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Jul-2013

A novel approach to cleaning 
furnace coils

A modified pigging operation aims to greatly reduce the time required for coke removal from furnace coils

RUPALI SAHU, SHYAM KISHORE CHOUDHARY, UGRASEN YADAV and M K E PRASAD
Technip KT India
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Article Summary
As the refining industry moves towards heavier and dirtier crudes, attention to maintaining longer run lengths for furnaces is increasingly important in reducing downtime. Large numbers of furnaces with different services and types require frequent cleaning due to fouling and coke deposition in the furnace tubes. A typical block flow diagram of refinery units and associated furnaces that require frequent cleaning is shown in Figure 1.

Fouling/coke formation is a function of fluid composition, residence time and temperature. Crude oil’s API value and viscosity play a major role in fouling and coke formation in furnace coils. The sodium, asphaltene, Conradson carbon residue (CCR) and calcium content of the operating fluid enhances fouling/coke formation. Operating parameters including a high furnace outlet temperature, low fluid mass velocity (high film temperatures), loss of velocity steam, uneven heat distribution (formation of hot spots inside the furnace), and fluid residence time above the cracking threshold result in fouling/coke deposition on the coils inside a furnace. Furnaces dealing with heavier process fluids — crude distillation unit furnaces, vacuum distillation unit furnaces, coker furnaces and visbreaker furnaces — are more susceptible to fouling and coke formation. 

The thickness of the coke deposits on the inner wall of a furnace coil can be calculated from the difference between the maximum tube metal temperature (TMT) and bulk fluid temperature based on the following equation from API 530:

Tm = Tb +  Δ Tf + ΔTf + ΔTc + ΔTw

Where Tm = TMT
Tb = bulk fluid temperature
ΔTf = temperature difference across the fluid film   
ΔTc = temperature difference across the deposited coke
ΔTw = temperature difference across the tube wall
With this method for estimating TMT, and available empirical correlations, refiners can plan decoking operations for a given fluid.

Methods of furnace coil cleaning
Increased pressure drop inside the coils and high TMTs are indications of fouling inside the furnace coils. Hence, cleaning of the furnace coils is required in case any one of — or a combination of — the following conditions is encountered:
• Increased pressure drop across the coils
• Increased TMTs
• Increased fuel consumption.

There are three generally accepted industrial practices to remove coke from the coils:
• Steam-air decoking
• On-line spalling
• Mechanical pigging.

Steam-air decoking
In steam-air decoking, a steam and air mixture is passed through the coke deposits inside the coil walls. Shrinkage and cracking of the coke occurs by heating the coils from the outside while steam and air flow through the coils. This results in chemical reactions of hot coke, steam and air to produce CO, CO2 and H2. Although this process is more effective than the on-line spalling process, because these gases are vented to the atmosphere, it is not friendly to the environment. Also, the coils are vulnerable to rupture during this procedure.

On-line spalling
The on-line spalling method was developed to increase the on-stream factors of units that process heavy and dirty feedstocks. On-line spalling is generally performed at pre-planned intervals or when high TMTs are observed in the furnace coils. Coke is removed by delivering thermal shocks to the coils while the heater is on-line. Spalling is carried out in one pass of a multipass heater while the other passes remain in operation. By varying the steam and boiler feed water flow rate on the fouled coil, coke breaks off the coil. This coke is then disposed of to a downstream coke drum. Thus, on-line spalling offers the advantage of allowing the furnace to continue in operation while the furnace tubes are being cleaned and has fewer environmental issues than steam-air decoking. However, on-line spalling may not remove all of the coke from the coils, and other methods such as steam-air decoking and mechanical pigging are still required to bring the furnace back to start-of-run conditions. The other disadvantage of this method is that the coils are susceptible to damage due to contraction and expansion during the spalling process.

Mechanical pigging
Mechanical pigging eliminates the problems associated with steam-air decoking and on-line spalling, such as venting of waste gases to the atmosphere and the vulnerability of coils to rupturing due to high-temperature operation. Mechanical pigging is the process of propelling a “pig” through a coil with the help of a pig launcher, for the purpose of cleaning or inspection of the coil. A pig is a device inserted into a pipe that travels freely through the pipe, driven by the motive fluid. The pigging assembly consists of the pig launcher/receiver, pigs, pumps and motive fluid storage tank. Pig launchers are temporary bores used to push the pig into the coil with the aid of water at a higher pressure. The pig launcher/receiver is placed at grade and the pig is launched into the coil somewhere near the pass control valve at grade or at a suitable location at grade. The number of pigging cycles is equal to the number of passes in the furnace. Pigging removes almost all the coke from the coils. It is a faster cleaning process and comparatively longer run lengths are achieved with respect to the other cleaning processes.

Comparison of different methods of furnace coil cleaning
The main features of each cleaning method and a qualitative comparison with respect to function, safety concerns, efficiency and so on are shown in Table 1.
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