Storage Tanks

Materials of Construction - Storage Tanks
September 28,

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Introduction

The material selected for sulphuric acid storage tanks will depend on a number of factors such as:

• Tank size
• Desired acid quality
• Costs
• Acid concentration
• Storage temperature

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Shell

The materials of construction will depend on the acid concentration, temperature and composition.  The table under the heading Recommended Materials lists the options for materials of construction.  By far the most common material of construction is carbon steel.

Where iron pickup needs to be minimized stainless steel can be used for the shell although at considerably higher costs.  An alternative to stainless steel is anodically protecting the tank.

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Linings

Linings can serve to minimize or eliminate corrosion of the tank and reduce contamination of the acid.

Phenolic Coatings

Baked phenolic coatings are use to minimize iron pickup and corrosion in tanks storing 93% H2SO4.  The life of this type of coating is severely limited in 98% to 99% service.  Phenolic coatings should never be exposed to alkaline neutralizations because alkali attacks phenolic coatings even in dilute concentrations.

The integrity of the lining is dependent on the quality of the surface preparation and the actual application of the lining.  All internal surfaces must be inspected, degreased and sandblasted.  A thermally cured phenolic coating #P403 of 0.178 mm to 0.254 mm (0.007 to 0.010 in.) thick is applied using a multi-coat procedure immediately following sandblasting. 

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Epoxy Novolac

In , DUROMAR Inc. developed a novolac epoxycycloaliphatic amine coating which is a 100% solids spray applied coating.  The coating called HPL -XC has application as a storage tank lining for 98% sulphuric acid at temperatures up to 60oC (140oF).  Testing of the product was under taken in the form of lined test coupons.  The test lasted for one year in 98% sulphuric acid at temperatures up to 60oC (140oF).  The lined coupon showed evidence of some attack but held up better than any other material tested.  Based on the results of the tests, a 16.5 m x 7.3 m (54 ft x 24 ft) storage tank at a plant in Florida was lined.

Surface preparation consisted of cleaning, washing, drying and sandblasting to a near-white finish with a 3 mil profile according to SSPC-SP10.  HPL -XC was applied in two coats to a dry film thickness (DFT) of 40 mils.  The lining was cured at a temperature of 60oC (140oF) for 24 hours using steam.

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The tank was drained for inspection after one year of service with 98% sulphuric acid at a maximum temperature of 49oC (120oF).  In general, the lining was in good condition with a thickness of 20 to 30 mils.  Based on these results another tank on site was lined with HPL -XC.  It is anticipated that the lining will need to be maintained every 3 to 5 years by brush blasting and the application of a single additional coat of HPL -XC.

Additional references

www.tnemec.com/projects/sulfuric-acid-tanks/

Chemline Case Study

Sulphuric Acid Tank Protected by Belzona

ARC S4+ and S2 Coatings

Glass-lined

Glass-lined storage tanks are used where the acid is extremely pure and no contamination of the acid is permitted.  The storage requirements for high quality acid are generally small so storage tanks are generally small.

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Recommended Materials

The following materials are recommended for sulphuric acid service.

When specified, plate material shall be:

• Impact tested required where ambient temperatures are low such as in northern climates.
• Fully killed as per API 650.
• Fine grain practice as per API 650.

Some investigators claim that copper bearing steels such as ASTM A285 Gr. C, S57 provide lower corrosion rates and resistance to hydrogen grooving but others feel that there is no advantage and the added cost of the steel is not warranted.

The following steels are not recommended for use in sulphuric acid storage tanks:

• Carbon steel to ASTM A36, A283, A285, A515
• High strength steels and low alloy steels

When carbon steel to ASTM A36 or A285 is specified, it shall be made using other than rimmed or capped steel practices.

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