Both go into the same borehole, both are uPVC, and both are sold by the metre — which is exactly why they get confused. But a column pipe and a casing pipe do opposite jobs, and using one in place of the other is an expensive mistake.
Casing pipe — holds the bore open
A casing pipe lines the bore. Once drilling is done, the hole will not stay open on its own — soil and loose strata collapse inward. The casing is lowered in to hold the wall, keep contaminated surface water out of the aquifer, and give the pump a clean shaft to sit in. Its enemy is external pressure, so it is engineered for collapse resistance and comes in larger diameters, typically 4″ to 12″. The relevant standard is IS 12818.
Column pipe — carries the water up
A column pipe hangs from the top and carries water upward from the submersible pump. Its enemy is tensile load — it has to bear the weight of the pump, the weight of the pipe itself, and the column of water inside it, all while resisting internal pressure. That is why column pipes use heavy threaded joints with square threads and rubber sealing, and why sizes stay smaller, usually 1″ to 4″, matched to the pump discharge.
How to choose each
Pick the casing by bore diameter and soil condition — loose or sandy strata need a heavier class. Pick the column by pump discharge size and installation depth: the deeper the pump, the higher the tensile class you need, because the load at the topmost joint is the sum of everything hanging below it.
Two things that go wrong on site
Two practical notes from site. First, never reuse a column pipe joint that has been overtightened — the thread deforms and the joint becomes the weak point at depth. Second, check that the casing is plumb before lowering the pump; a bore that is even slightly off vertical puts the column under bending load it was never designed for.