Convection section tube removal

EXHIBIT 7-17 Convection Section Tube Removal in 3 Box-Type Furnace... 

The inside of the convection tubes rarely foul, but occasionally the Hquid unsaturates in feedstocks tend to polymerize and stick to the walls and thus reduce the heat transfer. This soft coke is normally removed by mechanical means. In limited cases, the coke can also be burnt off with air and steam. Normally, the outside surface of the convection section fouls due to dust and particles in the flue gas. Periodically (6 to 36 months), the outside surface is cleaned by steam lancing. With Hquid fuel firing, the surface may require more frequent cleaning. 

Impedance to gas flow is the result of heavy fouling on tubes in the convective section. Problems of this type are most likely to occur with coals having high sodium contents, such as those found in low rank coal deposits in Western U.S. seams. Hard, bonded deposits can occur which are resistant to removal by the retractable sootblowers. 

Natural gas is derived from the town gas line and is available at battery limits at a pressure of 12 barg. It is introduced through the pressure regulator and flow controller to the feed desulfurization (DS) reactor, where sulfur compounds are removed up to 0.1 ppm. The DS feed is then mixed with steam in a ratio ranging from 3 to 5, preheated in the convection section and fed to the first reformer tube. 

Convection section This compartment houses rows )f primarily horizontal tubes located downstream fom the radiant section that are used for additional leating in a process unit, Pa.ssing the hot flue gases >ver these tubes can provide additional duty. Exam-ties include preheating process streams, steam super-leating, and additional steam generation. Provisions aust be made to remove these tubes for inspection, epair, Of replacement. 

A fixed soot blower, shown in Exhibit 7-26, is mounted directly on the convection section wall and is hard-piped as shown. A retractable soot blower, illustrated in Exhibit 7-27, allows the lance to be removed from the convection. seaion during operation. Some of the principal components are the support channel, the gear-driven carriage, the poppet valve (used to control the flow of the cleaning medium), and the lance with nozzles. Exhibit 7-28 depicts a soot blower in operation. As the lance enters the heater, the blowing medium cuts a path through the deposits until the lance reaches its apex. The lance then reverses rotation and is indexed so that on the retraction path it cleans surfaces not covered on insertion. The reversed rotation and indexing allow the soot blower to peel and strip all deposits efficiently and with less chance of heater tube erosion. 

The use of tubes of rectangular cross section provides a maximum amount of wall area and so facilitates the removal of heat to the surroundings, and the maintenance of the apparatus at somewhat below 4 , where the variation of the density of the liquid with temperature is very small, greatly diminishes convection effects. With these improvements, and a device for obtaining initially sharp boundaries, the macroscopic method has proved a valuable means for the study of electrophoresis, and for its utilization to separate particles moving with different speeds. 

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