what is the purpose of orifice plate in pipe lines?

There is a problem with terminology. The use of the words "pipe lines" can mean two different things.

Some people use it to refer to any piping system and others use the term to only refer to cross country pipes that carry fluids great distance.

In the first case, orifice plates can be part of a measure system or they can be used as a restriction to control or limit flow. This of course adds pressure drop to the circuit but that is normally not a concern because there is more than adequate pressure for the system to function.

In the second case, they are almost always used for measurement purposes. In cross country pipelines pressure drop is something that needs to be limited as much as possible because of the energy required to over come it. When transporting large volume of a fluid small pressure drop additions can be very expensive.

well u need to specify for which case u want orifice's explntn... bcoz mainly it is usd for measurement of flow... and this is the only thing dat an orifice plate does....