PFI Standard ES-24

(Reaffirmed - September 2010)

PIPE BENDING METHODS,

TOLERANCES,
PROCESS AND MATERIAL
REQUIREMENTS

Prepared by
Pipe Fabrication Institute Engineering Committee
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All PFI Standards are advisory only. There is no

agreement to adhere to any PFI Standard and their
use by anyone is entirely voluntary.

Copyright by

PIPE FABRICATION INSTITUTE

Dedicated to Technical Advancements and Standardization in
the Pipe Fabrication Industry
Since 1913
USA CANADA
511 Avenue of America’s, # 601 655, 32nd Avenue, # 201
New York, NY 10011 Lachine, QC H8T 3G6

WEB SITE
www.pfi-institute.org

Pi p e F a b r i c a t i o n I n s t i t u t e L i c e n s e e = ANCAP 5 9 4 6 2 4 1
No t f o r Re s a l e , 2 0 1 4 / 7 / 8 1 4 : 4 8 : 0 0
No r e p r o d u c t i o n o r n e t wo r k i n g p e r mi t t e d wi t h o u t l i c e n s e f r o m I HS
 PFI Standard ES-24
(Reaffirmed - September 2010)

Pipe Bending Methods, Tolerances,

Process and Material Requirement

METRIC CONVERSIONS

The conversion of quantities between systems of units involves a determination of the number of significant digits to be retained.
All conversions depend upon the intended precision of the original quantity and are rounded to the appropriate accuracy.
Pipe sizes together with applicable wall thicknesses are not shown with metric equivalents.
The SI (metric) values where included with the customary U.S. values in this Standard are the rounded equivalents of the U.S.
values and are for reference only.
Metric units were derived utilizing the following conversion factor:

Conversion Factor Conversion Factor

Inches to 25.4 Deg. Fahr. to 5/9 (Deg. F.– 32)
Mitlimeters Deg. Cent.
Lb/in2 to 0.0703
Kg/cm2

1. Scope Dn = Nominal outside diameter of pipe.

tn = Nominal wall thickness of pipes.
1.1 This standard covers methods, process tm = Minimum calculated wall thickness
requirements, tolerances and acceptance criteria for required by the applicable code.
shop fabricated pipe bends. T = Pipe wall thickness (measured or
minimum, in accordance with purchase
1.2 The bending methods described in this standard are specification).
a partial representation of commonly used R = Center line radius of bend.
processes and do not preclude the use of other
methods.
3. Bending Methods
2. Terminology 3.1 This standard covers bends formed by both hot and
cold bending methods. For this standard, a
2.1 The symbols and terms defined below are used temperature 100 degrees F below the lower critical
throughout this standard: temperature of the material is defined as being the
boundary between hot and cold bending.
BEND EXTRADOS
3.2 Unless otherwise specified by the governing code,
TANGENT
the bending procedure, including the
heating/cooling cycle and post bend heat treatment,
NT
N GE Dn is determined by the pipe material, diameter, wall
TA
thickness, bend radius and the required properties
after bending. Because of the many variables
R
involved, the bending procedure should be
BEND INTRADOS
determined by the fabricator.

FIGURE 2.1

D = Nominal pipe size.

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Pi p e F a b r i c a t i o n I n s t i t u t e L i c e n s e e = ANCAP 5 9 4 6 2 4 1
No t f o r Re s a l e , 2 0 1 4 / 7 / 8 1 4 : 4 8 : 0 0
No r e p r o d u c t i o n o r n e t wo r k i n g p e r mi t t e d wi t h o u t l i c e n s e f r o m I HS
 PFI Standard ES-24
(Reaffirmed - September 2010)

3.3 While the bending equipment used in many of the HEATING DEVICE
methods is generically the same, there may be
differences in bending procedures, material
allowances, hold and pull legs, wall thickness,
etc., between bending fabricators.

3.4 Hot bending methods

ANCHOR BOX
3.4.1 FURNACE BENDING:
In this method, the pipe is firmly packed HYDRAULIC CYLINDER
with sand and then heated in a furnace to a
temperature in the range of 2000 degrees F.
After removing from the furnace, one end of FIGURE 3.4.2
the pipe is retained in a holding device and a INCREMENTAL BENDING
bending moment is applied at the other end.
3.4.3 INDUCTION BENDING:
The radius of the bend is controlled by dies,
The induction bending equipment is
stops or templates as the pipe is bent. For
composed of three basic components
long radius bends and/or heavy wall pipe,
consisting of a bed, a radial arm, which is
the sand filling operation may not be
set at the required radius, and an induction
necessary.
heating system. The pipe is placed in the
HOLDING SHOES bed and the front tangent is clamped to the
radial arm. The induction heating system
heats a narrow circumferential band around
the pipe to the appropriate bending
temperature. When this temperature is
reached, the pipe is continuously moved
through the heating coil while a bending
moment is applied to the heated area. After
passing through the coil, the pipe may be
RADIUS STOPS either forced or naturally cooled as required
(OPTIONAL)
by the appropriate qualified bending
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PULLING DEVICE
procedure.

FIGURE 3.4.1 INDUCTION COIL

FURNACE BENDING

3.4.2 INCREMENTAL BENDING:

The incremental bending equipment is
composed of an anchor box, a hydraulic
cylinder, and a moveable heating device.
The pipe is clamped in the anchor box and BED
the front tangent is connected to the
RADIAL
hydraulic cylinder. The heating device heats ARM
a narrow circumferential band on the arc to
the proper bending temperature. A force is
then applied by the hydraulic cylinder to
bend the small increment a predetermined FIGURE 3.4.3
amount. The heating device is then moved INDUCTION BENDING
to successive segments where the process is
repeated until the required arc is attained.
After bending each increment, the heated
area is cooled as required by the appropriate
bending procedure.

Pi p e F a b r i c a t i o n I n s t i t u t e L i c e n s e e = ANCAP 5 9 4 6 2 4 1
No t f o r Re s a l e , 2 0 1 4 / 7 / 8 1 4 : 4 8 : 0 0
No r e p r o d u c t i o n o r n e t wo r k i n g p e r mi t t e d wi t h o u t l i c e n s e f r o m I HS
 PFI Standard ES-24
(Reaffirmed - September 2010)

3.5 Cold bending methods

3.5.1 ROTARY DRAW BENDING:

In this method, the pipe is secured to a
bending die by a clamping die. As the
bending die rotates, it draws the pipe against
the pressure die and, if necessary to prevent
wall collapse, over an internal mandrel. The
pressure die may remain fixed or move with
the pipe.

PRESSURE DIE

CLAMP FIGURE 3.5.3

ROLL BENDING

4. Welds in Bends
4.1 In some instances it is not practical to utilize pipe
ROTATING BEND DIE of sufficient length to satisfy the required arc
length of the bend. When it becomes necessary to
join lengths of pipe resulting in a circumferential
FIGURE 3.5.1
butt weld in the arc of a pipe bend, the following
ROTARY DRAW BENDING
practices should be considered:
3.5.2 RAM BENDING:
In ram bending, the pipe is held by two 4.1.1 Pipes to be welded should be selected to
supporting dies and a force is applied by provide the best uniformity possible at the
means of a hydraulic ram to a forming shoe mating ends. Pipe wall thickness shall not be
located at the center of the workpiece. The less than the design minimum plus bend
supporting dies rotate on their mounting thinning allowance (see section 7.0).
pins so that they follow the pipe and 4.1.2 End preparation for welding shall be in
maintain external support throughout the accordance with the qualified welding
operation. procedure to be used. Internal counterboring
should be avoided wherever possible. During
SUPPORT
DIE
fit-up of the joint, the pipes should be rotated
HYDRAULIC RAM
or aligned as necessary to provide the least
amount of I.D. and/or O.D. mismatch and the
best transition across the weld.
4.1.3 The welding procedure must be qualified in
accordance with the governing Code for the
FORMING SHOE thermal exposures, (if any) excepted in
bending and heat treatment.
4.1.4 After completion of the circumferential butt
FIGURE 3.5.2 weld, but before bending, the O.D. and I.D.
RAM BENDING (where accessible) of the weld should be
ground to remove excess weld reinforcement
3.5.3 ROLL BENDING: and blended smoothly into the base metal.
In roll bending, three forming rolls of
approximately the same diameter arranged in 4.1.5 It is good practice to examine the
a pyramid are used. The two fixed rolls circumferential butt weld by radiography
oppose the adjustable center roll. The pipe is prior to and after bending, whether or not
passed through the rolls with the position of such radiography is required by the
the adjustable roll controlling the bend applicable Code.
radius.

--`,``,`,`,```,```,,,,,,`-`-``,```,,,`---

Pi p e F a b r i c a t i o n I n s t i t u t e L i c e n s e e = ANCAP 5 9 4 6 2 4 1
No t f o r Re s a l e , 2 0 1 4 / 7 / 8 1 4 : 4 8 : 0 0
No r e p r o d u c t i o n o r n e t wo r k i n g p e r mi t t e d wi t h o u t l i c e n s e f r o m I HS
 PFI Standard ES-24
(Reaffirmed - September 2010)

5. Linear and Angular Tolerances 1

2
3
5.1 Bends shall be provided with a total angularity
tolerance of ± .5 degrees as determined by the
intersection of the tangent centerlines measured A
by appropriate equipment.

5.2 When the fabricator is required to provide bends

cut to a specified center-to-end dimension it shall
be to the tolerances specified in PFI ES-3.

5.3 If intermediate portions of the bend profile are FIGURE 6.2

essential, their tolerances shall be a matter of APPLICATION OF PIPE WALL BUCKLING
agreement between the purchaser and the TOLERANCES
fabricator.
Note 1 – Depth of average crest to valley is the sum of the
outside diameters of the two adjoining crests divided by two,
5.4 See fig. 9.3 for an explanation of terminology
minus the outside diameter of the valley.
regarding bend tolerances.
( OD )1 ( OD ) 3
6. Form Tolerances Depth ( OD ) 2
2
6.1 The ovality of a pipe bend shall not exceed the Note 2 – Ratio of distance between crests to depth is:
ovality required by the governing code. If there is A 12
no governing code, the difference between the
maximum and minimum diameters shall not Depth (per Note 1) 1
exceed 8% of the average measured outside
diameter of the straight portion of the pipe unless 6.3 Buckles which exceed the above tolerances will
by mutual agreement between the purchaser and be subjected to corrective action to bring them
the fabricator. Where operating conditions require within tolerance.
less ovality it may be necessary to use larger
radii, heavier pipe walls or a specific bending 6.4 If operating conditions require tighter tolerances
method that will provide a closer control of on buckles, it may be necessary to use larger
ovality. radii, heavier pipe walls or a specific bending
process.
6.2 Since there are occasions when buckles cannot be
avoided, the following restrictions should apply: 6.5 To determine what bends can be produced with a
satisfactory degree of quality, the Pipe
(a) All wave shapes shall blend into the pipe Fabrication Institute has conducted studies on
surface in a gradual manner. carbon steel and low alloy steel hot bends to
(b) The maximum vertical height of any wave, determine minimum recommended bend radii for
measured from the average height of two various ratios of outside diameters to wall
adjoining crests to the valley, shall not thickness. The resulting bending range
exceed 3% of the nominal pipe size. (See determined by these studies for each of the
Figure 6.2, Note 1) bending processes is shown in Figure 6.5.1 and
6.5.2.
(c) The minimum ratio of the distance between
crests as compared to the height between
crests and the included valley should be 12
to 1. (See Figure 6.2, Note 2)

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Pi p e F a b r i c a t i o n I n s t i t u t e L i c e n s e e = ANCAP 5 9 4 6 2 4 1
No t f o r Re s a l e , 2 0 1 4 / 7 / 8 1 4 : 4 8 : 0 0
No r e p r o d u c t i o n o r n e t wo r k i n g p e r mi t t e d wi t h o u t l i c e n s e f r o m I HS
 PFI Standard ES-24
(Reaffirmed - September 2010)

(2) Enter 29.1 on the Dn/T axis of Figure 6.5.1

and move vertically to the intersection with
7
the bending range boundary.
6 (3) Then move horizontally to determine the
5 minimum recommended radius to diameter
ratio which equals approximately 4.5. For
4 practical purposes, bending radii are seldom
3
expressed in terms of fractional numbers,
but rather in terms of whole integers
2 multiplied by the nominal pipe size. Hence,
the recommended bending radius would be
1
5 x 12 = 60".
0
Example B: Determine the minimum
0 10 20 30 40 50 60 70
permissible wall thickness required
for induction bending 22" O.D.
FIGURE 6.5.1 carbon steel pipe per ASTM A 53-
FURNACE BENDING RANGE Grade B at a 3D bend radius.
(1) Enter 3 on the R/Dn axis of figure 6.5.2 and
7
move horizontally to the intersection with
the bending range boundary.
6
(2) Then move verticality to determine the
5 minimum recommended diameter to wall
thickness ratio which equals approximately
4 45. i.e. Dn/tn = 45 or tn = Dn/45 = 22/45 =
.489".
3
6.7 Figure 6.5.1 is based on extensive experience in
furnace bending carbon and low alloy steel pipe.
2 Since stainless and non-ferrous materials have
higher coefficients of expansion than carbon and
1 low alloy steels, a greater reduction in the density
of the sand fill occurs as these materials are being
0
heated to the bending temperature. As a result,
0 25 50 75 100 125 150 the sand fill does not provide the same rigidity
against flattening and buckling as it does when
FIGURE 6.5.2
carbon or low alloy steel pipe is being bent.
INDUCTION AND INCREMENTAL BENDING RANGE Because of this fundamental difference, special
consideration must be given to the selection of
6.6 Two examples are given for the determination of the minimum bending radius by the design
minimum recommended wall thickness and engineer.
bending radius combinations for a given pipe
size. 6.8 Cold bending ranges can vary significantly with
the process and degree of specialized tool used.
Example A: Determine minimum permissible Figure 6.8.1 can be used to select the type of bend
bending radius required for furnace or the process required.
bending 12"-Extra Strong carbon
steel pipe per ASTM A 106-Grade
B.
(1) Determine the diameter to wall thickness
ratio. 12" X-Stg. is 12.75" O.D. with a .500"
nominal wall. Under ASTM A 106, the
minimum wall is .438". Therefore Dn/T =
12.75/.438 = 29.l.

Pi p e F a b r i c a t i o n I n s t i t u t e L i c e n s e e = ANCAP 5 9 4 6 2 4 1
No t f o r Re s a l e , 2 0 1 4 / 7 / 8 1 4 : 4 8 : 0 0
No r e p r o d u c t i o n o r n e t wo r k i n g p e r mi t t e d wi t h o u t l i c e n s e f r o m I HS
 PFI Standard ES-24
(Reaffirmed - September 2010)

12 The formula used to calculate the third column is:

11 AW L
tm
(2nd column value) – ( 1
ND

10 DR DRE ),
BE

9 RY N t
LL

TA MA
RO

8 RO ITH RE
L where t =Nominal pipe wall thickness minus
D
AN
&

W
7 LM manufacturing tolerance, and tm is derived from
M

AL
I-B
RA

6 ULT
WIT
HM
BALL
equation (3) of B31.1, par. 104.1.2. The most
5 AW H M ULTI-
YD
R
RAW
WIT
DIE conservative values over that range of pipe sizes
4 TAR RY D IPER
RO ROTA L AND W and wall thicknesses where used.
3 R E
MAND
2
1 TABLE 7.2.1
0 FURNACE BENDING
0 25 50 75 100 125 150
Bend Minimum Thickness Factored
FIGURE 6.8.1 Radius Prior to Bending Thickness
COLD BENDING RANGE
6D 1.06 tm 1.03 tm
7. Material Allowances 5D 1.08 tm 1.04 tm
4D 1.14 tm 1.09 tm
7.1 The following recommendations for material 3D 1.25 tm 1.19 tm
allowances will vary with the bending method,
material, pipe size, bend radius and the bend TABLE 7.2.2
fabricator's equipment. It is suggested that the INDUCTION AND INCREMENTAL BENDING
purchaser contact the fabricator for specific
information. Bend Minimum Thickness Factored
Radius Prior to Bending Thickness
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7.2 To compensate for wall thinning from bending, the

second column, (“Minimum Thickness Prior to 6Dn 1.06 tm 1.03 tm
Bending”), in Tables 7.2.1, 7.2.2, 7.2.3, and 7.2.4 can 5Dn 1.08 tm 1.04 tm
be used as a guide for ordering pipe for the appropriate 4Dn 1.10 tm 1.05 tm
process. ASME B31.1 Power Piping, equations 3Dn 1.14 tm 1.08 tm
102.4.5(3B, 3C & 3E), and B31.3 Process Piping, 2Dn 1.22 tm 1.13 tm
equation 304.2.1(3c & 3e) define a minimum thickness 1.5Dn 1.30 tm 1.19 tm
at the extrados of the bend that is less than the
minimum thickness required for straight pipe. TABLE 7.2.3
Conversely, the minimum thickness for the intrados of ROTARY DRAW BENDING
the bend needs to be proportionally thicker than that
required for straight pipe. The third column, (“Factored Bend Minimum Thickness Factored
Thickness”), of these tables lists the minimum required Radius Prior to Bending Thickness
thickness prior to bending factored for the allowable
thinning at the extrados of the bend. Experience 6Dn 1.09 tm 1.06 tm
indicates that a minimum thickness of straight pipe 5Dn 1.14 tm 1.10 tm
shown in tables should be sufficient to meet the 4Dn 1.20 tm 1.15 tm
minimum thickness requirements after bending. 3Dn 1.28 tm 1.22 tm
Interpolation is permissible for bending to intermediate
radii. TABLE 7.2.4
RAM AND ROLL BENDING
The code formula for calculating the minimum
thickness required for the bend extrados, as defined in Bend Minimum Thickness Factored
B31.1 2007 is: Radius Prior to Bending Thickness
R
4 1
PDo 6Dn 1.08 tm 1.05 tm
tm A where I Do
SE R 5Dn 1.10 tm 1.06 tm
2 Py 4 2 4Dn 1.13 tm 1.08 tm
I Do
3Dn 1.17 tm 1.11 tm

Pi p e F a b r i c a t i o n I n s t i t u t e L i c e n s e e = ANCAP 5 9 4 6 2 4 1
No t f o r Re s a l e , 2 0 1 4 / 7 / 8 1 4 : 4 8 : 0 0
No r e p r o d u c t i o n o r n e t wo r k i n g p e r mi t t e d wi t h o u t l i c e n s e f r o m I HS
 PFI Standard ES-24
(Reaffirmed - September 2010)

7.4 Because induction heated bends are produced

7.3 Since the start of the bend is somewhat dependent on under pressure, a compression of the material takes
factors not completely within the control of the bender, place which results in more footage of straight pipe
some additional length must be added to insure that being required than that determined from the
overall center-to-end dimensions can be met. To developed length. To provide for this compression
compensate for this factor, the purchaser should add 6 the purchaser of induction bends should add 5% to
inches to the total of the required straight tangent the theoretical arc length. The compression of the
lengths. material may leave a hump, or bump, at the
beginning of the bend on the inside radius which is
not detrimental to the bend.

Nominal pipe size 1" thru 2" 2½" thru 3½" 4" thru 48"
Pulling End 6" 8" 2D (10" min)
Holding End 6" 8" 1.5D (10" min)
Tangent Between Bends (Tm) 6" 9" 1D (12" min)*
*Note: 9" min for 4" pipe. 10" min for 5" pipe.

TABLE 7.5.1
MINIMUM TANGENT LENGTHS FOR FURNACE BENDS

7.5 In order to produce a bend properly, minimum

straight tangents are required on both ends of the VARIES
arc for holding and/or pulling purposes.
Characteristic minimum tangent lengths for the

R
various processes are given in Tables 7.5.1, 7.5.2,
and 7.5.3. Definition of these tangent lengths can
R

be seen in Figure 7.5.4. Minimum tangent lengths

required for incremental bending should be TM
VARIES
obtained from the fabricator. Longer tangents
should be utilized wherever possible and should
be considered in the piping design. In cases
where tangents must be kept short, it should be FIGURE 7.5.4
realized that an out-of-round condition may exist TANGENT LENGTHS
at the pipe ends.
8. Material Requirements for Hot Bending
Nominal Pipe Size 2" thru 24" 26" thru 66"
8.1 Pipe to be bent should be protected from the
Front Tangent OD+ 6" OD+ 6"
Rear Tangent 72" 132"
contamination of harmful materials such as the
Tangent Between Bends (Tm) 24" 60" low melting temperature alloys of copper, brass
and lead.
TABLE 7.5.2 8.2 Stainless steel pipe suspected of being
MINIMUM TANGENT LENGTHS contaminated with ferrous metals or other
FOR INDUCTION BENDING harmful materials should be cleaned prior to
bending to remove the contaminants.
Nominal Pipe Size 1" thru 10"
Front Tangent 2XD 8.3 Stainless steel pipe should be furnished in the
Rear Tangent 3XD solution annealed condition.
Tangent Between Bends (Tm) 2XD

TABLE 7.5.3
MINIMUM TANGENT LENGTHS FOR COLD BENDING

--`,``,`,`,```,```,,,,,,`-`-``,```,,,`---

Pi p e F a b r i c a t i o n I n s t i t u t e L i c e n s e e = ANCAP 5 9 4 6 2 4 1
No t f o r Re s a l e , 2 0 1 4 / 7 / 8 1 4 : 4 8 : 0 0
No r e p r o d u c t i o n o r n e t wo r k i n g p e r mi t t e d wi t h o u t l i c e n s e f r o m I HS
 PFI Standard ES-24
(Reaffirmed - September 2010)

9. Ordering Information 9.3 Suggested Bending Tolerances:

9.1 The Pipe Bend Order Form, (Form 9.1), provides Radius of Bend --------------- 1% of Dim “C”
the recommended information required for the Degree of Bend --------------- 0.5 Degree
purchase pipe bends. Plane of Bend ----------------- 1.0 Degree
9.2 The applicability and acceptance criteria of each Flat Plane of Bend ------------ 1% of Dim “C”
field, as defined by the Pipe Bend Order Form, Linear Dimensions ----------- 1/8”, 12” & under
shall be determined by the applicable codes, 3/16”, over 12”
specifications and fabrication requirements. Ovality in Bent Area --------- 8% of Diameter
after Bend
Ovality at End Prep. ---------- ASME B16.9

9.4 The miscellaneous remarks area and the space

provided at the bottom of the pipe bend order
form can be used to address additional
requirements such as, Piece Marking, Centerline
Scribing, Bend Heat Treatment, Butt Welds in
Lineal Bends Area, etc.
Dimension
Lineal Tangent
Dimension Length

Radius of
Bend (Dim C) Plane of Bend
(Angle between
Two Bend Planes)

Plan View End View

Ovality Tolerence :
Minor The difference
Dia. between the major
and minor axis (percent)
Tangent Line

Flat Plane
Nom. Major
of Bend
Dia. Dia.

Degree of
Bend
Ovality
(2 x size)
Tangent Line
Elevation
Figure 9.3
Tolerences

Pi p e F a b r i c a t i o n I n s t i t u t e L i c e n s e e = ANCAP 5 9 4 6 2 4 1
No t f o r Re s a l e , 2 0 1 4 / 7 / 8 1 4 : 4 8 : 0 0
No r e p r o d u c t i o n o r n e t wo r k i n g p e r mi t t e d wi t h o u t l i c e n s e f r o m I HS
 PFI Standard ES-24
(Reaffirmed - September 2010)

PIPE BEND ORDER FORM

FORM 9.1

Pi p e F a b r i c a t i o n I n s t i t u t e L i c e n s e e = ANCAP 5 9 4 6 2 4 1
No t f o r Re s a l e , 2 0 1 4 / 7 / 8 1 4 : 4 8 : 0 0
No r e p r o d u c t i o n o r n e t wo r k i n g p e r mi t t e d wi t h o u t l i c e n s e f r o m I HS