Sulfuric Acid Plant Audit Checklist: What to Verify at a Producer Site should be structured around process boundaries rather than equipment lists. A technically defensible verification sequence begins at sulfur receipt and continues through melting, filtration, process air drying, sulfur combustion, catalytic conversion, absorption, cooling, storage, and finished-product loading. At each boundary the auditor compares field measurements with design baselines, vendor data sheets, maintenance records, analytical methods, and permit limits. The following paragraphs identify verification points that require direct observation or instrument confirmation, with emphasis on parameters that influence product acidity, acid mist emission, corrosion rate, catalyst differential pressure, and sampling integrity.

Raw Sulfur Unloading, Melting, and Filtration Line

Bulk sulfur unloading requires confirmation that free water and soil contamination are rejected at the receiving hopper or pit. The sulfur feed specification is checked against the burner vendor’s values; many designs require moisture by Karl Fischer titration to remain below 0.05% by mass because excess water increases downstream acid mist and accelerates refractory wear. Ash content is usually limited to 0.03% by mass or tighter where high-purity acid is produced. Molten sulfur is transferred through steam-jacketed lines and maintained between 135 °C and 150 °C. A low-point skin temperature below 120 °C indicates tracing failure and a possible solid-sulfur plug. Sulfur filtration through leaf or basket filters is verified by differential pressure; sustained pressure differential above the filter vendor’s maximum, commonly 0.7 bar, indicates blinding and requires switching to a clean filter. The melter overflow, settling tank, and filter vent lines are inspected for blockages and for sulfur dust accumulation.

Filter mesh aperture is typically specified in the 50 µm to 75 µm range for sulfur burners. The auditor should also review the molten sulfur storage tank nitrogen blanket pressure and the relief vent, because molten sulfur at temperatures above 165 °C increases in viscosity and can generate H₂S in the tank headspace. Steam tracing traps and condensate return are checked for water hammer and failed thermostatic elements. If the plant receives prilled or granulated sulfur, transfer dust and static charge control are reviewed at the bag dump station, and the hopper grounding resistance is verified against the site’s hazardous area classification records. No acceptance inspection is complete without comparing the sulfur supplier’s certificate of analysis to the laboratory retain of the previous 3 to 6 deliveries.

At the drying tower, the circulating acid strength is held between 93.0% and 96.0% H₂SO₄ by weight, confirmed by total acidity titration according to ASTM E223-16. If the concentration drifts below the band, water partial pressure above the acid rises and moisture carryover into the converter increases. Outlet process air moisture is measured with chilled-mirror dew-point sensors or quartz-crystal microbalance analyzers; modern sulfur-burning plants typically maintain outlet moisture below 50 mg/Nm³ as water. The packed bed pressure drop is trended against the tower vendor’s design curve, and the acid distributor is inspected during the next turnaround for missing or displaced distribution points. Direct observation of drying acid temperature, normally limited to 50 °C to 80 °C, prevents excessive acid vapor entrainment into the process air.

At the sulfur furnace outlet, the process gas SO₂ concentration is normally maintained between 10.0% and 12.0% by volume, measured by a calibrated NDIR analyzer. Combustion air flow is cross-checked against the sulfur feed rate and furnace temperature. The waste heat boiler outlet temperature and steam drum level are reviewed because carryover of boiler solids or too low a gas temperature can create sticky deposition on the first converter bed. A large oxygen excess above design increases downstream SO₂ conversion demand and can reduce the plant’s energy recovery per ton of acid produced. The auditor verifies the burner management system purge, the sulfur flow totalizer, and the acid plant air blower surge controls.

Does Converter Bed Thermometry Align with Pressure-Drop Signatures?

The converter audit begins with thermocouple loop calibration and a review of radial temperature spread. In a sulfur-burning double-absorption plant, the first catalyst bed outlet is normally 600 °C to 630 °C at design feed rate, while the first bed inlet is controlled near 420 °C. The final bed inlet is usually held between 410 °C and 430 °C to balance equilibrium and kinetic requirements. The alkali-promoted vanadium pentoxide catalyst loses activity over time, so the rate of rise in first-bed gas outlet temperature relative to SO₂ conversion is compared with the catalyst supplier’s activity curve. Excessive radial spread across a bed indicates gas maldistribution, damaged support grids, localized crusting, or poor catalyst loading. Pressure-drop transmitters are compared with the loaded catalyst mass and sieve analysis. A single shallow bed commonly operates between 1.5 kPa and 3.5 kPa differential at design flow, but the true acceptance band is the converter vendor’s model value. The auditor checks whether bed pressure readings are compensated for temperature and whether the impulse lines were purged, because acid condensation in the lower stage impulse lines is a known source of false high readings. Catalyst core samples from top, middle, and bottom are screened for crush strength, dust fraction, and active component distribution using the supplier’s methods; pronounced fines at the bed top suggest thermal cycling or moisture exposure.

Absorption tower verification focuses on acid strength, temperature, distributor integrity, mist eliminator pressure drop, and cooler heat balance. The final absorber circulated acid is maintained at 98.5% to 99.5% H₂SO₄; the intermediate absorber may operate at the lower end of that band. Acid concentration is calculated from density and temperature and is checked against ASTM E223-16 or ISO 910:1977 where oleum-bearing streams are present. Recirculation inlet temperature is normally held between 70 °C and 90 °C; lower temperatures reduce absorption rate and increase mist, while higher temperatures increase vapor pressure and can accelerate downstream corrosion. The tower differential pressure and the candle mist eliminator differential pressure are read against the clean-commissioning baseline. A mist eliminator differential above 250 mm H₂O or a rising trend indicates fiber-bed plugging, liquid entrainment, or mechanical failure. Acid cooler performance is verified by comparing the measured heat removal with the design duty at current flow and temperatures. Shell-and-tube anodically protected coolers are common; the protection rectifier voltage and current should remain within the supplier’s operating envelope, and water chemistry is checked for conditions that promote scale deposition on stainless steel heat-transfer surfaces.

Cooling water pH, conductivity, and biocide records are reviewed only if the cooler is water-cooled. The acid flow transmitter and low-flow trip are functionally tested, and the acid pump discharge pressure is compared with the pump curve. A discharge pressure lower than the curve at a given flow suggests erosion, wear ring clearance increase, or air entrainment in the suction. The acid dilution water system is inspected at the mixing tee; the dilution point must be located where turbulent mixing occurs to avoid localized overheating of pipe or tank walls.

When Oleum or Liquid SO₃ Storage Is Included in the Audit Boundary

Oleum verification adds checks for free SO₃ content, heating, agitation, and moisture exclusion. Free SO₃ in commercial oleum grades is commonly 20%, 25%, or 65% by weight; total acidity and free SO₃ are determined by ISO 910:1977 or equivalent. Oleum storage tanks are heated and sometimes recirculated to prevent stratification or freezing. The audit verifies that the heating medium temperature does not exceed the vessel lining or alloy supplier’s maximum, because hot oleum is more aggressive to carbon steel than ambient concentrated sulfuric acid. Transfer piping is checked for low-velocity sections and dead legs that can cool and plug. The loading area ventilation, water exclusion, and neutralizing agent availability are confirmed. Because dilution of oleum with water is strongly exothermic, water must never be introduced into a closed oleum circuit without a pressure-relief path. The auditor records the relief valve set pressure, the scrubber pH, and the absence of incompatible soft metals in contact with oleum service.

Finished acid storage is evaluated against NACE SP0294 for concentrated sulfuric acid tanks and against the site’s mechanical integrity program. Tank inlet piping is designed to reduce acid velocity and turbulence near the carbon steel floor; average acid velocity in carbon steel piping is usually limited to 0.9 m/s to 1.2 m/s unless corrosion allowances and materials were specifically selected for higher flow. Ultrasonic thickness testing of the shell and floor follows API 653 intervals based on the measured corrosion rate. Hydrogen evolution from acid attack on carbon steel requires open vents, flame arrestors, and elimination of ignition sources. The auditor checks high-level shutdown transmitters, tank dike integrity, leak detection sumps, and emergency isolation valves under OSHA 29 CFR 1910.119 for covered processes. For loading racks, the audit confirms that the loading arm or hose is rated for acid service, that gaskets are PTFE or another acid-resistant fluoropolymer, and that the high-level interlock closes the product valve before the tank volume reaches the safe fill margin. Loading lines are checked for static bonding and for residual pressure after shutdown.

The following matrix consolidates the audit’s primary instrument and laboratory checks. The acceptance bands are typical; permit, vendor, or site-specific values govern where they differ.

Process safety verification covers converter interlocks, acid pump low-flow trips, and high-temperature shutdowns. The auditor checks the safety requirement specification, proof test records, and logic solver diagnostics for the sulfur furnace high-temperature trip and the drying tower low-acid-strength alarm. The acid circulation pump low-flow switch is tested against actual pump curve data; a bypassed or defeated low-flow trip creates an unsafe condition if loss of coolant overheats the diluter or absorber. Relief devices on oleum tanks and acid piping are inspected for set pressure, seal integrity, and discharge routing to a neutralization or scrubbing vessel. The audit record notes whether the facility uses remote-operated isolation valves with spring-return actuators in loading operations and whether those valves close on high level or loss of air signal. These checks are evaluated against the plant’s functional test interval, not merely the drawing revision.

Stack Emissions and Mist Carryover Are Read Through EPA Method 8

Stack verification reviews the sampling train setup, calibration gas certificates, and the analytical method used for acid mist, SO₂, and visible emissions. In U.S. jurisdictions, sulfuric acid mist is measured by EPA Method 8; 40 CFR 60 Subpart H sets an acid mist limit of 0.15 kg per metric ton of 100% acid produced for affected facilities and an SO₂ limit of 2 kg per metric ton for sulfur-burning plants. The SO₂ analyzer sample system is checked for moisture removal, sample line heat tracing, and zero/span drift. If the plant uses a caustic or ammonia tail gas scrubber, the audit verifies recirculation pH, pressure drop, and reagent consumption against the design range. Stack opacity is trended alongside mist eliminator differential pressure, as a rise in opacity without an SO₂ increase often indicates mist eliminator breakthrough or absorber distributor damage. Continuous emissions monitoring data are compared with quality assurance records under 40 CFR Part 60 Appendix F or the local equivalent. The auditor also checks whether the stack test ports meet the physical access criteria of the reference method and whether traverse point calculations are current.

An audit cannot close the producer-site verification without reviewing the on-site laboratory and shipment certification process. The laboratory should demonstrate reproducible total acidity, iron, sulfur dioxide, and appearance using ASTM E223-16. For downstream users with tight inorganic impurity limits, the certificate of analysis should include the customer-specified methods from ISO or ASTM and the actual numerical result for each lot, not a pass/fail notation. Product transfer samples are taken from each loading line after recirculation and compared with the retained shipping sample. The audit verifies that sampling ports are accessible, that sample containers are dry and compatible, and that retained samples are stored for the period required by the supplier’s quality plan or commercial contract. Batch-to-batch variance in acid concentration is evaluated from the last 12 months of certificates; a standard deviation above the producer’s stated precision indicates inadequate blending after absorption or drift in density compensation. This final verification ties product grade, process control, and sampling integrity into the same audit record.