Page 107 - Pipeline Rules of Thumb Handbook

94 Pipeline Rules of Thumb Handbook

Spacing of pipe supports

For chart on deflection of horizontal pipe lines, see next page

When a horizontal pipeline is supported at intermediate Example: What is the maximum distance between sup-
points, sagging of the pipe occurs between these supports, the ports for a 4≤ Standard pipeline assuming a pitch or average
amount of sag being dependent upon the weight of the pipe, gradient of 1≤ in 30 feet?
fluid, insulation, and valves or fittings which may be included Using the chart on the opposite page, find the point where
in the line. If the pipeline is installed with no downward pitch, the diagonal dotted line for an average gradient of 1≤ in
pockets will be formed in each span in which case condensa- 30 feet intersects the diagonal solid line for 4≤ pipe. From
tion may collect if the line is transporting steam. In order to this point, proceed downward to the bottom line where the
eliminate these pockets, the line must be pitched downward maximum span is noted to be approximately 22 feet.
so that the outlet of each span is lower than the maximum sag.
Crane has conducted tests to determine the deflection of Code for Pressure Piping: The Code for Pressure
horizontal standard pipelines filled with water, in pipe sizes Piping, ASA B 31.1, makes the following statements relative
3
/ 4 ≤ to 4≤ inclusive, the results of which have indicated that for to installations within the scope of the Code:
pipes larger than 2≤ and with supports having center to center
“605 (g) Supports shall be spaced so as to prevent excessive
dimensions greater than 10 feet, the resultant deflection is
sag, bending and shear stresses in the piping, with special
less than that determined by the use of the formula for a uni-
consideration given to those piping sections where flanges,
formly loaded pipe fixed at both ends. For pipe sizes 2≤ and
valves, etc., impose concentrated loads. Where calculations
smaller, the test deflection was in excess of that determined
are not made, suggested maximum spacing of hangers
by the formula for pipe having fixed ends and approached, for
or supports for carbon steel piping operating at 750°F
the shorter spans, the deflection as determined by the use of
and lower are given in Table 21a (see the table below).
the formula for pipelines having unrestrained ends.
“Where greater distance between supports, concentrated
Page 96 gives the deflection of horizontal standard pipe-
loads, higher temperatures, or vibration considerations are
lines filled with water, for varying spans, based upon the
involved, special consideration should be given to effects of
results obtained from tests for sizes 2≤ and smaller, and upon
bending and shear stresses.”
the formula for fixed ends for the larger sizes of pipe. The
“623 (b) The design and spacing of supports shall be checked
deflection values given on the chart are twice those obtained
to assure that the sum of the longitudinal stresses due to
from test or calculation, to compensate for any variables
weight, pressure, and other sustained external loading
including weight of insulation, etc.
does not exceed the allowable stress (S value) in the hot
The formula given below indicates the vertical distance that
condition.”
the span must be pitched so that the outlet is lower than the
maximum sag of the pipe. Suggested Maximum Spacing Between Pipe Supports For
Straight Runs of Standard Wall and Heavier Pipe (At
2
144 Sy
h = 2 2 Maximum Operating Temperature of 750F)
36 S - y
Nominal Maximum Nominal Maximum
where: Pipe Size Span Pipe Size Span
h = Difference in elevation of span ends, inches Inches Feet Inches Feet
S = Length of one span, feet
y = Deflection of one span, inches 1 7 8 19
1 9 10 22
2
By eliminating the inconsequential term “-y ” from the 1 / 2 10 12 23
2
denominator, the formula reduces to: 1
2 / 2 11 14 25
h = 4 y 3 12 16 27
1
3 / 2 13 18 28
The pitch of pipe spans, called the Average Gradient, is a ratio 4 14 20 30
between the drop in elevation and the length of the span. This 5 16 24 32
is expressed as so many inches in a certain number of feet. 6 17 .. ..
4y Notes: The values in the table do not apply where there are concen-
Average Gradient =
S trated loads between supports such as flanges, valves, etc.
The dotted lines as shown on the chart on the opposite page Spacing is based on a combined bending and shear stress of
are plotted from the above formula and indicate average gra- 1500psi when pipe is filled with water and the pitch of the line
dients of 1≤ in 10¢, 1≤ in 15¢, 1≤ in 20¢, 1≤ in 30¢, and 1≤ in 40¢. is such that a sag of 0.1 inch between supports is permissible.
Reprinted with permission—Crane Company