Lined Pipe for Sulfuric Acid Service

Sulfuric acid is the largest-volume industrial chemical in the world, and how it treats steel depends entirely on how much water is in it. Marspec Technical Products fabricates PTFE-, PVDF- and polypropylene-lined pipe spools and supplies the lined fittings and valves that take the concentration question off the table, from 98 percent acid at the unloading rack to 10 percent at the neutralization tank. For a quote on a spool list or valve takeoff, call 251-625-4949 or contact us online.

Concentration decides everything

Above roughly 90 percent, sulfuric acid is oxidizing and nearly anhydrous, and carbon steel survives in it because a film of iron sulfate forms on the wall. That film is the basis of every steel acid tank, and it is fragile: it washes away above about 2 feet per second, weakens past about 120 F, and never forms once the acid is diluted. Between roughly 10 and 90 percent the acid is reducing and plain steel has no practical use; 304 and 316 stainless corrode through much of that range; the old answers were lead, high-silicon iron, nickel alloys and acid brick. Dilute acid is still aggressive to steel, only slower. A liner does not care about the film, the velocity or the concentration, and it keeps iron out of the acid, which matters for electronic-grade and battery-grade product.

Liners, concentration by concentration

• PTFE (minus 20 F to 450 F): suitable at every concentration from dilute through 98 percent, including hot acid within the liner's limit. We quote it for concentrated acid transfer, dilution loops, fresh and spent alkylation acid, and any line that sees more than one concentration. For oleum and fuming acid, send us the conditions.
• PVDF (0 F to 275 F): a strong choice for dilute and intermediate acid, for abrasive acid slurries such as the gypsum-laden streams of a phosphoric acid plant, and for high-purity acid. It is not for hot concentrated sulfuric acid and is attacked by fuming acid.
• Polypropylene (0 F to 225 F): available where a specification calls for it on dilute acid in pH control, neutralization, water treatment and effluent lines, though we recommend and quote PTFE for those lines too. Concentrated acid is an oxidizer and polypropylene is not suited to it.

If a line sees two concentrations, choose the liner for the harsher one. Liner thicknesses, flange ratings and bolt torque are on our technical details page; for anything near an edge, send us concentration, temperature, velocity and solids content and we will recommend a liner in writing.

Dilution heat, freezing and temperature maintenance

Two thermal quirks shape the piping. First, diluting sulfuric acid releases a great deal of heat, on the order of 880 kJ per kilogram of concentrated acid, enough to flash water and spatter. Wherever dilution happens, at a tee, in a mixing loop or in a tank, the liner sees a transient temperature well above the bulk; we build dilution spools in PTFE and ask for the dilution ratio when we quote. Second, concentrated acid freezes at surprisingly high temperatures. Ninety-eight percent acid solidifies a few degrees above 32 F, while 93 percent (66 degree Baume) acid stays liquid well below the freezing point of water, so 93 percent is the usual winter grade and outdoor 98 percent lines are traced or jacketed.

For those lines we supply clamp-on steam heaters, electric tracing with heat-transfer compound, and steam-jacketed lined pipe and fittings. Match the heat to the liner: 15 psig saturated steam is already about 250 F, above polypropylene's limit and close to PVDF's, so jacketed and steam-heated acid lines are PTFE-lined, and thermostatically controlled electric tracing suits a PVDF or polypropylene line that only needs to stay above freezing.

Flanges, vacuum and expansion in acid service

Sulfuric acid is non-volatile, so liner permeation matters far less than with hydrochloric acid or chlorine; our housings are vented anyway, and the vents earn their keep on hot dilute acid, where water vapor is the permeant. Lined spools seal flare to flare without a separate gasket; against alloy, cast iron or glass-lined equipment use a PTFE envelope gasket, torque to the technical details table and retorque after the first hot cycle. Acid concentrators and vacuum evaporators pull the liner inward, so we confirm vacuum ratings, which depend on size and liner, for every spool list that mentions vacuum. On hot headers the housing grows like any steel line, and PTFE-bellows expansion joints take that movement without loading the flares. One more advantage: with no sulfate film to protect, the 2 feet per second ceiling disappears and the line can run at ordinary liquid velocities.

Valves, accessories and where this acid flows

We supply PTFE- and PFA-lined ball valves and PTFE-sleeved plug valves for isolation at unloading racks and headers; lined diaphragm valves for dosing dilute acid; lined butterfly valves for larger dilute-acid circuits; lined check valves at acid pump discharges; and lined sampling valves for drawing samples without opening a flange. No alloy trim is wetted, and automated packages are assembled and tested in our shop. Alongside the valves: lined Y and basket strainers to catch the iron sulfate that acid carries out of steel tanks, PTFE spacers and spectacle blinds, sight glasses, dip pipes for tank unloading and solid PTFE pipe for small-bore and vent lines. Glass-lined reactors and columns are also sound for sulfuric acid at any concentration, within glass's limits on thermal shock and impact, and we supply them alongside lined pipe for the transfer service.

Fertilizer and phosphate plants are the biggest users, making acid in contact plants and consuming it to attack phosphate rock; mining and metals operations leach copper, nickel and uranium with it and run acid plants on smelter gas; refineries use it as alkylation catalyst and move fresh and spent acid to and from regeneration; steel mills and galvanizers pickle in it; water plants dose it for pH; and chlor-alkali plants dry chlorine with it. Our pages for fertilizer and phosphate plants, mining and metals, refineries and petrochemical plants, steel pickling and metal finishing and water and wastewater treatment cover those systems, and the nitric and mixed acids page covers nitration mixtures.

Every lined spool and fitting we ship, 1/2" through 12" in carbon steel or 304/316 stainless housings with ANSI B16.5 flanges, complies with ASTM F1545; fittings conform dimensionally to ANSI B16.5 and are welded to ASME B31.3 by welders certified to ASME Section IX, and each lined part is spark tested at a minimum of 10,000 volts. Custom spools go to 20 ft. More than half a century in the business, shipping nationwide from Fairhope, Alabama. Send the acid concentration, temperature and isometrics: call 251-625-4949 or contact us online.

FAQ: Lined Pipe for Sulfuric Acid

Our 98 percent acid line is carbon steel and has lasted for years. Why line it?

Steel works in 98 percent acid as long as the acid stays dry, cool and slow. It fails where those conditions break: at dilution points, downstream of a leaking steam coil, in a recirculation loop above 2 feet per second, or when the plant switches to 93 percent for winter. A lined line is indifferent to all of that and keeps iron out of the acid.

Can PVDF-lined pipe carry 93 percent sulfuric acid?

At ambient temperature PVDF is used for concentrated acid, but it is not for hot concentrated sulfuric acid, and fuming acid attacks it. For 93 to 98 percent acid that sees any heat we quote PTFE, and we want the maximum temperature before recommending either liner.

What liner handles the acid and gypsum slurry in a phosphoric acid plant?

PVDF, for its abrasion resistance and strength, within its 275 F limit. Our slurries and abrasive service page covers wall thickness and velocity for that duty, and we will want the solids loading and particle size before quoting.

How do you keep a 98 percent acid unloading line from freezing in a northern plant?

Electric tracing under heat-transfer compound, or clamp-on steam heaters, with the control set a little above 40 F; the liner only needs the acid liquid, not warm. Jacketed PTFE-lined pipe is the answer where acid must be held warmer for viscosity. We size the heat to the liner, not the pipe.