Lined Pipe for Caustic Soda Service

Carbon steel handles caustic soda, right up until it does not. Near room temperature a steel line carries 50 percent sodium hydroxide for decades; warm it past roughly 120 to 150 °F (49 to 66 °C), depending on concentration and whether the welds were stress relieved, and caustic stress corrosion cracking takes over. Marspec Technical Products fabricates PTFE- and polypropylene-lined pipe spools and supplies the lined fittings and Swissfluid lined valves for caustic at every strength, with the tracing and jacketing that keep strong caustic liquid outdoors. Call (251) 625-4949 or contact us online for a quote.

Why line a pipe for a chemical steel tolerates

Three reasons, in the order we hear them. Iron: steel corrodes slowly in caustic, but it does corrode, and the result is rust-tinted liquor that fails specification for membrane-grade product and for food, pharmaceutical, rayon and bleach-making customers; a lined line puts no metal in the stream. Temperature: evaporator discharge, dilution loops and steam-heated caustic routinely run past the cracking threshold, and the choices are stress-relieved steel with inspection, nickel-alloy piping, or a liner. Mixed service: the line that carries 50 percent caustic today may carry hypochlorite, brine or acid wash tomorrow, and a PTFE-lined system takes all of them. Hot strong caustic also attacks glass, so glass-lined equipment is wrong above roughly pH 12 at temperature; that work goes to a PTFE liner.

Picking the liner

PTFE is the liner we recommend and quote for sodium hydroxide at every strength: it covers ambient transfer, hot and mixed services on one liner specification, and in a finished spool the liner is a small part of the cost. Polypropylene-lined pipe is available for caustic transfer, tank and dosing lines wherever a customer specification requires it, and its orange color is easy to identify in the field.

PTFE, rated from minus 20 °F to 450 °F (minus 29 °C to 232 °C), is the liner wherever the caustic is hot: evaporator product lines, dilution loops, steam-traced or jacketed strong caustic, spent caustic from refineries and crackers that carries sulfides and hydrocarbons, and any line that alternates between caustic and acid. It is also the liner for caustic mixed with solvents or oxidizers.

PVDF is the one liner we steer away from here; it is not used above about pH 12. Its resin maker does not recommend it in caustic soda at any strength, so every caustic line we quote is PTFE. If a PVDF-lined acid system must take an occasional caustic wash, send us strength, temperature and duration.

Crystallization, freezing and temperature maintenance

Caustic soda crystallizes as hydrates rather than simply freezing, and the crystallization temperature climbs steeply with concentration. Fifty percent caustic begins to crystallize near 54 °F (12 °C), so an outdoor 50 percent line in most of the country needs heat for part of the year. Seventy-three percent caustic solidifies well above 140 °F (60 °C) and must be kept hot from the evaporator to the point of use. Dilute caustic at 20 to 30 percent stays liquid to much lower temperatures, which is why some plants dilute at the unloading rack rather than trace. Crystals in a dead leg or valve cavity plug sample lines, bind stems and crack instruments.

We build heat into the lined line three ways. Steam-jacketed lined pipe and fittings with a PTFE liner hold hot 50 and 73 percent caustic at process temperature from evaporator to loading rack. Clamp-on steam heaters warm an existing lined line without a jacket. Electric tracing under heat-transfer compound is the usual freeze protection. For jacketed or traced service, we recommend specifying PTFE: 15 psig steam is about 250 °F (121 °C) and will cook a polypropylene liner. Insulate every traced line, trace valves and instrument branches too, and slope the line to drain.

Flange practice, vacuum and expansion

Caustic is non-volatile, so liner permeation is a minor concern, though the housings are vented like all of our lined pipe. The sealing surface is the flared liner face, so lined-to-lined joints need no gasket; at a steel or stainless tank nozzle, use a PTFE envelope gasket, torque to the values on our technical details page and retorque after the first warm cycle. Caustic that crystallizes between flare and flange face will lift the flare, so traced lines get traced flanges, and PTFE spacers belong wherever a nozzle face is recessed. Vacuum shows up in evaporators and in unloading-pump suction on a cold morning, so flag those lines when you send the line list. On jacketed and traced headers the housing grows with heat, and PTFE-bellows expansion joints absorb it without stressing the flanges. Custom spools to 20 ft keep the flange count down in a service that eats bolting when it weeps.

Valves, accessories and the industries that run on caustic

For isolation we quote PFA-lined ball valves and PFA-lined cylindrical plug valves, 1/2" through 2" full port and 3" through 6" reduced port, cavity-free, with no in-line adjustment; with no body cavity to trap caustic that will crystallize, the plug valve is our first suggestion on 50 percent service. Lined diaphragm valves meter caustic to pH control, scrubbers and clean-in-place systems, lined butterfly valves cover the larger dilute lines, lined check valves protect dosing pumps, and lined sampling valves draw a sample from a line that is corrosive and, hot, a burn hazard. Actuated packages leave our shop assembled and tested. With them: lined Y and basket strainers to catch crystals ahead of pumps and control valves, sight glasses, PTFE spectacle blinds for positive isolation, dip pipes and spargers for storage and neutralization tanks, and solid PTFE pipe for small-bore sample and vent runs.

Chlor-alkali plants make caustic, concentrate it in evaporators and ship it at 50 percent. Pulp mills consume it in white liquor, bleach-plant extraction and chemical recovery; alumina refineries digest bauxite in hot strong caustic; refineries and ethylene plants scrub acid gases with it and then handle the spent caustic; food and dairy plants circulate it in clean-in-place loops; water plants use it for pH; and battery materials plants use it to convert lithium sulfate to lithium hydroxide and to precipitate cathode-precursor hydroxides. See our pages on chlor-alkali and chemical plants, pulp and paper mills, refineries and petrochemical plants, food and beverage plants and water and wastewater treatment, and the chlorine and sodium hypochlorite page for the bleach that caustic becomes.

Our lined pipe and fittings run 1/2" through 12" in carbon steel or 304/316 stainless housings with ANSI B16.5 class 150 flanges, rotating as standard. 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 gets a minimum 10,000-volt spark test. More than half a century in plastic-lined piping, shipping nationwide from Fairhope, Alabama. Send the line list, concentration and temperature; call (251) 625-4949 or contact us online.

FAQ: Lined Pipe for Caustic Soda

Can we use polypropylene-lined pipe for 50 percent caustic?

It is available wherever a customer specification requires it, but we recommend and quote PTFE, which also covers the lines that run hot, are jacketed or traced, carry spent caustic with hydrocarbons or alternate with acid.

Our 50 percent caustic unloading line plugs every winter. What do you recommend?

Trace it. Fifty percent caustic crystallizes near 54 °F (12 °C), so electric tracing under heat-transfer compound with insulation, carried over the valves, blinds and sample points, keeps the line open. For jacketed or traced service, we recommend specifying PTFE. Swap any ball valve with a cavity for a lined plug valve, and slope the line so it drains to the tank.

Can we steam-jacket a lined line for 73 percent caustic?

Yes, with a PTFE liner, which is rated to 450 °F (232 °C). Seventy-three percent caustic must stay well above 140 °F (60 °C) to remain liquid, and jacketed PTFE-lined pipe and fittings, with tracing and insulation carried over the valves, are the normal design.

Why not just use stainless steel for hot caustic?

304 and 316 handle caustic at ambient temperature, but strong caustic approaching 200 °F (93 °C) puts them in the caustic stress-corrosion-cracking zone, and chlorides carried in diaphragm-cell caustic make it worse. A PTFE-lined carbon steel line takes the same conditions without the cracking risk and without adding nickel or chromium to the liquor.