Thursday 10 March 2011

INSULATION


Thermal insulation is necessary in order to reduce valuable heat losses. Proper application of insulation show effective operation of process equipment and conserves valuable heat

Selection of insulation

There are only few standardized test procedures for comparing insulating materials. Only one method, based on determining insulation thermal conductivity, has been recommended by American Society for Testing Materials(ASTM)
The method requires skilled manipulation in order to obtain reproducible results. In the selection of insulation not only precise laboratory measurement but also it is necessary to base upon previous experience. The following factors must be investigated to evaluate insulating material. The effect of moisture and changes in temperature on the properties must be carefully considered

Physical Properties

a)Thermal conductivity
b)Specific heat
c)Co-efficient of expansion
d)Compression tensile and impact strength.
e)Hardness
f)Fatigue resistance
g)Fire resistance

Chemical Properties

a)Analysis
b)Corrosive nature
c)Effect of chemicals in pipe or vessel being considered.

Insulation Characteristics

a)Toxicity
b)Action on skin
c)Overall workability

Low temperature Insulation 

The principal difference in the erection of low temperature insulation is to provide some means of preventing the water vapours entering the insulation. Since the inner surface of the insulation is at lower temperature than the outer surface, the flow of water vapours is towards the inside, where it freezes and causes a marked increase in the conductivity of insulation and also causes the deterioration. A vapour barrier can be installed to minimize this movement of water vapours. Materials which serve as vapour barriers include vapour proofing felt, which is finished with a fabricated asphalt sealing compound followed by a water-proof jacket. Cork insulation is of tin finished with several coasts of asphalt followed by roofing paper which is applied with additional amounts of asphalt as a final layer over the paper

TYPES OF LOW TEMPERATURE INSULATION

 Cellular glass

Insulating material, made of thousands of glass bubbles moulded into a rigid structure of extremely light weight serves as low temperature insulation since it cannot absorb water. It is suitable for temperatures from –350 to 800F, but it requires protection against abrasion.

Mineral Cork

Mineral wool fibers with asphalt binder are used for low temperature insulation ranging from below -150F to 150F because of its following characteristics
a)       Good resistance to water.
b)      Moderate resistance to vapours.
c)       Moderate compressive and tensile strength.
d)      Easily fitted at atmospheric temperature.
e)       Flexibility

Polystyrene (Expanded)

Styrene, polymerized and molded, include many bubbles of air, forms a light and serviceable low temperature (-200 to 200f). It is used because of its following notable characteristics.
a)       Moderate compressive and fair tensile strength.
b)      Flexibility
c)       It can withstand considerable expansion and contraction.
d)      Excellent resistance to vapors
e)       Excellent resistance to water.

On the basis of above discussion the following insulations have been recommended

For De-methanizer, and E/E Splitter

Mineral Cork with following characteristics.
Temperature limits (oF)            min.-150 to max 150.
Conductivity (Btu/hr(sq.ft.)(F/in)0.305-0.325.
Density (lb mass/cu.ft) ---15.

For De-ethanizer

Polystyrene (expanded with following properties)
Temperature limits oF…         min.-200 to max.200
Conductivity… Btu/(hr)(sq.ft)(Lin.F/in)0.23-0.4

Density lbm/ft3…2.


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