Injection valves allow for passage of liquid from a feed line into a process line, an injection valve has an integral check valve. This check valve does not allow process fluid to enter or travel back up the injection line.
Withdrawable injection lance (Also called Corporation Stops)
Withdrawable lances are like an injection valve; however, they have additional features. They allow for the injector to be removed without having to turn off the process flow. They incorporate a shut-off valve and a form of a radial seal. The quill can be retracted while sealed until it reaches a point at which a shut-off valve (usually a ball valve) can be closed, and then the quill can be safely removed without leakage. They usually have a locking mechanism to ensure the quill does not slide out under system pressure and a hard stopping point (via chain or cable) to stop the quill from sliding all the way out when being serviced.