Furnaces - Equipment Description

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Furnaces - Equipment Description

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in in this document which is not already 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, Engineering Services, Saudi Aramco.

Chapter : Ve Vessels File Reference: MEX10505

For additional information on this subject, contact J.H. Tho m as o n 87 5-2 23 0

Engineering Encyclopedia

Vessels Furnaces - Equipment Description

M OD UL E C O MP ON EN T

P AGE

FURNACE CHARACTERISTICS .......................................................................... 1 Types and Terminology................................................................................ 1 Burner Arrangements ........................................................................ 6 Sulfur Furnace .................................................................................. 7 Furnaces in Use in in Saudi Aramco ..................................................... 9 Major Furnace Components Componen ts ....................................................................... 10 DOCUMENTS USED BY SAUDI ARAMCO...................................................... 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



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 the combustion of fuel and is transferred to fluids contained in tubular coils. Since the composition of the fluids being processed and the operating conditions vary widely between units, 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 is transferred to the process fluid in the tubes primarily by radiation from high-temperature flue gas. In the convection section, heat is transferred from the flue gas to a bank of tubes, mainly by convection.

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


1



TYPICAL FURNACE TYPES

Source: API Standard 560, Fired Heaters for General Refinery Services , 1st Edition, January 1986. Reprinted courtesy of the Am erican Petroleum Institute.

FIGURE 1


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

SAUDI ARAMCO VERTICAL CYLINDRICAL FURNACES

Stack Flue Gas Crossover Convection Section Coils

F-102

F-101 Inspection

Damper Control

Doors

Radiant Section

Burner

Draft Gauge

Snuffing Steam Header

FIGURE 2


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VERTICAL CYLINDRICAL FURNACE - INTERNAL VIEW

Draft Gauge Stack Stack Temperature Indicator

Damper Draft Gauge

Header Box

Breeching

Coil Inlet

Convection Section Shield Section Crossover TI Crossover Piping Draft Gauge

Side View Radiant Section

Radiant Tubes

Burners Coil Outlet

Radiant Tube Circle

Elevation

Plan View

FIGURE 3


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Most vertical cylindrical furnaces have a horizontal convection section located above the radiant 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-radiant furnaces are very inefficient and usually are used only for very small furnaces or when the furnace 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, also containing horizontal tubes, extends over the entire length of the radiant section. Larger box furnaces can have two radiant boxes with one common convection section, as shown in Figure 1f. Cabin furnaces are very similar to box furnaces. An internal view of a cabin furnace is shown in Figure 4.

CABIN FURNACE - INTERNAL VIEW Stack

Draft Gauges

Breeching

Tube Supports

Damper Stack Temperature Indicator

Sootblower Locations

Header Box

Coil Inlet

Corbelling Convection Section Shield Tubes

Draft Gauge Radiant Section

Coil Outlet Burners

End View

Side View

FIGURE 4


5



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

TYPICAL BURNER ARRANGEMENTS - ELEVATION VIEW


FIGURE 5


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Arbor, or “hoop” tube, coils are also used in box furnaces (Figure 1a). These coils consist of a series of vertical tubes located on each side of a radiant section, connected at the top by a large semicircular tube section. This arrangement permits the use of a large number of parallel flow paths. It is used in noncoking, all-vapor process flow services, where a low pressure drop is desired. Arbor coils are used in the Plant 488 and 493 Rheniformer furnaces. In this furnace, separate radiant zones are provided for the preheat and each reheat service. These zones are separated by free-standing brick walls. The flue gases from all the radiant zones pass through a common convection section.

Sulfur Furnace A sulfur furnace is a different type of furnace, which is used in a sulfur recovery plant. It consists of a refractory-lined combustion chamber, in which the H2S in the acid gas feed stream is partially combusted with air under carefully controlled conditions. The effluent from the combustor flows directly through a fire-tube waste heat boiler, where steam is generated, and then to the remainder of the unit for sulfur recovery and further processing. A sketch of a typical sulfur combustor furnace is shown in Figure 6.


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TYPICAL SULFUR FURNACE

20'-0 3/4" Length

7/8" End Plate

4 1/2" Thk. Insulating Castable 9" Thk. Firebrick A

Detail 1

Waste Heat Boiler

A

1/2" Shell Reaction Furnace

Elevation Refractory Anchor

(Waste Heat Boiler)

Refractory Air Inlet Shell

Reaction Furnace Tubes Tube Sheet

Detail 1

Acid Gas Inlet

Section A-A

FIGURE 6


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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

Duty (Design) MBtu/hr

Type

Ras Tanura 015 Two Stage Crude F-100A&B Atmospheric F-200A&B Vacuum 493 Prefractionator F-101 Lt Ends Reboiler F-102 Hvy Ends Reboiler Naphtha Hydrotreater F-201 Hydrotreater Charge F-202 Desulfurizer Reboiler Rheniformer F-301 No. 1 Reactor Heater F-302 No. 2 Reactor Heater F-303 No. 3 Reactor Heater F-304 No. 4 Reactor Heater Convection Section Superheater Steam Generation Boiler Feedwater Total for Rheniformer

2 Cell Box 2 Cell Box

250.7 124.6

VC VC

45.2 64.4

VC VC

32.0 27.0

Arbor Arbor Arbor Arbor

84.0 47.0 28.0 14.0

=

13.0 79.8 17.2 283.0

Uthmaniyah R34 F-101 F-102 F-103

Reaction Furnace Reaction Furnace Reaction Furnace

65.0 65.0 65.0

FIGURE 7


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

FURNACE COMPONENTS

21

18

Process In

13 11

3

12 8 19

7

15

14

2 9

4

6 17 18

13

16

Process Out

22 10

1 5 20

Legend: 1. 2. 3. 4. 5. 6.

Access door Arch Breeching Bridgewall Burner Casing

7. 8. 9. 10. 11. 12.

Convection section Corbel Crossover Tubes Extended surface Return bend

13. 14. 15. 16. 17. 18.

Header box Radiant section Shield section Observation door Tube support Refractory lining

19. 20. 21. 22.

Tubesheet Pier Stack/duct Platform


FIGURE 8


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The process coil consists of a series of tubes which are connected at the ends by headers. The process fluid usually enters the furnace at the top of the convection section, where both the process and flue gas temperatures are the lowest. It passes down through the convection section, following several parallel flow paths (passes), and exiting at the bottom of the convection section. The convection section tube bank consists of several tube rows, with each row consisting of several tubes. In the upper rows, the tubes have an external extended surface to improve heat transfer with the flue gas. This extended surface can be either fins or studs, and these are illustrated 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 the convection section. Headers can be either return bends or plug-type headers. Plug-type headers are used only in cases where access to the inside of the coil is required for inspection or cleaning. This access is not required in most cases. The shield section consists of the bottom two rows of tubes in the convection section. These tubes shield the next rows of tubes from direct radiation. After leaving the convection section, the fluid passes through crossover pipes to the radiant section. In most furnaces, the radiant tubes are arranged in single rows that are located a few inches from refractory walls.


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In some furnaces, an auxiliary coil is included in the convection section to provide heat to a secondary process service. In addition to heat required by the process services, more heat can often be recovered from the flue gas by adding a steam generating section to the convection section. In the case of the Plant 488 and 493 Rheniformer furnaces, the process coils are located only in the radiant section, and the entire convection section is used for steam generation. Throughout the furnace, the tubes are supported by tube supports, which are attached to the furnace structure. In the convection section, large supports which extend across the width of the 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 discussed in greater detail in another module. Arbor coils are supported by the inlet and outlet manifolds which are usually located below the furnace floor. Tube guides are also provided to restrict movement of the tubes. Most of Saudi Aramco's furnaces are designed to fire either gas or oil fuel, although some are designed to fire only gas. Oil fired furnaces usually require sootblowers to periodically clean the convection section tubes, particularly when extended surface tubes are used. Sootblowers direct 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 is caused by the natural draft in the furnace, which is caused by the difference in temperature between the flue gas inside the furnace and stack, and the outside air. The rate of air flow is adjusted so that only the amount of air required for good combustion is admitted to the furnace. Greater quantities of air-flow waste energy. Combustion takes place in the furnace radiant section. Heat is transferred from the burning fuel and hot flue gases to the radiant tubes by direct radiation. The radiant section is sized to ensure that there is no direct flame impingement on the radiant tubes. The flue gases then flow through the convection section around the tube bank. Corbels are provided 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 the stack. In some cases, the flue gases from two or more furnaces are combined and exit through a common stack. Dampers are provided in the ducting or stack to regulate the available draft in the furnace.


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Some furnaces use a combustion air preheater to improve efficiency. The most common type of air preheater system is shown in Figure 10a. This system uses a heat exchanger to transfer heat from the flue gas leaving the furnace to the air to be used for combustion. Because of the increased combustion air, and flue gas pressure drops, forced and induced-draft fans are needed. Another type of air preheat system is shown in Figure 10b. In this system, waste heat 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. An external structure supports all the furnace components. The furnace casing is designed for minimum air infiltration, since any air entering the furnace, except through the burners, does not contribute to good combustion. However, since the furnace normally operates with a slight negative pressure (draft) inside, the structure is not subject to the same internal pressures as boilers. Therefore, an expensive pressure-tight structure is not required. Observation doors are provided in the casing to permit viewing the burners, the combustion process, and all the radiant section tubes.


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DOCUMENTS USED BY SAUDI ARAMCO Standards and Codes Several Standards and Codes are used to cover the minimum requirements of new equipment purchased by Saudi Aramco. Additional documents are used to record details of the equipment actually 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 (fired heaters). It is based on and includes API Standard 560, Fired Heaters for General Refinery Services, which contains the minimum requirements for the design and fabrication of process furnaces. AES-F-001 also includes specific additions and revisions to API 560 to meet Saudi Aramco'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 furnace supply and design. The most important are listed below: •

API Recommended Practice 530, Calculation of Heater-Tube Thickness in Petroleum Refineries. 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 the instruments 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, and inspection personnel with important information about certain pressure-containing and rotating equipment in the plants. This information is presented in a consistent format and includes safe operating limits for the equipment, protective devices, and any special safety precautions required. SIS are initially prepared for new equipment, and then revised when existing equipment is re-rated. For furnaces, SIS are required for the furnace tubes, using Saudi Aramco Form 2731 - Fired Heaters which is presented in Work Aid 1. Use of the SIS is covered in MEX 105.06.


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Fired Heater Data Sheets This document is provided by the furnace manufacturer and includes a summary of the furnace's physical characteristics and its predicted operating performance. It is usually part of the manufacturer's original proposal and is subsequently modified to incorporate any changes that occur as the furnace detail design progresses. The final issue should reflect the as-built furnace. Data sheets are contained in Saudi Aramco Form 8009 (6 pages). This form is equivalent to a general form for the data sheets contained in the Appendix of API Standard 560. Unless the use of Saudi Aramco Form 8009 is insisted upon, manufacturers will often use their own data sheets, 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 the furnace design and construction, including many of the manufacturer's drawings. Adequate information should be included to permit necessary inspection and maintenance of the furnace. The plant record book should also contain information on components purchased by the manufacturer from subsuppliers, such as burners, fans and drivers, sootblowers, safety valves, and instruments.

Drawing Files Drawings covering furnace details should be available for reference and to permit furnace inspection and maintenance. These should be available in the plant record books and/or in the inspection record books. As modifications are made to the furnace over its life, these changes should 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 maintenance and 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 determining future maintenance requirements.


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WORK AID 1

USE PHOTOSTAT OF SAUDI ARAMCO 2731-ENG


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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 1978 issues.


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GLOSSARY air preheater

A heat exchanger which heats the air required for combustion by exchanging heat with the flue gases leaving the convection section, or with another fluid of higher temperature.

arch

The flat or sloped portion of the radiant section opposite the floor.

box

The burners and tubes are enclosed in the fire box, which consists of a structure, refractory lining, and tube supports.

breeching

The hood which collects the flue gas at the convection section exit, for transmission to the stack or the outlet duct work.

bridgewall temperature

The temperature of the flue gas leaving the radiant section. The term comes from the old horizontal box heaters, where a bridgewall physically separated the radiant and convection sections.

burner

A device for mixing fuel and air for combustion.

casing

A steel sheathing which encloses the heater box and makes it essentially air-tight.

cell

A portion of the radiant section, separated from other cells by tubes or a refractory wall. Also called a “zone”.

center wall

A refractory wall in the radiant section, which divides it into two separate cells.

coil

A series of straight tube lengths connected by 180° return bends, forming a continuous path through which the process 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 by convection.


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corbel

A projection from the convection section sidewall to prevent flue gas from flowing up the side of the convection section, between the wall and the nearest tubes, 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 duct and to control draft in a heater. A typical damper consists of 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 inside the heater, usually expressed in inches of water.

excess air

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

extended service

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

fire box

A term used to describe the structure which surrounds the radiant coils and into which the burners protrude.

flue gas

A mixture of gaseous products resulting from combustion of the fuel.

forced draft

Use of a fan to supply combustion air to the burners and to overcome the pressure drop through the burners.

header

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

header box

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


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heat duty

The total heat absorbed by the process fluid, usually expressed in MBtu/hr (million Btu per hour). Total furnace duty is the sum of heat transferred to all process streams, including auxiliary services such as stream superheaters and drier coils.

heat fired

The total heat released in the furnace, equal to gross fuel times lower heating value (LHV) of the fuel. Usually expressed in MBtu/hr.

hip section

The transition zone at the top of the radiant section in cabin type furnaces. The wall of this section is usually at a 45° angle.

induced-draft

Use of a fan to provide the additional draft required, in addition to that supplied by the stack, to draw the flue gas through the convection section, and any downstream heat recovery equipment (i.e., air pressure).

inspection doors

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

manifold

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

natural draft

System in which the draft required to move combustion air into the furnace, and flue gas through the furnace and out the stack, is provided by stack effect alone.

observation doors

Openings in the radiant section floor and at selected points along the walls, to permit viewing of tubes, supports, and burners. Radiant section tubes located adjacent to a furnace wall have only one side directly exposed to a burner flame. Radiation to the backside of the tubes is by reflection/radiation from the refractory wall.

one-side fired tubes

pass

A coil which transports the process fluid from furnace inlet to outlet. The total process fluid can be transported through the furnace by one or more parallel passes.

radiant section

The section of the furnace in which heat is transferred to the furnace tubes primarily by radiation from hightemperature flue gas.


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setting

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

shield section

The first two tube rows of the convection section. These tubes shield the balance of the convection section from direct radiation.

sootblower

A steam lance (usually movable) in the convection section for blowing soot and ash from the tubes, using high-pressure steam.

stack

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

stack effect

The difference (buoyancy) between the weight of a column of high-temperature gases inside the furnace and/or stack, and the weight of an equivalent column of external air, usually expressed in inches of water per foot of height.

stack temperature

The temperature of the flue gas as it leaves the convection section, or air preheater, directly upstream of the 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 more tubes.

two-side fired tubes

Radiant section tubes which are exposed on both sides to direct radiation from the burners.


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Furnaces - Equipment Description

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