Lined Pipe for Hydrochloric Acid Service
Hydrochloric acid gives carbon steel and stainless steel no place to hide! It attacks steel at every concentration without forming a protective film, and 304 and 316 stainless go active and pit in it. The answers have always been exotic metals or a plastic barrier, and for more than half a century Marspec Technical Products has supplied the barrier: PTFE- and PVDF-lined steel pipe spools, fittings and valves that carry HCl for a fraction of the cost of tantalum or nickel-molybdenum alloy. Call 251-625-4949 or contact us online with your line list.
What hydrochloric acid does to metal
Commercial acid is sold at 31 to 37 percent, and by-product acid from vinyl plants runs near 32 percent. HCl forms a constant-boiling mixture with water at about 20 percent, boiling near 230 F, and it is corrosive to steel at every point on that curve. Inhibited 10 to 15 percent acid is tolerated for a few hours of pickling, and that is all plain steel will do. The alloys that survive are expensive and particular: nickel-molybdenum alloys resist all concentrations to the boiling point but corrode rapidly once ferric or cupric ions or dissolved chlorine contaminate the acid, as they usually do in a working plant; titanium is attacked; tantalum and zirconium hold up and are priced accordingly. A fluoropolymer liner is indifferent to those contaminants, which is why lined carbon steel is the default for HCl unloading, transfer, dilution and absorber piping.
Liner choice for HCl
PTFE
PTFE is our standard liner for hydrochloric acid at every concentration, rated from minus 20 F to 450 F. It handles 37 percent acid, the azeotrope, hot regenerated pickling acid and the chlorine-and-organics mixtures off chlorination reactors. Its one weakness here is permeation, covered below, so PTFE-lined HCl lines get vented housings and scheduled flange-face inspection.
PVDF
PVDF is rated from 0 F to 275 F and resists hydrochloric acid across the concentration range. It is the strongest of the three liners, which helps in larger sizes under vacuum, and HCl permeation through PVDF is generally lower than through PTFE at the same wall, so it is often specified for warm concentrated acid within its temperature limit. Keep it away from HCl mixed with amines, ketones or esters.
Polypropylene
Polypropylene (0 F to 225 F) is available where a specification calls for it on dilute hydrochloric acid at ambient and moderate temperatures: pH adjustment, ion-exchange regeneration, effluent neutralization. It is not for concentrated acid at temperature or for acid carrying free chlorine, and even on the dilute services we recommend and quote PTFE, since in a finished spool the liner is a small part of the cost.
Permeation, venting, vacuum and flanges
Why HCl housings are vented
Hydrogen chloride is a small molecule and it permeates every fluoropolymer, faster as temperature and concentration rise. What gets through condenses with permeated water vapor against the steel housing. If the housing is sealed, that acid corrodes the steel behind the liner, lifts the liner and collapses it. Vent holes through the steel let the permeant escape, and a rust stain at a vent is a maintenance signal, not a failure. Our lined pipe and fittings are vented as standard, and on HCl we recommend heavier liners in long hot runs and flange-face inspection at every opportunity, since that is where permeated acid shows up first. Vacuum, common in HCl absorbers and strippers, loads the liner a second way; vacuum ratings by size and liner are on our technical details page.
Gaskets, torque and thermal growth
A lined spool seals liner to liner: the flared face on each end is its own gasket, so a lined-to-lined joint needs none. Against alloy, glass-lined or rubber-lined equipment, use a PTFE envelope gasket, with a PTFE spacer if the mating face is recessed. Bolt torque matters more in HCl than elsewhere, because an under-torqued flare weeps fume that attacks the bolting; use the torque table on the technical details page and retorque after the first thermal cycle, since fluoropolymers cold-flow. Rotating flanges are standard, fixed by request. The housing carries the load, so thermal growth is that of a steel line: take it up with PTFE-bellows expansion joints on hot headers, not at the flanges.
Valves, accessories and glass-lined equipment for HCl
For on-off duty at unloading stations and headers we quote PFA- or PTFE-lined ball valves or PTFE-sleeved plug valves; the plug's wiping action keeps the seat clean in acid carrying iron chloride solids. Lined diaphragm valves dose dilute acid without a cavity to trap it, lined butterfly valves cover the larger scrubber-water sizes, lined check valves protect acid pumps, and lined sampling valves draw a sample without opening a flange. No metal trim touches the acid, and automated packages leave our shop with actuators, positioners and switches mounted and tested. Around the valves: PTFE-lined strainers, spectacle blinds, sight glasses, dip pipes, and solid PTFE pipe for small-bore sample and vent lines.
Upstream of the transfer lines, HCl is a classic glass-lined service. We also supply glass-lined reactors, columns, receivers and glass-lined pipe with split-ring flanges; borosilicate glass shrugs off hydrochloric acid at any concentration, which is why chlorination reactors are so often glass. Reactors in glass, transfer lines and valves in PTFE is the natural split. The firm exclusion is fluoride: any HCl carrying hydrofluoric acid or fluorosilicates stays out of glass, as our hydrofluoric acid page explains.
Where HCl piping goes to work
Chlor-alkali and vinyl plants make and consume the most of it, as by-product acid from ethylene dichloride cracking, and absorb it in falling-film absorbers under vacuum. Steel mills pickle strip in hot HCl and regenerate the spent liquor in roasters that return acid of roughly 18 percent to the line. Ore leaching, electronic-grade HCl in wafer fabs, catalyst regeneration in refineries and pH control in water plants round out the list. See our pages for chlor-alkali and chemical plants, steel pickling and metal finishing, mining and metals, semiconductor and high-purity plants and refineries and petrochemical plants, and the chlorine and sodium hypochlorite page, since wet chlorine is HCl in the making.
Lined pipe and fittings run 1/2" through 12" in carbon steel or 304/316 stainless housings with ANSI B16.5 class 150 flanges, with custom spools to 20 ft. Products 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 part is spark tested at a minimum of 10,000 volts before it ships nationwide from Fairhope, Alabama. Send the line list, concentration, temperature and any vacuum conditions and we will return a liner recommendation and a price. Call 251-625-4949 or contact us online.
FAQ: Lined Pipe for Hydrochloric Acid
Why do PTFE-lined HCl lines rust at the vent holes, and is that a failure?
A rust streak at a vent hole means hydrogen chloride permeated the liner, met moisture at the steel wall and left through the vent as designed. It is a cue to inspect flange faces and liner condition at the next outage, not a leak. A vent hole that has been plugged or painted over is the real problem, because trapped acid then corrodes the housing and lifts the liner.
Can lined pipe handle 37 percent acid at summer unloading temperatures?
Yes. PTFE and PVDF both carry 37 percent hydrochloric acid at ambient and warm conditions well inside their limits. For long runs of hot concentrated acid we lean toward PVDF or heavy-wall PTFE for lower permeation, and we ask for the actual temperature before we quote.
Which valve pattern holds up best on HCl carrying iron chloride solids from pickling?
PTFE-sleeved plug valves. The plug wipes its own seat on every stroke and has no cavity to collect sludge. Lined ball valves do well in clean acid, and lined diaphragm valves are the choice for dosing and throttling.
We have glass-lined reactors; can lined steel transfer lines bolt straight to them?
Yes, that is the usual arrangement. Use a PTFE envelope gasket at the glass-to-lined-steel joint, match the drilling (ANSI B16.5 or DIN on the glass side), and keep any fluoride-bearing stream out of the glass.