Lined Pipe for Water and Wastewater Treatment

The water is the easy part of a treatment plant. The chemicals that make it safe, sodium hypochlorite, ferric chloride, sulfuric acid, caustic soda, fluosilicic acid and sodium bisulfite, between them rust through carbon steel, pit stainless and gas-bind pumps. Marspec Technical Products fabricates PTFE-, PVDF- and polypropylene-lined steel pipe spools and supplies the lined fittings, valves and accessories chemical feed systems are built from, with spools shipping nationwide from Fairhope, Alabama. Call (251) 625-4949 or contact us online with a chemical list and a sketch.

More than half a century in plastic-lined piping has put us in front of every chemical a treatment plant feeds.

What a treatment plant pumps, and which liner carries it

A surface water plant doses coagulant, usually ferric chloride, ferric sulfate, alum or polyaluminum chloride, adjusts pH with sulfuric acid, caustic or lime, oxidizes with permanganate or chlorine, disinfects with sodium hypochlorite, forms chloramine with aqueous ammonia, fluoridates with fluosilicic acid and feeds orthophosphate for corrosion control. A membrane plant adds antiscalant, sulfuric acid, sodium bisulfite ahead of the membranes, citric acid and caustic for cleaning, and brine on the reject side; softeners and nitrate removal beds regenerate with salt brine.

Wastewater adds ferric chloride or alum for phosphorus, methanol, glycerin or acetate for denitrification, caustic or magnesium hydroxide for alkalinity, hypochlorite and bisulfite for disinfection and dechlorination, peroxide, nitrate or ferric for sulfide and odor control, magnesium chloride and caustic for struvite recovery, acid and caustic neutralization, and caustic scrubbers on digester gas.

Sodium hypochlorite: the line that bites back

Bulk 12.5 percent hypochlorite loses strength two ways, to chlorate and to oxygen, and the oxygen reaction fills pipe with gas. It runs faster as temperature and concentration rise, and it is catalyzed by nickel, copper, cobalt and iron, which is why the wetted path should contain no metal at all, not even a spring in a check valve. The gas air-binds metering pumps and, trapped between two closed valves, builds pressure that has split pipe and ruptured valve bodies. So a hypochlorite line is laid out to vent back to the tank, without dead legs or horizontal pockets where gas collects. It can freeze in deep cold; trace it only to keep it from freezing, and never overheat it, since heat speeds its decay. We recommend PTFE-lined spools for bulk-strength hypochlorite, because commercial bleach carries excess caustic that attacks PVDF over time, with PVDF an option only for dilute or on-site hypochlorite at pH 12 or below, and polypropylene wherever a customer specification requires it. A lined diaphragm valve with the PTFE/FPM diaphragm is the first choice, since hypochlorite permeates the PTFE face and the FPM backing resists it; where a quarter-turn valve is wanted, we supply a lined ball valve vented to the upstream side so the cavity cannot trap off-gassing liquid. Our chlorine and sodium hypochlorite page goes deeper.

Liner by stream

• Ferric chloride, ferric sulfate, alum and polyaluminum chloride: PTFE at feed temperatures, with PVDF or polypropylene where a specification calls for it. Ferric chloride pits 316 stainless quickly, and the others still attack carbon steel; with the liner doing the work, the housing can stay carbon steel
• Sulfuric acid, delivered at 93 percent and diluted at the point of use: PTFE from the dilution point through the dilute feed lines, with polypropylene available for dilute acid at ambient temperature where a specification calls for it; see our sulfuric acid page
• Caustic soda at 25 or 50 percent: PTFE, with polypropylene available where a specification calls for it, never PVDF, which caustic attacks at any strength; see our caustic soda page
• Fluosilicic acid at 23 percent: it attacks glass, silica and steel, so unloading and feed lines are PTFE-, PVDF- or polypropylene-lined, as our hydrofluoric acid page explains
• Bisulfite, citric acid, antiscalant, methanol and polymer: PTFE, or polypropylene where a specification calls for it
• Peroxide, permanganate and chlorine dioxide: PTFE, with PVDF an option when cool
• Aqueous ammonia: PTFE, or polypropylene where a specification calls for it; amines and caustic attack PVDF, so we prefer PTFE for ammonia
• Lime, magnesium hydroxide, carbon and struvite slurries: PVDF with lined diaphragm or plug valves, polypropylene where a specification calls for it
• Softener brine, RO concentrate and seawater: PTFE, which also takes chlorinated brine that runs warm, with PVDF or polypropylene available where a specification calls for them; see our brine and seawater page

Where lined steel fits next to plastic pipe

Small-bore metering lines are often plastic tubing, and that is fine. Lined steel earns its place in the 1" through 12" runs plastic pipe handles badly: tanker unloading, transfer from bulk storage to day tanks, headers in galleries and under roadways, pump discharge manifolds, outdoor runs in sunlight, and anywhere a solvent-cemented joint has failed on hypochlorite. We fabricate custom lined pipe spools up to 20 ft and supply lined fittings from 1/2" through 12" in PTFE, PVDF or polypropylene, in carbon steel or stainless housings, flanged to ANSI B16.5. Liner temperature limits, liner thicknesses, flange dimensions and torque tables are on our technical details page.

Valves

Lined diaphragm valves for throttling and isolation on hypochlorite, ferric and acid; lined ball valves, vented when the service off-gasses; lined check valves at metering pump discharges and injection points; lined butterfly valves for large-bore concentrate and brine; and lined sampling valves on bulk headers. We supply Swissfluid PFA- and PTFE-lined valves and build automated valve packages in house; Swissfluid USA supports its national distribution network from its facility in Stafford, Texas.

Accessories and winter

Lined Y and basket strainers ahead of metering pumps and injection quills, sight glasses so an operator can see a feed line moving, PTFE spacers and spectacle blinds for positive isolation at unloading stations, expansion joints and dip pipes for day tanks are on our piping accessories page. Fifty percent caustic crystallizes near 54 °F (12 °C), which is why many plants buy 25 percent, and an outdoor caustic or polymer line in a northern winter needs clamp-on heaters or electric tracing with heat-transfer compound from our temperature maintenance line. Hypochlorite is the exception: keep it cool and shaded.

Where this work is

The scale varies by city. The Stickney plant outside Chicago treats up to 1.4 billion gallons a day, one of the largest in the world, and Baltimore's Back River and Patapsco plants run some of the largest nutrient removal systems in the country on methanol and ferric. Tampa Bay operates the largest seawater desalination plant in the Southeast, California runs the Carlsbad plant and the reuse systems of Orange County, El Paso in Texas has one of the largest inland desalination plants anywhere, and Pittsburgh's ALCOSAN plant is among the largest on the Ohio.

Built to spec, quoted from a sketch

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 gets a minimum 10,000-volt spark test before it ships. A feed system does not need full isometrics for a price: send the chemical, concentration, line size, a rough layout and the flange standard, and we will confirm the liner and quote it. Call (251) 625-4949 or contact us online. We ship nationwide from Fairhope, Alabama, and fabricate custom spools in our own shop.

FAQ: Lined Pipe for Water and Wastewater Treatment

Which liner do you recommend for 12.5 percent sodium hypochlorite feed?

PTFE for bulk-strength hypochlorite, since commercial bleach carries excess caustic that attacks PVDF over time, with PVDF an option only for dilute or on-site hypochlorite at pH 12 or below, and polypropylene wherever a customer specification requires it. Pair it with lined diaphragm valves or vented lined ball valves and lay the line out to vent back to the tank.

Can one lined header carry both ferric chloride and sodium hypochlorite at different times?

We recommend against it: give hypochlorite a dedicated line. Ferric chloride is strongly acidic, and hypochlorite releases chlorine gas when it meets acid, so one missed flush becomes a release. Ferric chloride runs in PTFE, or PVDF where a specification calls for it, and bulk hypochlorite runs in PTFE.

Is lined steel pipe used for fluosilicic acid at a fluoridation system?

Yes. Fluosilicic acid attacks glass, silica and steel, and PTFE-, PVDF- and polypropylene-lined pipe are all unaffected at feed strength and temperature. We supply the spools, lined diaphragm valves with the PTFE/FPM diaphragm, and lined swing check valves for unloading, storage and feed.

How do we keep an outdoor 50 percent caustic line from plugging in winter?

Trace it. Fifty percent caustic crystallizes near 54 °F (12 °C), so we supply electric tracing or clamp-on steam heaters with heat-transfer compound that mount outside the lined spool and hold the line above that point. Many plants switch to 25 percent caustic for the same reason. For jacketed or traced service, we recommend specifying PTFE. Keeping the liner inside its rating, even when the line is not flowing, depends on sizing the steam pressure or tracer output, and we need your application data to make that recommendation.