Sulphuric Acid Resistant Centrifugal Fan

Overview

The aggressiveness of sulphuric acid is governed by concentration and temperature together. The same fan that works in 98 percent acid at ambient temperature will be destroyed by 30 percent dilute acid, so the concentration window and temperature limit must be fixed before selecting materials.

Concentration and Temperature Window

Concentrated sulphuric acid passivates carbon steel at ambient temperature, while dilute acid attacks almost all common metals.

Location	Material / process	Basis
Concentrated, ambient	Carbon steel with epoxy or vinyl ester coating	Reliable while the passivating film is stable
Dilute acid	PP, PVDF or FRP lining	Non-metallic materials resist dilute acid
Severe or hot duty	Lead lining, PTFE lining or titanium	Further upgrade by concentration and temperature
Shaft	316L sheathed or acid resistant alloy	Protects against acid vapour along the shaft
Sealing	PTFE mechanical seal	Prevents leakage and protects bearings

Operating and Process Controls

Location	Material / process	Basis
Concentration monitoring	Monitoring ahead of the fan	Avoids the aggressive concentration range
Avoid dilution	Prevent water vapour entering the stream	Dilution drives accelerated attack
Temperature limit	Cap the inlet temperature	Corrosion rate rises with temperature
Drainage	Low point drain in the casing	Prevents liquid accumulating
Shutdown protection	Wash and keep dry when stopped	Reduces acid attack during standstill

Applications

Applications include sulphuric acid plant tail gas, acid making from metallurgical gas, fertiliser plants, battery charging mist and acid tank breathing.

Failure is usually pitting and weld corrosion in locally diluted zones rather than uniform thinning, so stagnant areas and dead corners must be avoided.

Where the medium also contains fluoride or chloride, the rules for sulphuric acid no longer apply and the mixed acid has to be assessed separately.

Acceptance and Verification

Standard	Title	Purpose
HG/T 20679	Design code for external anti-corrosion of chemical equipment and piping	Anti-corrosion design for chemical media
GB/T 1236	Industrial fans - Performance testing using standardized airways	Air flow, pressure and efficiency testing
ISO 12944-5	Paints and varnishes - Corrosion protection of steel structures by protective paint systems - Part 5	Coating system and durability class
GB/T 9239.1 / ISO 21940-11	Mechanical vibration - Balance quality requirements for rigid rotors	Impeller balancing grade
JB/T 10563	General purpose centrifugal fans - Technical specifications	General technical conditions and works acceptance

Please provide the concentration range, medium temperature, moisture content and air flow. Size and motor power are calculated for the actual duty.

FAQ

Can carbon steel really be used for concentrated sulphuric acid?

At ambient temperature, high concentration and low moisture, carbon steel forms a ferrous sulphate film and can be used with a protective coating. If temperature rises or concentration falls, that film breaks down and non-metallic or upgraded materials are required.

How should dilute sulphuric acid duty be specified?

Dilute acid attacks metals strongly, so PP, PVDF or FRP are the usual choice, or lead or PTFE lining. The specific grade still has to be confirmed for the actual concentration and temperature.

Why is concentration fluctuation more dangerous than a fixed value?

Corrosion rate is very sensitive to concentration, and process upsets push the equipment briefly into the aggressive range. The technical agreement should state a range rather than a single value, backed by monitoring and interlock.

Related pages: Anti-corrosion fan range  PP plastic fan  Rubber and plastic lined fan