Lined Pipe for Brine and Seawater Service

Salt water is the most ordinary corrosive in the world and still one of the hardest on stainless steel. Chloride ions pit, crevice-corrode and stress-crack the 300-series alloys that look so sensible on a drawing, and carbon steel simply rusts away. Marspec Technical Products fabricates PTFE-, PVDF- and polypropylene-lined steel pipe for brine and seawater service and supplies lined valves, strainers and tracing to match. Call (251) 625-4949 or contact us online with your line list and we will quote it.

Why chlorides beat stainless steel

Seawater carries roughly 19,000 parts per million of chloride; reverse-osmosis concentrate runs near double that; saturated sodium chloride brine, at about 26 percent salt, is close to 160,000. Against those numbers the published thresholds for austenitic stainless are sobering. In fresh water, 304 is satisfactory below about 200 ppm of chloride and marginal from 200 to 1,000 ppm, where 316 is preferred, and pitting rises sharply above a critical temperature; in seawater, at roughly 19,000 ppm, 316 pits and crevice-corrodes at ambient temperature, and crevice corrosion under a gasket or a deposit starts lower still. Above roughly 50 °C, chloride stress corrosion cracking of both grades becomes a real risk at every weld. The alloys that do hold up in warm, aerated chloride water are titanium, copper-nickel and super-duplex stainless within its temperature limits, all priced accordingly. A plastic liner takes chloride out of the materials question: the steel housing never sees the water, and the liner has no passive film to break down.

Three liners, three kinds of brine

• Polypropylene, rated 0 °F to 225 °F (−18 °C to 107 °C), is available where a specification calls for it on sodium chloride, potassium chloride, calcium chloride and sodium sulfate solutions that are neutral or alkaline and free of oxidizers: brine saturation and purification, salt and potash circuits, food-grade brine, cooling seawater at moderate temperature.
• PVDF, rated 0 °F to 275 °F (−18 °C to 135 °C), takes over when the brine carries chlorine, hypochlorite or bromine, or has been acidified with hydrochloric acid, though chlorine lowers its limit to about 200 °F (93 °C) and bromine to about 150 °F (66 °C). Polypropylene is not suitable for depleted chlor-alkali brine, which carries dissolved chlorine; we recommend PTFE for it and for electrochlorinated seawater, with PVDF an option. PVDF is also the hardest, most abrasion-resistant of the three liners, useful where brine carries salt crystals.
• PTFE, rated -20 °F to 450 °F (-29 °C to 232 °C), is the liner we recommend and quote for brine service: it covers hot brine, chlorinated brine at temperature, bromine and mixed or uncertain streams. When a line sees brine one day and acid the next, PTFE needs no second opinion.

All three come in lined pipe and fittings from 1/2" through 12", with carbon steel or 304/316 stainless housings and custom spools to 20 ft. Send us the composition, temperature and any chlorine or acid content and we will confirm the liner.

Chlor-alkali brine, desalination brine, oilfield brine

The chlor-alkali brine loop

A membrane cell plant moves brine in a loop that changes character at every step. Saturated brine is purified at a pH of 9 to 11 with caustic and soda ash, filtered, polished through chelating resin that is regenerated with hydrochloric acid and caustic, then acidified with HCl to a pH of 4 to 6 before the cells. The cells return depleted brine at about 17 percent salt, roughly 200 g/L, carrying dissolved chlorine, and that stream is acidified again to a pH of 2 to 4 ahead of a vacuum dechlorination tower before it goes back to saturation. PTFE serves the whole loop, alkaline purification side and acidified, chlorinated, warm return side alike, with PVDF an option on the return side and polypropylene available for the purification side where a specification calls for it; because the dechlorinator pulls vacuum, the liner's vacuum rating by size matters as much as its chemistry. Companion pages cover chlorine and sodium hypochlorite, hydrochloric acid and caustic soda service in the same plant.

Desalination and produced water

Seawater reverse-osmosis plants dose sodium hypochlorite at the intake, ferric chloride as coagulant, sulfuric acid for pH, sodium metabisulfite to strip chlorine before the membranes and antiscalant ahead of the high-pressure pumps, then clean the membranes with acid, caustic and peracetic acid. Every one of those chemical lines is lined-pipe service, and the concentrate header, at roughly 35,000 ppm chloride, is where 316 fails first. Large seawater mains are usually fiberglass or polyethylene; lined steel earns its place where pressure rating, support spans and mechanical protection call for steel, around pumps and inside buildings. Oilfield produced water and well-acidizing returns combine high chloride with carbon dioxide, hydrogen sulfide and spent hydrochloric acid, which is PTFE service, and lithium brines bring the same chloride load to the battery materials industry.

Flanges, housings, valves and tracing for salt service

Inside a lined joint the liner faces seal against each other, so there is no gasket crevice for chloride to concentrate in. The outside of the pipe is a different matter on a coastal site or under salt drip: carbon steel housings are primed for a field topcoat, and we supply 316 stainless housings where the exterior environment or washdown justifies them; on traced or insulated lines ask us, since warm stainless under wet, salty insulation can crack. Rotating flanges are standard; liner temperature limits, liner thicknesses, flange dimensions and torque tables are on our technical details page.

For valves, PTFE-lined butterfly valves are the economical choice on 6" through 12" brine mains, with PTFE seats rather than elastomers where chlorine is present. Lined ball valves handle on-off duty, PFA-lined diaphragm valves meter hypochlorite, bisulfite and acid at the dosing skids, PFA-lined cylindrical plug valves (1/2" through 2" full port and 3" through 6" reduced port, cavity-free, with no in-line adjustment) suit brine that carries salt fines, lined check valves protect pump discharges and lined sampling valves draw a clean chloride sample. All are on our plastic-lined valves page, and we assemble and test automated packages in-house.

Lined Y and basket strainers ahead of pumps and membranes catch salt crystals and shell fragments, sight glasses show when a saturated line is starting to crystallize, and PTFE spectacle blinds isolate an intake train for cleaning. Saturated brine drops salt as it cools, and sodium sulfate solutions crystallize far more steeply, so near-saturated lines outdoors are traced or jacketed, and seawater lines in northern plants are traced against freezing. Electric tracing, clamp-on steam heaters and heat-transfer compound are sized to the liner's limit; see our piping accessories page. For jacketed or traced service, we recommend specifying PTFE.

Industries, standards and a quote

Brine and seawater lines run through chlor-alkali and chemical plants, water and wastewater treatment and desalination works, food and beverage plants that brine and pickle, mining and metals operations that process potash, salt and lithium, and coastal refineries and petrochemical complexes that cool with seawater. Our lined pipe 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 every lined spool and fitting gets a minimum 10,000-volt spark test. We have been in plastic-lined piping for more than half a century and ship nationwide from Fairhope, Alabama. Send the brine analysis and the line list, then call (251) 625-4949 or contact us online.

FAQ: Lined Pipe for Brine and Seawater

Can we use polypropylene-lined pipe for chlor-alkali brine?

On the saturation and purification side it is available, since that brine is alkaline, cool and free of chlorine, but we recommend and quote PTFE: depleted brine leaving the cells carries dissolved chlorine and is acidified with HCl, so PTFE covers both sides of the loop on one liner, and in a finished spool the liner is a small part of the cost.

Why did our 316 stainless seawater line pit when the chloride level seemed low?

Pitting of 316 is reported at only a few hundred ppm of chloride once the water is warm, and crevice corrosion under gaskets and deposits begins cooler still; seawater has roughly 19,000 ppm. A lined steel line removes the alloy from contact with the water, which is a cheaper fix than moving to super-duplex or titanium.

Do brine lines need heat tracing?

Saturated and near-saturated brine sheds salt as it cools, and sodium sulfate brines crystallize sharply, so outdoor saturated lines are commonly traced or jacketed. Dilute brine and seawater only need tracing where freezing is possible. For traced lines we recommend specifying PTFE, and we size the electric tracing or clamp-on heaters to stay inside its temperature rating.

Which valves do you suggest for the chemical dosing skids at a desalination plant?

PFA-lined diaphragm valves for hypochlorite, bisulfite, acid and coagulant metering, since they have no packing and throttle cleanly, with lined check valves on each pump discharge and lined ball valves for isolation, with a vented ball (vent facing upstream) on the hypochlorite and peracetic acid lines. Butterfly valves with PTFE seats cover the larger brine and concentrate headers.