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HP/IP/LP cylinders of the steam turbine are core pressure-bearing housing components in the steam turbine system, which are respectively used in the steam expansion process in the high-pressure section, intermediate-pressure section, and low-pressure section.
Their main function is to form a stable steam flow channel, withstand high-temperature and high-pressure steam loads, and ensure precise fit and long-term stable operation between the rotor system and stator components.
According to different pressure grades and unit capacity requirements, the structural form, thickness design, and manufacturing techniques of the cylinder block will be specially optimized to meet the requirements for strength and rigidity under different working conditions.
Our HP, IP, and LP casings for steam turbines are engineered to safely contain and direct high-temperature, high-pressure steam flows. Material selection follows a graded approach based on service conditions—chrome-moly or chrome-moly-vanadium alloy steels for HP and IP sections, and carbon or low-alloy steels for LP sections. This material strategy provides resistance to creep, thermal fatigue, and sustained pressure loads under continuous high-demand operation, protecting internal components from steam leakage and mechanical degradation.
The casing design incorporates double-shell or multi-cylinder configurations with strategically positioned steam inlets, extraction ports, and support structures. This arrangement accommodates thermal expansion during startup, load variation, and shutdown, reducing thermal stress and distortion. Wall thickness transitions and flange geometries are guided by finite element analysis to promote uniform temperature distribution, avoid localized overheating, and preserve dimensional stability over the equipment lifecycle.
Each casing is machined on large-scale CNC boring and milling equipment to maintain tight tolerances on joint surfaces, diaphragm grooves, and gland seal fits. Horizontal joint flanges are hand-scraped to achieve metal-to-metal contact, while split surfaces are designed for robust bolting and sealing. This approach minimizes inter-stage steam leakage, supports turbine efficiency, and facilitates reliable reassembly during maintenance intervals.
| Parameter Item | HP Casing | IP Casing | LP Casing | Remarks |
| Steam Conditions | High pressure & temperature | Reheat intermediate | Low pressure / Vacuum | Designed per turbine class |
| Design Pressure | ≥ 12 MPa - 30 MPa+ | ≥ 4 MPa - 8 MPa | ≤ 1 MPa / Vacuum | Specification per drawing |
| Construction Type | Single/Double shell | Double shell | Double/Single flow welded | Based on capacity |
| Main Connections | Main steam / HP exhaust | Reheat steam / IP exhaust | Exhaust / Extraction | Custom per heat balance |
| Machining Equipment | Large CNC boring/milling | Large CNC boring/milling | Floor boring / Positioner | Ensures tolerances |
| Sealing Type | Flange bolting / Horizontal joint | Flange bolting / Horizontal joint | Flange / Welded seal | Leakage prevention |
| Quality Standard | JB/T 9632 / HEI / Customer spec | JB/T 9632 / HEI / Customer spec | JB/T 9632 / HEI / Customer spec | Includes NDT |
| Inspection Items | Dimension/Material/NDT/Hydrotest | Dimension/Material/NDT/Hydrotest | Dimension/Material/NDT/Vacuum test | NDT per code |
HP/IP/LP casings are usually made of high-quality alloy steel or high-strength carbon steel materials, for example:
· Low-alloy high-strength steel
· High-temperature resistant alloy steel
· Thick-plate welded structural steel
These materials have the following features:
· Good high-temperature strength performance: They can maintain stable mechanical properties under high-temperature steam environments.
· Excellent creep resistance: It is not easy for plastic deformation to occur under long-term high-temperature conditions.
· Good welding performance: Widely used in multi-layer and multi-pass welding of large thick-walled structural components.
· Higher fatigue resistance: They can withstand periodic thermal stress and mechanical stress changes.
All raw materials are subject to incoming inspection in accordance with relevant standards to ensure that their chemical composition and mechanical properties meet the design requirements.
The reheat casing of a steam turbine is a large thick-walled welded structural component with high machining accuracy requirements and a complex manufacturing process. The main process flow is as follows:
· Material blanking and pretreatment: According to the drawing requirements, steel plates or forgings are cut. Ultrasonic Testing (UT) is then carried out, and preheating treatment is performed when necessary.
· Welding: The HP/IP/LP turbine shell adopts a segmented assembly and welding structure. Multi-layer and multi-pass welding technology is used during welding to control the interlayer temperature and reduce welding deformation. After welding, NDT will be conducted, and heat treatment will be carried out as required to eliminate residual stress.
This process is the core quality control stage, which directly affects the overall strength and sealing performance of the outer casing.
Welding
Welding
· Rough turning: Rough turning of the outer circle, end face, and mating surfaces.
· Semi-finish turning: Process the internal flow channel profile, flange surface, and locating surfaces.
· Finish turning: Precisely machine the mating surfaces of the upper and lower cylinders to ensure flatness. Finish boring of the inner hole is carried out to control coaxiality.
Turning
Milling
To ensure transportation and storage safety, the cylinder adopts a hierarchical protective packaging method:
· Inner protection: Apply anti-rust oil and cover key machined surfaces with anti-rust paper or moisture-proof materials.
· Cushioning and fixation: Use wooden supports or special support structures to secure the product, preventing deformation or collision during transportation.
· Outer packaging: Pack the cylinder in customized wooden cases or steel frames, suitable for sea transport and long-distance transportation.
We provide completely customizable packaging options tailored to your specific needs. Every aspect, including the level of protection, cushioning techniques, fixation methods, and the design of the outer case, can be adjusted to ensure suitability for various transport conditions and project demands.
