why are the pipelines cylindrical shape??

One simple answer - the circular cross-section maximizes the cross-sectional area for a given amount of construction material.

Not only that, but you also remove the stress concentrations in the corners.

as already mentioned and additionally

a) Uniform stress on every point of a section (explained as no stress concentration)

b) Pipes Easier to manufacture

c) Uniform flow (less turbulence) - due to uniform velocity gradient from wall to center, in a say rectangular cross section the flow lines will be much more complex.

d) Calculations hence are much easier.

e) Minimum wall minimises friction

and other myriad factors.

1. A round pipe has the minimum surface area exposed (maximum flow for surface area) to the water so there is less drag on the water at it flows down the pipe (i.e. less friction) - so you need less energy to pump the water, or conversely you will drain a higher flowrate under gravity
2. Pipes are round because circles are the most efficient shape when handling pressure. When water is flowing through the pipe, there is an internal pressure. With a circular shape, the pressure force is evenly distributed around the entire circumference of the pipe. With squares, triangles, or any other shape with sharp corners, the pressure forces concentrate at these locations. Therefore, the edges or corners would have to be reinforced, adding weight to the pipe and increasing the complexity of manufacturing; pressures on a concave surface have less stress than on a flat surface (the square sides would need to be a lot thicker than the round pipe wall thickness)
3. Square pipes would have crevices (folds) on the inside where the corners are. Bits of debris and minerals would build up in these crevices over time leading to corrosion problems, or slowing down the efficiency of the pipe and probably eventually blocking it.
4. Lastly, easier fit up when building pipe and also rounds are easier to bend.

GA for you guest. Please register as you clearly understand pipe flow and stresses. Are you a piping design engineer by the way?

Ha-ha - how ironic ......... I just copied and pasted the above answer from my previous answer @

http://cr4.globalspec.com/thread/25721

and the same registering point has been made.

I am an all-round design engineer into piping, vessels, mechanical - moonlighting as a general maintenance engineer.

I have since thought of another reason for round piping

• It gives an almost infinite number of angular possibilities when fitting elbows, etc. That is you can have it rotated at 10°, 15°, 27°, 39°, etc.... without having to make countless number of rotated square fittings.
  

Generally, if it were any other cross-sectional shape it wouldn't be a called a <...pipeline...>; it would be a "duct", a "transit", or a "conduit".