using a protractor or compass for accuracy. Always
Therefore, a straight section of pipe 89 9/32 inches long,
draw your lines toward the inside of the bend. Measure
after proper bending, would fit the dimension AC of
a distance along line DE that is equal to the bend radius
(point E). At point E, draw a line parallel to line AB.
Repeat the same process for the other leg of your piping
Flange Bolthole Layout
section to get line FG, point G, and a line parallel to line
BC. The point where the two lines you have constructed
When you get ready to lay out boltholes in a flange,
cross is the center of the bending circle and should be
check your plan for the number of boltholes needed and
the diameter of the pitch circle. A pitch circle is the
2. Lay out the bend section by constructing lines
circumference of the circle that will connect the center
that are at right angles from lines AB and BC that cross
points of the boltholes. A pitch chord is the distance
from the center of one bolthole to the center of the next.
at point X. Label these points H and I. Now set your
compass to the bend radius and strike an arc that touches
Here are the steps that you should use to lay out the
points H and I on lines AB and BC. This arc is your bend
boltholes in any given flange:
1. Scribe the pitch circle using the radius given in
3. Use a protractor to determine angle X. Angle
X should be 40 degrees. Use the formula (bend section
2. Determine what the pitch chord is to be. This
= 2 × pi × radius × angle + 360) to determine the total
may be done geometrically or mathematically, or it may
length of the bend section. In this case, the bend section
be listed in the plan.
equals 2 × 3.1416 × 24 × 40 ÷ 360 or 16.76 inches.
3. Divide the circle into the desired number of
4. To find the TDL, you still need to know the
equal parts as called for in the plan.
length of the remaining straight sections. You divide
When the circle has been accurately divided, the
angle X (40 degrees) by 2 for an angle of 20 degrees.
points on the circumference of the circle will locate the
Now refer to table 16-1 to find the tangent for an angle
centers of the boltholes. You can locate the center of the
of 20 degrees or 0.3640. Multiply the tangent (0.3640)
boltholes with a pair of dividers by adjusting the points.
by the bend radius (24) and you get 8.74 inches or the
But this trial-and-error method is time consuming and
distance from BH and BI. Next subtract 8.74 inches
not very accurate. Use the proper geometric
from the distance of AB (54 inches) and from BC (36
construction described in the following paragraphs, and
inches) or 45.26 inches for AB and 27.26 for BC,
you can locate the bolthole centers more accurately and
in less time.
5. The final step is to add the length of the legs to
The following problems illustrate the constructions
the length of the bend section (45.26" + 27.26" + 16.76"
used to lay out flange boltholes.
= 89.28") to get the TDL in inches. At this point, you
may convert the decimal measurement into a fraction
PROBLEM I--Lay out a five-hole flange. (See
for easier use in measuring a section of pipe.
Figure 16-5.--Layout for a five-hole flange.