Two-Phase Flow Lines

In what context?

Actually i am doing internal pressure wall thickness calculations as per ASME B31.3. From line list Process specified as 2-Phase flow(Gas-liquid and vise versa).

thats why i asked whether there is any additional care or any requirement is needed for 2 phase flow fluids.

In what context does the external pressure have any impact on the two-phase flow within the pipe?

may be due to change phase and change in temperature.

The external pressure will have no influence on the phases within the pipe. The external pressure might have an influence on the piping wall thickness. For example, a pipe that is at the bottom of the sea might need less wall thickness than the same pipe carrying the same fluid in open air.

The external temperature might have an effect on the phases within the pipe, though that was not asked about!

I'm not sure that the term "two-phase flow" is consistently defined in the literature.

One regime would be a mixture of vapor and liquid of the same substance flowing in the same direction in a pipe. This is common in recirculating refrigerant systems. External pressure is not usually in question, unless in a submarine, say.

For refrigerants draining from condensers into receivers, there may be two-phase countercurrent flow, with liquid moving downward and vapor upward. No special pressure conditions are involved, but the piping must grade strictly downward, and the velocities must be limited. External pressure is normally not an issue here, either. This type of flow is colloquially called "sewer flow."

Speaking of which, sewers are often buried. Their internal pressure is atmospheric plus maybe a bit of vertical head, so internal pressure is not normally an issue. However, external pressure from the earth in which the sewer is buried is a relevant issue.