Casing Design & Construction
Containing Steam Around a Spinning Rotor
Where Module 2.1 covered the rotating rotor, this module covers the stationary structure that surrounds it: the casing. The casing contains steam pressure, holds the stationary nozzle diaphragms in fixed position relative to the rotor, and must do all of this while accommodating significant thermal expansion during every startup and shutdown — all without compromising the tight internal clearances the turbine depends on.
Horizontally Split Construction
Nearly all large steam turbine casings are horizontally split — built as an upper half and a lower half joined along a precision-machined horizontal joint, rather than as one continuous enclosed shell. The lower half stays in place, supporting the rotor's bearing pedestals and typically carrying steam inlet and exhaust connections. The upper half is designed to be lifted off as a complete unit, exposing the entire rotor and internal steam path for inspection or blade work.
This design exists entirely for maintainability: a horizontally split casing lets technicians access and inspect every stage of blading without disturbing the lower half's rotor support or precise alignment. The horizontal joint itself is a precision-fitted interface, closed with a large number of fitted (often dowelled) bolts tightened in a specific sequence and to specific tension values — uneven or incorrect bolt-up can distort the casing enough to shift internal clearances, which is why joint reassembly follows a controlled procedure rather than simple "tighten until snug" bolting.
Horizontally split construction trades a small increase in design complexity (a precision-fitted joint that must seal and hold alignment) for a large practical benefit: the ability to inspect and maintain internal components without a full teardown of the rotor support structure.
Double-Wall (Barrel) Construction for HP Casings
Some HP casings, particularly on high-pressure units, use a double-wall (barrel) design: an inner casing directly contains steam pressure and temperature, surrounded by an outer casing that shares structural containment duty. Splitting containment between two walls reduces the thickness — and therefore the thermal mass and temperature gradient — either wall needs on its own compared to a single-wall design rated for the same pressure. Thinner walls heat and cool more uniformly, directly reducing the thermal stress concerns covered in Module 1.5.
Centerline Support and Thermal Growth
Casings are mounted on support feet or keys positioned at the casing's horizontal centerline — the same elevation as the rotor's centerline. This positioning is deliberate: as the casing heats and thermally expands during startup, centerline support allows that expansion to occur symmetrically outward from the centerline, keeping the casing (and therefore the internal clearances it defines) concentric with the rotor throughout the transient. Support positioned anywhere else would allow the casing to grow asymmetrically, progressively shifting clearances as temperature rises.
Casing distortion — whether from uneven bolt-up, support problems, or thermal gradients — doesn't announce itself directly. It shows up indirectly, as rubs, unusual vibration, or clearance problems that trace back to the casing not maintaining its designed shape and position relative to the rotor.
Lagging, Insulation, and Material Selection
Casings are wrapped in insulation (lagging) for personnel protection, heat loss reduction, and — often overlooked — more uniform casing metal temperature, which reduces gradient-driven distortion. Material selection follows the same section-specific logic established for rotors in Module 2.1: HP and IP casings, exposed to the highest steam temperatures, typically use chrome-moly alloy steel selected for high-temperature strength and creep resistance, while LP casings operating at much lower temperature can use simpler carbon steel construction.