Top picture in the attachment (hopefully the attachment works)

Force = Area * Pressure = 150^2 * 0.7854 * 10 = 176715 N

Area = OD^2 - ID^2 * 0.7854 = (170^2 - 150^2) * 0.7854 = 5026.56 mm

Stress = F/A = 176715 / 5026.56 = 35.15 N/mm^2

Now if the heavy base was replaced by another cap (bottom of attachment) would this mean the force doubles through the cross sectional???? As you have the same pressure on both caps??

Some clarification on this would help me sleep!! lols. If you need anymore details please do ask.

Best Regards
Jonnox

 

[desertfox]

Hi Jonnox

Firstly there is no attachment however what your describing is a pressure vessel so the stresses are more complicated then you think.
In the cylindrical portion you have hoop stress and longitudinal stress on the basis of thin walled vessel theory subjected to internal pressure, the formula for hoop stress is :-

σ = P*d/(2*t) where P= internal pressure
d = internal diameter of pipe
t = pipe wall thickness

So the hoop stress is 75 N/mm^2 more than double your longitudinal stress of 35 N/mm^2 and for the cylindrical part the hoop stress is most important.
Now moving on to the end caps and these could be a real problem because they will be subject bending stresses from the internal pressure so end cap thickness is extremely important,this also means the welds on the end caps are also critical from at point of view that you want the end caps to remain welded to the pipe.
My advice is find a local pressure vessel company and discuss your requirements with them, the last thing you want is a pressure vessel failure and someone getting hurt.

 

[desertfox]

Here is a link to a site showing stresses in a thin walled vessel

http://www.colorado.edu/engineering/CAS/courses.d/Structures.d/IAST.Lect03.d/IAST.Lect03.pdf

 

[LittleInch]

Your attachment is there and does work. You don't ignore the other stresses but what you are interested in is axial stress.

Does the axial force double - No.

In the single end cap case the end cap force is resisted by the force from the welded end, otherwise it would shoot off into space. In your second case this force is simply replaced by the end cap force and becomes a standalone vessel.

Always remember that for every force three is an equal and opposite reaction....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Jonnox]

Hi All,

Thanks for your reply's very help information!!!

I know not to ignore the other stress just wanted to single this one out and complete understand it. I do now, thanks again all.

Best Regards
Jonnox

 

[yazguli]

Is there any rule for when to use blind caps or blind flanges in headers (say parallel pumps headers)?

 

[LittleInch]

If you want to add more piping easily, use a flange, If you think that this is the end of the header and you don't want to , or can't, extend it then use a cap as it's cheaper.

personally I would always go for a flange - you never know when you need to get access and a flange is far easier to remove than a cap

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way