Sulfuric acid is the most widely used industrial chemical. It is also one of the most deceptive — its corrosion behavior depends entirely on concentration. A material that resists 98% sulfuric acid may dissolve instantly in 10% sulfuric acid. Pump selection for acid service is not a one-size-fits-all exercise.
This selection example walks through a typical acid transfer application in a chemical plant. Operating parameters are representative, not a real customer project.
Example Application: Sulfuric Acid Unloading and Transfer
The example is a tank truck unloading system transferring 96% sulfuric acid from road tanker to a 50 m³ storage tank. The pump runs intermittently — 2–3 hours per day, during unloading operations.
| Parameter | Value | Notes |
|---|---|---|
| Fluid | Sulfuric acid (H₂SO₄), 96% concentration | Concentrated, near-anhydrous |
| Flow rate | 30 m³/h | Truck unloading within 2 hours |
| Head | 18 m | Lift to tank height + piping loss |
| Temperature | 20–35°C | Ambient — exothermic dilution if water contacts acid |
| Specific gravity | 1.84 | High — affects pump power |
| Viscosity | 25 cP at 25°C | Much higher than water — affects performance curve |
| Solids | None | Clean acid |
Pump Type Evaluation: What Works for Concentrated Acid
Three pump types are commonly used for concentrated sulfuric acid transfer. Each has trade-offs.
| Factor | Centrifugal Pump ( lined ) | Magnetic Drive Pump | Air-Operated Diaphragm |
|---|---|---|---|
| Flow range | 5–100 m³/h (ideal for 30 m³/h) | 1–50 m³/h (borderline at 30 m³/h) | 1–20 m³/h (too small) |
| Sealing | Mechanical seal — leakage risk | Sealless — zero leakage | Seal-less — but slower |
| Material options | Cast iron (for 98% acid!) or PTFE-lined | PVDF / PFA-lined | PTFE / PVDF |
| Maintenance | Mechanical seal replacement every 1–2 years | Bearing and containment shell every 2–3 years | Diaphragm replacement every 6–12 months |
| Capital cost | Low (cast iron is cheap) | Medium | Medium |
| Best for | Large flow, intermittent duty | Leakage-sensitive, continuous duty | Small flow, variable duty |
Selection conclusion: For 30 m³/h of 96% sulfuric acid transfer, a PTFE-lined centrifugal pump is the workhorse choice. But if leakage to the environment is unacceptable (plant has zero-emission policy), upgrade to a fluoroplastic magnetic drive pump. The extra cost is justified by zero leakage risk.
Material Selection: Concentration Is Everything
This is the critical point in sulfuric acid pump selection. Material compatibility is NOT the same across all concentrations:
| Material | 96–98% H₂SO₄ (concentrated) | 30–60% H₂SO₄ (dilute) | <10% H₂SO₄ (very dilute) |
|---|---|---|---|
| Cast iron | ✅ Good (forms protective FeSO₄ film) | ❌ Bad (rapid corrosion) | ❌ Bad |
| 316L stainless | ⚠️ Conditional (passive film) | ❌ Bad (pitting) | ❌ Bad |
| Hastelloy C | ✅ Good | ✅ Good | ✅ Good |
| PVDF | ✅ Good (up to 80°C) | ✅ Good | ✅ Good |
| PTFE (Teflon) | ✅ Universal — works at all concentrations | ✅ Universal | ✅ Universal |
| Cast steel (carbon) | ⚠️ Conditional | ❌ Bad | ❌ Bad |
Key insight: Cast iron works for 98% concentrated sulfuric acid because the acid reacts with iron to form a protective iron sulfate film on the surface. This film prevents further corrosion. But the same cast iron dissolves rapidly in dilute acid — there is not enough oxidizer to form the protective film.
For this application (96% H₂SO₄), cast iron is actually a reasonable and economical choice. But if the concentration might change (e.g., dilute acid production in the same plant), use PTFE-lined or Hastelloy — it works across all concentrations.
Mechanical Seal vs Sealless Design
Sulfuric acid fumes are dangerous — even a small mechanical seal leak releases corrosive mist into the pump room. Seal selection:
Single mechanical seal: Acceptable for 96% acid in a well-ventilated pump room. Seal faces: SiC / SiC. Elastomer: FKM (Viton) — resists concentrated acid. Avoid EPDM, which swells in concentrated sulfuric acid.
Double mechanical seal with Plan 53B: For zero-emission applications. Barrier fluid (process-compatible oil) leaks inward to the pump, not outward. Double the seal cost, but eliminates environmental risk.
Magnetic drive (sealless): Best for maximum safety. No dynamic seal at all. The containment shell separates the wet end from the atmosphere. For acid service, the containment shell is PFA-lined or Hastelloy C.
For this intermittent unloading application, a single mechanical seal in a ventilated pump room is acceptable. For continuous process transfer, upgrade to magnetic drive.
Viscosity Correction: Don't Forget the 25 cP
Sulfuric acid at 96% is 25 cP — about 25 times more viscous than water. Pump performance curves are published on water. At 25 cP, the pump experiences:
- Head reduction: ~5–8% lower than water curve at the same flow
- Capacity reduction: ~3–5% lower
- Efficiency reduction: ~8–10% lower
- Power increase: ~10–15% higher (due to viscous shear + high SG)
When sizing the motor, calculate: hydraulic power = (flow × head × SG × viscosity factor) / (3600 × efficiency). For this application: (30 × 18 × 1.84 × 1.15) / (3600 × 0.60) = 5.3 kW. Motor size with service factor: 7.5 kW. Do not size the motor as if pumping water — it will overheat and trip.
Use the viscosity correction calculator to adjust the pump performance curve for viscous service.
Final Configuration Summary
| Selection Item | Recommendation |
|---|---|
| Pump type | PTFE-lined centrifugal (or cast iron for 96% only) |
| Flow / head | 30 m³/h at 18 m |
| Speed | 2,950 rpm (2-pole motor) |
| Wetted material | PTFE-lined casing, PFA impeller |
| Shaft seal | Single mechanical seal, SiC/SiC faces, FKM elastomer |
| Motor | 7.5 kW, 3-phase, 50 Hz, TEFC |
| Baseplate | Polypropylene or epoxy-coated steel (acid fumes corrode bare steel) |
Safety and Installation Requirements
1. Eye wash and safety shower: Within 10 meters of the pump station. Sulfuric acid splashes require immediate flushing.
2. Secondary containment: The pump and piping sit in a concrete dike or steel pan that can hold 110% of the largest tank. Acid spill into the ground is an environmental violation.
3. No water contact: Never flush the pump with water after acid service. Water + concentrated acid = violent exothermic reaction. Flush with dilute acid or process-compatible solvent.
4. Grounding: Pump and piping grounded to prevent static buildup during acid transfer. Acid is non-conductive at high concentration — static sparks can ignite acid fumes.
For material compatibility across acid concentrations, the material corrosion guide lists wetted material performance. For chemical pump selection and quotation, submit your fluid data — we will recommend pump type, material, and sealing arrangement.