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Department Editor: Kate Torzewski

Acid Storage

Most common acids can be stored inhorizontal or vertical ASME-typetanks, as shown in the figures to the

right, or vertical API-type tanks. Horizontal,

carbon-steel ASME-type tanks of 10,00040,000 gal capacity should have a platethickness of 3/8 in. with dished heads ofthe same thickness. The thickness includesa corrosion allowance of 1/4 in., whichprovides a tank life of 1520 years.

Sulfuric acid (H2SO4)Storage.This acid is prone to enter intoreactions that generate hydrogen, so inaddition to keeping the vessel ventedadequately, exclude potential sources ofignition from the vicinity.

Materials of construction.Carbonsteel is satisfactory for concentratedtechnical grades of sulfuric acid atnormal atmospheric temperature. H2SO4solutions that are more dilute corrodecarbon steel severely. To avoid inadver-tent dilution of concentrated acid, keepacid away from contact with moist air.Polyvinyl chloride pipe is recommendedfor ordinary sulfuric acid, but for oleum,Type 316 stainless steel or carbon-steellined with a fluorocarbon is best.

Phosphoric acid (H3PO4)Storage.The tank bottom should be rolledto a height of 3 in. (upward). This allowswelds to freely expand or contract. Cornerwelds should be avoided, as undue stressescan occur and aggravate corrosion [2].Depending on the acid grade, the freezingpoint varies and may necessitate heating toavoid freeze-up in storage. In any case, toavoid corrosion, high-pressure steam shouldnot be used; steam coils located severalinches below the bottom of the tank arerecommended. The space below the tankbottom should be enclosed to permit heat-ing of the air to 50C, and the tank wallsshould be insulated.

Materials of construction.Tanks canbe fabricated of Type 316 extra-low-carbonstainless steel, rubber--lined carbon steel orfiberglass-reinforced plastic. Carbon steelshould not be used, as it will corrode.

Hydrochloric acid (HCl)Storage.HCl of all strengths should bestored in tanks similar to those men-tioned above. Containment areas shouldbe provided around tanks, and storagefacilities should include a pressure- andvacuum-relief service, primary andredundant level indicators, a high-levelalarm, an overflow line, an emergencyblock valve at the tank outlet nozzle anda vent-fume scrubber.

Materials of construction.Thesestorage tanks should be fabricated of rub-ber-lined carbon steel, glass-lined carbonsteel or fiber-reinforced polymer (FRP). Softnatural-rubber compounds are used as lin-ers for concentrated acid storage tanks attemperatures up to 60C with a minimumlining thickness of 3/16 in. Semi-hardrubber is used for lining equipment andpiping for acid up to 70C with FRP tanksof vinyl-ester resin.

Nitric acid (HNO3)Storage.Storage tanks for HNO3of lessthan 95 wt.% concentration should bedesigned for at least a slight pressure andvacuum, permitting the venting of nitrogen-oxide fumes to collection and disposalequipment, such as a scrubber or a flare.When locating the tank vent and overflowpipe, consider that escaping vapors andliquid can corrode exterior welds as theacid is diluted with atmospheric moisture.

Materials of construction.For concen-trations up to 95 wt.% at ambient tempera-ture, storage units should be fabricated ofType 304L stainless steel. For concentra-tions of 95 wt.% and above, Type 3003aluminum alloy should be used. Acid inthe range of 5255 wt.% should be storedin tanks of Type 347 stainless steel usingNo. 12 gage sheet. Above 90 wt.%, cor-rosion allowance in the tank-wall thicknessmay be necessary. Glass-lined carbonsteel tanks are satisfactory for all acidgrades.

Hydrouoric acid (HF)Storage.Because of anhydrous HFs highvapor pressure, tanks are designed for aminimum pressure of 60 psig and haveX-rayed and stress-relieved welds. Tanksholding 70 wt.% HF are also designed per

ASME code, or for lower pressure, as itsvapor pressure is much lower than that of an-hydrous HF. These tanks should be equippedwith a relief device, and discharge piping

should be routed to a scrubber. Aqueous HFtanks should have a vent, with the vent linealso going to the scrubber.

Materials of construction.Carbon-steelstorage tanks can be used for anhydrous HFat temperatures up to 66C and 70 wt.%HF. Acid of concentrations greater than 60wt.% may be handled in steel up to a tem-perature of 38C. In steel tanks, hydrogenblistering may be caused by the accumula-tion of H2, so periodic tank inspections arerequired to evaluate blistering.

References

1.Grossel, S., Safe Efficient Handling of Acids,Chem. Eng.December 1998, pp. 104112.2.Anon., Phosphoric Acid, Rhone-Poulene Basic

Chemicals Co., Shelton, Conn. (1992).

2A*

Weld lines

* Minimum width of the shell course at each saddle. Width may be increased if more economical for shell course layout.

Knuckle radius line

2A*

DImension all nozzlelocations from this line

Standard flangedand dished heads

Manhole Vent

AA

Top plate of saddlewelded to shell

6 in. typical

Nominal capacity level

Bottomoutletbaffle

Channellegs

Submergedfill pipe

Groundingclip

Nameplate

R=

OD

(approx)

15

in.

OD