Quantcast Section 2. Thermal Expansion of Pipes

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Section 2.
THERMAL EXPANSION OF PIPES
1 OVERVIEW.  T h e length of a pipe increases as temperature rises; for
example, when a piping system is taken to operating temperatures from a cold
c o n d i t i o n .  Table 7-3 shows the expansion of steel, wrought iron, and copper
p i p e per 100 feet of length for different temperature changes. From the
t a b l e , it may be noted that a steel pipe of 100 feet in length expands 2.88
inches when the temperature changes from 0F to 360F (a temperature change of
360F) .  I f this movement is restricted in any manner, for example, by pipe
a n c h o r s , equipment connections, or Inflexible changes of direction, the full
s t r e n g t h of the expanding pipe will be exerted against the restriction. In
t h e case cited, this force, for a 12-inch pipe, would equal 232,000 pounds.
I f not properly relieved, such expansion can damage or cause failure of pipe-
lines and equipment.  T h e elongation of a lengthy heating line is conveniently
handled by expansion loops and bends, and expansion joints. Where possible,
piping systems should be designed to provide for expansion of branch lines
i n s i d e buildings to prevent any effect on mains.
2 EXPANSION LOOPS AND BENDS.  W h e n e v e r possible, it is better to provide for
p i p e expansion by changing the direction of pipe runs, or by using expansion
bends or loops.  Expansion loops are factory fabricated and may be furnished
i n sections to facilitate handling and delivery. They absorb the pipe
e x p a n s i o n by introducing U-type or Z-type loops in the pipeline, and they do
n o t require maintenance.
3 EXPANSION JOINTS.  Expansion joints are commonly used where space
r e s t r i c t i o n s prevent the use of expansion loops and bends.  E x p a n s i o n joints
m u s t be installed only in accessible locations to facilitate maintenance.  The
f o l l o w i n g are examples of the usual types of expansion joints:
q The mechanical slip joint
q The
bellows
type
joint
q The
flexible
ball
joint
3 . 1 M e c h a n i c a l S l i p J o i n t .  M e c h a n i c a l slip joints consist of a female member
w h i c h slides over a male member, while the joint is kept tight by means of
p a c k i n g .  T h e packing may be the semiplastic type, injected into the packing
s p a c e by means of a built-in packing gun (plunger and cylinder assembly); or
t h e c o i l t y p e , i n s t a l l e d in a conventional stuffing box.  M e c h a n i c a l slip
joints require periodical lubrication, usually supplied through special
l u b r i c a t i o n f i t t i n g s .  When used, maximum traverse of piping should be limited
t o less than 8 inches.  F i g u r e 7 - 1 I l l u s t r a t e s a m e c h a n i c a l s l i p j o i n t t h a t
u s e s semiplastic packing.
3 . 2 Bellows Type Joint.  T h e b e l l o w s t y p e j o i n t , i l l u s t r a t e d i n f i g u r e 7 - 2 ,
a b s o r b s pipe expansion and contraction by the flexing of a metal bellows.  The
j o i n t consists of a corrugated thin-walled tube of an appropriate metal
( g e n e r a l l y stainless steel) clamped between flanges. The pipes should be
supported and guided in such a way that misalignment is reduced to a minimum.
A l s o , external tie rods with limit stops are usually provided to protect the
j o i n t a g a i n s t o v e r t r a v e l .  S o m e types of joints have rings to help the
c o r r u g a t i o n s in resisting the fluid pressure.  B e l l o w s or corrugations for
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