25.2 Heater Tube Failures
Heater tubes are designed to operate at a particular pressure an temperature. The design pressure of the tube is not the inlet operating pressure of the heater. The design tube pressure is the heater charge pump dead-head, or shut-in, pressure, as discussed in Chap. 29. The design temperature of the tube is not the heater outlet process operating temperature. The design tube temperature is the anticipated or calculated maximum tube skin temperature (at end-of-run conditions), which is simply the temperature of the exterior metal surface of the tube. Many plants call this temperature the tube metal indication (TMI).
The calculated tube skin temperature is mainly a function of the fouling resistance assumed inside the tube. The greater the assumed fouling resistance, the higher the design tube skin temperature, and the thicker the tube wall. In a sense, then, we partially assume the design tube thickness, on the basis of experience, for a particular plant service.
A typical process heater tube diameter is 4 to 10 in. Tube thickness is usually between 0.25 and 0.50 in. Heater tubes are often constructed
out of chrome steel. A high chrome content is 13 percent. The chrome content increases the heat resistance of the tube. A tube with a 11 to 13 percent chrome content can normally withstand a skin temperature of up to 1300 to 1350°F. A low-chrome-content tube of perhaps 3 percent may be limited to 1200°F tube metal temperature. Naturally, the pressure, thickness, and diameter of the tube, all affect its maximum
skin temperature limitations.
For added corrosion and temperature resistance, the nickel alloy content of tubes and sometimes the Moly (molybdenum) content as well are increased. Tubes with a high nickel content are classified as 300 series stainless steels. A 0.5 percent silicon content is used to
enhance the tube's oxidation or exterior scaling resistance.
25.2.1 High-Temperature Creep
When the tube metal temperature exceeds a value of 1300 to 1400°F, it becomes plastic. This means that the pressure inside the tube causes the tube diameter to expand. This is called high-temperature creep. As the diameter of the tube bulges and expands, the tube walls become progressively thinner and ultimately too thin to constrain the pressure inside the tube, and the tube bursts. Large-diameter tubes operating at higher pressures and with a thin wall thickness fail at a relatively low tube skin temperature.
Tubes seldom fail because of external oxidation, and tubes rarely "burn up." They fail because of high-temperature creep, which causes the tube to expand and burst. Thus, the fundamental cause of tube failure is a high localized temperature, which is called a "hot spot."
Heater tubes are designed to operate at a particular pressure an temperature. The design pressure of the tube is not the inlet operating pressure of the heater. The design tube pressure is the heater charge pump dead-head, or shut-in, pressure, as discussed in Chap. 29. The design temperature of the tube is not the heater outlet process operating temperature. The design tube temperature is the anticipated or calculated maximum tube skin temperature (at end-of-run conditions), which is simply the temperature of the exterior metal surface of the tube. Many plants call this temperature the tube metal indication (TMI).
The calculated tube skin temperature is mainly a function of the fouling resistance assumed inside the tube. The greater the assumed fouling resistance, the higher the design tube skin temperature, and the thicker the tube wall. In a sense, then, we partially assume the design tube thickness, on the basis of experience, for a particular plant service.
A typical process heater tube diameter is 4 to 10 in. Tube thickness is usually between 0.25 and 0.50 in. Heater tubes are often constructed
out of chrome steel. A high chrome content is 13 percent. The chrome content increases the heat resistance of the tube. A tube with a 11 to 13 percent chrome content can normally withstand a skin temperature of up to 1300 to 1350°F. A low-chrome-content tube of perhaps 3 percent may be limited to 1200°F tube metal temperature. Naturally, the pressure, thickness, and diameter of the tube, all affect its maximum
skin temperature limitations.
For added corrosion and temperature resistance, the nickel alloy content of tubes and sometimes the Moly (molybdenum) content as well are increased. Tubes with a high nickel content are classified as 300 series stainless steels. A 0.5 percent silicon content is used to
enhance the tube's oxidation or exterior scaling resistance.
25.2.1 High-Temperature Creep
When the tube metal temperature exceeds a value of 1300 to 1400°F, it becomes plastic. This means that the pressure inside the tube causes the tube diameter to expand. This is called high-temperature creep. As the diameter of the tube bulges and expands, the tube walls become progressively thinner and ultimately too thin to constrain the pressure inside the tube, and the tube bursts. Large-diameter tubes operating at higher pressures and with a thin wall thickness fail at a relatively low tube skin temperature.
Tubes seldom fail because of external oxidation, and tubes rarely "burn up." They fail because of high-temperature creep, which causes the tube to expand and burst. Thus, the fundamental cause of tube failure is a high localized temperature, which is called a "hot spot."
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