DETERIORATION MECHANISMS OF NON PRESSURE PARTS OF BOILERS / HEATERS

Non-pressure parts, including refractory linings of heaters, burners, supporting structures, and casings, may also be damaged from overheating. Usually, such overheating is caused by improper operating conditions or is a result of deterioration of other protective parts. For example, if the refractory lining of a heater is permitted to deteriorate from normal wear, erosion, spalling, or mechanical damage, it will no longer protect the outer heater casing and structural supports adequately, and such parts may in turn begin to deteriorate rapidly.

Tube Hangers and Supports

Hangers and supports are made from heat resistant alloys chosen for their high-temperature strength, creep properties, and resistance to corrosion. Most of this hardware is originally made from castings although wrought materials tend to be installed for replacements due to good availability of plate and bars. The form of the material, cast vs. wrought, and the grade of material influence the deterioration mechanisms.

Tube hangers and supports deteriorate for several reasons including stress/creep rupture, mechanical damage, corrosion and poor quality castings. Similar to previous discussions about stress and creep rupture of tubes, hangers and supports fail from an excessive combination of stress and temperature. Components made from a casting generally have better stress rupture/creep properties than the wrought equivalent. Therefore, extra attention should be given to those wrought components. Especially vulnerable are those fabricated for an unplanned replacement of a cast component. Corrosion of the supports and rods can reduce the cross-section enough to elevate the stress level to promote failure. Corrosion can occur from high-temperature oxidation, fuel ash, and acid attack (during turndown). Oxidation can be avoided by proper alloy selection. Fuel ash and acid turndown attack from sulfur in deposits can be severe depending on the fuel quality.

Mechanical damage from vibration in service or mechanical impact during maintenance work can crack the components. Castings are particularly susceptible to mechanical impact damage since they tend to have poor resistance to impact loads. In addition, some alloys can change metallurgically from long-term exposure at elevated temperatures to become brittle at room temperature.

Poor casting quality can be the root cause for premature failures. Casting defects like voids and cracks, can initiate failure from other mechanisms like stress rupture or mechanical damage. These cast components do not usually receive significant inspection after casting. Some purchasers have found it necessary to require supplemental radiographic inspection to assure themselves of acceptable components.

Casing and Structural Steel

Corrosive agents are produced in the combustion of fuels that contain sulfur.

Deterioration from sulfur will occur on cold steelwork when it has been exposed to the heater gases as a result of deterioration of the refractory or insulating linings or if a heater is operated under a positive pressure. It is imperative that the outer casing of heaters be maintained in a tight condition. When flue gases are permitted to permeate to the atmosphere at various locations, they deposit sulfurous acid on the casing and metal parts that are below the dew point. Such deposits are acidic, accelerating corrosion of the casing and the refractory supports. Most fired heaters are designed to operate at negative pressure. Operation at positive pressure results in flue gas leakage and shell corrosion.

The rate of deterioration caused by climatic conditions primarily depends on whether the atmosphere is dry, humid, or salty and on the industrial fumes that may be present. Deterioration resulting from a humid atmosphere may not be due to geographic location but may be the result of the location of the heater within the refinery. Location near cooling ponds or towers when the prevailing winds are toward the heater may cause deterioration.

The types of deterioration resulting from climatic conditions are rusting of exposed or unpainted steelwork, general deterioration of painted surfaces, and erosion and further deterioration of the external housing of a heater. If the external housing is allowed to deteriorate, rain or other moisture will enter the openings and deteriorate the internal refractory, insulation, and steelwork, especially when the heater is out of service for any reason (see API RP 571).

Structures
Foundation settlement may be a serious cause of deterioration in boilers because of the severe stress that may be set up in the complicated interconnection of parts, in the external piping, and especially in the refractory linings and baffling. Excessive loads on the boiler by the connection of large pipe lines may cause damage to the boiler foundation and pressure parts.

Settlement of foundations may also result from heat transmission from the firebox and subsequent drying of the soil.

In zones with seismic activity, earthquakes may cause severe damage. The damage will be somewhat similar to that caused by foundation settlement and may be particularly severe to refractory linings. Not only will refractory linings be affected but internal, firebrick gravity walls (also know as bridgewalls or centerwalls) may also be damaged and have a need for inspection. Vibrations from high and moderate winds, earthquakes, burner operating instability, and high flue-gas flow across tube banks can damage various parts of boilers as follows:

a.    Stacks may be so damaged that they overturn.
b.    Air and flue-gas ductwork may be damaged, resulting in cracks at corners or connections.
c.    Expansion joints may crack.
d.    Guy lines may loosen or break.
e.    Piping and tubing may be overstressed and fail.
f.    Anchor bolts of stacks may be overstressed and fail.