Plungers are generally manufactured as bypass and non-bypass. In the non-bypass form, the plunger falls more slowly into the wellbore. This is a problem for the more efficient seal types because of the increase of the shut-in time involved. If the well is poor and will require a lengthy shut-in for a pressure build-up, then the use of a more efficient seal is preferred. There are three types of bypass plungers; shift rod, short dart and the two-piece shuttle ball plunger.
This full length bypass (open center) plunger has a 12mm rod through the center of the (one piece) plunger that acts as a long valve stem. This valve stem shifts into the open position when the plunger strikes the surface spring. This internal valve opening allows the plunger to fall back down the tubing to the bottom spring. The internal valve closes when plunger sits down upon the bottom-hole assembly. The flow area of the bypass is limited and therefore the well is not allowed to flow while the plunger is falling. Even so, the fall rate exceeds that of a solid plunger.
An open center, bypass plunger with a much shorter valve stem than the shift-rod plunger in the paragraph above, the valve stem of the short dart projects from the underside (only) of the plunger. The valve closes when the valve stem strikes the bottom spring. The flow area up through the hollow center of the dart-style bypass plunger is larger because of the absence of the full length shift rod. Therefore, this plunger falls faster than the shift rod plunger. In low flow-rate wells, this bypass type can fall even while the well is flowing, if the rate is not too high. However, in the absence of an internal shift rod, this type of plunger requires a shift rod (stinger) mounted in the lubricator. Therefore, the plunger must have a female fishneck instead of the standard male fishneck.
Two Piece Ball Plunger
The bypass plunger with the fastest fall rate is the ball plunger (shuttle ball plunger). This plunger incorporates a large diameter ball to close off the open valve seat area at the bottom of the plunger. This ball is independent of the plunger body and falls rapidly into the tubing. The plunger body follows the ball into the tubing and joins the ball at the down-hole spring. The up- flowing gas pushes the combined two-piece plunger to the surface. See Using the Disc Plunger.