scope:

Foreword

This supplement covers changes adopted at the 1991 Standardization Conference as reported in Circ PS-1952 and approved by letter ballot.

Page 10. Add Paragraphs 4.4 to 4.4.10 to Section 4.

4.4 Allowable Pump Setting Depths. The formulas for the determination of the maximum allowable pump setting depth (ASD) are presented in this section.

4.4.1 Limitations. The limitation for the ASD is determined by the maximum allowable stress generated in the working barrel of the pump. Depending on the pump type, this maximum stress can be generated by 1) Burst pressures, 2) Collapse pressures, or 3) Axial loads, with the critical modes listed below:

4.4.2 Burst Mode. In the Burst mode, there exists a differential pressure from the Barrel I.D. to O.D., typically during the downstroke, as shown in Figure 4.4A.

The ASD formula in the Burst mode is a derivative of the Barlow formula for tangential stress as given in API Bulletin 5C3, Formula 3.1.1 (Ref. 1):

S = Endurance Limit (PSI)

t = Min. Barrel Thickness (Inches) at Thread Root or at the Extension Coupling Bore

Do = Min. Barrel O.D. (Inches)

S.F. = Service Factor (S.F. < 1.0)

F.S. = Design Factor of Safety

S.G. = Fluid Specific Gravity

4.4.2.1 Alternate Formula, if Barrel Thickness at Thread Root Unknown.

tw = Barrel Thickness at Bore = (Do - Di)/2

Di = Bore Diameter

4.4.2.3 Assumptions.

a. The maximum differential pressure exists across the barrel, i.e. with zero pressure on the outside of the barrel.

b. The radial stresses are negligible.

c. Pressure increases due to fluid compression on the downstroke are neglected.

4.4.3 Axial Mode. In the Axial mode, there exists a tensile load on the Barrel created by the hydrostatic load of the fluid in the tubing acting on the Standing Valve, as shown in Figure 4.4B and 4.4E. Tubing weight hanging below a tubing pump should also be considered in combination with the Burst pressure. These loads are negligible if the tubing is anchored or if the load is supported by the seating nipple (bottom holddown).

The ASD formula is a derivative of the standard Tensile Stress (Sa) formula (Sa = F/At; F = P*Ai + TBG, WGT)

Ai = Internal Area of Barrel or Extension Coupling at Bore (in2) =.785(Di)2

At = Cross Sectional Area of Barrel or Extension Coupling at Thread Root (in2)

S = Endurance Limit (psi)

K = Stress Concentration Factor

P = Internal Hydrostatic Pressure at Standing Valve (psi) = H * g

H = Height of Fluid Above Pump (ft)

g = Fluid Gradient (psi/ft) =.433 psi/ft (S.G. = 1.0)

S.G. = Fluid Specific Gravity

4.4.3.1 Alternate Formula, if Cross Sectional Area of Barrel at Thread Root Unknown.

Aw = Cross Sectional Area of Barrel at Bore (in2)

4.4.4 Collapse Mode. In the Collapse mode, there exists a differential pressure from the barrel O.D. to I.D., during the upstroke, as shown in Figures 4.4C and 4.4D.

The ASD formulas in the Collapse mode are derivatives of the formulas given in API Bulletin 5C3, Section 1, depending on the maximum D/t of the Barrel housing:

Sy = Barrel Yield Strength (psi)

For D/tw values greater than (D/t)pt, refer to formula 1.1.3.1, 1.1.3.2, and 1.1.4.1 in 5C3 for the applicable Collapse pressure formula.

A complete set of values for factors A, B, C, F, G can be found in API Bulletin 5C3, Tables 1.1.2.2 & 1.1.3.1.

4.4.4.1 Assumptions

a. The differential pressure across the barrel is at a maximum, with zero pressure inside the barrel.

b. Radial stresses are negligible.

c. The length-to-diameter ratio (L/D) is greater than 8.

d. For Traveling Barrel pumps, the Barrel is weaker than the plunger, pull tube, or other elements exposed to collapse pressure.

4.4.5 Factors of Safety. It is generally recommended that a minimum design Factor of Safety (F.S.) of 1.25 be used to account for material and dimension variations.

An additional service factor (S.F.) should also be used for each application and each manufacturer, depending on the particular well conditions and usage. Consideration should be made for at least the following pump service problems which are known to affect pump service life.

1) Corrosion

2) Fluid Pound

3) Gas Pound

4) Gas Lock

5) Sand Problems

6) Scale Problems

7) Usage (Cycles/Day; Total Cycles, Etc.)

For additional information, see API RP 11AR (Ref.6). This Service Factor (S.F.) should be less than or equal to one.

4.4.6 Stress Concentration Factor (K). Due to the cyclical loads, a stress concentration value should also be applied to the barrel threads. This value is typically a function of the thread root radius and diameter. For standard 'V' type machined threads, a value of 2.8 is recommended. Typical values can be found in Ref. 4.

4.4.7 Endurance Limit (S). Due to the cyclic nature of the loads, it is recommended that the fatigue Endurance Limit(S) at the maximum number of cycles be used for the maximum stress in the Burst and Axial modes.

For many materials, the Endurance limit(S) is unavailable from suppliers, and therefore the Ultimate Tensile Strength (Su) should be substituted using suggested correction factors of 2.5 for steel & 3.0 for brass.

Steel S = Su/2.5.........4.4.6.1

Brass S = Su/3.0.........4.4.6.2

Typical Ultimate Tensile Strength values are given below. These values should be used unless the actual Tensile Strength or Endurance Limit of the material is known.

4.4.8 Dimensional Data, See Table 4.4.8.

4.4.9 Examples

4.4.9.1 Burst or Axial Mode Example. Determine the maximum setting depth of a 2-3/8 × 1-½ RWA pump, made of low carbon steel (oil = 10 degrees API):

Step 1: By Table 4.4.1, failure mode is Burst or Axial, use the minimum value determined from formulas 4.4.2.1 and 4.4.3.1.

Step 2: Calculate ASD1:

Step 3: Calculate ASD2: Assume weight below pump negligible.

Step 4: Compare ASD1 to ASD2, and select lower value:

ASD2

4.4.9.2 Collapse Mode Example. Determine the maximum setting depth of a 3-½ × 2-¼ RHB pump, made of Admiralty Brass material (oil = 10 degrees API):

Step 1: By Table 6.1, the failure mode is Collapse.

Step 2: Calculate D/tw: D/tw = 2.75/.25 = 11.00

From Table 4.4.4.1, for an H-60 equivalent barrel, (D/t)yp = 16.4 and less. Therefore D/tw < (D/t)yp and ASD3 applies.

Step 3: Calculate ASD3: Sy = 60000. Table 4.9.

4.4.10 References

1. API Bulletin 5C3, Bulletin on Formulas and Calculations for Tubing, Drill Pipe and Line Pipe Properties, Fifth Edition, July 1989.

2. API 11AX, Specification for Subsurface Sucker Rod Pumps and Fittings, Third Edition, June 1, 1989.

3. Fatigue of Materials; J. Y. Mann; Melborne University Press, 1967, Table I, P. 50.

4. Stress, Strain, and Strength; R. Juvinall; McGraw-Hill 1967, Table I13.1, P.251.

5. NACE Standard MR0176-76; Metallic Materials for Sucker Rod Pumps for Hydrogen Sulfide Environments; Copyright 1976, National Association of Corrosion Engineers.

6. API RP11AR, Recommended Practice for Care and Use of Subsurface Pumps, Third Edition, June 1, 1989.