2 replies to this topic

[Anantha Kumar]

I was wondering, Why is pipe always circular not a square & Rectangular shape?..

 

if we use square & rectangular pipe, what will happen inside of pipe?..

[sgkim]

Hi, Kurar:

 

1) Circular cross section has the maximum inside area to perimeter ratio than any other type of polygons, so the piping material and installation costs would be minimum.  

2) Compared with any other shapes, the minimum deformation would be noticed for circular cross sectional pipe when internal or external pressure is applied.  

3) When in pipe run, any kind of directional, elevation change or branching is possible by using various standard pipe fittings.      

 

Considering the view points like the above 1), 2) and 3), no other type of conduit seems to be superior to circular pipe.

 

~Stefano G Kim   

[Art Montemayor]

Kurar:

 

The 3 reasons that Stefano has pointed out are all valid and important when considering the use of a circular cross-section for a fluid conductor in industry.

 

However, I've always thought that there are 2 additional - and more important - values to consider:

 

First, recall your basic equation in Strength of Materials that states that the wall thickness of a cylindrical vessel (or pipe) is basically:

 

Thk = DP x R / [(JE x SA) + 0.4 x DP)]

 

Where,

DP = Design Pressure, psig
R = Radius of vessel or pipe, inches
JE = Joint Efficiency, fraction
SA = Allowable Stress, psi

 

The strongest configuration to withstand stress that God or nature has given us is a circular one.  Related to this is the fact that the strongest geometric shape to withstand pressure (internal stress) is the sphere.  When you calculate the required wall thickness of your supposed rectangular cross-section you will find that the amount of metal wall thickness is far superior to that of the circular one.   This calculation is more complicated than the above.   And, as Stefano has noted, the circular one won't deform and weaken.  The circular configuration takes so much less wall thickness for a given pressure that a nominal wall thickness can be applied that is conservative and serves to cover a relatively large range of pressures.  Additionally, because of its natural shape, the circular one keeps its pressure-resistant shape regardless of the internal pressure.  Check this out and you will find that this gives you a very safe range of pressures that any schedule of pipe gives you.  This is a very valuable safety feature that is not offered by a plane wall - which requires an extremely thick wall for a given pressure.  This also applies to external pressures as well and is the reason that submarines nor diving bells are not designed nor built with rectangular hulls.

 

Secondly, the circular configuration allows the manufacture of piping to be done automatically and in relatively large lengths - and even in welded seam pipe, this is done quickly and automatically.  Seamless pipe is automatically extruded and is inherently more resistant to pressure than welded seam pipe due to the lack of having a seam subject to welding defects.

The subject of stability is very important when installing and using fluid piping in industry.  Any other configuration besides circular is going to require bracing - whether internal or external.  This invokes a complicated need for installation and upkeep.  A rectangular object will deform when subjected to external forces or stress and requires bracing - just like a normal rectangular house does.  This, plus the exorbitant wall thickness required makes the use of such fluid transport objects as impractical.

 

Only in very low internal pressure application - such as ducts for air conditioning or low pressure blower applications does the application of rectangular fluid piping become justified - and these applications are justified due to the rather large flow area offered with the resulting low pressure drop.  But even then, the need for bracing and support becomes an issue as always with this type of conductor.

 

I hope I have succeeded in expressing the common, rational, and engineering sense that takes place in considering the best cross-section for piping from a common sense point of view.