Why do we use pipelines for transmitters in steam service and tubes in any other service?

Pipe as well as tubes can be used for steam line transmitters. However pipe walls being thicker provide more corrosion allowance.

For steam services, with high temp and pressure needs IBR standard. Max. Working pressure is 153 kg/cm2 at 38 degC for 1/2” O.D tube as per design standard. So tube is not suitable.