14 Precision 316L/S30408 Ozone Generator Pressure Vessels for Mitsubishi Electric: ASME U + GB Multi-Code Fabrication for Water Treatment Applications

Lmart has manufactured 14 ozone generator pressure vessels for Mitsubishi Electric Corporation (三菱電機) across multiple projects, including the Oji Paper water treatment installation. The vessels — models OG-760N, OG-1040N, SZN-S6670, SZN-S7167, and SZN-S7570 — are built in SA240 316L, S30408, and S31603 to both ASME VIII Div.1 (U-stamp) and Chinese GB standards, with wall thicknesses as thin as 3.4 mm and diameters ranging from DN168 to DN1200.

Customer	Mitsubishi Electric Corporation — 三菱電機株式会社
Industry	Water treatment — ozone generation for municipal and industrial applications
Country	Japan (OEM) — systems installed domestically and internationally
Equipment	OG-760N × 6, OG-1040N × 4, SZN-S6670 × 2, SZN-S7167 × 1, SZN-S7570 × 1 (14 units total)
Design Codes	ASME VIII Div.1 (U-stamp) and GB/T 150
Key Materials	SA240 316L (ASME), S30408 (GB), S31603 (GB — equivalent to 316L)
End User (notable)	Oji Paper Co., Ltd. (王子製紙 — OG-1040N, 4 units)
Service	Ozone generation — pressure vessels housing high-voltage electrode assemblies

Customer Background and Industry Context

Mitsubishi Electric Corporation (三菱電機株式会社), headquartered in Tokyo, is one of the world’s largest electronics and electrical equipment manufacturers, with annual revenues exceeding ¥5 trillion and operations spanning power systems, industrial automation, transportation, building systems, and environmental solutions. Their ozone generation division designs and manufactures large-scale ozone systems for water and wastewater treatment, pulp bleaching, and industrial oxidation processes.

An ozone generator is fundamentally a pressure vessel that houses high-voltage electrode assemblies. Dried air or oxygen is fed into the vessel under pressure, passes through the gap between concentric electrodes where a high-voltage corona discharge converts O&sub2; to O&sub3;, and exits as ozone-enriched gas for injection into the water treatment process. The pressure vessel must maintain gas-tight integrity while providing precise dimensional tolerances for the electrode gap — typically measured in millimeters — and electrical isolation between the high-voltage electrodes and the grounded vessel shell.

The ozone environment is aggressively oxidizing. Ozone attacks carbon steel, copper, and most elastomers rapidly. Only specific grades of stainless steel (316L/316), titanium, and specialized polymers survive in concentrated ozone service. For this reason, every wetted surface of the pressure vessel must be 316L or equivalent, with interior surface finish controlled to prevent ozone decomposition at surface irregularities and to facilitate cleaning of any oxide deposits that form during operation.

Oji Paper Co., Ltd. (王子製紙), one of the project end users, is Japan’s largest paper manufacturer. Pulp and paper mills use ozone for ECF (elemental chlorine-free) bleaching — replacing chlorine-based chemicals with ozone to reduce chlorinated organic compound discharge. This application requires large ozone generating capacity, hence the four OG-1040N units (DN1035, 2,930 kg each) supplied for the Oji Paper project.

Challenges and Engineering Pain Points

Precision Tolerances for Electrode Assembly Housing

Unlike conventional pressure vessels where internal dimensions are governed by process flow requirements, ozone generator vessels must maintain precise cylindrical tolerances to support the concentric electrode assembly. The electrode gap — the annular space between the inner (high-voltage) electrode and the outer (ground) electrode — must be uniform around the circumference and along the length of the vessel to produce consistent ozone concentration across the discharge zone. Out-of-roundness in the vessel shell directly affects the electrode gap uniformity, which in turn affects ozone production efficiency and energy consumption. For the OG-1040N at DN1035 and 6 mm wall thickness, the shell roundness tolerance is significantly tighter than standard pressure vessel requirements.

Thin-Wall Stainless Construction Under Pressure

The ozone generators use remarkably thin walls for pressure vessel construction — 6 mm for the OG and SZN-S6670 models, 4 mm for the SZN-S7570, and just 3.4 mm for the SZN-S7167. These wall thicknesses reflect the relatively low operating pressures of ozone generators (typically 0.1–0.3 MPa gauge) but create fabrication challenges. Welding 3.4 mm 316L without burn-through, distortion, or excessive heat input that would sensitize the material requires controlled-parameter TIG welding with backing gas protection. Shell rolling at 3.4–6 mm thickness in stainless steel demands careful handling to avoid surface scratches, dents, or work-hardening that would compromise the precision cylindrical form.

Ozone-Compatible Interior Surface Finish

The interior surfaces of an ozone generator vessel must be free from contamination, grinding marks, weld spatter, and any surface condition that could promote ozone decomposition or trap residual cleaning agents. While ozone generators do not require mirror polishing to the Ra ≤ 0.4 μm standard used in pharmaceutical vessels, the interior surface must be consistently smooth and passivated. Any carbon steel contamination (from tools, grinding discs, or handling equipment used on carbon steel in the same workshop) will cause localized corrosion in the ozone environment and contaminate the ozone product gas. This requires strict material segregation protocols during fabrication — dedicated stainless steel tooling, separate grinding and polishing consumables, and clean-room-like handling practices.

Dual-Code Certification: ASME U-Stamp + GB

Mitsubishi Electric supplies ozone generators to both international (ASME-code) and Chinese domestic (GB-code) markets. The 14 vessels span both code systems: SA240 316L for ASME U-stamp vessels and S30408/S31603 for GB vessels. While the material grades are functionally equivalent (316L ≅ S31603, 304 ≅ S30408), the documentation, marking, testing, and third-party inspection requirements differ between codes. ASME vessels require Authorized Inspector involvement, U-1 data reports, and ASME-specific material certifications; GB vessels require Chinese regulatory documentation and local inspection authority sign-off. Managing both systems for the same vessel design demands dual-track quality management.

Why Mitsubishi Electric Selected Lmart

Precision thin-wall stainless fabrication. Ozone generator vessels demand fabrication precision beyond typical pressure vessel work — thin walls (3.4–6 mm), tight roundness tolerances, and clean stainless steel handling throughout. Lmart’s experience with thin-gauge stainless vessels and interior surface quality control meets the dimensional and cleanliness standards that ozone electrode assembly requires.

Dual ASME/GB certification. Mitsubishi Electric’s ozone generators are installed in both ASME-code and GB-code jurisdictions. Lmart’s concurrent ASME U-stamp and Chinese GB manufacturing licenses allow production of both code variants from the same workshop, using the same WPS/PQR base and fabrication procedures with code-specific documentation overlays. This eliminates the need for two separate vessel vendors.

Repeat order reliability. The 14 units represent multiple orders across different models and time periods. Mitsubishi Electric’s continued selection of Lmart for successive ozone generator models indicates satisfaction with dimensional consistency, documentation quality, and delivery reliability across production batches.

316L material handling discipline. Ozone service requires verified absence of carbon steel contamination on all wetted surfaces. Lmart’s workshop maintains material segregation protocols for stainless steel fabrication — dedicated tools, separate work areas, and PMI verification — that prevent the cross-contamination issues common in mixed carbon steel / stainless steel production environments.

Engineering and Manufacturing Solution

The 14 ozone generator vessels span five models with different dimensions and code requirements, but share common fabrication principles driven by the ozone generation application.

OG-760N (6 units). The most frequently ordered model, with four units in S31603 (GB) and two in SA240 316L (ASME U). DN761, 6 mm wall, 2,555 mm length, 1,230 kg each. This is Mitsubishi Electric’s mid-range ozone generator suitable for municipal water treatment plants with moderate ozone demand. The relatively small diameter (761 mm) simplifies shell rolling precision but the 6 mm wall requires careful weld parameter control to avoid distortion that would affect electrode gap uniformity.

OG-1040N (4 units, Oji Paper). The largest model in this order, at DN1035, 6 mm wall, 2,700 mm length, and 2,930 kg in S30408 (GB). Four units were supplied for the Oji Paper pulp bleaching application. The larger diameter provides greater electrode surface area and correspondingly higher ozone output per unit. At DN1035 in 6 mm stainless, achieving the roundness tolerance required for electrode gap uniformity demands precision shell rolling with intermediate measurement and correction.

SZN-S6670 (2 units). The largest-diameter model at DN1200, 6 mm wall, 3,400 mm length, and 3,200 kg in SA240 316L (ASME U). These represent a high-capacity configuration — the DN1200 shell provides the largest electrode assembly envelope in this product range. At 1.2 meters diameter with only 6 mm wall, the diameter-to-thickness ratio exceeds 200:1, making this vessel susceptible to ovality under its own weight during horizontal handling and requiring internal stiffening or careful support during fabrication stages.

SZN-S7167 (1 unit). A compact model at DN168, 3.4 mm wall, 1,085 mm length, and just 146 kg in SA240 316L (ASME). This is the smallest vessel in the project — essentially a small-diameter tube section with dished heads, used as a pilot-scale or auxiliary ozone generator. Despite its small size, it carries the full ASME U-stamp documentation requirement, including AI inspection and U-1 data report.

SZN-S7570 (1 unit). A mid-size model at DN396, 4 mm wall, 1,950 mm length, and 385 kg in SA240 316L (ASME). The 4 mm wall thickness in 316L at this diameter represents a moderate fabrication challenge — thin enough to require controlled-parameter welding but thick enough to allow standard TIG procedures without special backing arrangements.

Complete Equipment Specifications

#	Model	Qty	Weight (kg)	DN (mm)	THK (mm)	Length (mm)	Standard	Material	Remarks
90	OG-760N	2	1,230	761	6	2,555	ASME U	SA240 316L
91	SZN-S6670	2	3,200	1,200	6	3,400	ASME U	SA240 316L
92	OG-1040N	4	2,930	1,035	6	2,700	GB 150	S30408	Oji Paper project
93	OG-760N	2	1,230	761	6	2,555	GB	S31603
94	SZN-S7167	1	146	168	3.4	1,085	ASME	SA240 316L
95	SZN-S7570	1	385	396	4	1,950	ASME	SA240 316L
96	OG-760N	2	1,230	761	6	2,555	GB	S31603

Project Execution, Quality Control, and Delivery

The ozone generator vessels required quality control procedures adapted to the unique demands of precision thin-wall stainless fabrication for electrode housing service.

Key QC milestones:

• Material procurement: SA240 316L (ASME) and S30408/S31603 (GB) plate sourced from Mitsubishi Electric-approved mills with full chemical and mechanical certification, including intergranular corrosion resistance testing per ASTM A262 (ASME) or GB/T 4334 (GB)
• Material segregation: all fabrication performed in dedicated stainless steel work zones with dedicated tools, grinding consumables, and handling equipment; carbon steel contact eliminated throughout the fabrication chain
• Shell rolling: precision rolling with intermediate roundness verification at ≥ 3 cross-sections per shell course; ovality controlled to Mitsubishi Electric’s electrode gap tolerance specification (tighter than standard GB/ASME requirements)
• Welding: TIG welding with argon backing gas on all interior welds; controlled heat input parameters documented per weld joint; weld bead profile inspected for smoothness and absence of undercut or convexity that would interfere with electrode installation
• NDE: 100% radiographic testing on all longitudinal and circumferential welds; DPT on all accessible surfaces including interior weld toes
• Interior surface preparation: mechanical polishing followed by acid pickling and passivation; surface verified for absence of carbon steel contamination by ferroxyl test; surface roughness measurement at representative locations
• Dimensional survey: internal diameter measured at multiple stations for roundness compliance; overall length, nozzle positions, and flange face dimensions verified against Mitsubishi Electric’s electrode assembly interface drawings
• Hydrostatic test per applicable code (ASME or GB); pneumatic leak test where specified for gas-tight integrity verification
• ASME U-stamp with AI witness for ASME-code units; GB regulatory documentation for domestic-code units
• Clean packaging: interior dried, preserved with dry nitrogen purge where specified, and sealed with vapor barrier protection for shipment

Completed vessels were shipped to Mitsubishi Electric’s assembly facility, where electrode assemblies, electrical connections, and control systems are integrated before the complete ozone generators are delivered to end-user sites.

Results and Business Impact

Fourteen ozone generator vessels across five models and two design codes, delivered across multiple order cycles. For Mitsubishi Electric, Lmart functions as a reliable pressure vessel fabrication partner for their ozone product line — providing the specialized thin-wall stainless capability and dual-code certification that ozone generator manufacturing demands.

The Oji Paper project (4 × OG-1040N) demonstrates Lmart’s involvement in significant end-user installations. Oji Paper is Japan’s largest paper manufacturer, and the four OG-1040N units represent a major ozone generation capacity installation for pulp bleaching — a technically demanding application where ozone quality and system reliability directly affect production output and environmental compliance.

Ozone generator vessels represent a niche but technically differentiated market segment for Lmart. The precision tolerances, thin-wall construction, and strict material cleanliness requirements set these vessels apart from standard industrial pressure vessel work. Successfully serving Mitsubishi Electric — a global technology leader with exacting quality standards — validates Lmart’s capability for precision stainless steel fabrication across a range of applications beyond conventional petrochemical and energy equipment.

Customer Voice

Mitsubishi Electric has placed multiple orders for ozone generator vessels across several model variants, reflecting confidence in Lmart’s ability to maintain the dimensional precision and material cleanliness that ozone generation equipment requires.

“Shell roundness has been consistently within our electrode gap specification across all batches. The stainless steel cleanliness protocol eliminates the contamination issues we experienced with previous vendors.”

— Mitsubishi Electric ozone division (paraphrased from vendor review)

Note: Lmart respects client confidentiality. Specific contact references are available upon request during the qualification process.