Turbine Casings

The casing shape and construction details depend on whether it is a High Pressure(HP) or Low Pressure (LP) casings. For low and moderate inlet steam pressure up to 120 bar, a single shell casing is used. With a rise in inlet pressure the casing thickness as to be increasing. Handling such heavy casing is very difficult also the turbine as to slowly brought up to the operation temperature. Otherwise undue internal stress or distortions to the thick casing may arise. To over this for high pressure and temperature application double casing is used. In the double casing inner casing is for High pressure and the outer casing is for hold the low pressure.

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Most of the turbine have casings with horizontal split type. Due to horizontal split it easy for assembling and dismantling for maintenance of turbine. Also, maintain proper axial and radial clearance between the rotor and stationary parts.

Usually, the turbine casings are heavy in order to withstand the high pressures and temperatures. It is general practice the thickness of walls and flanges decrease from the inlet to exhaust end due to the decrease in steam pressure from inlet to exhaust.

Large casings for low-pressure turbines are of welded plate construction, while smaller L. P. casings are of cast iron, which may be used for temperatures up to 230°C.

Casings for intermediate pressures are generally of cast carbon steel able to withstand up to 425°C. The high-temperature high-pressure casings for temperatures exceeding 550°C are of cast alloy steel such as 3 Cr 1Mo (3% Chromium + 1% Molybdenum.) The turbine casings are subjected to maximum temperatures and under constant pressure. Hence the material of casing shall subject high  “Creep”.to read more about material properties)

The casing joints are made of steam tight by matching the flange faces very exactly and very smoothly, without the use of gaskets. Dowel pins are used to secure exact alignment of the casing flange joints.

The casing contains grooves for fixing the diaphragms (for impulse turbines) or for the stationary blades (reaction turbines).

to read more about impulse and reaction turbine