Lined Pipe for Hydrofluoric Acid Service

Hydrofluoric acid is the one common process chemical that takes glass, silica and most alloys off the table at once. It etches glass-lined equipment, attacks stainless steel as soon as there is water in it, attacks even nickel-copper alloy 400 once the acid is aerated, and finds any pinhole in a coating. Fluoropolymer-lined steel pipe is the standard answer for aqueous HF because PTFE and PFA are among the very few materials HF does not touch, and the steel housing around them carries the pressure and takes the abuse. Marspec Technical Products fabricates PTFE- and PVDF-lined pipe spools and supplies lined fittings and valves for HF service, with spools shipped from Fairhope, Alabama. Call (251) 625-4949 or contact us online with the concentration, temperature and line sizes.

HF is also one of the most dangerous fluids in a plant, so the piping around it is specified conservatively: inspected flanges, spark-tested liners, no exposed metal in the wetted path, and valves that do not weep through packing.

What HF does to the usual materials

Hydrofluoric acid weaker than roughly 60 to 70 percent corrodes carbon steel rapidly (stronger acid and anhydrous HF are handled in steel, as described below) and attacks 304 and 316 stainless aggressively; even high-nickel alloys depend on concentration, temperature and aeration. Glass and most ceramics dissolve in it, which rules out glass-lined pipe, glass-lined reactors and sight glasses with glass windows. Fluoride ions are the problem, not just the acid: fluosilicic acid, ammonium bifluoride, acidic fluoride rinse water and spent nitric-hydrofluoric pickle liquor all behave the same way toward glass and alloys. A fluoropolymer liner sidesteps the whole list. PTFE is unaffected by hydrofluoric acid at any concentration across its working range, and because it is a liner inside steel, the line keeps a full pressure rating and the ruggedness of flanged steel.

Liner selection for hydrofluoric acid

PTFE: the default

A PTFE liner handles HF from dilute rinse water to concentrated acid, from -20 °F to 450 °F (-29 °C to 232 °C), mixed with nitric or sulfuric acid or not. It is the liner we recommend for concentrated HF, hot HF, mixed acids and any line where the concentration swings. HF permeates PTFE even at ambient temperature, and faster as temperature and concentration rise. Our PTFE liners meet or exceed the ASTM F1545 minimum at every size and run well above it from 1-1/2" up, so permeant takes longer to reach the housing and arrives more slowly. The vent holes drilled as standard in every housing relieve permeant pressure between the liner and the housing to prevent liner collapse. The flanges get 100% woven PTFE spray shields, which have no glass, or polypropylene shields on lines near ambient. A PTFE-coated shield is built on glass cloth and a stainless one is 316 stainless, and HF attacks both.

PVDF: dilute acid near ambient temperature

PVDF resists hydrofluoric acid well and is widely used for it in semiconductor fabs and metal finishing, where the acid is dilute to moderate and the temperature is near ambient. Its working range is 0 °F to 275 °F (−18 °C to 135 °C), and its higher strength and smoother bore make it the better choice where the line also carries abrasive solids or must stay clean for high-purity work. Above moderate concentration at elevated temperature we specify PTFE instead. Send us the concentration and temperature together; the pair decides.

Polypropylene: dilute and cool only

Polypropylene-lined pipe is available for dilute HF at ambient temperature, which covers some rinse and neutralization lines, but it has the narrowest window of the three, 0 °F to 225 °F (−18 °C to 107 °C), so for concentrated acid, mixed acids with nitric and anything warm we recommend PTFE. On a dilute fluoride wastewater line we still recommend and quote PTFE; polypropylene is available where a specification calls for it.

How an HF line is built

The wetted path on a lined HF line is liner to liner: the PTFE flare on each spool and fitting seats against the flare on the next, so the acid never reaches the steel flange or a gasket of some other material. That is the main reason lined pipe is preferred over coated or alloy pipe in HF service. We fabricate custom lined spools from 1/2 inch through 12 inch, up to 20 ft, in carbon steel or 304 or 316 stainless housings, and supply lined elbows, tees, reducers and laterals to the same ANSI B16.5 flange pattern. Lined spectacle blinds and PTFE spacers give positive isolation for maintenance, lined strainers protect pumps, and lined expansion joints take thermal movement where a loop will not fit; all are on our piping accessories page. Every lined spool and fitting is spark tested at a minimum of 10,000 volts before it ships. Vacuum matters where HF is drawn from tank cars or recovered 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.

Valves for hydrofluoric acid

The valve question in HF service is the pipe question with packing added: the stem seal must not weep acid into the air. PFA-lined ball valves with a lined ball and PTFE seats give tight quarter-turn shutoff on transfer and unloading lines with no wetted metal. 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 quarter-turn isolation with no cavity to trap acid. Lined diaphragm valves are the choice for throttling and for lines with fluoride solids, since the diaphragm is the stem seal and there is no packing at all. HF permeates the PTFE face of the diaphragm, so on those lines we supply the PTFE/FPM diaphragm, whose FPM backing resists it, rather than the standard PTFE/EPDM. Lined swing check valves protect pumps and injection points, and lined sampling valves draw a sample without opening the line. All of our lined valves come flanged to match the lined pipe, and we assemble automated packages in-house, with actuators, positioners and switches mounted and tested, for HF block valves that must close from the control room.

Where hydrofluoric acid runs

HF alkylation units in refineries use anhydrous HF as the catalyst and handle it in carbon steel and Monel; the fluoropolymer-lined piping is on the wet side, in the acid relief neutralization, KOH scrubber and acid-soluble oil systems where water is present. Fluorochemical and fluoropolymer plants make and consume HF in bulk. Semiconductor fabs run dilute and buffered HF for oxide etch and cleaning; the electronic-grade acid runs in solid natural PVDF or PFA, and lined pipe there is for bulk chemical supply, acid waste and neutralization. Stainless and specialty alloy mills pickle in nitric-hydrofluoric mixed acid, which is PTFE territory. Phosphate fertilizer plants scrub fluosilicic acid out of their phosphoric acid evaporators and sell or neutralize it. Aluminum smelters handle fluoride in cryolite bath and scrubber water, and glass plants etch with HF. Our pages on refineries and petrochemical plants, semiconductor and high-purity plants, steel pickling and metal finishing and fertilizer and phosphate plants cover each of these in turn, and HF users include plants in Texas, Louisiana, Kentucky, West Virginia, New Jersey and Arizona.

Anhydrous HF and the limits of lining

Anhydrous hydrogen fluoride, with essentially no water, is handled in carbon steel because the steel forms a protective fluoride film, and the industry's materials practice for it is its own discipline. Lined pipe is not a drop-in there. The lined system belongs wherever water enters: wet HF, HF vapor scrubbing, neutralization, acid blending and the fluoride wastewater downstream. If a line crosses from anhydrous to aqueous service, that boundary is where the specification changes, and we are glad to talk through it with your materials engineer.

Built to the standard your HF spec calls for

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 every lined spool and fitting is spark tested before it leaves the shop. We have fabricated plastic-lined piping for more than half a century and ship nationwide from Fairhope, Alabama. Send the line list with concentration, temperature, pressure and flange standard, and we will recommend the liner and quote the spools, fittings and valves together. Call (251) 625-4949 or contact us online.

FAQ: Lined Pipe for Hydrofluoric Acid

Which liner should I specify for 49 percent hydrofluoric acid?

PTFE. PTFE is not attacked by HF at any concentration, though HF permeates it; put 100% woven PTFE spray shields on the flanges, or polypropylene near ambient (HF attacks the glass cloth in PTFE-coated shields and the 316 stainless of metal ones). PVDF is an option for dilute or moderate HF near ambient temperature; send us the concentration and temperature together and we will confirm.

Can glass-lined pipe or a glass sight glass be used in HF service?

No. Hydrofluoric acid dissolves glass and silica, so glass-lined pipe, glass-lined vessels and glass-window sight glasses are excluded from any fluoride-bearing stream. Use PTFE-lined pipe and a lined flow indicator with a fluoropolymer window instead.

Why do lined pipe housings have vent holes?

HF is a small molecule and permeates PTFE even at ambient temperature, faster when hot. Every spool and fitting housing has drilled vent holes as standard; they relieve that permeant pressure between the liner and the housing to prevent liner collapse.

What valve is best for hydrofluoric acid isolation?

PFA-lined ball valves and PFA-lined cylindrical plug valves are the usual isolation valves, with lined diaphragm valves, fitted with the PTFE/FPM diaphragm, where the line throttles or carries fluoride solids, because the diaphragm doubles as the stem seal. All come flanged to match lined pipe, and we assemble automated packages in-house.