Seamless and Welded Pipe

It's irrelevant. Pipe is rated according to various schedules. Simply select the required pipe from the tables in the schedules.

Who in their right mind would pressurise a pipe to its ultimate tensile limit, whatever the theoretical strength <rhetorical question>?

By "seamless" pipe, do you really mean "tube"? Either can be used to transport fluids, but tubes are also used in structural applications, like for support columns or roll bars. Welded pipe would not be used for structural applications.

Welded pipe is less expensive to manufacture and needs less rigorous testing. It can be used in low pressure systems such as potable water or fire sprinkler systems. Seamless pipe is used for systems where pressure will be above a given pressure, such as 175 psi in some industries. Seamless pipe will work in very high pressure systems (over 1000 psi), but wall schedule will increase (schedule 80 v/s schedule 10). In ANY situation, your pipe type (ASTM rating) and wall thickness schedule MUST come from a code book where many thousands of hours of testing has taken place to determine the requirements.

seamless pipes are more strong because they have similar composition through it's length...

but welded pipes are not in that case.. it is difficult to produce 100% defect free weld.. so minor defect today will be major in future...

there are 2 types of pipes because it depend on the need, application and productivity....

seamless pipes cannot be produced easily. welded pipe may be produced of less cost...

physical and chemical properties of welded pipe is different in parent metal and weldment....it depends on electrode and base material used....

welded pipes are also used in pressure parts like drum,coils provided the welding is carried out according to standards and code.......

and i need some more details to give a clear idea......

There is little if any difference in chemical resistance between seamless and welded pipe made from the same material. For example, welded carbon steel pipe has the same chemicl resistance as seamless carbon steel pipe.....assuming the weld material is also carbon steel (and it should be).

As you have read in other replies, seamess pipe is stronger than welded pipe but quite a bit more expensive, especially in larger diameters. It's used when design constraints (usually pressure and safety requirements) justify the extra expense.

Choice of material (carbon steel vs. alloy vs. stainless steel, etc) is governed by the service conditions, not seamless vs. welded.

Am I missing something in ASME B31.3? I've seen a few responses saying that welded pipe is for low pressure systems.

You need a thicker wall for welded pipe vs. seamless but that doesn't mean only low pressure.

Welded pipe is cheaper than seamless but you add one more spot for failure with welded pipe. Some forms of corrosion occur at welds.

What's your service conditions (fluid, temperature, pressure, etc.)? Maybe we can give some recommendations.