SECTION 3.0
Materials of Construction
Detailed MOC specifications for all plant sections with material selection rationale, applicable standards, and lining requirements for the 3,000 MTPD Sulphuric Acid Plant.
Material Selection Philosophy
Material selection for sulphuric acid plants is governed by the corrosion characteristics of H₂SO₄ at various concentrations and temperatures. Concentrated acid (93–98.5%) is handled in carbon steel due to the formation of a protective passive film. Dilute acid (<70%) is highly corrosive to CS and requires HDPE/FRP or alloy materials. High-temperature gas service uses alloy steels (Cr-Mo) for creep resistance. All materials comply with ASME, IS, and ASTM standards as applicable.
3.1 Material Summary by Type
| Material | Grade / Standard | Primary Usage | % by Weight |
|---|---|---|---|
| Carbon Steel (CS) | IS 2062 Gr.B / SA 516 Gr.70 / SA 106 Gr.B | Shells, vessels, piping, structural | 65% |
| Stainless Steel 304/321 | SA 240 / SA 213 / SA 312 | Converter internals, filter elements, HE tubes | 8% |
| Alloy Steel (Cr-Mo) | SA 213 T11/T22 | WHB tubes, superheater tubes, economizer tubes | 5% |
| SX Alloy / HiSiCr | ASTM A518 / proprietary | Acid pumps, tower distributors, nozzles | 3% |
| Acid-Resistant Bricks | IS 4860 / DIN 51071 | Furnace, towers, stack, pits lining | 12% |
| FRP / HDPE | IS 14402 / ASTM D3350 | Cooling tower, dilute acid piping, drains | 4% |
| Refractory / Insulation | IS 8 / IS 2042 | Furnace, gas ducts, converter insulation | 3% |
3.2 Sulphur Handling & Melting
| Equipment | MOC | Selection Rationale | Lining |
|---|---|---|---|
| Sulphur Melter | CS (SA 516 Gr.70) shell, Sch 80 coils | Sch 80 coils (vs Sch 40 at IFFCO) for better heat transfer. CS adequate for molten sulphur at 135–145°C. | None required |
| Sulphur Filter | CS body, SS 304 filter elements | SS 304 filter leaves resist sulphur deposits and allow easy cleaning. CS body adequate for non-corrosive service. | None |
| Molten Sulphur Pit | RCC with acid-resistant brick lining | RCC provides structural integrity. Brick lining protects against occasional acid spillage. | Acid-resistant bricks with carbon-filled mortar |
| Sulphur Feed Pumps | CS body, SS 316 shaft | CS adequate for molten sulphur. SS shaft prevents galling at seals. | None |
| Sulphur Handling (Conveyor, Hopper) | CS / SS 304 (Bevcon Wayors) | Bevcon proprietary design for bulk sulphur handling. CS structure with SS contact parts. | None |
3.3 Combustion & Gas Handling
| Equipment | MOC | Selection Rationale | Lining |
|---|---|---|---|
| Sulphur Furnace | CS shell (SA 516 Gr.70) — Spaarkon / Enmax | CS shell withstands structural loads. Brick lining protects from 1,100°C gas temperature and SO₂ attack. | Acid-resistant firebrick (230mm) + insulating brick (115mm) |
| Main Air Blower (MAB) | CS casing, SS impeller — Howden | Dried air is non-corrosive. Howden centrifugal blower with SS impeller for fatigue resistance. | None |
| Gas Ducts (Hot) | CS with refractory lining | Hot SO₂ gas at 400–1,100°C requires refractory protection. CS provides structural support. | Castable refractory (50–100mm) |
3.4 Converter System
| Equipment | MOC | Selection Rationale | Lining |
|---|---|---|---|
| Converter Shell | CS (SA 516 Gr.70) — Spaarkon / Enmax | Operates at 400–620°C. CS adequate as SO₂/SO₃ gas is dry and non-corrosive. 3+2 configuration. | Internal insulation between beds |
| Converter Internals | SS 321 (Ti-stabilized) | SS 321 resists sensitization at 400–620°C. Ti stabilization prevents intergranular corrosion. | N/A |
| Catalyst Support Grids | SS 304 / SS 321 | Must withstand catalyst weight and thermal cycling. SS provides creep resistance. | N/A |
| V₂O₅ Catalyst (5 beds for 99.92%) | Vanadium pentoxide on silica carrier — MECS / Topsoe | 5 beds (3+2 DCDA) with extra bed for 99.92% conversion vs 99.86% benchmark. Cs-promoted in lower beds. | N/A |
3.5 Heat Exchangers & WHB
| Equipment | MOC | Selection Rationale | Lining |
|---|---|---|---|
| Hot Interpass HE | CS shell, SS 321 tubes | Gas-gas exchanger at 400–500°C. SS 321 tubes resist oxidation and sensitization at elevated temperatures. | None |
| Cold Interpass HE | CS shell, SS 321 tubes | Handles SO₂/SO₃ gas at 300–420°C. SS 321 provides corrosion resistance above acid dew point. | None |
| Waste Heat Boiler | SA 213 T22 tubes, SA 516 Gr.70 shell — Thermal Systems | T22 (2.25Cr-1Mo) tubes provide creep strength at high temperatures. Thermal Systems Hyderabad package. | None (fire-tube design) |
| Superheaters | SA 213 T22 tubes, alloy steel headers — Thermal Systems | Steam at 480°C and 65 bar(g) requires high-temperature creep-resistant alloy steel. | None |
| Economizers | SA 213 T11 tubes (1.25Cr-0.5Mo) — Thermal Systems | Moderate temperature service (200–350°C). T11 provides adequate strength and oxidation resistance. | None |
3.6 Absorption Towers & Acid Circuit
| Equipment | MOC | Selection Rationale | Lining |
|---|---|---|---|
| Drying Tower | CS shell, Zecor-Z internals — Spaarkon | 93–98% H₂SO₄ at 40–80°C. Zecor-Z (Lewis proprietary alloy) for acid-wetted internals. Brick lining for shell. | Acid-resistant bricks (2 layers) with carbon-filled mortar |
| IAT / FAT Towers | CS shell, Zecor-Z / Lewmet internals — Spaarkon | 98.5% H₂SO₄ at 80–110°C. Lewmet alloy for high-temp acid service. Brick lining essential. | Acid-resistant bricks + ceramic saddle packing |
| Tower Distributors | Lewmet / Zecor-Z (Lewis proprietary) | Lewis proprietary alloys provide superior resistance to hot concentrated H₂SO₄ vs standard SX alloy. | N/A |
| Acid Circulation Pumps | L-14 Iron / Lewmet — Chemlin / Chas Lewis | L-14 Iron (Lewis proprietary high-silicon iron) is the premium material for pumping hot concentrated acid. | N/A |
| Acid Coolers | CS shell with anodic protection, Zecor-Z tubes | Anodic protection maintains passive film on CS. Zecor-Z tubes resist acid-side corrosion. | Anodic protection system |
| Combination Pump Tank | CS shell, Zecor-Z / Lewmet internals | Integrated pump tank design minimizes piping. Acid-resistant alloy internals for long service life. | Acid-resistant brick |
| Acid Piping (concentrated) | CS (IS 1239 / ASTM A106) | CS resistant to 93–98.5% H₂SO₄ at velocities < 1.2 m/s. Most economical choice. | None (velocity-controlled) |
| Acid Piping (dilute/drain) | HDPE / FRP | Dilute acid (<70%) is highly corrosive to CS. HDPE/FRP provides excellent resistance. | N/A |
3.7 Stack & Emission Control
| Equipment | MOC | Selection Rationale | Lining |
|---|---|---|---|
| Plant Stack (60m) | CS shell with acid-resistant brick lining | Exhaust gas contains traces of SO₂ and acid mist. Brick lining protects CS from acid condensation. | Acid-resistant bricks (full height) |
| Start-up Stack | CS (unlined) | Used only during start-up with dry gas. No acid condensation risk during short-term operation. | None |
3.8 Utilities & Storage
| Equipment | MOC | Selection Rationale | Lining |
|---|---|---|---|
| Acid Storage Tanks | CS with acid-resistant lining — SIL–Sparkon / Enmax | Large volume storage of 98% H₂SO₄. CS adequate with proper lining for long-term storage. | Acid-resistant brick or rubber lining |
| Deaerator | CS shell, SS 304 trays — Forbes Marshall | Handles hot deaerated water. CS shell standard. SS trays resist erosion from steam/water contact. | None |
| Cooling Tower | FRP structure, PVC fills — Paharpur / M-Square | FRP provides corrosion resistance in wet environment. PVC fills standard for induced draft towers. | N/A |
| BFW Pumps | CS/SS 304 — KSB / KBL | Handles treated boiler feed water. SS provides resistance to any residual dissolved oxygen. | None |
3.9 Critical MOC Decisions — Pending Confirmation
CS for Converter Shell & Gas Ducting
A major notable change vis-à-vis SAP-C (existing plant) is the proposal to use Carbon Steel (CS) as the material of construction for the Converter shell and hot gas ducting. This requires documented confirmation from both the Licensor and the Client before acceptance of this MOC.
Additionally, ducting with refractory lining may not be optimal since the outside diameter will increase significantly to accommodate the lining thickness, impacting support design and layout.
Status: Awaiting Licensor & Client Sign-off
FRP Construction for Cooling Tower
The proposed FRP (Fibre Reinforced Plastic) construction for the Cooling Tower requires documented confirmation from the Vendor and the Client, given the very large capacity required for the SAP plant (~6,898 m³/hr) and the still larger capacity needed for the downstream Turbine Generator condenser cooling.
FRP structural integrity at this scale needs to be validated against wind loads (50 m/s basic wind speed at Paradeep) and seismic Zone III requirements.
Status: Awaiting Vendor & Client Confirmation
Key Corrosion Considerations
Iso-Corrosion Zones for H₂SO₄ on CS:
- < 0.5 mm/yr: 93–98.5% at <80°C
- 0.5–2 mm/yr: 70–93% (transition zone)
- > 2 mm/yr: <70% or >100°C
Critical Velocity Limits (CS in conc. acid):
- Pipe flow: < 1.2 m/s (to maintain passive film)
- Pump impeller tip: < 15 m/s
- Acid flow over weirs: < 0.5 m/s
- Tower distributor: < 0.3 m/s