| One of the most feared phrases in
the water well industry is “It's stuck”.
Every day hundreds, maybe thousands, of submersible
motors are removed from wells without incident. However, every once
in a while something causes one to hang up or keeps it from being
removed.
Some of the common causes are shale or rocks that
fall from above the pump and motor. These rocks wedge between the
pump or motor and the well casing. They don't have to be very big,
just wedged in the wrong place. Sometimes carefully moving the motor
up and down will free the unit. Another common reason for hang-ups
is when a safety cable or the drop wire comes loose from the top
and coils itself around everything down below. This can happen if
the wire is not properly supported. Again moving the motor up and
down carefully might free the unit.
One of the more uncommon reasons for a motor to
remain stuck happened several years ago. An earthquake caused a
horizontal shift in the earth's crust. This shift was great enough
to break the well casings and offset them by several inches. I don't
know how to get a motor out of this one.
From time to time, we also receive calls from contractors
who have had to abandon 4” PVC cased wells due to the well
casing forming around a 4” motor.
The contractor and his customer are frustrated because
of the difficulty and expense associated with the process of restoring
water service.
There is a misconception in our industry that the
sole cause of the problem is the heat from the motor causing the
PVC to shrink around the motor. Although the heat from the motor
does weaken the PVC, it alone is not the root cause of the problem.
CertainTeed, a leading manufacturer of PVC well casing located in
Valley Forge, PA (610-341-7000), has spent considerable time addressing
this problem. Through their laboratory and field work, they developed
technical bulletins that deal with the subject in more depth.
Based on conversations with CertainTeed, a review
of their bulletins, our lab testing, and phone conversations with
contractors who have experienced the problem, we conclude that the
solution to the problem is more dependent upon well construction
than it is on the motor.
This infrequent problem seems to occur primarily
in areas of loamy soil or when the drill cuttings are used as back
fill. Saturated with water, this sludge becomes hydraulic fluid
pressing on the PVC casing when the water level outside the casing
is higher than the pump drawdown level.
Certain Teed determined the potential for PVC casing
collapse can be eliminated by installing a firm and dimensionally
stable backfill.
If the wells are properly backfilled, this pressure
is distributed downward instead of inward.
Lab experiments show a 4” pump/motor running
under shut-off or deadheaded conditions could soften the 4”
well casing, but it would not cause the casing to melt, shrink or
deform without external pressure. Even though the PVC will soften
from the heat of the motor, particularly in the case of a locked
pump or a “dead head” condition, it is the hydraulic pressure
outside the PVC casing which causes it to form around the pump and
motor.
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