Fouling heater tubes

Many of the conservation measures require detailed process analysis plus optimization. For example, the efficient firing of fuel (category 1) is extremely important in all applications. For any rate of fuel combustion, a theoretical quantity of air (for complete combustion to carbon dioxide and water vapor) exists under which the most efficient combustion occurs. Reduction of the amount of air available leads to incomplete combustion and a rapid decrease in efficiency. In addition, carbon particles may be formed that can lead to accelerated fouling of heater tube surfaces. To allow for small variations in fuel composition and flow rate and in the air flow rates that inevitably occur in industrial practice, it is usually desirable to aim for operation with a small amount of excess air, say 5 to 10 percent, above the theoretical amount for complete combustion. Too much excess air, however, leads to increased sensible heat losses through the stack gas. 

A typical firebox temperature is 1500°F. Thus, the heater tubes can reach 1300°F on loss of the process flow, even though the fuel flow has been immediately stopped. Tubes with a low chrome content may bend and distort as a result of such overheating. Even at 1000°F, residual liquid left in the tubes when flow is lost may thermally degrade to a carbonaceous solid or heavy polymer that fouls the interior of the tubes. 

For fired heaters subject to creep problems, make sure that the tube metal temperature was considered in materials selection, hi the absence of better information, assume the fireside temperature is 100°F (38°C) higher than the process temperature. (If tube-side fouling is anticipated [e.g., coke formation], assume the tube metal temperature is 150°F [85°C] higher than the process temperature.) If necessary, make a note on the template to ensure that creep is accommodated during design of heater tubes, in accordance with API 530 [23]. 

An important observation made by Atkins [1962] was that in fired heater tubes fouling deposits appeared as two layers an outer porous (or tarry layer) and a hard crust of deposit next to the tube wall (see Fig. 11.6). 

A2.2.4 Cooling System—In the normal operation of the JFTOT, some cooling is necessary to remove heat going into the bus bars by conduction from the hot heater tube. Cooling water is circulated through each bus bar using either laboratory tap water (Models 202, 203, and 215 JFTOT) or an internally circulated and radiator cooled liquid system (Models 230 and 240). The only precautions with these systems is to monitor them to be sure they are working and to avoid use of coolants that contain contaminants or salts that may eventually foul the system. 

Asphaltene content. High content of asphaltenic molecules in the feed will reduce conversion. Asphaltenes pass through the furnace relatively unaffected at moderate severities. Under severe conditions, the asphaltenes present in the feed tend to be broken down, which in turn leads to asphaltenes precipitation, resulting in an unstable visbroken residue unsuitable for preparing fuel oil. When this happens, the process lines and the furnace heater tubes are fouled, provoking incomplete combustion of the fuel oil and an early shutdown of the units (shorter run length). 

If fouling of the coils is not the problem, water losses can be reduced with a vapor recovery exchanger mounted on top of the heater shell. It consists of thin tubes that condense the water vapor. Vapor losses can also be reduced by altering the composition of the heat medium or, in drastic cases, by changing the heat medium. 

If this problem—the sudden loss of flow, followed by the premature restoration of flow—occurs repeatedly over a period of a few hours, then layers of fouling deposits or coke are accumulated inside the tubes until a heater shutdown becomes unavoidable. This sort of failure is called a stuttering-feed interruption. 

Flame impingement. This is often caused by dirty burner tips, lack of combustion air, poorly designed burners, high burner tip pressure, improper adjustment of the burner, or improper draft. I have seen a heater in Cartagena, Colombia, with the flames being forced outward against the upper radiant wall tubes. The problem was an extreme positive pressure in the firebox, due to excessive pressure drop of the flue gas in the fouled convective section. 

TEMPERATURE HIGH Ambient Conditions Fouled or Failed Exchanger Tubes Fire Situation Cooling Water Failure Defective Control Valve Heater Control Failure Internal Fires Reaction Control Failures Heating Medium Leak into Process Faulty Instrumentation and Control... 

The value of Uc = 22.0 Btu/hr. sq.ft. °F does not take into consideration the metal resistance. The metal resistance is assumed to be small enough that it may be neglected without introducing any appreciable error. Also, for this example, fouling factors are not considered. It is assumed that the tubes are clean. However, if the particular type of service for the heater is such that fouling factors are critical, they should be included. 

Check steam leakage or leaking tubes and fouled surfaces in indirect heaters... 

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