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-, PVDF- and polypropylene-lined steel pipe for nitric and mixed acid service and supplies the lined valves and accessories to go with it. Send us the stream and we will tell you which liner, or 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 and storage up to roughly 95 percent are routinely handled in 304L. The trouble starts with heat, contamination and mixtures. Hot concentrated acid attacks grain boundaries and weld heat-affected zones. A few hundred parts per million of chloride or fluoride turns passivation into pitting. 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. The alloy answer is tantalum or zirconium. A PTFE liner inside ordinary steel pipe answers the same 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. 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, but whatever reaches the steel housing attacks it, which is why our lined pipe and fittings have vented housings that let permeant escape instead of blistering the liner. For hot or fuming service we may recommend 304 or 316 stainless housings so a weeping vent hole does not thin a carbon steel shell.

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 rating. Polypropylene is a polyolefin, and oxidizers degrade polyolefins; published data accept 10 percent nitric at room temperature and reject 40 percent and above. We do not recommend it beyond dilute, ambient rinse and neutralization lines.

Venting, gaskets, vacuum and the other details

• Vented housings on every spool and fitting, with vent holes and 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.
• Fluoropolymer liners expand more than steel, 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. Vacuum ratings by size and liner are on our technical details page, with flange ratings and torque.
• 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. PTFE-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. Sleeved plug valves 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. Tracing is rarely needed on pure nitric, which freezes far below any weather in the country, but 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.

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 nitric etches and mixed-acid cleans run in small sizes at high purity; 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.

Everything we ship 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 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 with vented housings. 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, which is exactly why the pickle bath removes scale. 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 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 degrades in oxidizing service, so 40 percent and stronger acid is out regardless of temperature.

What valves do you recommend for nitration mixed acid?

Sleeved plug valves or PTFE-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.