Coal

Wallpapering Concerns

Issue 10 and Volume 103.

The June 1999 article, “New Welding Technology Eases Wallpapering,” contains some questionable information on wallpapering and implies that the technique is slow, difficult to do and uses alloys that have relatively short service lives.

Lining of FGD equipment with thin 1/16-inch to 1/8-inch sheets, or wallpapering as it has become known, has been shown to be a relatively fast, cost-effective way of using the high-nickel alloys and the higher-alloyed stainless steels which are required in the most corrosive areas encountered in FGD service. The lower-alloyed stainless steels such as Types 316L and 317L generally are used only in the thicker, solid plate form because sheet thicknesses usually do not offer a significant cost advantage and because thicker sections of these alloys are preferred from a corrosion-resistance standpoint.

Corrosion in FGD equipment can be caused not only by sulfur acids, but also by chlorides and even fluorides that are present in coal. Both pitting corrosion and crevice corrosion are the major types of attack found in FGD systems, and if the service life of the alloys being used is limited by corrosion, the user should select a more-corrosion resistant alloy. Proper selection of alloys will give the desired length of service.

Wallpapering procedures are set forth in NACE International Recommended Practice RP0292-92 (now RP0292-98) and call for an overlap of about one inch for joints between lining sheets. Generally, welding of stainless steels and nickel alloys using proper filler metals is done using the computerized, synergic, pulsed arc, gas metal arc welding process. Welds produced by this method are high quality with good penetration and a smooth, low profile. Welding of stainless steels and nickel alloys is different but it is not difficult.

W.L. Mathay
Consultant, Nickel Development Institute
Pittsburgh, Pa.