Powell Manufacturing & Fabrication (St. Louis, Mich.) manufactures 800 million lbs of household and industrial-grade bleach per year. The piping and fittings in the plant's manufacturing and waste treatment area must stand up to chlorinated and alkaline compounds at a wide range of temperatures and pressures.In the manufacturing areas, liquid chlorine reacts with 22% sodium hydroxide to form sodium hypochlorite at pressures to 80 psia. Reactor temperatures rante from 0°F at the chlorine inlet, to 130°F.
The treatment section presents similar problems. The gas scrubbers routinely contact sodium hydroxide at concentrations to 25% and sodium hypochlorite concentrations to 20%. Temperatures reach 200°F.
The effects of the chemicals, temperatures and pressures are most visible in steel fittings. Stress cracking occurs in elbows, tees, 45-degree joints and concentrics in both sections of the plant. According to Clarence Powell, company president, fittings would last anywhere from two weeks to two months before requiring replacement.
Unable to accept the continual replacement of fittings or the potential for leaks, Powell Manufacturing looked for more-resistant materials. Fittings made from polyvinyl chloride (PVC) and chlorinated polyvinyl chloride (CPVC) were considered first.
PVC cannot be used at temperatures above 100°F so it was immediately eliminated. CPVC has a higher temperature resistance than PVC but becomes brittle at low temperatures. The material won't survive chlorine-inlet temperatures in the process area or ambient temperatures in the gas scrubbers, which are located outdoors.
The company than examined various fluoropolymer linings for the steel fittings. Polytetrafluoroethylene was easily permeated by chlorine. When this happened cracking occurred just as it did in the unlined fittings.
Polyvinylidene fluoride was tried next. The material withstood the chlorine, but couldn't hold up against the sodium hydroxide at process operating temperatures.
Ethylene chlorinated tetrafluoroethylene was resistant to chemicals and temperatures in the two areas but, the linings peeled away from the fittings in the low-pressure transfer lines. Faced with the prospect of replacing linings as frequently as it once replaced fittings the company continued its search.
Finally, Powell Manufacturing tried TEFZEL, an ethylene-tetrafluoroethylene copolymer made by DuPont (Wilmington, DE). This material resists chlorine and sodium hydroxide and temperatures franging from -148 to 300°F. The firm lined all its fittings, and the pipes in the bleach plant with 1/8-inch thick TEFZEL.
For pipes the TEFZEL is extruded in tubes with diameters 2-4% larger than the pipe diameter. The tubes are then compressed slightly and pulled through the pipe, whereupon the they expand to fit snugly against the pipe walls. For fittings, a proprietary molding process creating a permanent bond between the lining and the steel is used.
Since TEFZEL fittings were installed last year not one has had to be replaced. The company has since installed lined piping with diameters to 18 inches and fittings with diameters to eight inches.