Lined Pipe for Nitric and Mixed Acid Service

Nitric acid is the acid that makes stainless steel look good right up until the day it does not. Pure acid at moderate strength is one of the few corrosives 304L handles well, so plants are surprised when the same alloy fails on a nitric-hydrofluoric pickle line or a nitration header. Marspec Technical Products fabricates PTFE- and PVDF-lined steel pipe for nitric and mixed acid service, with polypropylene for dilute rinse and neutralization lines where a specification calls for it, and supplies the lined valves and accessories to go with it. Send us the stream and we will tell you which liner. Call (251) 625-4949 or contact us online.

What nitric acid does to steel and alloys

Carbon steel is not usable in nitric acid at any concentration; published data put its corrosion rate above a millimeter a year across the board. Stainless is the opposite story at first, because nitric acid is a strong oxidizer and holds 304L passive: commercial 65 percent acid is routinely stored and piped in 304L at ambient temperature. Acid stronger than roughly 85 to 90 percent, and fuming acid, attack 304L and are stored in aluminum or high-silicon stainless. The trouble starts with heat, contamination and mixtures. Hot concentrated acid attacks grain boundaries and weld heat-affected zones. Fluoride in the acid, even at low levels, breaks down that passivity. And the mixed acids industry actually uses are formulated to defeat the passive film: nitric-hydrofluoric pickle liquor exists to strip scale off stainless, aqua regia dissolves gold and platinum, and nitration acid is hot, dehydrating sulfuric acid carrying nitric or oleum. For hot pure nitric acid the alloy answer is tantalum or zirconium, but neither survives the fluoride in nitric-hydrofluoric pickle, and zirconium is attacked by aqua regia. A PTFE liner inside ordinary steel pipe handles all of these streams for a fraction of the cost.

Liner by liner

PTFE: the liner for the whole range

PTFE is chemically indifferent to nitric acid at every concentration, including red and white fuming grades, and to aqua regia, nitric-HF mixtures and nitration acid, over our full rating of -20 °F to 450 °F (-29 °C to 232 °C). The behavior to design around is permeation: at high concentration and temperature, nitric vapor and nitrogen oxides pass slowly through PTFE. The liner is unharmed, and the vent holes drilled as standard in every spool and fitting housing relieve that permeant pressure to prevent liner collapse. For hot or fuming service we may recommend 304 or 316 stainless housings.

PVDF and polypropylene: dilute and ambient only

PVDF resists dilute nitric acid and moderate concentrations at ambient temperature, and its hardness helps where pickle liquor carries scale and grit. Concentrated acid is marginal, fuming nitric attacks it, and mixtures rich in hot concentrated sulfuric acid are outside its range, so PVDF stays on the dilute, cool end of nitric service within its 0 °F to 275 °F (−18 °C to 135 °C) rating. Polypropylene is a polyolefin, and oxidizers degrade polyolefins; resin data accept dilute nitric acid at room temperature but not strong or warm acid. It is available only where a specification calls for it, on dilute, ambient rinse and neutralization lines.

Venting, gaskets, vacuum and the other details

• Drilled vent holes in every spool and fitting housing, as standard, to relieve permeant pressure and prevent liner collapse; flange faces checked on the inspection round.
• Flange faces seal liner to liner with no gasket. Where a lined flange meets a glass-lined nozzle or an alloy vessel, use a PTFE envelope gasket; elastomer gaskets do not belong in a nitric header.
• The steel housing sets the thermal growth, as in any steel line, so hot nitric lines get lined expansion joints with PTFE bellows and real anchoring.
• Absorption towers, spent-acid stills and vacuum unloading put nitric lines under vacuum, so mark those lines on the line list. Liner temperature limits, liner thicknesses, flange dimensions and torque tables are on our technical details page.
• PTFE cold-flows under bolt load, so hot acid joints are retorqued after the first thermal cycle.

Spools are fabricated to your isometrics in sizes 1/2" through 12" and lengths up to 20 ft, rotating flanges standard, in the same liner as the elbows, tees and reducers that complete the run.

Valves and accessories for nitric service

Valve selection in nitric acid comes down to keeping every wetted surface fluoropolymer. PFA-lined ball valves handle on-off duty on transfer and loading lines. PFA-lined diaphragm valves are our pick for throttling and dosing, since the diaphragm is the only moving wetted part and nothing packs or weeps. PFA-lined cylindrical plug valves, 1/2" through 2" full port and 3" through 6" reduced port, cavity-free, with no in-line adjustment, give tight shutoff on mixed-acid feed to nitrators and pickle tanks. PTFE-lined butterfly valves cover the larger sizes, provided the seat is PTFE rather than an elastomer. Lined check valves protect dosing pumps, and lined sampling valves draw an acid-strength sample without opening the line. Automated packages are assembled and tested in-house. The range is on our plastic-lined valves page.

Around the valves go lined Y strainers ahead of metering pumps, PTFE spectacle blinds for positive isolation when a pickle tank or nitrator comes down, and sight glasses to confirm flow. Commercial 60 to 68 percent nitric stays liquid in all but extreme cold, but acid near 54 percent freezes at about 0 °F (−18 °C), so check the freezing point for the strength you run. Sulfuric-rich nitration acid thickens in a cold pipe rack, and we supply electric tracing and clamp-on heaters that keep it moving without exceeding the liner's limit. For jacketed or traced service, we recommend specifying PTFE.

Where nitric and mixed acids are handled, and how to order

The heaviest users we build for are steel pickling and metal finishing lines, where nitric-HF pickling, bright dipping and passivation baths run on it; fertilizer and nitrogen plants making ammonium nitrate and nitrophosphate; semiconductor fabs, where lined pipe carries bulk acid supply, acid waste and neutralization while the electronic-grade nitric and mixed acids run in solid natural PVDF or PFA; pharmaceutical and fine chemical plants running nitrations; and mining and metals operations that refine precious metals in aqua regia. The companion streams have their own pages on hydrofluoric acid service and sulfuric acid service.

Nitration and nitric acid chemistry are also classic glass-lined reactor services. Borosilicate glass fused to a steel shell resists nitric acid across the concentration range and cleans easily between batches, so the sensible split is reactors and receivers in glass, transfer lines and valves in PTFE and PFA. Glass is never used with hydrofluoric acid or fluoride-bearing streams, which makes a nitric-HF pickle circuit fluoropolymer end to end, and it does not tolerate hot strong caustic, thermal shock or impact. We supply glass-lined pipe and equipment, including reglassing.

Our lined pipe and fittings comply 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 spool and fitting passes a minimum 10,000-volt spark test. We have lined pipe for more than half a century and ship nationwide from Fairhope, Alabama. Send the acid composition, concentration, temperature and any vacuum condition with the line list, then call (251) 625-4949 or contact us online.

FAQ: Lined Pipe for Nitric and Mixed Acids

Can PTFE-lined pipe carry fuming nitric acid?

PTFE is chemically resistant to red and white fuming nitric acid, and we quote it for that service. Because nitric vapor and nitrogen oxides permeate the liner more readily at high concentration and temperature, send us the operating conditions so we can advise on housing material and inspection intervals.

Why is my 304L piping fine on nitric acid but failing on the nitric-HF pickle line?

Nitric acid alone keeps 304L passive, but hydrofluoric acid and fluoride ions destroy that passive film. That is what lets the pickle bath strip oxide scale from stainless, and it attacks a 304L line just as readily. The mixture needs a fluoropolymer liner, and because fluorides also attack glass, PTFE-lined pipe and valves are the standard answer rather than glass-lined.

Is polypropylene-lined pipe acceptable anywhere in a nitric system?

Only where a specification calls for it, on dilute, ambient streams such as rinse water and neutralization lines, and we would want to see the concentration before quoting it. Nitric acid is an oxidizer, and polypropylene is not suitable for strong or warm nitric acid, which goes in PTFE.

What valves do you recommend for nitration mixed acid?

PFA-lined cylindrical plug valves or PFA-lined ball valves for shutoff and PFA-lined diaphragm valves for feed control, all with fluoropolymer wetted surfaces and PTFE seats rather than elastomers. Tell us the sulfuric-to-nitric ratio and temperature and we will match the lining and confirm the trim.