Understanding the term expansion & contraction
When a body is heated it will expand & when it cooled it contracts. So body
expands & deforms when heated & cooled. Change in temperature of a free
body causes body to expand & contract without inducing stress. When the
deformation of the body is restricted by means of any external force, there
will be huge chances of stress induction. Such induced stresses are called
temperature stresses. These may be tensile or compressive in nature.
Here L =
Original length of the steel bar
∆t =
Change in metal temperature deg C
ɑ =
Coefficient of thermal expansion
Temperature
strain e = Free deformation / Original length = ∆L / L
E = L ɑ
∆t / L = ɑ ∆t
Temperature
stress σ= Young’s modulus X Strain = E X e
Temperature
stress σ = E ɑ ∆t
Stress
induced in a rigid or constrained body
P =
Force exerted by a rigid support of constraint
We have
σ = P / A
P = E ɑ
∆t A
Expansion in Boilers
Boiler
is made up of plates, tubes, pipes and simple steel of various grades depending
upon the duty conditions. Depending on the service such as cold air/hot
air/cold flue gas/hot flue gas/cold water/hot water/saturated steam/super heated
steam, thermal expansion movement of steel materials takes place to different
extent in Boiler. Ignorance of thermal expansion movement of boiler components
in design/installation may lead to failure of boiler components. The damage to
boiler components can be costly affecting human life in some cases.
There are two types of expansions in Boilers
Absolute
Expansion: Boiler whole mass expands.
Differential Expansion: Individual parts expansion. There are places where there is a
relative expansion movement, which can cause stress in those parts.
In boilers expansion pointers are attached to
all the pressure parts header to under stand the direction & value of
expansion.
Expansion
pointers are used for verifying the expansion movement of the boiler. These are
attached to the drum ends/bottom or top header ends. When the boiler is under
commissioning stage the expansion must be monitored. Depending on the anchor
points in X-axis and Y-axis, the expansion is predicted by designers. The same
is counter checked at site. Deviations in the form of non-uniform expansion
should be checked.
Modern
boilers expand towards bottom, during start ups & shutdown it is very
important to observe the Boiler expansion.
Expansion
of the metals depends on;
- Change in metal temperature
- Length & area of the materials
- Coefficient of expansion of the materials
- Boilers all headers bottom space should be free from obstacles
- For air & flue gas ducts expansion bellows may be of metal or fabric should be provided
- Expansion loops for all steam lines should be provided
- Spring supports & hangers for steam line & hot water lines
- Rocker washers for steam drum & super heater headers supports
- Provision of stay bolts
Precaution to be
taken for free expansion:
- After shutdown, ensure all foreign material from Boilers & ductings have been removed completely
- Ensure all temporary supports & platforms have been removed from boilers & ducting
- Ensure there is no welding between any pressure part & hot ducts with platform or other or beams which can restrict the expansion
- Ensure enough space is available for all bottom headers for free expansion
- Ensure steam line drains line are free to expand with steam lines
- Ensure there are no any uneven expansions
Examples:
A square rod of size 20mm X 20 mm in cross section &
2000 mm in length is allowed to expand by fixing both of its ends. Determine
the force developed if the rod is heated from 25 degree c to 150 deg C.
L = 2000 mm
A =20 X 20 = 400 mm2
E = 2 X 105 Mpa
ɑ = 12 X 10-6 0C
∆t = 150-25
= 125 0C
We
have P = σA
P = E ɑ
∆t A
P =2
X 105 X 12 X 10-6 0C X 125 X 400 = 30000 N
Force developed at the end P = 30 KN
Force developed at the end P = 30 KN
What is the expansion of Boiler
side water wall panel , whose total length is 18 meters & metal
temperature is 295 deg c. Consider atmosphere temperature 30 deg c & ɑ = 12 X 10-6 0C
Expansion of side water wall panel ∆L
=L ɑ ∆t = 18 X = 12 X 10-6 X (295-30) =0.068mm = 68mm
- Damages to the restricted part
- Damage to the refractory & sealing
- Leakages
- Secondary failure
- Tube/pipe puncture
- Explosions
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