Pulling on the rod string to unseat a stuck pump can cause accidental overload, excessive rod stretch and/or tensile breakage. Excessive stretch indicates the load exceeded the yield strength; breakage occurred as the load increased above the yield strength to the tensile strength. If the sucker rods are permanently stretched by overload, there most likely will be localized, external and perhaps internal damage that may lead to early failures. Consequently, these affected rods should be removed from service.
Calculating the maximum allowable pull can prevent stretching and tensile failure.
EXAMPLE
Reduce the minimum yield strength rating of the type rod in question by 5% (an arbitrary safety factor), then multiply the reduced yield strength by the cross sectional area of the smallest rod in the string (convert PSI to pounds); add to this number (#’s) the weight of all rods above the top smallest rod. Do not exceed this total number of pounds in steady hook load pull. Also, do not jerk or pull with surges at this level of load.
CALCULATIONS
Type of Rod API C
Minimum yield strength 60,000 PSI
Smallest rod ¾”
500 feet 7/8” rods above top ¾” rod
- 60,000 PSI X .95 = 57,000 PSI
- 57,000 PSI X 0.442 sq. inches = 25,194 lbs.
- Weight of 7/8” rods above top ¾” rod 500 feet X 2.224 lb. /ft =
1,112 lbs. - Maximum allowable pull in pounds 25,194 + 1,112= 26,306 lbs.
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