Mex 30201
Mex 30201
Mex 30201
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 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.
Contents Pages
INTRODUCTION................................................................................................................ 1
INTRODUCTION
This module provides an introduction to the steam generating equipment used by Saudi Aramco,
including an overview of typical plant steam systems.
The types of boilers and boiler components used by Saudi Aramco are described.
This module also presents the types of documents used by Saudi Aramco to specify boilers for
design and purchase, and to record their inspection and maintenance history.
A typical steam system, shown in Figure 1, provides thermal energy in the form of steam to a
number of consumers. The key elements in this system are the steam generators, the distribution
system, and the users. Supply of boiler feedwater (BFW) to the steam generator and recovery of
condensate for recycling are also included in the steam system.
1 Boiler
2 Deaerator
3 BFW Pump
4 Continuous Blowdown Facilities
5 Intermittent Blowdown Facilities
6 Pressure Reducing Station
7 Desuperheater
8 Distribution Piping and Valving
9 Condensate Return System
SV System Safety Valves
The steam generator can be either a part of a process unit using waste or surplus heat, or a
separate offsite facility burning fuel in a boiler. The fuel can be gas, oil, or coal. Steam can also
be generated by recovery of waste heat from the exhaust of a gas turbine.
Waste or surplus process heat can come from furnace flue gases, process coolers (heat
exchangers), or hot product rundown.
The steam distribution network consists of piping and valves that interconnect between the
producers and consumers of steam. Steam can be produced and consumed at various pressure
levels. These different pressure levels are usually connected via pressure reducing and steam
desuperheating stations, so that surpluses of steam at higher pressure levels can be utilized at
lower pressure levels.
Although not standardized, commonly used steam pressure levels in Saudi Aramco plants range
from 60 to 625 psig. Most plants have a high-pressure (HP) level of 600 to 625 psig, plus one or
more lower pressure levels. For example, steam pressure levels at Abqaiq are 625 and 60 psig,
and at Ras Tanura the steam pressure levels are 600, 225, 150, and 60 psig. These 60- to 225-
psig levels are referred to as medium-pressure (MP) levels. Usually there is also a low-pressure
(LP) level at 15 psig.
The steam system begins with the supply of treated water to a deaerator, where dissolved gases
such as oxygen and carbon dioxide are removed prior to introduction of the water into the boiler.
These gases are undesirable because of their corrosive attack on metal surfaces. The water is
deaerated by heating it to its saturation temperature and then scrubbing it with steam to carry
away the dissolved gases. The deaerator operating pressure is normally the same as the LP steam
system (15 psig). Boiler feedwater pumps are used to deliver BFW from the deaerator to the
boiler steam drum. Two or more pumps are provided to ensure a reliable supply of water to the
boiler.
Wherever possible, condensed steam from the steam users is collected in a condensate system and
returned to the deaerator for recycling.
BOILER CHARACTERISTICS
Types
The three main types of boilers used by Saudi Aramco are water-tube, firetube, and electric.
Water-tube boilers are illustrated in Figures 2 through 5. These are the largest and most common
boiler types. A firetube boiler is illustrated in Figure 6, and an electric boiler is illustrated in
Figure 7.
Water-tube boilers have no real pressure or size limitations. The basic configuration is a firebox
surrounded by tubes filled with water. Located at the top of the water-tube boiler is a drum in
which steam is separated from the water. Water circulates from the steam drum down through
the water tubes and back to the steam drum. This usually occurs because of natural circulation,
although some boilers have forced circulation systems.
The smaller-sized water-tube boilers are usually shop assembled and delivered to the field in a
complete unit. These units are referred to as package boilers. The maximum size of package
boilers depends largely on transportation limits. Larger units can be shipped in a few modules
which are assembled in the field. Many Saudi Aramco boilers are package units, with the largest
having a capacity of about 600,000 lb/hr of steam at a design pressure of about 880 psig. Typical
package boilers are shown in Figures 2 and 3. Larger sized water-tube boilers are field erected. A
typical field-erected boiler is shown in Figure 4.
Steam
Drum
Burners
Mud
Drum
Source: Steam, Its Generation and Use, 38th Edition, © 1972. With permission from Babcock & Wilcox.
A gas turbine waste heat boiler is illustrated in Figure 5. These boilers are usually a special
design. They can be designed as unfired units operating on waste heat only, or as supplementary
fired units, with gas or liquid fuel, to increase steam production. Because of the type of
supplementary firing used, the boiler shown in Figure 5 can also be referred to as a duct-fired
waste heat boiler. Boiler size is dependent on the size of the gas turbine. They typically produce
100,000 to 400,000 lb/hr of steam at 150 to 600 psig.
Firetube boilers are generally of small size (under 100,000 lb/hr and more commonly under
50,000 lb/hr) and low pressure (under 250 psig and more commonly 150 psig or lower). The
steam is usually not superheated. The name "firetube" comes from the arrangement where hot
combustion products flow inside tubes that are located in a water-filled cylindrical shell. Firetube
boilers are less expensive than water-tube boilers of the same size and pressure rating. Firetube
boilers are usually packaged and skid mounted.
Electric boilers are often used for small steam loads at locations outside of main plants, for
example, mechanical shops, dining halls, and hospitals. Electric boilers are usually very small in
size (generally less than 25,000 lb/hr) and low pressure (less than 100 psig). In a resistance boiler,
electric resistance heating elements are immersed in a water bath that is enclosed in an outer
pressure vessel. Heat input and steam output are regulated by controls which select the number
of heating elements to be energized.
Automatic
Blowdown
Vacuum Breaker
Control
Sight Glass
Heating
Elements
Fuses, Terminal
Insulation Blocks, and
Contactors
Strainer
Gate Valves
With permission from Chromalox.
A summary of boiler types and their typical ranges of applications is shown in Work Aid 1. This
can be used to make preliminary boiler selections for study purposes. Figure 8 lists the major
boilers in Saudi Aramco plants.
Juaymah
F-103 1976 P 34 VP-18 Comb. Eng'g 300 200 400 Gas
F-104 1976 P 34 VP-18 Comb. Eng'g 300 200 400 Gas
F-105 1976 P 40 VP-26W Mitsubishi 530 700/625 725 Gas/Oil
F-106 1976 P 40 VP-26W Mitsubishi 530 700/625 725 Gas/Oil
F-108 1976 P 40 VP-26W Mitsubishi 530 700/625 725 Gas/Oil
F-109 1976 P 40 VP-26W Mitsubishi 530 700/625 725 Gas/Oil
Juaymah Industrial
- 1974 P 33 A15 Comb. Eng'g 200 200 400 Gas
- 1974 P 33 A15 Comb. Eng'g 200 200 400 Gas
FIGURE 8
The major components of a typical Saudi Aramco boiler are shown in Figure 9, which is an
illustration of Ras Tanura HP Boiler No. 10. The upper drum (steam drum) provides space for
separating steam from water. It also provides liquid holdup capacity (typically from 10 to 60
seconds) to allow for a dynamic response to load changes without losing liquid in the water tubes.
Steam drum internals for this type boiler are shown in Figure 10. Included are baffles and screens
to assist in separating the steam and water mixture entering from the tubes. Other devices, such
as cyclone separators, are also used in some boilers to separate the steam. Other drum internals
include feedwater piping, blowdown piping, and chemical injection piping.
Upper Drum
Upper Right Side Header
Superheater Tube
Baffle Plate
Air Duct
Fire Brick
Supply Tube
Superheater Header
Boiler Side Tube
Lower Drum
Most water-tube boilers have a lower drum (mud drum) for collecting sediment and impurities.
This drum also acts as a lower header for connection of the water tubes.
The water circuit between the drums consists of downcomers and risers. A downcomer is a tube
in an unheated or less heated area of the boiler. A riser is a tube in the heated or hotter section of
the boiler. Water tends to flow by natural circulation from the steam drum to the mud drum via
the downcomers and then back to the steam drum via the risers. Evaporation takes place in the
risers, and the steam/water mixture weighs less than the water in the downcomers. In some
boilers, circulation is forced by pumping.
Most water-tube boilers have wall, floor, and roof tubes surrounding the firebox. Water flows in
these tubes from the mud drum to the steam drum. In addition to absorbing heat, these tubes cool
the boiler enclosure and reduce the amount of refractory required. In Figure 9, the floor, front wall,
and roof tubes are shown forming a continuous flowpath from the mud drum to the steam drum. A
refractory layer is often placed on the floor tubes to reduce the heat transfer to these tubes. The
sidewall tubes are connected to bottom and top headers, which in turn are connected to the drums
by supply and relief tubes. Adjacent tubes in these walls are often welded to connecting steel strips
to form a continuous membrane wall, which is illustrated in Figure 11. This construction permits a
pressure-tight enclosure.
Outer Casing
Insulation
Combustion Zone
Most water-tube boilers also have steam superheaters. When steam is separated from water in the
steam drum, it is saturated at the drum pressure. This steam is routed through the superheater
tubes to raise the steam temperature above saturation temperature. The superheater is usually
located at the exit of the firebox, where the flue gas temperature is high enough for efficient heat
transfer. However, screen tubes are often used just ahead of the superheater, to shield the
superheater from direct radiation from the hot combustion gases.
Headers are used to distribute the steam to the parallel flowpaths used in the superheater, and to
collect it. As shown in Figure 9, these headers are located at the bottom of the superheater so
that the entire superheater coil is drainable (a Saudi Aramco requirement).
Safety valves are usually installed both on the steam drum and at the boiler outlet to protect both
the boiler and the superheater from overpressure.
Boilers can be designed to fire nearly any fuel, either solid, liquid, or gas, or combinations of each.
Saudi Aramco's boilers fire gas and oil fuels, and these will be considered in this course.
Combustion air is supplied to the boiler by a forced draft fan. This air enters the boiler through a
windbox, where it is distributed to the burners. The air mixes with the fuel in the burners, and
combustion takes place in the combustion zone of the boiler. There are many types and
configurations of burners, depending upon the fuels fired and the size and design of the boiler.
Heat is transferred from the burning fuel and hot gases to the water wall tubes surrounding the
combustion zone. These flue gases then flow past the superheater tubes and through the boiler
bank tubes. Baffles are often used in the boiler bank to define the flue gas flow- path. The flue
gases then exit the boiler through a flue gas duct and stack.
To improve efficiency, many boilers have economizers. These are heat exchangers that transfer
heat from the flue gas leaving the boiler to the boiler feedwater. This reduces the stack
temperature.
Some boilers may also use a combustion air preheater to improve efficiency. This is also a heat
exchanger and is used to transfer heat from the flue gas leaving the boiler to the air that is used for
combustion.
Figure 12 is a simplified flow plan of a boiler and its auxiliaries. It shows how these components,
and those described in the following pages, fit into the overall steam generation system.
Some boilers require attemperators downstream of the steam superheater to control the boiler
steam outlet temperature at various loads. The superheater is designed to provide a specific
steam temperature at design conditions. At other conditions, this temperature may be exceeded
and is controlled by spraying high-quality water into the outlet steam to cool, or attemperate, it.
Oil-fired boilers usually require sootblowers to periodically clean the boiler tubes. Soot-blowers
direct a blast of steam at tubes in the dirtiest part of the boiler to blast combustion products off
the tube surfaces. This cleans the tubes and increases heat transfer.
Boilers have blowdown connections in both the steam and mud drums to remove concentrated
sediment and impurities that result from evaporation of the boiler water. Blowdown from the
steam drum is usually continuous, and the amount is set to control impurity levels below specified
maximums. The blowdown rate may be increased or decreased occasionally, depending upon
boiler water analysis. Blowdown from the mud drum is usually intermittent, based on experience.
Mud drum blowdown primarily removes sediment.
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 boiler's history during its operating life.
This specification is mainly used for the purchase of new boilers. It covers the minimum Saudi
Aramco requirements for industrial water-tube boilers, including the following:
• Mechanical design requirements, including the type of construction to be used, steam drum
internals, burners and fans, platforms, stacks, and ducts.
• Inspection and testing.
• Required connections and auxiliary equipment.
• Boiler performance.
• This specification also states that the proposed boiler type and size must have been
satisfactorily demonstrated, based on at least five years operating experience.
This specification also references other documents that cover certain aspects of the boiler supply
and design. The most important are listed below:
This code covers the design, fabrication, and testing of boiler pressure parts. Application of this
code to boiler design will be covered in the next module.
These specifications cover the instruments and controls required for safe and satisfactory
operation of the boiler.
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 boilers, SIS are required for the pressure parts covered by the ASME code (tubes, headers,
and drums). Since specific forms have not been developed for boilers, the most applicable SIS are
used, as follows:
This SIS is used for all boiler tubes. Work Aid 2 is a copy of this SIS.
This SIS is used for boiler drums and headers. Work Aid 3 is a copy of this SIS.
This document is provided by the boiler manufacturer and includes a summary of the boiler'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 boiler detail design progresses. The final issue should reflect the as-built boiler.
Details of the boiler and its auxiliary equipment are contained in several sources.
• Plant Record Books. The contractor prepares a plant record book for the boiler. This
contains details of the boiler design and construction. Adequate information should be
included to permit necessary inspection and maintenance of the boiler. The plant record
book should also contain information on components purchased by the manufacturer of the
boiler from sub-suppliers, such as burners, fans and drivers, safety valves, and instruments.
• Drawing Files. Drawings covering boiler details should be available for reference and to
permit boiler inspection and maintenance. Modifications made to the boiler throughout its
life 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 maintenance and repairs. Any modifications to the boiler should also be
documented in this file. This file should provide a valuable history of boiler performance
and assist in determining future maintenance requirements.
GLOSSARY