Could anyone explain me the reasons for using RB valves on a pipeline and on which type of pipelines? I know only as the cost and should not be used on a pigable pipeline.

It would be very unusual, if not dangerous, to need one on a pipeline for most products.

Here's some of the applications where they might be more useful,
http://www.terrauniversal.com/desiccators-dry-boxes/automatic-rb-valve.php

There's not much reason to use them other than lower cost.  Since most applications where you would use them are probably going to be on/off service, their generally increased pressure drop when open would probably not be very useful either, except perhaps in vents, drains, or bypasses.  In pipeline applications, where the bulk of the cost is NOT for valves/ball valves, their cost advantage to the project would be insignificant and come at the risk of hanging a pig, or for replacing them for easier pigging, or for instrumented pigging at a later date, if somebody had used them in the mainline.

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