Exchanger Train Fouling

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Heat transfer in crude exchangers often declines because of tube-side fouling. The difference in crude preheat may be 50°F for a dirty vs a clean exchanger train.

A sudden drop in furnace feed temperature is commonly caused by a slug of bottoms sediment and water (F3S&W) in crude charge. This happens when a tank in the crude supply system has been cleaned. The silt and waxy dirt were flushed down the crude line to the refinery. A floating suction in the refinery’s crude charge tank minimizes the effects of these incidents.

Certain low-sulfur waxy crudes rapidly foul heat exchanger tubes. An easy method that has worked in some services to restore lost heat-transfer capacity partially is as follows:

  1. Block in and bypass the tube side of the fouled exchanger.
  2. Continue shell-side flow. This heats the tubes. Waxy deposits are melted off. Harder deposits are spalled off due to thermal shock.
  3. After 20 minutes, restore normal flow.

For one set of exchangers that preheated crude to 250°F, this method doubled the observed heat-transfer coefficients.

Low-crude side velocity also promotes fouling. If the calculated velocity is less than 2-3 ft/sec, consider doubling the number of tube-side passes. Inadequate desalter operation or excessive use of caustic will salt up ex­changers.

Anti-foulant chemicals may have a noticeable—but not a major—effect in reducing exchanger fouling. High velocities, exclusion of tank-washing bottoms from the refinery crude, and good desalting are the paramount factors.

Excessive tube-side pressure drop due to fouling can cause the channel head pass partition baffle to fail. (Consult the TEMA data book for exchanger details.2) This pass partition baffle prevents the crude from bypassing the tube bundle. However, as the tube-side AP rises, this baffle will eventually fail and lead to a sudden loss in the preheat exchanger duty. The maximum allowable pressure difference across this baffle should be listed on the ex­changer data sheet.

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