Heater tube failures

It was easy to spot two failed tubes during an inspection of the heater after the fire. One of the 4-inch (10 cm) diameter tubes was swollen like a bubble with a split that was about 2.5 inches (6.4 cm) wide and 4.5 inches (11.4 cm) long. The other tube failure was smaller. The failures were about 6 ft. (1.9 m) from the bottom bend of the tube. 

Burner to tube clearance is very important in heater design because flame radiation is directly proportional to the square of the distance to the tube. Small burner to tube clearance can result in flame impingement, hot spots, and tube failure. That is why most heater failures ean be traeed to flame impingement due to burners plaeed too close to the tubes. For example, eonsider a 5 ft burner to tube clearanee versus 3 ft... 

A fired heater is not operated uniformly over the entire run as it eould run light in turndown operation and harder in full capacity and toward the end of run for reaction heaters. To estimate the effects of changing tube wall temperature, corrosion rates, and pressure, a metaUurgic examination can be applied to estimate the remaining life of tubes. Knowing the tube life not only prevents premature tube failure, but also identihes the need for metal upgrade if the operating skin temperature increases over time. 

A bulging tube indicates a thin area of a tube. If the diameter of a tube were to uniformly increase by 20 percent, then the thickness of the tube would decrease by 20 percent. But tubes rarely expand uniformly. They expand mainly on that side of the tube that is hottest. Hence, a tube bulge that increases the overall tube circumference by 20 percent typically reduces the thickness of the tube in the area of the bulge by 40 percent. For many tubes, this reduction in thickness is sufficient to cause tube failure. There is no theoretical basis for these statements. It is just a summary of what I have seen when a section of tubing is cut from a heater for failure analysis (see Fig. 30.3). 

In the North American market, water heaters are almost always made with the cold water inlet and hot water outlet lines coming out of the top of the tank. The hot water outlet opens right into the top of the tank and so draws off the hottest water. The hot water has risen to the top of the tank because of its lower density. The cold water on the inlet side is directed to the bottom of the tank by a plastic dip-tube. In some models the dip-tube is curved or bent at the end to increase the turbulence at the bottom of the tank. This is to keep any sediment from settling on the bottom of the tank. As sediment— usually calcium carbonate or lime—precipitated out of the water by the increased temperature builds up, it will increase the thermal stress on the bottom of a gas-fired water heater and increase the likelihood of tank failure. On electric water heaters the sediment builds up on the surface of the elements, especially if the elements are high-density elements. Low-density elements spread the same amount of power over a larger surface of the element so the temperatures are not as high and lime doesn t build up as quickly. If the lower elements get completely buried in the sediment, the element will likely overheat and burn out. 

Fires involving liquid process streams are the most common heater loss. Most involve a ruptured process stream tube leading to a firebox fire or a pool fire under or near the heater. The two most common causes are failure of the tubes due to overheating and rupture of the tubes as the result of a fire box explosion. 

All the obstacles in the path of distillation progress, however, were not equipment fabrication and design problems. It was discovered very early in the running of sour crudes that the shell still corroded severely at the vapor-liquid interface line and in that portion of the shell in contact with vapors. At the same time severe corrosion in pipe stills and tube stills, along with overheating and coking, resulted in expensive equipment failures. These problems started metallurgists on a chain of developments which produced the corrosion- and heat-resistant alloys used in modern oil heaters and the alloy liners used in distillation columns. 

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. 

Figure 21.3 Heater tube cross section close to failure.

Dry-point deposits. We sometimes see that a certain heater tube will glow a light red and fail for no apparent reason. The particular tube position in the firebox seems to be far more subject to failure than its neighbors. The tube is not located in an area of flame impingement, and the tubes upstream and downstream are a nice dark red (see Table 21.1). 

Within 20 to 35 minutes after the heater was fired, a fire-water sprinkler system tripped. A heater flame failure alarm occurred a short time later. Witnesses stated flames were over 50 ft. (15 m) high in approximately five seconds after the tube ruptured. The fire damages... 

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... 

Radiant heat flux is defined as heat intensity on a specific tube surface. Thus, heat flux represents the combustion intensity and is analogous to how hard a fired heater is run. More specifieally, keeping the firing rate within safe limits is equivalent to maintaining the peak heat flux at less than the design limit because high firebox temperatures could cause tubes, tube-sheet support, and refiractory failures. What is the peak flux and why is it so important to keep it within the limit These questions will be answered next. 

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