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 requires 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 ball and sleeve plunger.
Captured Shift Rod
This full-length bypass plunger has a robust 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 rapidly down the tubing to the bottom spring while flowing. The internal valve closes when the 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 at a rate much greater than 300-400 MCFD while the plunger is falling. Even so, the fall rate exceeds that of a solid plunger.
Short Dart
The Dart Plunger is 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 spring assembly at the bottom of the tubing. 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 captured shift rod plunger. In wells flowing at less than 700 MCFD, this bypass type can fall while the well is flowing. However, because there is no 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 and Sleeve Plunger
The bypass plunger with the fastest fall rate is the ball plunger (ball and sleeve plunger). This plunger incorporates a large diameter ball to close off the open valve seat area at the bottom of the plunger (sleeve). 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 where the shift-rod stinger knocks the ball free of the sleeve.
PAD BALL PLUNGERS
The useful range of the Pad/Ball Plunger lies between the speed of the ball and sleeve plunger and the sealing efficiency of the pad plunger. The Pad/Ball Plunger is very similar to the ball and sleeve plunger but with a better seal against gas/liquid slippage.
The Pad/Ball Plunger has all of the familiar moving parts of a standard, non-bypass plunger and is therefore subject to the mis-treatment of high speed, hard running. It is used in low gas to liquid ratio wells that don’t have the gas volume required to run the less efficient sleeve style plunger. Pad/Ball Plungers are available in 2-3/8” and 2-7/8”. A 3-1/2” Pad/Ball Plunger is available as a special order.