Furnaces - Equipment Description

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramcosemployees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,or disclosed to third parties, or otherwise used in whole, or in part,without the written permission of the Vice President, EngineeringServices, Saudi Aramco.

Chapter : Vessels For additional information on this subject, contactFile Reference: MEX10505 J.H. Thomas on 875-2230

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Furnaces - Equipment Description

Engineering Encyclopedia Vessels


Saudi Aramco DeskTop Standards

MODULE COMPONENT PAGE

FURNACE CHARACTERISTICS .......................................................................... 1

Types and Terminology................................................................................ 1

Burner Arrangements........................................................................ 6

Sulfur Furnace .................................................................................. 7

Furnaces in Use in Saudi Aramco..................................................... 9

Major Furnace Components ....................................................................... 10

DOCUMENTS USED BY SAUDI ARAMCO...................................................... 14

Standards and Codes .................................................................................. 14

AES-F-001 Process Fired Heaters .................................................. 14

AES-A-005 Safety Instruction Sheets............................................. 14

Fired Heater Data Sheets ............................................................................ 15

Saudi Aramco Equipment Files.................................................................. 15

Plant Record Books ........................................................................ 15

Drawing Files.................................................................................. 15

Inspection Record Books ................................................................ 15

WORK AID 1 ........................................................................................................ 16

REFERENCES....................................................................................................... 17

GLOSSARY .......................................................................................................... 18



Saudi Aramco DeskTop Standards 1

FURNACE CHARACTERISTICS

Furnaces are used mainly to heat process fluids and usually are parts of specific process units.Process furnaces consist of an internally-insulated enclosure, in which heat is liberated by thecombustion of fuel and is transferred to fluids contained in tubular coils. Since thecomposition of the fluids being processed and the operating conditions vary widely betweenunits, many furnace arrangements are used to satisfy the individual process unit requirements.Furnace size is almost always denoted by the amount of heat absorbed.

Furnaces are also referred to as fired heaters. These two terms are used interchangeably.

Furnaces consist of a radiant section and a convection section. In the radiant section, heat istransferred to the process fluid in the tubes primarily by radiation from high-temperature fluegas. In the convection section, heat is transferred from the flue gas to a bank of tubes, mainlyby convection.

Types and Terminology

Furnaces are generally classified by the type of structural configuration and radiant coilarrangement used. The choice of furnace type depends mainly on the furnace size and theprocess coil requirements. Figure 1 illustrates typical furnace types in use today. The maintypes of furnaces used by Saudi Aramco are vertical cylindrical and box furnaces (Figures 1a,e, f).




TYPICAL FURNACE TYPES

Source: API Standard 560, Fired Heaters for General Refinery Services , 1st Edition, January 1986. Reprintedcourtesy of the American Petroleum Institute.

FIGURE 1




Vertical cylindrical furnaces are the most common type in use in the petroleum processingindustry. They are usually used for heat duties up to about 100 M Btu/hr, although largerones are sometimes used. Figure 2 shows two vertical cylindrical furnaces at the Ras Tanurarefinery. The interior of a typical vertical cylindrical furnace is shown in Figure 3. In theradiant section, tubes stand or hang vertically in a circle around the floor-mounted burners.Thus, firing is parallel to the radiant section tubes.

SAUDI ARAMCO VERTICAL CYLINDRICAL FURNACES

Stack

Flue Gas Crossover

ConvectionSectionCoils

F-101

DraftGauge

BurnerSnuffingSteamHeader

InspectionDoors

F-102

RadiantSection

DamperControl

FIGURE 2




VERTICAL CYLINDRICAL FURNACE - INTERNAL VIEW

Draft Gauge

DamperDraft Gauge

Convection Section

StackTemperatureIndicator

Stack

Breeching

Crossover TIShield Section

DraftGauge

RadiantTubes

Burners

Coil Outlet

Elevation

Header Box

CrossoverPiping

Side View

Plan ViewRadiant Tube Circle

Coil Inlet

Radiant Section

FIGURE 3




Most vertical cylindrical furnaces have a horizontal convection section located above theradiant section. Flue gases flow upwards through the convection bank and then to the stack.

Some vertical cylindrical furnaces do not have convection sections. These all-radiantfurnaces are very inefficient and usually are used only for very small furnaces or when thefurnace is operated very infrequently.

Box type furnaces are used for larger duties. In box furnaces with horizontal radiant tubes,the tubes are located on the radiant section sidewalls and roof. The convection section, alsocontaining horizontal tubes, extends over the entire length of the radiant section. Larger boxfurnaces can have two radiant boxes with one common convection section, as shown inFigure 1f. Cabin furnaces are very similar to box furnaces. An internal view of a cabinfurnace is shown in Figure 4.

CABIN FURNACE - INTERNAL VIEW

Stack

Draft Gauges Damper

Stack TemperatureIndicator

Coil Inlet

Corbelling

Shield Tubes

CoilOutlet

Draft Gauge

ConvectionSection

RadiantSection

Tube Supports

SootblowerLocations

Burners

End View

Header Box

Breeching

Side View

FIGURE 4




Burner Arrangements

Several burner arrangements can be used with box furnaces, as shown in Figure 5. In SaudiAramco's furnaces, the burners are located in the furnace floor, firing upward, or in thesidewalls below the tubes, firing horizontally against a center wall (Figures 5a and c). Ineither case, firing is perpendicular to the tubes.

TYPICAL BURNER ARRANGEMENTS - ELEVATION VIEW


FIGURE 5




Arbor, or hoop tube, coils are also used in box furnaces (Figure 1a). These coils consist ofa series of vertical tubes located on each side of a radiant section, connected at the top by alarge semicircular tube section. This arrangement permits the use of a large number ofparallel flow paths. It is used in noncoking, all-vapor process flow services, where a lowpressure drop is desired.

Arbor coils are used in the Plant 488 and 493 Rheniformer furnaces. In this furnace, separateradiant zones are provided for the preheat and each reheat service. These zones are separatedby free-standing brick walls. The flue gases from all the radiant zones pass through acommon convection section.

Sulfur Furnace

A sulfur furnace is a different type of furnace, which is used in a sulfur recovery plant. Itconsists of a refractory-lined combustion chamber, in which the H2S in the acid gas feedstream is partially combusted with air under carefully controlled conditions. The effluentfrom the combustor flows directly through a fire-tube waste heat boiler, where steam isgenerated, and then to the remainder of the unit for sulfur recovery and further processing. Asketch of a typical sulfur combustor furnace is shown in Figure 6.




TYPICAL SULFUR FURNACE

4 1/2" Thk. InsulatingCastable

Detail 1 A

WasteHeat Boiler

A

Refractory

ShellAir Inlet

Reaction Furnace

(Waste HeatBoiler)

ReactionFurnace

Tubes

Tube Sheet

Acid Gas Inlet

Detail 1

Section A-A

9" Thk. Firebrick

7/8" End Plate

1/2" Shell

Refractory Anchor

Elevation

20'-0 3/4" Length

FIGURE 6




Furnaces in Use in Saudi Aramco

A partial list of the furnaces in Saudi Aramco plants is contained in Figure 7.

SAUDI ARAMCO FURNACES

Plant/ Unit Furnace Service Type

Duty(Design)MBtu/hr

Ras Tanura015

Two Stage CrudeF-100A&B Atmospheric 2 Cell Box 250.7F-200A&B Vacuum 2 Cell Box 124.6

493Prefractionator

F-101 Lt Ends Reboiler VC 45.2F-102 Hvy Ends Reboiler VC 64.4

NaphthaHydrotreater

F-201 Hydrotreater Charge VC 32.0F-202 Desulfurizer Reboiler VC 27.0

RheniformerF-301 No. 1 Reactor Heater Arbor 84.0F-302 No. 2 Reactor Heater Arbor 47.0F-303 No. 3 Reactor Heater Arbor 28.0F-304 No. 4 Reactor Heater Arbor 14.0Convection Section

Superheater 13.0Steam Generation 79.8Boiler Feedwater 17.2

Total forRheniformer

= 283.0

UthmaniyahR34

F-101 Reaction Furnace 65.0F-102 Reaction Furnace 65.0F-103 Reaction Furnace 65.0

FIGURE 7




Major Furnace Components

The major components of a typical furnace are shown in Figure 8, which is an illustration of ahorizontal tube box furnace. These components comprise the three major elements of thefurnace: the process coil, the combustion/flue gas system, and the structure.

FURNACE COMPONENTS

Process Out 15

1016

13

4

9

192

15

8 7

13

18

11

12

Process In

20

17

22

14

21

3

18

6

Legend:

1. Access door 7. Convection section 13. Header box 19. Tubesheet2. Arch 8. Corbel 14. Radiant section 20. Pier3. Breeching 9. Crossover 15. Shield section 21. Stack/duct4. Bridgewall 10. Tubes 16. Observation door 22. Platform5. Burner 11. Extended surface 17. Tube support6. Casing 12. Return bend 18. Refractory lining


FIGURE 8




The process coil consists of a series of tubes which are connected at the ends by headers. Theprocess fluid usually enters the furnace at the top of the convection section, where both theprocess and flue gas temperatures are the lowest. It passes down through the convectionsection, following several parallel flow paths (passes), and exiting at the bottom of theconvection section.

The convection section tube bank consists of several tube rows, with each row consisting ofseveral tubes. In the upper rows, the tubes have an external extended surface to improve heattransfer with the flue gas. This extended surface can be either fins or studs, and these areillustrated in Figure 9.

TYPES OF EXTENDED SURFACES

a) Serrated Fins b) Solid Fins c) Studs

FIGURE 9

The headers which connect the tubes are located in header boxes at each end of theconvection section. Headers can be either return bends or plug-type headers. Plug-typeheaders are used only in cases where access to the inside of the coil is required for inspectionor cleaning. This access is not required in most cases.

The shield section consists of the bottom two rows of tubes in the convection section. Thesetubes shield the next rows of tubes from direct radiation.

After leaving the convection section, the fluid passes through crossover pipes to the radiantsection. In most furnaces, the radiant tubes are arranged in single rows that are located a fewinches from refractory walls.




In some furnaces, an auxiliary coil is included in the convection section to provide heat to asecondary process service. In addition to heat required by the process services, more heat canoften be recovered from the flue gas by adding a steam generating section to the convectionsection. In the case of the Plant 488 and 493 Rheniformer furnaces, the process coils arelocated only in the radiant section, and the entire convection section is used for steamgeneration.

Throughout the furnace, the tubes are supported by tube supports, which are attached to thefurnace structure. In the convection section, large supports which extend across the width ofthe convection section are used to support several rows of tubes. In the radiant section,horizontal tubes are supported by small supports which resemble fingers. These are discussedin greater detail in another module. Arbor coils are supported by the inlet and outletmanifolds which are usually located below the furnace floor. Tube guides are also providedto restrict movement of the tubes.

Most of Saudi Aramco's furnaces are designed to fire either gas or oil fuel, although some aredesigned to fire only gas. Oil fired furnaces usually require sootblowers to periodically cleanthe convection section tubes, particularly when extended surface tubes are used. Sootblowersdirect a blast of steam at the tubes to clean the surface and increase heat transfer.

Combustion air is supplied to the burners, where it is mixed with the fuel. This flow of air iscaused by the natural draft in the furnace, which is caused by the difference in temperaturebetween the flue gas inside the furnace and stack, and the outside air. The rate of air flow isadjusted so that only the amount of air required for good combustion is admitted to thefurnace. Greater quantities of air-flow waste energy. Combustion takes place in the furnaceradiant section. Heat is transferred from the burning fuel and hot flue gases to the radianttubes by direct radiation. The radiant section is sized to ensure that there is no direct flameimpingement on the radiant tubes.

The flue gases then flow through the convection section around the tube bank. Corbels areprovided in the convection section sidewalls to prevent flue gas from bypassing the tube bank.The flue gas then flows from the convection section, through the breeching and ducting, to thestack. In some cases, the flue gases from two or more furnaces are combined and exit througha common stack. Dampers are provided in the ducting or stack to regulate the available draftin the furnace.




Some furnaces use a combustion air preheater to improve efficiency. The most common typeof air preheater system is shown in Figure 10a. This system uses a heat exchanger to transferheat from the flue gas leaving the furnace to the air to be used for combustion. Because of theincreased combustion air, and flue gas pressure drops, forced and induced-draft fans areneeded. Another type of air preheat system is shown in Figure 10b. In this system, wasteheat from another source is used to heat the furnace combustion air.

AIR PREHEAT SYSTEMS


FIGURE 10

The furnace enclosure consists of a metal casing plate with an internal refractory-lining. Anexternal structure supports all the furnace components. The furnace casing is designed forminimum air infiltration, since any air entering the furnace, except through the burners, doesnot contribute to good combustion. However, since the furnace normally operates with aslight negative pressure (draft) inside, the structure is not subject to the same internalpressures as boilers. Therefore, an expensive pressure-tight structure is not required.Observation doors are provided in the casing to permit viewing the burners, the combustionprocess, and all the radiant section tubes.




DOCUMENTS USED BY SAUDI ARAMCO

Standards and Codes

Several Standards and Codes are used to cover the minimum requirements of new equipmentpurchased by Saudi Aramco. Additional documents are used to record details of the equipmentactually purchased and installed, and to record the furnace's history during its operating life.

AES-F-001 Process Fired Heaters

This Engineering Standard is mainly used for the purchase of new process furnaces (firedheaters). It is based on and includes API Standard 560, Fired Heaters for General RefineryServices, which contains the minimum requirements for the design and fabrication of processfurnaces. AES-F-001 also includes specific additions and revisions to API 560 to meet SaudiAramco's specific requirements. Included in these standards are:

Basic process and mechanical design considerations.

Mechanical design requirements, including the design of tubes and tube supports,refractory, burners and fans, structure, platforms, stacks and ducts.

Inspection and testing.

Required connections and auxiliary equipment.

This standard also references other documents that cover specific aspects of the furnacesupply and design. The most important are listed below:

API Recommended Practice 530, Calculation of Heater-Tube Thickness in PetroleumRefineries. This document covers procedures for the design of furnace tubes.Application of these procedures is covered in MEX 105.06.

AES-J-600, 603 Process Heater Burner Safety System. These standards cover theinstruments and controls required for safe operation of the furnace combustion system.

AES-A-005 Safety Instruction Sheets

The purpose of the Safety Instruction Sheets (SIS) is to provide operating, maintenance, andinspection personnel with important information about certain pressure-containing androtating equipment in the plants. This information is presented in a consistent format andincludes safe operating limits for the equipment, protective devices, and any special safetyprecautions required. SIS are initially prepared for new equipment, and then revised whenexisting equipment is re-rated.

For furnaces, SIS are required for the furnace tubes, using Saudi Aramco Form 2731 - FiredHeaters which is presented in Work Aid 1. Use of the SIS is covered in MEX 105.06.




Fired Heater Data Sheets

This document is provided by the furnace manufacturer and includes a summary of thefurnace's physical characteristics and its predicted operating performance. It is usually part ofthe manufacturer's original proposal and is subsequently modified to incorporate any changesthat occur as the furnace detail design progresses. The final issue should reflect the as-builtfurnace.

Data sheets are contained in Saudi Aramco Form 8009 (6 pages). This form is equivalent to ageneral form for the data sheets contained in the Appendix of API Standard 560. Unless theuse of Saudi Aramco Form 8009 is insisted upon, manufacturers will often use their own datasheets, which are very similar to this.

Saudi Aramco Equipment Files

Details of the furnace and its auxiliary equipment are contained in several sources.

Plant Record Books

The contractor prepares a plant record book for the furnace. This contains details of thefurnace design and construction, including many of the manufacturer's drawings. Adequateinformation should be included to permit necessary inspection and maintenance of thefurnace. The plant record book should also contain information on components purchased bythe manufacturer from subsuppliers, such as burners, fans and drivers, sootblowers, safetyvalves, and instruments.

Drawing Files

Drawings covering furnace details should be available for reference and to permit furnaceinspection and maintenance. These should be available in the plant record books and/or in theinspection record books. As modifications are made to the furnace over its life, these changesshould be recorded on the original drawings or on new drawings.

Inspection Record Books

Inspection Record Books should include inspection results and a record of past maintenanceand repairs. Any modifications made to the boiler should also be documented in this file.This file should provide a valuable history of furnace performance and assist in determiningfuture maintenance requirements.




WORK AID 1

USE PHOTOSTAT OF SAUDI ARAMCO 2731-ENG




REFERENCES

Saudi Aramco Standards

AES-A-005: Safety Instruction Sheets - Form 2731 Fired Heaters

AES-F-001: Process Fired Heaters

AES-J-600: Process Heater Burner Safety System

AES-J-603: Process Heater Burner Safety System

API Publications

Recommended Practice 530: Calculation of Heater-Tube Thickness in Petroleum Refineries

Standard 560: Fired Heaters for General Refinery Services

ASTM Publications

Material Specifications

Other Publications

Chemical Engineering Magazine, Fired Heaters, H. L. Berman, June-September 1978issues.




GLOSSARY

air preheater A heat exchanger which heats the air required forcombustion by exchanging heat with the flue gasesleaving the convection section, or with another fluid ofhigher temperature.

arch The flat or sloped portion of the radiant section oppositethe floor.

box The burners and tubes are enclosed in the fire box, whichconsists of a structure, refractory lining, and tubesupports.

breeching The hood which collects the flue gas at the convectionsection exit, for transmission to the stack or the outletduct work.

bridgewall temperature The temperature of the flue gas leaving the radiantsection. The term comes from the old horizontal boxheaters, where a bridgewall physically separated theradiant and convection sections.

burner A device for mixing fuel and air for combustion.

casing A steel sheathing which encloses the heater box andmakes it essentially air-tight.

cell A portion of the radiant section, separated from othercells by tubes or a refractory wall. Also called a zone.

center wall A refractory wall in the radiant section, which divides itinto two separate cells.

coil A series of straight tube lengths connected by 180 returnbends, forming a continuous path through which theprocess fluid passes and is heated.

convection section The portion of a heater, consisting of a bank of tubes,which receives heat from the hot flue gases, mainly byconvection.




corbel A projection from the convection section sidewall toprevent flue gas from flowing up the side of theconvection section, between the wall and the nearesttubes, thereby bypassing the tube bank.

crossover The interconnecting piping, either internal or external,between any two heater coil sections.

damper A device to regulate flow of gas through a stack or ductand to control draft in a heater. A typical damper consistsof a flat plate connected to a shaft which can be rotated,similar to a butterfly valve.

draft The negative pressure (vacuum) at a given point insidethe heater, usually expressed in inches of water.

excess air The percentage of air in the heater in excess of thestoichiometric amount required for combustion.

extended service Surface added to the outside of bare tubes in theconvection section to provide more heat transfer area.This may consist of cylindrical studs butt welded to thetube, or fins continuously wound around and welded tothe tube.

fire box A term used to describe the structure which surrounds theradiant coils and into which the burners protrude.

flue gas A mixture of gaseous products resulting from combustionof the fuel.

forced draft Use of a fan to supply combustion air to the burners andto overcome the pressure drop through the burners.

header The fitting which connects two tubes in a coil. Incommon usage, header refers to cast or forged 180U-bends (return bends).

header box The compartment at the end of the convection sectionwhere the headers are located. There is no flue gas flowin the header box, since it is separated from the inside ofthe furnace by an insulated tubesheet. Header boxes aresometimes also used in the radiant section.




heat duty The total heat absorbed by the process fluid, usuallyexpressed in MBtu/hr (million Btu per hour). Totalfurnace duty is the sum of heat transferred to all processstreams, including auxiliary services such as streamsuperheaters and drier coils.

heat fired The total heat released in the furnace, equal to gross fueltimes lower heating value (LHV) of the fuel. Usuallyexpressed in MBtu/hr.

hip section The transition zone at the top of the radiant section incabin type furnaces. The wall of this section is usually ata 45 angle.

induced-draft Use of a fan to provide the additional draft required, inaddition to that supplied by the stack, to draw the flue gasthrough the convection section, and any downstream heatrecovery equipment (i.e., air pressure).

inspection doors Openings in the convection section sidewalls to allowinspection of tubes, extended surfaces, and supports.

manifold A pipe connected to several parallel passes and used todistribute or collect fluid from these passes.

natural draft System in which the draft required to move combustionair into the furnace, and flue gas through the furnace andout the stack, is provided by stack effect alone.

observation doors Openings in the radiant section floor and at selectedpoints along the walls, to permit viewing of tubes,supports, and burners.

one-side fired tubes Radiant section tubes located adjacent to a furnace wallhave only one side directly exposed to a burner flame.Radiation to the backside of the tubes is byreflection/radiation from the refractory wall.

pass A coil which transports the process fluid from furnaceinlet to outlet. The total process fluid can be transportedthrough the furnace by one or more parallel passes.

radiant section The section of the furnace in which heat is transferred tothe furnace tubes primarily by radiation from high-temperature flue gas.




setting The furnace casing, brickwork, refractory, and insulation,including the tiebacks or anchors.

shield section The first two tube rows of the convection section. Thesetubes shield the balance of the convection section fromdirect radiation.

sootblower A steam lance (usually movable) in the convectionsection for blowing soot and ash from the tubes, usinghigh-pressure steam.

stack A cylindrical steel, concrete, or brick shell which carriesflue gas to the atmosphere and provides necessary draft.

stack effect The difference (buoyancy) between the weight of acolumn of high-temperature gases inside the furnaceand/or stack, and the weight of an equivalent column ofexternal air, usually expressed in inches of water per footof height.

stack temperature The temperature of the flue gas as it leaves theconvection section, or air preheater, directly upstream ofthe stack.

tube guide Device used to restrict the movement of tubes.

tubesheet A large tube support plate supporting a number of tubes.

tube support A metal part which supports the weight of one or moretubes.

two-side fired tubes Radiant section tubes which are exposed on both sides todirect radiation from the burners.

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Furnaces - Equipment Description