Tube metal temperature

FIG. 27-40 Elffect of departure from nucleate boiling (DNB) on tube-metal temperature. 

A typical process heater tube diameter is 4 to 10 in. Tube thickness is usually between V4 and V2 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. 

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... 

Compared to a fired reformer, catalyst tube damage is less likely. This is because direct flame impingement cannot occur, and flue gas distribution is not a concern. The maximum tube metal temperature is also limited to the temperature of the shell-side gases. Since die reformer burners have been eliminated, process control is less complicated. And the lack of a flue gas stream reduces emissions of NOx and C02 by as much as 60% to 75%203. 

The shutdown setpoint was found to have been mistakenly raised to the maximum of the instrument s setpoint range or 1600° F (870° C). (The tube-metal high temperature alarm should have been fixed at 830° F (443° C) and the shutdown temperature should have been 850° F (454° C).) The investigating team concluded the limited burner shutdown instrumentation had not been properly inspected and tested to insure the necessary high degree of reliability. Perhaps the instrumentation was troublesome. If just a high tube metal temperature warning alarm would have sounded, the loss may have been minimized. 

Heat exchangers are treated in a manner similar to columns (i.e., 12Cr clad shells and channels and 5Cr- Mo tubes are usually used above 550° F [288°C]). When selecting materials for exchangers, one must take into account crevices, changes in direction, and actual tube metal temperatures (since the tubes are exposed to fluids of different temperatures). 

This situation can be clearly seen when observing the time evolution of the tube metal temperature of the pyrolysis coils there is a fast initial increase and then a reduced asymptotic slope. Note that although the initial slope is initially related to the catalytic rate, it is also due to the relatively low thermal conductivity of the initial fibrous material as a result of the large void fraction. The thickness of this layer is in the order of 20-40 pm. The evolution of the fluid temperature over time either at the TLE outlet or in visbreaking processes and in delayed coking furnaces shows a very similar behaviour. 

Thermal conductivity, H/C ratio, specific volume and specific heat vary during the chemical evolution of the deposit. Unfortunately, there is very small quantity of data in the literature on thermal conductivity. In fact, what little there is refers to coke or bitumen and provides limited or sometimes contradictory information because of the high dependency on the structure and composition of the solid. More reliable data refer to disordered graphite, similar to an aged deposit, without hydrogen and with a low porosity. The available experimental data on the time evolution of pressure drop and tube metal temperature in pyrolysis coils of ethylene crackers only permit rough estimates of the overall and average thickness and thermal conductivity of the deposit. 

The bottom-fired reformer is classified into two types, one with the reforming gas flowing down the tube (as in top-fired and side-fired) and the other where the process gas flows up the reformer tubes. The burners are located on the floor on either side of two rows of reformer tubes. The flames are long and pencil thin. The system has a simplified air combustion distribution and single operating level. The system cannot handle more than 200-300 reformer tubes per radiant section and the tube metal temperatures at the process gas outlet are higher than the inlet. 

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]. 

The location of the maximum tube metal temperature changes as the heat flux profile changes. Retrofitting ultralow NO and the latest generation burners in short fireboxes can result in high metal temperatures for roof and shock tubes. 

TWTor Tube Metal Temperature (TMT) Identify root cause for high TWToperation. 

BWT bridge wall temperature TMT tube metal temperature TWT tube wall temperature... 

Sample connections, together with pressure, temperature and flow measurement points, are located at the inlet and outlet of the reformer and membrane modules to measure the performance of the RMM. A multipoint thermocouple is installed inside the first reformer tube in order to monitor the axial temperature profile along the heated catalyst length while two glass peepholes allow the reformer tube metal temperature to be measured by an infrared pyrometer. The control room is located in a safety area with a bird s eye view of the plant area. 

Tube metal temperature from installed thermocouples. 

If tube metal temperature data are not available, they can be calculated from process information and verified by correlation with the historic metal loss rates. This procedure is obviously one of second choice nevertheless, it has been adopted successfully many times. Alternatively, when the desired thermocouple tube metal temperature and tube wall thickness inspection data are available, one of the standard QA data procedures is to check that they are physically compatible. The authors have come across cases where either the thermocouple wall temperature data or the reported wall thickness data must be in error, as it was physically and thermodynamically impossible for them to co-exist. In such cases, the procedure is to compare the tube wall thinning history in question with an in-house database for all such similar alloys and compare the reported thermocouple data with calculated data from process information. It is generally clear which of the data sets is in major error. 

The coil with the higher gas outlet temperatures requires higher heat transfer coefficients and/or higher heat transfer surface-to-volume ratio in order to not exceed the tube metal temperature limitation. This is achieved by utilizing a coil with relatively small diameter outlet tubes. This design, however, results in other undesirable effects due to simultaneous heat, mass and momentum transfer. 

Where A max. is the coke thickness which coincides with the maximum allowable tube metal temperature or the maximum allowable pressure drop. 

In all reactor types, the desired conversion takes place on the catalyst. Side-reactions, such as carbon formation (refer to Chapter 5), are eliminated by use of the proper catalyst types, controlled catalyst bed inlet temperatures and a given H2O/C ratio depending on the t) e of feed. The role of the catalyst is in all cases to achieve equilibrium conversion and maintain low pressure drop and in tubular and convective reformers to keep low tube metal temperatures, too. 

For measuring hot (1,000-1-°F) furnace flue-gas temperatures, a velocity thermocouple should be used (see Chapter 15). Using an ordinary static thermocouple will indicate a lower-than-actual temperature due to the effects of reradiation from the thermocouple. Furnace tube metal temperatures are approximated visually ... 

An optical pyrometer can successfully be used to follow firebox tube metal temperatures. This instrument will read the color of the heater s tubes. In practice, this method will permit one to follow the Increase in tube metal temperature as the heater s run length progresses and the tubes get hotter. Since reading the optical pyrometer is somewhat subjective, it is best to have the same individual take all the readings. 

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. 

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