CAUSES OF TUBING FAILURES
The purpose of the following information is to focus attention on these facts:
- The environmental conditions within the boiler can be highly aggressive and alter the microstructure of tubing.
- Stresses caused by external loads, or induced by cold forming operations, uneven cooling or welding, may substantially lower the resistance of tubing to attack by certain corrosive media.
Immediately following are contributing factors that may cause failures.
Pitting is a type of extremely localized attack which can be difficult to detect. Pitting is a destructive form of corrosion that affects the water side of boiler tubes. Surface imperfections and deposits can serve as initiation sites for pitting, and a consequent breakdown of the protective scale. The corrosive penetration depends on factors such as temperature, oxygen concentration, and lack of flow of fresh solution to the pitted area.
mproper chemical cleaning procedures may also lead to pitting. Pitting can be prevented by:
- treating the water to eliminate the halide ions.
- providing a continuous flow of water to eliminate stagnant conditions
- minimizing the oxygen content in the water.
- following proper chemical cleaning procedures.
Sulfidation or sulfide corrosion is a problem often encountered if there are reducing conditions in coal and oil fired boilers. Sulfidation can become a problem when temperatures exceed 500F. Sulfides may form scale on the tubing surface and cause damage, because the scale is friable and subject to exfoliation; or the sulfides with their comparatively low melting point may fuse with the tubing surface and cause rapid integranular penetration.
SA213-T22 and T5 offer some resistance to sulfidation in the absence of hydrogen.
Departure from nucleate boiling (DNB) occurs when the bubbles of steam forming on the hot tube surface begin to interfere with the flow of water to the inside tube surface, and eventually coalesce to form a film of superheated steam over part or all of the heating surface. Consequently, the tube wall may be subjected to local overheating and eventual rupture.
Ribbed tubing was developed to inhibit the onset of DNB.