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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

A

s 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,

LPG treating unit Gasoline treating unit Naphtha hydrotreating unit

Crude oil

Crude furnace

Catalytic reforming unit Diesel hydrotreating unit

Crude distillation unit VGO furnace

VGO hydrotreating unit

Fluidised catalytic cracking unit

Products

Hydrocracking

Vacuum distillation unit Vacuum furnace

Coker unit Coker furnace

Visbreaker furnace

Resid processing (Visbreaker) unit

Figure 1 Furnaces in a refinery that require frequent cleaning

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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 + ∆ Tf + ∆Tf + ∆Tc + ∆Tw

Where Tm = TMT Tb = bulk fluid temperature ∆Tf = temperature difference across the fluid film ∆Tc = temperature difference across the deposited coke ∆Tw = temperature difference across the tube wall With this method for estimating TMT, and available empirical correlations, refiners

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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

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Comparison of different methods of coil cleaning Main features Steam-air decoking Function Remove coke from the coils

On-line spalling Mechanical pigging Remove coke while the furnace is on-line Remove coke from the coils. Measure coil to prolong the furnace run length prior thickness using “intelligent” pigs to pigging or decoking

Technique employed

Coke is burnt off the furnace coils in a controlled decoking process by circulating an air-steam mixture at elevated temperatures

Coke is removed by using a high-velocity Coke is removed by pumping a metal-studded steam flow while thermally contracting foam or plastic pig with water in the coils, and expanding the coils mainly by scrubbing coke from inside the coils

Off-line/on-line operation

Performed while furnace is off-line

Performed while the furnace is on-line

Performed while the furnace is off-line

Safety concern Potential for coil rupture Effluent generation/ Environmental concern as disposal air-steam mixture is vented to atmosphere

Potential coil damage due to expansion/contraction Coke is collected in coke drums

No damage to coils foreseen

Operating personnel Performed by operating crew

Performed by operating crew using a regular decoking sequence

Requires an external agency/vendor to perform pigging operations

Efficiency Furnace run length

Removes almost all the coke from the inside coils

May not remove all coke; decoking/ pigging still needed

Removes all coke from the inside coils

Fair run length (shorter than that achieved by pigging) is achieved since efficiency is less than pigging

An intermittent operation that helps in Largest furnace run length is achieved extending the furnace run length prior to pigging

Dirty water needs to be disposed off

Table 1

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, effi-

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ciency and so on are shown in Table 1.

Mechanical pigging: a recent trend

In the past, the key method used was steam-air decoking, where coke was burnt off the furnace coils in a controlled decoking process by circulating an air-steam mixture at elevated temperatures. On-line spalling was developed to increase on-stream days in visbreaker and coker units. Nowadays, mechanical pigging is used to remove deposits from the coil surface by pumping a pig with water in the same manner as in offshore and onshore pipeline transportation systems. Pigging has become the preferred method in the refining and petrochemical industries to remove coke and scale deposits from the walls of furnace coils because it is more effective and faster than

steam-air decoking and on-line spalling. The type of pig to be selected for a specific pigging operation depends on the following factors: • The purpose of pigging ■ Cleaning/decoking of coils (cleaning pigs) ■ Data recording (intelligent pigs) • Characteristics of process fluid, properties such as CCR, heavy metal content • Driving pressure of motive fluid • Pig velocity • Coil configuration ■ Coil diameter ■ Length to be covered ■ Bend radius.

Pigs for cleaning

A cleaning pig is a plastic foam cylinder with pins uniformly studded around its surface. When these pins come into contact with the inside wall of a coil they scrape coke and other

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Foam/plastic/metal pig Studs/grits/ protrusions on the pig’s surface

Battery module Sensors and recorders

analogue and digital tapes, solid-state memory devices, and so on. These data can be archived for comparison with past and future inspection data. Figure 3 shows the movement of an intelligent pig inside the bend of a coil during pigging.

Pigging techniques n-pass vertical cylindrical furnace arrangement

Figure 2 Cleaning pig through the bend

Figure 3 Intelligent pig through the bend

contaminants off the coil surface. As the pig travels through the coil, dirty water is routed to a collection reservoir. After the cleaning pig has been received from the coil, the remaining coke is flushed out of the coil with a high flow of water over several hours until clear water is received from the coil. The appearance of clear water continuously from the coils indicates to the operator that the coils are satisfactorily cleaned. As a result, effective cleaning can be done without causing harmful thermal fatigue, which happens with the spalling and steam-air decoking cleaning methods. Figure 2 shows the movement of a cleaning pig inside the bend of a coil during pigging.

Intelligent pigs for coil thickness surveying

Modern, intelligent pigs are sophisticated instruments that vary in their technology and complexity according to intended use. These are also called smart pigs and are used for health monitoring of the coil by measuring thickness and corrosion along the coil. Cracks and corrosion are often detected using magnetic flux leakage pigs. Some intelligent pigs use ultrasonic devices or electromagnetic acoustic transducers to detect coil deformation. These pigs consist of various built-in sensors and electronics that collect and store data while the pig is travelling in the coil. The electronics are sealed to prevent ingress of coil fluid in the pig. Data are stored on

To convection section

1

2

3

4

From process unit

Figure 4 n-pass furnace: arrangement of passes at grade

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n

The inlet header for an n-pass furnace branches into n separate passes upstream of the furnace. Of these, n/2 passes enter the furnace through one side of the convection section and the other n/2 passes enter the furnace through the other side. From the convection section, the coils enter the radiant section through internal or external cross-overs. The outlet of the radiant coils is at either side of the furnace. Figure 4 shows the arrangement of n passes at grade. Figures 5 and 6 show the arrangement of n passes at the convection inlet and radiant outlet, respectively.

Conventional pigging

The conventional pigging process consists of the following steps: • Using a pig launcher, the pig is inserted into one pass of the furnace • The pig travels inside the pass driven by the hydraulic force of motive fluid • Reverse pigging is carried out using a pump; the pig travels backwards and is received by the pig receiver • The above steps are repeated for other passes until all n passes are cleaned. Thus, n passes in the furnace will require n times to connect, launch and receive the pig.

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Figure 7 shows the conventional pigging process for an n-pass furnace. Although mechanical pigging is faster than other cleaning methods, there is scope to make it even faster. Considerable time is taken to clean each pass individually. This time taken by conventional pigging can be reduced by modifying the pigging operation as follows.

From process unit

n/ +1 2

n/ +2 2

n/ +3 2

n/ +4 2

n

To convection section

1

2

3

n/ 2

4

From process unit

Figure 5 n-pass furnace: arrangement of passes at the convection section inlet

Modified pigging scheme

A reduction in the number of cycles in conventional pigging can be achieved by interconnecting the various passes of the furnace. The convection inlets of various passes at grade are interconnected by temporary spools; similarly, the radiant outlets of various passes are interconnected by temporary spools. This modified arrangement reduces the number of pigging cycles, making this process faster than the conventional pigging process. This arrangement can: • Minimise the number of pigging cycles, in turn reducing the duration of a pigging operation • Reduce the quantity of modification work, especially at height (radiant outlet at top) • Launch and receive pigs at grade for ease of operation • Reduce the number of pig launchers/receivers. In this modified pigging procedure, pigs are launched from the inlet of the convection section of the first pass of a furnace. The outlet of the radiant section of this first pass will be temporarily connected by a piping spool piece to the second pass. This second pass inlet at grade is connected to a

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Heater outlet to process unit

n/ +1 2

n/ +2 2

n/ +3 2

n/ +4 2

n

From radiant section

1

2

3

n/ 2

4

Heater outlet to process unit

Figure 6 n-pass furnace: arrangement of passes at the radiant section outlet

Pass 1

Pass 2

Pass 4

Pass n

Loop-1 Loop-2 Loop-3 Loop-4

Loop-n

Launch pig

Pass 3

Receive pig

Figure 7 n-pass furnace: conventional pigging process

third pass through the pipe spool at grade and so on up to pass n/2 (see Figure 8). With this arrangement, the pig that was launched at the inlet of the first pass of the convection

section is received at n/2 pass in this loop at grade. A similar arrangement and pigging procedure is performed in the second loop from the n/2 +1 to nth pass. In this way, a pig will

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Interconnections at radiant top

Interconnections at radiant top

Radiant outlet at top

Pass 1

Pass 2

Pass 3

Pass 4

Pass n/ 2

Pass n/ +1 2

Pass n/ +2 2

Pass n/ +3 2

Pass n/ +4 2

Pass n

Furnace inlet valve at grade Launch pig

Interconnections at grade

Receive pig

Launch pig

Loop-1

Interconnections at grade

Receive pig

Loop-2

Figure 8 ‘n’ pass furnace: arrangement for modified pigging scheme

go through n/2 passes in a single cycle. Hence, n passes are pigged in two cycles. Figure 8 shows the pigging loops for n passes.

Steps of a modified pigging scheme

Pigging can be carried out in two loops, either sequentially or simultaneously. In the sequential approach, a single pig and setup is used in sequence for both Loop 1 and Loop 2. In simultaneous pigging, two pigs and their corresponding setups are used. The pigging steps for Loop 1 (pass 1 to pass n/2) are shown in Figure 9. The steps for Loop 2 are similar to those for Loop 1 except for the pass numbers (pass n/2+1 to pass n). A sequential description of this procedure is: Step 1 Pig 1 is launched through the inlet of pass 1 at grade. The pass 1 inlet at grade should be modified temporarily for pig launching

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Step 2 The pig travels the complete length of pass 1 from the inlet at grade to the radiant outlet. At the radiant outlet of pass 1, the pig enters pass 2 through an interconnection between the radiant outlets of passes 1 and 2 at the radiant top Step 3 The pig travels the complete length of pass 2 from the radiant outlet at the top to the inlet at grade. At the inlet of pass 2, the pig enters pass 3 through an interconnection between the inlets of passes 2 and 3 at grade Step 4 The pig travels the complete length of pass 3 from the inlet at grade to the radiant outlet at the top. At the radiant outlet of pass 3, the pig enters pass 4 through an interconnection between the radiant outlets of passes 3 and 4 at the radiant top Step 5 The pig travels through a series of passes via various interconnections at grade and at the radiant top and enters

the last pass (pass n/2) in the loop Step 6 The pig travels the complete length of pass n/2 from the radiant outlet at the top to the inlet at grade. This is the end point for the pig. Here, the pig is sent back to the loop by means of hydraulic force. The pig now travels backwards and is received from pass 1.

Precautions to be taken during pigging operations

The furnace coil design pressure is a function of the coil operating pressure. Different furnaces have different coil design pressures. Mechanical pigging incorporates water as the driving medium. Generally, water is pumped using a manually regulated pump. In case the pump shut-off pressure exceeds the furnace coil design pressure, a pressure safety valve should be installed in the line to protect the coil from over-pressure during a pigging operation.

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Step 1 Step 2, Pass 1 Step 3, Pass 2 Step 4, Pass 3 Step 5 Step 6, Pass n/2

Temporary interconnections at radiant top

Pig launcher

Pig receiver Water tank

Interconnections at grade Dirty water

Loop-1

Figure 9 Steps of a modified pigging scheme

Thermocouples at the furnace outlet are susceptible to damage during pigging and may cause hindrance to the pig. Hence, thermocouples should be removed before pigging and placed in their respective positions after pigging is completed.

Incorporation of new pigging techniques during furnace design

The following points should be taken into consideration during the design phase of a furnace to incorporate new pigging techniques as well as to ease operations while pigging: • The furnace should have an even number of passes • Spare interconnection spools should be provided so that there is no need to prepare or procure these while pigging • The thermowell flanges at the outlet of passes should be

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equal to the coil size so that they can be used for pass interconnections during pigging • Temperature transmitters should not be head-mounted to thermocouples and should be remote-mounted so that they can be removed during pigging • Sufficient space should be provided on radiant/convection platforms to accommodate personnel and material for safe pigging.

Conclusions

Mechanical pigging is the preferred method for furnace cleaning in view of the operating run length that it provides compared to other cleaning methods. Furnace downtime for cleaning is less during mechanical pigging compared to steam-air decoking. Compared to on-line spalling, the effectiveness of mechanical pigging is higher. Mechanical

pigging is the safest method of cleaning. Intelligent pigging is the only method by which a furnace coil’s thickness can be measured. Conventional pigging used in furnaces involves individual cleaning of each pass. Since the pigging auxiliaries are individually connected to the inlet and exit of each pass, downtime is greater during conventional pigging. The modified pigging scheme eliminates this problem as half the total number of passes is pigged in one go by interconnecting various passes. Thus, substantial time and labour are saved as there is no need to connect and receive pigs at each pass. By using a modified pigging scheme, furnace net downtime is reduced by approximately a half to one-quarter of the time taken for conventional pigging. Reducing the cleaning time for

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furnace coils with a modified pigging scheme increases the availability of corresponding process plant. References 1 Jegla Z, Design and operating aspects influencing fouling inside radiant coils of fired heaters operated in crude oil distillation plants, Heat Exchanger Fouling, Jun 2011. 2 Conticello R, Bernhagen P, Fired heater design & decoking techniques, NPRA Technology Forum, 2005. 3 Katala K A, Karrs M S, Advances in delayed coking heat transfer equipment, Hydrocarbon Processing, Feb 2009. 4 Roberts R D, Increased reliability/ reduced risk by applying intelligent pigging technology to inspect coils in

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process heaters, 4th Middle East NDT Conf, 2007. 5 Adams J, Coker furnace - online spalling, paper presented at AFM, 2012. 6 API 560, Fired heaters for general refinery services. 7 API 530, Calculation of heater tube thickness in petroleum refineries.

Rupali Sahu is Senior Engineer, Process & Technology Department with Technip KT India Ltd. She holds a bachelor’s degree in chemical engineering from MIET, Meerut, India. Email: [email protected] Shyam Kishore Choudhary is Principal Engineer, Process & Technology Department with Technip KT India Ltd. He holds a bachelor’s degree in chemical engineering from BIT, Sindri, India. Email: [email protected]

Ugrasen Yadav is Deputy General Manager, Refinery & Petrochemicals, in the Process & Technology Department of Technip KT India Ltd. He holds a master’s degree in chemical engineering from HBTI, Kanpur, India. Email: [email protected] M K E Prasad is Head of the Process and Technology Department with Technip KT India Ltd. He holds a bachelor’s degree in chemical engineering from Osmania University, Hyderabad, India. Email: [email protected]

LINKS More articles from the following categories: Corrosion/Fouling Control Fired Heaters Heat Transfer

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