Lined Pipe for Fertilizer and Phosphate Plants
A phosphate complex is a sulfuric acid plant, a phosphoric acid plant and a granulation plant on one fence line, and the streams between them carry fluoride, chloride, sulfate and abrasive solids at temperatures that pit 316 stainless and eat carbon steel outright. Nitrogen complexes add nitric acid, ammonium nitrate and UAN. Marspec Technical Products fabricates PTFE-, PVDF- and polypropylene-lined steel pipe spools and supplies the lined fittings, valves and accessories for that service, shipping nationwide from Fairhope, Alabama. Call 251-625-4949 or contact us online.
We have spent more than half a century in plastic-lined piping, and this page goes unit by unit through the streams in a fertilizer plant and the liner we would put in each.
Where the acid goes in a phosphate complex
Sulfuric acid: strong acid is easy, weak acid is not
A sulfur-burning acid plant makes 93 to 98 percent sulfuric acid, which is stored at ambient temperature in plain carbon steel because it passivates the surface. The trouble starts wherever the acid is diluted: the dilution point ahead of the phosphoric acid reactor, weak acid from the gas-cleaning section, and acid dosed into process water for pH control. Below roughly 70 percent, carbon steel corrodes quickly, and faster still as the acid gets weaker and hotter. PTFE-lined spools handle sulfuric acid across the whole concentration range within the liner's 450 °F limit, polypropylene-lined spools are available for dilute acid at ambient temperature where a specification calls for them, and we do not put PVDF in hot concentrated sulfuric acid. Our sulfuric acid service page has the detail.
Phosphoric acid: fluoride, chloride and gypsum
Wet-process acid is made by digesting ground rock in sulfuric acid, filtering off phosphogypsum and concentrating the filter acid, roughly 26 to 30 percent P2O5 from a dihydrate reactor, in vacuum evaporators to 52 to 54 percent merchant grade and on toward 70 percent superphosphoric acid. The acid is never clean: it carries free hydrofluoric and fluosilicic acid from the fluorine in the rock, chlorides that vary with the ore body, sulfate, and gypsum and silica fines. Chloride and fluoride together pit 316L stainless, and a fluoropolymer liner takes the alloy question off the table. PTFE is unaffected by fluoride-bearing acid at evaporator temperatures, and PVDF, the strongest of the three liners, is the pick for filter acid and clarifier underflow where gypsum abrasion sets the life of the line. Evaporators and flash coolers run under vacuum; vacuum ratings by size and liner are on our technical details page.
Fluosilicic acid and pond water
The fluorine driven off during evaporation is scrubbed from the vapor as fluosilicic acid, recovered at about 18 to 25 percent and sold for water fluoridation and aluminum fluoride production. It attacks glass, carbon steel and stainless alike. PTFE- and PVDF-lined spools carry it from the scrubber loop to storage and the loading rack, with lined diaphragm valves for throttling and lined check valves at the pumps; our hydrofluoric acid page covers the fluoride chemistry. Acidic process water from the gypsum stack, near pH 2 and loaded with P2O5 and fluoride, runs in PTFE-lined pipe at ambient temperature, as do the ammonia scrubber liquors in granulation; polypropylene is available for both where a specification calls for it.
Granulation, nitrogen and potash
Ammoniating phosphoric acid into MAP and DAP produces scrubber liquor that swings between acidic phosphoric acid and ammonium phosphate solution, with fluoride in both: PTFE-lined pipe takes the scrubber loop and the hot preneutralizer alike, with PVDF or polypropylene available at scrubber temperatures where a specification calls for them. In a nitrogen complex the ammonia travels in steel, but nitric acid, ammonium nitrate solution and UAN are lined-pipe media. UAN is corrosive enough to mild steel that producers ship it inhibited; PTFE-lined pipe carries it uninhibited, and polypropylene is available for UAN where a specification calls for it. Nitric acid belongs in PTFE, since it oxidizes polypropylene at strength; see our nitric and mixed acid page. Potash plants add hot saturated potassium chloride brine, which stress-cracks stainless and is routine duty for PTFE- and PVDF-lined pipe, covered on our brine page.
What a phosphate spool package looks like
Most of what we ship to a phosphate plant is 2" through 8" PTFE- or PVDF-lined pipe in carbon steel housings: evaporator feed and discharge spools, filter feed lines, fluosilicic acid loops, acid dilution headers and pond water returns, fabricated to isometrics in lengths up to 20 ft with rotating flanges so bolt holes line up on a rack that has shifted since it was built. We supply 1/2" through 12" in carbon steel or 304 and 316 stainless housings, with lined elbows, tees, reducers, laterals and instrument tees that conform dimensionally to ANSI B16.5. Hot fluoride-bearing acid permeates slowly through PTFE, so our housings are vented to let permeants escape instead of blistering the liner. See our lined pipe and fittings page.
Valves and accessories
- PTFE-lined sleeved plug valves on filter acid and evaporator circuits, where gypsum would score a ball and seat
- PTFE- and PFA-lined diaphragm valves for throttling scrubber liquor, fluosilicic acid and pond water
- Lined ball valves on clean acid headers, lined check and sampling valves, and automated packages assembled and tested in-house
- Lined strainers ahead of evaporator pumps and spray nozzles, PTFE-bellows expansion joints, spacers and spectacle blinds for isolating an evaporator during descaling, sight glasses, dip pipes and spargers
- Solid PTFE pipe for small-bore fluosilicic acid and instrument lines
Keeping it fluid
Superphosphoric acid is viscous at ambient temperature, 54 percent acid grows crystals as it cools, UAN 32 begins to salt out just below freezing, and 50 percent caustic crystallizes near 54 °F. We supply clamp-on steam heaters, electric tracing and heat-transfer compound for traced lines, and steam-jacketed lined pipe and fittings where a line must be held hot, matched to the liner's temperature limit.
Where the industry is
The Bone Valley district behind Tampa holds the largest concentration of phosphate mines and concentrate plants in the country, and the Aurora operation in North Carolina is the largest integrated phosphate mine and chemical plant in the world. Southeast Idaho feeds the plants around Soda Springs and Pocatello. Nitrogen production clusters at Donaldsonville below Baton Rouge, across Iowa and Nebraska and at Cheyenne in Wyoming, and potash is refined around Carlsbad in New Mexico.
Standards and how to order
Our lined 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 receives a minimum 10,000-volt spark test before it ships. Send a line list or isometrics with acid strength, temperature and solids loading, and we will confirm the liner and quote it. Call 251-625-4949 or contact us online. Custom spools fabricated in our own shop ship nationwide from Fairhope, Alabama.
FAQ: Lined Pipe for Fertilizer and Phosphate Plants
Which liner handles 54 percent phosphoric acid coming off the evaporators?
PTFE. Evaporator product is hot, under vacuum on the suction side and full of fluoride and chloride, and PTFE is the only one of the three liners rated for that combination. PVDF is a good choice for cooler filter acid where gypsum abrasion matters more than temperature.
Can lined pipe carry 23 percent fluosilicic acid to storage and loading?
Yes. Fluosilicic acid attacks glass and silica, so glass-lined equipment is out, but PTFE, PVDF and polypropylene liners are unaffected at ambient temperature. We supply vented housings, lined diaphragm valves for throttling and lined check valves for the transfer pumps.
We run both UAN and nitric acid at a nitrogen plant. Does one liner cover both?
PTFE covers both and is what we recommend when a header sees either stream. Polypropylene is available for UAN at ambient temperature but should not be used for nitric acid, which oxidizes it at strength.
Can you match the lined spools already on our evaporator rack during a turnaround?
Yes. Give us the face-to-face dimensions, size, flange rating and liner, and we fabricate replacements that bolt to the existing flanges, spark test them and ship from Fairhope, Alabama. Custom spools built to field dimensions are our normal work.