Sulfuric acid is the most produced industrial chemical on Earth. It is also the most deceptive when it comes to pump selection. The material that resists 98% sulfuric acid may dissolve instantly in 10% sulfuric acid. Get the material wrong, and the pump leaks acid within months. Get it right, and it runs trouble-free for a decade.
This guide covers pump selection for sulfuric acid service, organized by concentration — because that is the single factor that determines everything else.
Sulfuric Acid Concentrations: The Critical Distinction
Sulfuric acid's corrosive behavior changes completely with concentration. There is no "universal material" for all concentrations. You must know your exact concentration before selecting pump materials.
| Concentration | Typical Application | Corrosivity | Key Material |
|---|---|---|---|
| < 10% (very dilute) | Wastewater treatment, dilute acid cleaning | High | PTFE-lined or Hastelloy |
| 10–60% (dilute) | Fertilizer production, steel pickling | Very high | PTFE-lined, PVDF, or Hastelloy C |
| 60–85% (concentrated) | Chemical processing, battery acid | Medium | Cast iron works (passive film) |
| 93–98% (concentrated) | Petroleum refining, alkylation | Low (forms protective film) | Cast iron, carbon steel |
The counterintuitive fact: Concentrated sulfuric acid (98%) is less corrosive to cast iron than dilute sulfuric acid (10%). Why? Because concentrated acid reacts with iron to form a protective iron sulfate film on the surface. This film prevents further corrosion. Dilute acid does not form this film — it attacks iron continuously.
When a supplier asks "what acid are you pumping?", "sulfuric acid" is not enough. You must specify the exact concentration range.
Pump Type Selection by Concentration
Concentrated Acid (93–98%)
For concentrated sulfuric acid, cast iron pumps are the traditional choice. The protective film forms naturally. Capital cost is low. Maintenance is simple.
But cast iron pumps have two limitations:
- They cannot handle dilute acid — the film does not form
- They are not sealless — mechanical seal leaks are a risk
For zero-leakage service (environmental, toxic, or valuable product), upgrade to a fluoroplastic magnetic drive pump. The PFA/PVDF wetted end handles all acid concentrations. The magnetic drive means no shaft seal — zero leakage.
Dilute Acid (10–60%)
Cast iron does not work here. You need corrosion-resistant materials:
PTFE-lined pumps: Standard choice for dilute acid. A cast iron outer casing provides pressure containment. The PTFE liner contacts the acid. PTFE resists every concentration of sulfuric acid. Cost is medium.
Hastelloy C pumps: Premium option. Solid Hastelloy construction (no liner). Best for high-temperature or high-pressure dilute acid service. Cost is 2–3x PTFE-lined. But it lasts longer in aggressive service.
PVDF pumps: Plastic construction (PVDF). Chemical resistance is excellent. Pressure rating is limited (max 10 bar). For low-pressure dilute acid transfer, PVDF is cheap and reliable.
Mechanical Seal for Acid Service
The mechanical seal is the most failure-prone component in an acid pump. Seal selection for sulfuric acid:
Single mechanical seal: Acceptable for concentrated acid (98%) 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 leaks inward to the pump, not outward. Required for dilute acid service where leakage is more corrosive.
Magnetic drive (sealless): Best for maximum safety. No dynamic seal. The containment shell separates the wet end from the atmosphere. For acid service, the containment shell is PFA-lined.
Design Details That Matter
1. No water contact. Never flush the pump with water after acid service. Water + concentrated acid = violent exothermic reaction. The heat can crack the cast iron casing. Flush with dilute acid or process-compatible solvent.
2. Grounding and bonding. Sulfuric acid is non-conductive at high concentration. Static electricity builds up as the fluid flows through the pump. Ground the pump and piping to prevent static sparks.
3. Secondary containment. 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.
4. Eye wash and safety shower. Within 10 meters of the pump station. Acid splashes require immediate flushing.
Sizing and Motor Correction for Viscosity
98% sulfuric acid has viscosity of 25 cP at 25°C — 25 times more viscous than water. Pump performance curves are published on water. At 25 cP:
- Head reduction: ~5–8% lower
- Efficiency reduction: ~8–12% lower
- Power increase: ~10–15% higher
When sizing the motor, add 15% to the hydraulic power calculation. Do not size the motor as if pumping water — it will overheat and trip.
Use the viscosity correction calculator to adjust the performance curve for viscous acid service.
Selection Parameter Checklist
| Parameter | Concentrated (98%) | Dilute (10–60%) |
|---|---|---|
| Pump type | Cast iron centrifugal | PTFE-lined magnetic drive |
| Wetted material | Cast iron | PTFE-lined, PFA impeller |
| Mechanical seal | Single, SiC/SiC, FKM | Magnetic drive (sealless) |
| Flow range | 10–200 m³/h | 1–50 m³/h |
| Head range | 10–80 m | 10–60 m |
| Temperature limit | Up to 80°C | Up to 120°C (PTFE) |
Common Mistakes in Acid Pump Selection
Mistake 1: "Stainless steel" on the PO. 316L stainless is NOT corrosion-resistant in dilute sulfuric acid. It pits within weeks. Always specify the exact material grade, not just "stainless."
Mistake 2: Using cast iron for dilute acid. Cast iron works for 98% acid because of the protective film. In 10% acid, the film does not form. The cast iron dissolves. Specify PTFE-lined for dilute acid.
Mistake 3: Wrong elastomer. EPDM O-rings swell in concentrated sulfuric acid. FKM swells in dilute acid. Always specify the correct elastomer for your concentration. When in doubt, use PTFE encapsulated O-rings.
Mistake 4: No secondary containment. Acid spill into the ground is an environmental incident. The cost of cleanup is 100x the cost of a concrete dike.
For sulfuric acid pump selection by concentration, our fluoroplastic magnetic drive range handles all acid concentrations with zero leakage. For material compatibility across acid concentrations, the material corrosion guide lists wetted material performance. For acid pump quotation, submit your acid data — concentration, temperature, flow, and head.