Materials for Pipes :
Following materials are commonly used in manufacturing pipes to convey steam.
Wrought iron: for low pressures
Mild steel: for high pressures
Cast iron : for steam pressures upto 1 bar
Malleable iron, Copper, Brass and Galvanised Iron
Pipe fittings may also be made of these materials. Chromium, Nickel and Molybdenum are used as alloying metals. They increase the strength and resistance to corrosion. Generally, cast iron and bronze are used for low and medium pressures and temperatures; alloy cast iron and malleable iron are used for medium pressures and temperatures; cast steel and alloy cast steel are used for fairly high pressures and temperatures and forged steel is used for very high pressures and temperatures.
Specifications of Pipes :
Wrought iron and Mild steel pipes are marketed in standard sizes. These sizes are specified as follows :
• Upto 300 mm nominal inside diameter, pipes are designated by their diameters. Pipes above 300 mm, they are specified by outside diameter.
• Thickness of extra heavy (duty) pipes is usually 40% greater than standard pipes. Double extra heavy pipes are usually 212 to 3 times thicker than standard pipes. These pipes may be reamed to give smooth inner surface.
Expansion Joints :
When steam flows through the pipes, forces are produced due to expansion of pipes. To avoid rupture, the pipes must be allowed to expand considerably. To prevent undue stresses in pipe fittings, suitable bends or expansion joints are used in pipe line.
Slip expansion joints are used for high pressure steam service where long radius bends cannot be provided due to limited space or specific position. When using these joints, the pipe must be securely anchored to prevent failure of joints due to high steam pressure and at the same time permit the pipe in expanding freely in a joint.
Pipe Insulation :
Heat losses from pipes carrying steam at high temperature can be minimised to a great extent by using a proper insulating material covering the pipes. Some of the good insulating materials are : 85% magnesia and 15% of asbestos fibre, sponge felt, silicol etc., Heat losses can be reduced by 75 to 90% using such insulation material.
A good insulating material should have
- low coefficient of thermal conductivity
- ability to withstand vibrations, shocks and rough handling
It should not be too heavy, and affected by atmospheric conditions. It should not damage the pipe material. It should be cheap and readily available.