Pipe Bend Radius

Close Radius Pipe Bending and Forming 1.0 SCOPE This specification covers machine 1½D and 3D pipe bends (made at ambient temperatures). Substituting pipe bends for butt welded elbows will provide piping with higher integrity, faster fabrication, and lower cost.

2.0 BEND RADIUS 2.1 General

Pipe bends are classified according to the centerline radius (CLR) of the bend as a ratio to the nominal pipe diameter. For example, 4" N.P.S. pipe which is bent on a 6" CLR is classified as a 1½D Bend (1½ times the nominal pipe diameter). When bent on a 12" CLR, the bend is classified as 3D.

2.2 1½D Bends

Most cold bends are made on a 1½D radius. The major reason for choosing 1½D is that it is interchangeable with long radius butt weld elbows (a 4" Long Radius 90°Elbow is 6" center-toface). As a result, drawings do not need to be changed to permit bending. When modifying an existing pre-formed system, a standard weld fitting can be used or vice versa.

2.3 3D Bends As an alternative to 1½D bending, when flow restriction may be a concern, 3D bend radii may be utilized. Studies have also indicated potential energy savings using larger bend radii. 3D Bending may require special design consideration.

3.0 SIZE LIMITS Cold machine formed close radius bends are currently available as follows:

1/2 - 6" N.P.S Sch 5 - Sch 80 Cold close radius bending of Schedule 5 pipe is only possible with carefully selected pipe and tooIing.

4.0 MATERIALS 4.1 Carbon Steels Carbon steel pipe is suitable for machine cold bending without heat treatment to the bend radius limits indicated in the following table.

A587ERW

A-53 Grade BSeamless

A-53 Grade B A-106 Grade B ERW Seamless

Cold Bending

1-1/2D

3D - 6D

3D - 6D

3D - 6D

Heat Treatment

Normalized Hot Finished

Hot Finished

Hot Finished

Tensile Strength, Min. PSI

48,000

60,000

60,000

60,000

Yield Point, Min. PSI

30,000

35,000

35,000

35,000

Elong. in 2", Min. %

40

30

30

30

Carbon % Max.

0.15

0.3

0.3

0.3

Manganese %

0.27 - 0.63 1.20 Max.

1.20 Max.

0.27 - 1.06

Sulpher % Max.

0.058

0.06

0.06

0.058

Phosphorous % Max.

0.048

0.05

0.05

0.048

Aluminum % Min.

0.02

Not Specified

Not Specified

0.10 Min. Silicon

Grain Size

Fine Grain

Not Controlled

Not Controlled

Not Controlled

ASME B31.3 - 93 Allowable Stress Values KIPS / Sq. In. -20 deg. F to 400 deg. F

13.60

20.00

17.00

20.00

4.2 Alloys

Alloy pipe suitable for machine cold bending without heat treatment to 1½D or 3D bend radius. All other materials should be inquired. ALLOY

SPECIFICATION

Stainless Steel

A-312, Types 304, 304L, 316, 316L, 309, 310, 317, 321, 347

Nickel Alloys

Nickel 200, Alloy 400 (Monel®), Alloys 600, 601, 625, 690 (Inconel®) Alloys 800, 825 (Incoloy®) Alloys C-276, B-2 (Hastelloy)

Aluminum

3003, 5083, 6061-T4, 6061-T6, 6063-T6

Titanium

Grade 1, Grade 2

Zirconium

702 Grade

Monel®, Inconel®, and Incoloy® are trademarks of Inco Alloys International, Inc. Hastelloy® is the trademark of Haynes International

5.0 BEND SPECIFICATIONS 5.1 Bends A pipe bend made to this specification and verified for pressure design in accordance with ASME B31.3 shall be suitable for the same service as the pipe from which it is made.

5.2 Out of Roundness Flattening of a bend, is measured by the difference between the maximum and minimum outside diameter at any cross section. For 1½D bends, out of roundness shall not exceed 5 percent of the nominal outside diameter for internal pressure, and 3 percent for external pressure. For 3D bends, out of roundness shall not exceed 3 percent. (See FIG. 1)

5.3 Dimensional Tolerances

Degree of bend is to be held to plus or minus 1 degree. Overall spool length will be held to plus or minus 1/8 inch. All centerline-to-centerline or centerline-to-end face dimensions will be plus or minus 1/8 inch. (See FIG. 2)

5.4 Wall Thinning Wall thinning of 1½D and 3D bends, as measured by the difference between the actual thickness of the pipe "A" and the minimum thickness "B" on the outside of the bend shall not exceed 18 percent of the actual thickness for 1½D bends and 12 percent for 3D bends. (See FIG. 3) Maximum allowable working pressure should be calculated using the following formula: For 1 1/2D Bending

For 3D Bending

1P= 2SE[.82t -c] D

1P= 2SE[.88t -c] D

P - Maximum Allowable Working Pressure SE - Allowable Stress t - Actual Pipe Wall Before Bending c - Corrosion Allowance D - Outside Diameter of the Pipe

5.5 Wrinkling Light wall stainless coupled with pipe wall variation near the outer limits of mill specifications may occasionally wrinkle. (See FIG. 4) In these cases, peak to valley depth will be limited to: Pipe Size

Max. Depth

1/2" - 2"

1/32"

3"

3/64"

4"

1/16"

6"

3/32"

TABLE 1 - CLOSE RADIUS BENDING TABLES Piping designers should refer to the Close Radius Bending Tables as a guide to maximizing the use of bending in their piping layout. These minimum dimensional values are established to allow adequate clamping during bending. Closer center-to-center or center-to-face dimensions are obtained by cutting pipe and/or adding welds as required. Minimum forming dimensions for degree of bend other than those shown are available upon request.

1-1/2 DIA. BENDING TABLE

PIPE RADIUS

90

DEG. BENDS

45

SIZE R

A

B

C

D

1/2" *

1-1/2"

57" 1/2"

7"

3/4" *

1-1/2"

57" 1/2"

1" *

1-1/2"

11/2" *

DEG. BENDS F

G

4-5/8" 6"

5-1/4"

6-1/8"

7"

4-5/8" 6"

5-1/4"

6-1/8"

57" 1/2"

7"

4-5/8" 6"

5-1/4"

6-1/8"

2-1/4"

8"

10-1/4"

66" 11/16"

7-5/8"

815/16"

2"

3"

11" 8"

14"

9-1/4"

61/2"

10-1/2"

121/4"

21/2"

3-3/4"

1310" 3/4"

17-1/2"

119/16"

73/4"

13-1/8"

155/16"

3"

4-1/2"

1312" 1/2"

18"

107/8"

9"

12-3/4"

153/8"

4"

6"

1614" 1/4"

22-1/4"

123/4"

93/4"

15-1/4"

183/4"

8"

A or D - Plain or Beveled End

E

B or E - Flanged End

C, F, G - Center-to-Center

Minimum Center-to-Center, and Center-to-Face Dimensions to allow forming. Some Center-to-Center bends may require a weld due to the plane of bend and/or the distance from the centerline of the bender mandrel to the shop floor. * 12" Center-to-Center is required for ½", ¾" 1" N.P.S. and 13-½" Center-to-Center is required for 1-½" N.P.S., if the plane of bend is below the horizontal plane (due to bender limitations). 1½D pipe bends in 2½" N.P.S. are not available in Carbon Steel. material. Please contact APEX Piping Systems with questions regarding any minimum forming dimensions at (302) 995-6136. or fax to (302) 995-1257. 3 DIA. BENDING TABLE

PIPE RADIUS

90

DEG. BENDS

45

DEG. BENDS

SIZE R

A

B

C

D

1/2" *

1-1/2"

57" 1/2"

7"

3/4" *

2-1/4"

1" *

3"

7"

11/2" *

4-1/2"

10- 91/4" 3/16"

2"

6"

14"

21/2"

7-1/2"

3"

9"

18" 16"

27"

12- 102116-1/2" 3/4" 11/16" 3/4"

4"

12"

2219" 1/2"

34-1/4"

1512" 1/4"

6"

18"

3134" 1/2"

51-1/2" 21"

6-7/8" 10"

14-3/4"

1020" 15/16"

A or D - Plain or Beveled End

E

F

G

46" 5/8"

5-1/4"

61/8"

56" 1/4"

6-1/2"

81/4"

76-1/2" 9-1/2" 5/8"

121/8"

10- 71/2" 7/16"

161/2"

231/2"

13"

20-1/4"

271/4"

30-1/2" 1"

B or E - Flanged End

C, F, G - Center-to-Center

Minimum Center-to-Center, and Center-to-Face Dimensions to allow forming. Some Center-to-Center bends may require a weld due to the plane of bend and/or the distance from the centerline of the bender mandrel to the shop floor. * 12" Center-to-Center is required for ½", ¾" 1" N.P.S. and 13-½" Center-to-Center is required for 1-½" N.P.S., if the plane of bend is below the horizontal plane (due to bender limitations). Closer dimensions for 6" N.P.S. bends are available depending on pipe material. Minimum forming dimensions for ¾" and 2-½" N.P.S. are available upon request. Please contact APEX Piping Systems with questions regarding /The formula comes from elementary beam theory. The value of R is the radius of the deflection curve of an otherwise straight run of pipe. It doesn't have anything to do with formed runs like long radius bends which involve plastic deformation and it doesn't have anything to do with fluid flow considerations. In your basic strength of materials you find that a straight beam under load bends into a curve, and in doing so one side compresses a little while the other side stretches a little. If you do the

arithmetic, you find that the strain = r/R where r is the distance from the centroid of the section and R is the radius of curvature of the deflection curve. Since stress = strain x elastic modulus you get your formula by substituting y/R for strain so Sb = Es x r/R which is your formula. / I am not able to figure out how the r/R can be treated as the strain. The basic equation on bending being, M/I = f/y = E/R, where f is stress and y is the distance from neutral axis. Equating the last 2, R = Ey/f, rewritten as R = ED/2f, D being the OD of the pipe /The formula simply states that the stress due to deformation of the pipe under applied transverse load depends on the elastic modulus, the outside radius of the pipe and the radius of the deflection curve. The assumptions are that the pipe is straight and remains elastic when it curves and that the material behavior is linear and elastic. It doesn't account for local buckling under compressive bending stress--you have to figure that separately--and it doesn't apply to the process of forming elbows or other bends from straight pipe. If you want to figure what the radius of curvature is, you need to know the loading and support conditions for the pipe run and the cross-sectional properties for the pipe itself. Again using elementary strength of materials, the radius of curvature of the pipe is the elastic modulus x moment of inertia)/ bending moment = Es x I/ M. As a results you get Sb = (Es x r)(M/Es x I) = Mr/I. This is the usual bending stress formula for a pipe run supported at intervals. I don't have a copy of B31.8 so I don't know why they specified the bending stress is terms of R rather than the bending moment, M. Could be there are layouts where a radius of curvature is assumed in design, but the radius of curvature isn't necessarily constant for any loading condition. /I am quoting a calculation on permissible elastic bend limits from the Aramco pipeline standard AES-L-450 referencing ASME B31.4 para 419.6.4 stress values 26-inch OD pipe, 6.35 mm wall, Grade X52 D = 0.660 m, E = 200 000 MPa, SMYS = 358.5 MPa Design factor 0.72, maximum temperature 77°C, minimum tie-in temperature 38°C. _Calculation_: Sh (Hoop stress) SMYS =

=

0.72

258.1 MPa

St (Temperature stress) 38) = 91.3 MPa Sc (Max. combined stress) SMYS = 322.6 MPa

= =

2.34 (77 -

0.9 *

Sb (Max. bending stress) St = 50.7 MPa R (Min. bend radius) ED/(2Sb)

=

Sc - 0.7 Sh =

=

1300 m

Angle per 30 m = 30 /(2R) radians = 0.66 deg Allow for misalignment radians = 0.25 deg Permit change of slope radians = 0.41 deg

=

0.0115 =

0.0044 =

0.0071

Max. change of slope 0.0071 x 30000 /BTW, this thread is a really great example of the confusion results from picking an equation out of the blue and plugging in numbers without understanding what the equation means or why it's in the Code. The practice of hunting around for equations without thinking critically about what they're doing eventually results in a huge bite on the ass./ I have another one used for off-shore pipe laying by stringing off a barge. It also uses the same approach. I note that the calcs do not account for the compression buckling (wrinkling).