Bottom Hole Springs for Continuous Flow Plungers
Bottom hole springs are generally positioned at the end of tubing for vertical wells, or at a deviation angle of 45 to 50 degrees for horizontal wells. As the plunger falls in the tubing, it gathers speed. The spring acts as a stop and decelerates the plunger upon impact. Clean-out or bar stock plungers are typically the fastest falling type of non-bypass conventional plungers. These plungers can fall at 400 feet per minute.
The kinetic energy stored in the plunger when it strikes the bottom hole spring is determined by the equation:
The kinetic energy stored in the plunger when it reaches the bottom hole spring should be absorbed by the spring, without damaging the spring. Or in other words, the stress induced in the spring wire should not exceed the spring wire material stress carrying capabilities.
The energy absorbing potential of a wire style compression spring is determined by the equation:
As an example, an 8 lb clean-out plunger falling at 400 feet per minute impacts the bottom hole spring with KE of 5.52 ft lbf. A continuous flow plunger of the same weight, falling at 2,000 fpm, impacts the bottom hole spring with KE of 138 ft lbf (25 times more energy!). Springs designed to withstand the rigors of conventional plungers may fail quickly when subjected to the impact of continuous flow plungers.
Given a spring rate of 200 lb/in; the conventional plunger traveling at 400 feet per minute would compress the spring approximately 0.8 inch, while the continuous run plunger traveling at 2000 feet per minute would require at least 4 inches of spring deflection to decelerate the plunger!
Preferred operation of plunger lift wells requires a cushion of liquid covering the bottom hole spring to slow the velocity of the plunger prior to its impact. While often achieved with conventional plungers, continuous run plungers cycle so rapidly that, at times, there may not be sufficient liquid covering the spring to cushion the impact. Having a properly designed spring to accept this higher fall velocities allows the spring to remain functional throughout its design life.
In addition to the required spring deflection, the stress induced in the spring wire at that deflection should not exceed 30% to 45% of the minimum wire tensile strength. Greater induced stresses can reduce the effective life of the spring. When the stress is maintained in the allowable design range, the number of impacts without significant wire breakage should be 100,000 to 1,000,000 (dependent on a variety of other variables). To further complicate matters, the spring design equations do not effectively model shock forces – yet some spring materials are better than others when absorbing shock forces.
The basic equation for stress induced in the spring wire is:
Using the above equations for a spring with a spring rate of 200 lb/in; 1.00 inch mean diameter; 0.250
inch wire diameter – the stress in the wire is calculated as:
o 1 inch defection; S = 45,755 psi
o 4 inch deflection; S = 183,020 psi
If the spring is made from Chrome Silicon (suitable for Impact Loads – see below), the allowable design
stress is 30% to 45% of the minimal tensile strength – or 30% to 45% of 235,000 psi (70,500 psi to
105,750 psi). This spring, deflected 1 inch (8 lb plunger falling a little faster than 400 fpm) is in the
acceptable design range. However, when deflected 4 in (8 lb plunger falling at 2,000 fpm), the stresses in
the wire (183,020 psi) exceed the maximum allowable design stress (105,750 psi).
Thus, it’s expected this spring will have a much higher failure rate when impacted with a continuous flow
plunger falling at 2,000 feet per minute than when impacted with a conventional plunger falling at a
much slower velocity.
When using continuous flow plungers, it’s important to match the plunger with the proper bottom hole