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Developing of a monitoring system for evaluating and investigating the fired
heater performance
Article in Asia-Pacific Journal of Chemical Engineering · May 2019
DOI: 10.1002/apj.2314
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Received: 18 October 2018
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DOI: 10.1002/apj.2314
RESEARCH ARTICLE
Developing of a monitoring system for evaluating and
investigating the fired heater performance
Mahmoud R. Elsaid1,2 | Abeer M. Shoaib1
| Ahmed A. Bhran1,3
| Mostafa E. Awad1
1
Department of Petroleum Refining and
Petrochemical Engineering, Faculty of
Petroleum and Mining Engineering, Suez
University, Suez, Egypt
2
Egyptian Projects Operation and
Maintenance Company (EPROM),
Alexandria, Egypt
3
Chemical Engineering Department,
College of Engineering, Al Imam
Mohammad Ibn Saud Islamic University
(IMSIU), Al Riyadh, Saudi Arabia
Correspondence
Abeer M. Shoaib, Department of
Petroleum Refining and Petrochemical
Engineering, Faculty of Petroleum and
Mining Engineering, Suez University,
Suez, Egypt.
Email: abeer.shoaib@suezuniv.edu.eg
Abstract
Energy costs represent about 65% of the running cost of a chemical, petrochemical, or refining plant. Furnace fuel represents the largest percent of this
cost as it consumes large amounts of fuel to produce the necessary heat duty.
Therefore, it is important for fired heaters to have an efficient system for
monitoring operational parameters to reach the optimum performance and
minimum stack emissions with an acceptable safety levels. One of the most
critical operational parameters to be measured is the tube metal temperature
(TMT) inside the radiation section. Excessive TMT can accelerate tube creep,
hydrogen attack, and external and internal corrosion of the tube wall.
The objective of this work is to develop a program capable of calculating precisely and continuously TMT instead of using external pyrometers that measure it only at certain times for heaters not equipped with thermocouples.
The program can also be a predictive tool for estimating the changes of TMT
at various temporarily conditions such as raising the fired heater capacity.
Four case studies were investigated; the results showed a good agreement
between the actual results and the proposed program results with a maximum
deviation lower than 6%, which indicates the validity of the introduced
program.
KEYWORDS
fired heater, heater performance, tube metal temperature
1 | INTRODUCTION
A fired heater is a direct‐fired heat exchanger, which
raises a flowing fluid within coils of aligned tubes by
the hot gases produced from liquid or gaseous fuel
combustion. They could also be called process heaters
or furnaces, depending on their use. Two basic patterns
of fired heaters exist, which are box‐type or vertical cylindrical heaters.1-4 In this equipment, fuel is burned to
release heat into an open space, to be transferred into
fluids passing through tubes, which are arranged through
the walls and the combustion chamber roof.5 Energy
consumption used by heaters and furnaces corresponds
to about 65% to 90% of energy consumed in refining
and petrochemical industries.6
Even small improvement in efficiency can save thousands of dollars. Around 45% to 55% of the total heat
release in a fired heater is transferred to the fluid to be
NOMENCLATURE: BWT (°F), bridge wall temperature; C.C (ft), center‐to‐center spacing; cp (kcal/(kg K)), specific heat; DCS, distributed
control system; HCU, hydrocracker unit; LHV (Btu/lb), lower heating value; NHT, naphtha hydrotreater; Pacol, paraffin converter to olefin unit;
QR (Btu/hr), radiation heat absorbed; TMT (°C), tube metal temperature; Tg (°C), temperature of flue gas
Asia‐Pac J Chem Eng. 2019;e2314.
https://doi.org/10.1002/apj.2314
wileyonlinelibrary.com/journal/apj
© 2019 Curtin University and John Wiley & Sons, Ltd.
1 of 11
2 of 11
heated in the radiation section, whereas around 25% to
45% of the total heat release could be transferred to
the convection section or lost through the stack by the
flue gases.7
In petroleum refineries, as well as in other process
industries employing tube‐type process heaters, it is desirable to monitor the temperature at various points inside
the furnace. For this purpose, tube metal temperature
(TMT) thermocouples are utilized as a criterion for controlling process heater operations. More specifically, the reliable measurement of TMT is necessary in order to
prolong tube life by preventing overheating.8-11 Infrared
pyrometers could be used to measure temperature when
the conventional sensors cannot be applied.12-15 Many
mathematical‐based equations and models have been
introduced over the years to facilitate the determination
of such temperature.16-18 Dugue19 has reviewed how the
operation and design of fired equipment had been
developed over the previous 50 years to address growing
requirements on energy efficiency, safety, environmental
performance, and availability at an agreeable cost.
As known, the process heater tube materials become
weaker and less able to withstand internal pressure as
the temperature of the tubes increases. Thus, to provide
safe operation and satisfactory tube life and avoid
process tube ruptures, the tube wall or tube skin temperatures must be limited. The objective of the present
work is to create a computer‐based program that is able
to monitor and calculate the average TMT, accurately
and continuously, based on Lobo–Evan equation. The
introduced program has been validated and applied to
different case studies to determine maximum deviation
between devices reading and program results.
2 | THE P ROBLEM STATEMENT
The present work was planned to investigate the performance and operational issues of the hot‐oil‐fired heater
of LAB unit in the Egyptian Linear Alkyl Benzene
Company located in Alexandria, Egypt. A natural gas‐
fired heater with duty of 88.2 Gcal/hr is used to heat
hot oil. It is used to provide heat input to operating
equipment inside petrochemical plant having 48 burners
occupied with air preheat system; it has been put in
service since 2009. This heater does not have TMT
thermocouples installed on tube passes; thus, in 2012,
an attempt to measure the TMT using infrared pyrometers has been carried out. Heater tube layout is described
in hot oil heater data sheet (see Supporting Information).
The design TMT is 425°C. A test done to measure tube
skin temperature using infrared pyrometer gave readings
in the range of 447–497°C, which is much higher than
ELSAID
ET AL.
that of design TMT, although the heater is operating at
only 70% of design duty.
This indicates the possibility of inaccuracy of the
results obtained by the infrared pyrometer. There are
many reasons that may interpret these inaccurate results
for TMT; one of them considered that the infrared energy
reflected back to the measuring device does not come
only from the tube, but also from the refractory, adjacent
tubes, and hot gases. In addition, the infrared beam is not
in direct contact with the tube due to presence of flame
between the measuring device and the tube. This consequently will affect the pyrometer device final reading.
To overcome the inaccurate results obtained from the
infrared pyrometer, it was recommended to shut down
the heater for 20 days in order to install thermocouples
on tube passes. However, this suggestion was rejected
because of the unaffordable higher cost of this proposal
(about 600,000$) and the difficulty to drain about
3,500 tons of hot oil with a zero percentage of hydrocarbons inside all 448 tubes. Thus, the final decision taken
was to assume that the results obtained by infrared
pyrometer was correct and begin to take actions to reduce
the TMT. One of the actions taken was to reduce the
heater heat duty, which affects the normal operation of
overall plant.
3 | Methodology
Regarding the above case study, it is first needed to
develop a method to monitor TMT accurately and continuously at the same time. Therefore, this study introduces
a computer‐based program for calculating the average
TMT with maximum ±6% error. This consequently avoids
the need to install high‐cost thermocouples or to use inaccurate infrared pyrometers. The introduced program
could be used as a predictive tool that can calculate any
further change of average TMT if it is desirable, for example, to increase the furnace capacity. Furthermore, it can
be applied also on fired heaters that are already equipped
with tube metal thermocouples for providing correct
readings in case of thermocouple failure or malfunction.
The program modeling and validation will be discussed
in the following subsections.
3.1 | Modeling
This program is mainly designed using excel software and
is programmed using visual basic to be easier, flexible,
and comfortable for users. For understanding the concept
of the program, it is needed first to discuss radiation
section heat balance and its controlling equations.
ELSAID
ET AL.
3 of 11
3.1.1 | Radiation section heat flux
equation
Process fluid side
Radiation heat absorbed is considered simply as radiation
section duty, which can be calculated using Equation (5):
The proposed program calculates the heat absorbed by
the tubes in the radiant section by applying Lobo–Evan's
equation20 as presented below in Equation 1.
QR
¼ 0:173
αAcp F
"
Tg
100
4
þ
Ts
100
4 #
þ 7 Tg − Ts ;
(1)
where QR is the net heat transferred by radiation in British thermal unit per hour. Tg is the mean temperature of
the hot gases in the furnace in Rankin. Ts is the mean
TMT in Rankin. α is the relative effectiveness factor of
the tube bank, Acp is cold plane area of the tube bank
in square feet, and F is an overall exchange factor for
correcting flame emissivity.
In addition to the above heat flux equation, a heat
balance equation is required to be developed for understanding the heat transfer inside radiation section as
shown in Equation (2):
QR ¼ QLHV þ QFuel þ QAir
þ QSteam − QRadiation loss − QFlue
gas loss ;
(2)
where QLHV is the heat liberated in British thermal unit
per hour by using fuel gas lower heating value, QFuel is
the sensible heat of fuel gas in British thermal unit per
hour, QAir is the sensible heat of combustion air in British
thermal unit per hour, QSteam is the sensible heat of steam
used for oil atomization in British thermal unit per hour,
QRadiation loss is the heat loss in firebox through furnace
refractory walls as a percentage of QFuel in British thermal unit per hour. QFlue gas loss is the heat leaving radiation section with flue gases in British thermal unit per
hour. The datum temperature to calculate the sensible
heat of the above terms was chosen to be 60°F.
The input net heat to the fired heater can be expressed
by Equation (3):
Qnet ¼ QLHV þ QFuel þ QAir þ QSteam :
(3)
The following subsections consider the definition and
calculation of each term in radiation section heat flux
found in Equation (1).
QR ¼ m cp ðT out − T in Þ;
(5)
where m, cp, Tout, and T in are process fluid flow rate
inside tubes in radiation section (lb/hr),average specific
heat of process fluid (Btu/lb oF), outlet temperature of
process fluid from radiation section (°F), and inlet temperature of process fluid to radiation section (°F),
respectively. Regarding the above equation, a continuous
monitoring is required to determine the process fluid
physical properties such as temperature, flow rate, and
composition at inlet and outlet of radiation section.
Flue gas side
Using heat balance equation in radiation section, the heat
absorbed in radiation section can be calculated using
Equation (6) through knowing bridge wall temperature
(BWT), fuel gas flow rate, and composition.
QR ¼ Qnet − QRadiation
loss
− QFlue
gas loss :
(6)
This method is used when physical properties of
process fluid are unavailable due to change in phase,
laboratory analysis is unavailable, or process fluid is a
mixture of components.
3.1.3 | Relative effectiveness factor, α
Because the tube bank does not absorb all the heat radiated to the cold plane, an absorption effectiveness factor,
α, can be used to correct the cold plane area, depending
on the arrangement of the tubes, outside diameter, and
center‐to‐center spacing.
The relation between the effectiveness factor and the
center‐to‐center/outside diameter ratio presented by
Mekler and Fairall21 has been replotted using Microsoft
Excel. For a single row in front of a refractory wall, use
Total One Row. For two rows in front of a refractory wall,
use Total Two Rows. The program used the fitting
equations (derived by curve fitting) that have R2 higher
than 0.99 for determining easily the relative effectiveness
factor. Equations and replotted curves are presented in
Figure A‐1 (see Supporting Information).
3.1.4 | Cold plane area, Acp
3.1.2 | Radiation heat absorbed
Heat absorbed in radiation section can be calculated by
two methods, depending on the availability of data,
whether it is for process fluid side or flue gas side.
The cold plane area could be defined as the area of a
plane through the tube centerlines, whether they are in
a curved plane, such as in a cylindrical pattern, or in a
row side by side. In the case of tubes fired from both
4 of 11
ELSAID
sides, as when the tubes are positioned in the center of
the chamber, the tubes absorb direct radiation from both
sides. For most tube panels, the furnace width would be
equal to the center‐to‐center spacing of the tubes times
the number of tubes. The equivalent tube length is the
length exposed to the radiation. For tubes with the return
bends inside the firebox, the length may be taken as the
distance from the centerline of the return on one end to
the centerline of the return on the other end.
For a firebox with the tubes down the center or other
patterns that result in the tubes being fired from both
sides, the cold plane area would be twice the projected
area. However, for most fired heaters, the cold plane area
can be calculated applying Equation (7).
Acp ¼ N tubes * C:C * Leq ;
(7)
where Ntubes, C.C, and Leq are number of tubes exclusive
of the shield tubes, center‐to‐center spacing in feet, and
length of tube exposed to the radiation in feet,
respectively.
3.1.5 | Overall exchange factor, F
Because the flue gas in the firebox is a poor radiator,
Equation (1) must be corrected using an exchange factor,
which is dependent on the emissivity of the gas and the
ratio of refractory area to cold plane area.
The absorptivity of 0.9 was chosen for tube surface; this
is a typical absorptivity value for oxidized metal surfaces.
The overall radiant exchange factor, F , as a relation of
gas emissivity was extracted from curves presented by
Meckler and Fairall.21 This relationship was replotted
using Excel to find the corresponding equations that can
be used easily to determine the overall radiant exchange
factor (Figure A‐2, Supporting Information). All the fitted
equations relating the overall exchange factor with gas
emissivity are quadratic. The R2 values of these equations
are very close to 1; this consequently confirms the validity
of the values of overall exchange factors estimated by
these equations used in the proposed program.
3.1.6 | Flue gas emissivity
The gas emissivity can be described by the curve
presented by Lobo and Evans.20 Variations in tube wall
temperatures between 600°F and 1,200°F cause less than
1% deviation from these curves. This illustrate the minor
effect of the tube wall temperature on the gas emissivity.
Flue gas emissivity can be correlated as a function of
P * L product and the flue gas temperature Tg.
ET AL.
P * L is the product of the partial pressure of the
carbon dioxide and water times the mean beam length.
The only constituents that normally exist in the flue gas
and contribute significantly to the radiant emission are
carbon dioxide and water.
Relationships of gas emissivity versus P * L product at
different flue gas temperatures were replotted using
Microsoft Excel, and the equations fitted well. These relations were derived using polynomial type (Figure A‐3,
Supporting Information).
3.1.7 | Mean beam length
The mean beam length is the average depth of the
blanket of flue gases in all directions for each of the
points on the bounding surface of the furnace. It is used
instead of a cubical measure of the volume.
Mean beam length for different radiation section
dimension ratios and for both box and cylindrical fired
heater types is fully illustrated in Wimpress.22
3.1.8 | Mean temperature of hot gases Tg
For a well‐mixed radiant section, the mean temperature
of hot gases is assumed equal to the temperature leaving
the radiant section, that is, the BWT for most applications. However, in heaters of high temperature, with a
long narrow firebox and firing wall, the Tg controlling
radiant transfer may be in the range of 200 to 300°F
higher than the exit temperature.23
3.2 | Program interface
The proposed program is designed to be used easily and
could be applied for different fired heater designs. Consequently, the program offers for the user three scenarios,
which make the program more comfortable.
1. For heaters that have no enough data about fluid
conditions at radiation section such as temperature,
specific heat, and flow rate or when the fluid is evaporated inside the heater and changes its phase, the
program can calculate heat absorbed in radiation section by only knowing the BWT.
2. For heaters not equipped with thermocouples used to
measure flue gas temperature at radiation section
exit, the program can calculate BWT by only knowing process fluid conditions at radiation section.
3. For heaters that have enough data about process
fluid conditions at radiation section and with a
ELSAID
ET AL.
5 of 11
predetermined BWT, the program then will move to
next step to calculate TMT.
The introduced program provides the user with simple
graphs to select easily the configuration of tubes against
refractory wall from four different layouts. Beside the
calculation of the average TMT, the program is also able
to compute the following parameters to provide a
complete analysis of the heater performance:
• Overall heat balance on the entire heater.
• Thermal and fuel efficiency of heater.
• Fluid mass velocity inside radiation tube and compare
it with design value.
• Monitor operating conditions for heater and draw
curves for time intervals.
• Perform an economical study regarding the fuel cost
and heat losses.
• Determine remaining tube life by calculating
rupture–creep stress at radiant tubes.
• Perform complete flue gas analysis including SOx,
NOx, and carbon monoxide percentage.
the curve between air specific heat and temperature
were collected from an online source.24 Figure A‐4 in
Supporting Information indicates more details.
• To calculate sensible heat of fuel gas, another
equation is derived to calculate average specific heat
of each of the fuel gas components (kJ/kg K) in terms
of average temperature. The derived equation is of a
general formula presented in Equation (9).
CPfg ¼ aT 3 þ bT 2 þ cT þ d;
where CPfg is the fuel gas components specific heat, T is
the average temperature in Kelvin.
Table 1 represents the constant values derived for each
fuel gas component, in addition to R2 for each fuel gas
component equation. Specific heat of fuel gas components against temperature data are collected from an
online source.25
• The excess air could be represented by the following
equation:
%excess air ¼ 0:0239 X 3 þ 0:02032 X 2
þ 5:4746 X − 0:081;
3.3 | Derivation of program equations
Program equations used for TMT calculation can be
divided into two categories as discussed in the following
subsections.
3.3.1 | Physical properties
(10)
where X denotes oxygen analyzer reading. For more
details, please see Figure A‐5 in Supporting Information.
TABLE 1 Equation (9) constants correspond to each fuel gas
Equations used in calculating physical properties for air,
fuel gas components, and flue gas components are in different unit systems, which finally will be converted to SI
unit system using the program. Each physical property
as a function of temperature was replotted using excel
program to derive the corresponding equations for each
component. This consequently will facilitate the proposed
program to work with minimum error.
• To calculate sensible heat of air, it is first required to
derive an equation for calculating average specific
heat of air (kJ/kg K) in terms of average temperature
(Kelvin). The derived equation is as follows:
Air heat capacity ¼ 4*10−7 T 2 þ 2*10−5 T
þ 1:0056;
(9)
(8)
where T is the inlet air temperature in Kelvin. The estimated R2 for the derived equation is 0.999, which verify
the validity of the derived equation. Data used to draw
component
d
R2
0.0034
1.2027
0.9965
0
0.0042
0.5073
0.9999
0
0
0.0043
0.4007
0.9997
Butane
0
0
0.0042
0.4907
0.9998
Iso butane
0
0
0.0044
0.3859
0.9996
Pentane
0
0
0.004
0.5164
0.9994
Iso pentane
0
0
0.0042
0.4409
0.9998
Component
a
b
Methane
0
0
Ethane
0
Propane
Hexane
0
0
Nitrogen
2E‐10
1E‐7
Hydrogen disulfide
0
Carbon dioxide
c
0.0039
0.5554
0.9995
‐0.0001
1.0625
1
0
0.0004
0.8871
0.9902
0
0
0.0008
0.6128
0.9971
Sulfur dioxide
0
0
0.0004
0.518
0.9973
Ethylene
0
0
0.0034
0.5294
0.9992
Propylene
0
0
0.0036
0.4924
0.9998
Hydrogen
2E‐9
‐2E‐06
0.0008
14.321
0.9994
6 of 11
ELSAID
• To calculate flue gas emissivity as a function of BWT
(°F) and product of (partial pressure CO2 &
H2O × mean beam length) in atmosphere‐feet,
another set of equations have been derived from a
curve introduced by Shawabkeh.28 Table 4 indicates
the derived equations and their constants and R2
value for each listed equation.
• To calculate overall exchange factor as a function of
flue gas emissivity and the ratio of effective refractory
area (AR) to effective area cold plane (αAcp), an additional equation has been derived from a curve introduced by Kern27 as shown in Table 5.
• A set of equations have been derived from API 530[30]
to calculate Larson–Miller parameters as a function of
rupture stress for the most common metal type used
in fabrication tubes inside radiation section as listed
in Table 6. It should be noticed that The determination of Larson–Miller parameters will help in calculating the remaining tube life inside radiation section
• To calculate heat loss corresponding to flue gas exiting
stack or radiation/convection section, Universal Oil
Product Company has developed equations for each
flue gas component enthalpy (Btu/lb) in terms of
temperature in Rankin. Table 2 lists the derived
constants for each of the flue gas component.
The general form of this equation is presented below:
Enthalpy ¼ a þ b* T þ c*10−4 T 2 þ d*10−7 T 3
þ e*10−11 T 4 þ f *10−15 T 5 :
(11)
• To convert relative humidity of air into number of
water moles entering with combustion air, it is
needed first to derive an equation to calculate
water vapor pressure in air (P0water measured in
atmosphere) in terms of its inlet temperature
(T, degrees Celsius) using data obtained from
source.26 For more details, please review Figure
A‐6 in Supporting Information. The derived equation for determing the water vapor pressure is
shown below:
P0 water ¼ 9*10−9 T 4 –2*10−7 T 3 þ 3*10−5 T 2
þ 0:0002 T þ 0:0071:
3.4 | Program flow chart
(12)
This article is mainly talking about calculating the average TMT inside the radiation section; it is recommended
to utilize the inputs for the program to perform a complete heater performance analysis to ensure that the
heater operate safely and effectively as mentioned above.
However, it will be difficult to mention here how all other
outputs for the proposed program has been calculated in
detail; Figure A‐7 in Supporting Information shows the
program flow chart, which gives a brief picture about
the inputs, operating parameters calculated, and final
outputs.
3.3.2 | Factors calculation
To calculate effectiveness factor (absorptivity α) for the
most common tube layout against refractory wall in terms
of center‐to‐center spacing and outside diameter, a number of equations have been derived from the corresponding
figures presented by Kern27 to facilitate calculations.
The general equation used for different configurations is
shown in Equation (13).
Absorptivity α ¼ a X 6 þ b X 5 þ c X 4 þ d X 3
þ e X 2 þ f X þ g;
4 | R ESULTS A ND DISCUSSION
(13)
where X is the tube spacing to tube outside diameter ratio.
Table 3 presents the constants for Equation (3), R2, and
error percentage for each of the derived equations.
TABLE 2
ET AL.
This section considers the results of the proposed program and its validity through application on different
case studies.
Enthalpy equation constants for each flue gas component
Component
a
b
c * 10−4
d * 10−7
e * 10−11
f * 10−15
Nitrogen
−0.934
0.2552
−0.1779
0.1589
−0.32203
0.15893
Oxygen
−0.982
0.22749
−0.3731
0.4831
−1.85243
2.47488
4.778
0.11443
1.0113
−0.2649
0.34706
−2.463
0.45739
−0.52512
1.394
0.11026
0.33026
Carbon dioxide
Water vapor
Sulfur dioxide
−0.1314
0.64594
−2.0276
2.3631
0.0891
−0.77313
1.29287
ELSAID
ET AL.
TABLE 3
7 of 11
Absorptivity α equation constants, R2, and error percentage
Tube layout
a
Total two row
0
b
c
d
3 * 10−5
−0.0013
e
0.0182
−0.112
−0.525
Total one row
−0.00007
0.0022
−0.0269
0.1659
Direct to first row
8 * 10−5
−0.0027
0.0344
−0.2351
Direct to second row
−0.00003
0.0013
−0.0188
TABLE 4
Flue gas emissivity = AX2 + BX + C, where X = bridge wall
temperature in degrees Fahrenheit and Y=P * M.B.L in
atmosphere‐feet
Where A = EY + F
E
2 * 10−6
F
R2
−0.0114
0.9533
H
R2
0.1365
0.8272
J
R2
0.3664
0.9959
0.8773
R2
g
Error (%)
0.2158
0.8755
0.9995
0.025
0.6341
0.7505
0.9999
0.005
2.33
0.9998
0.01
−0.767
0.999
0.05
−1.902
−0.591
0.143
Flue gas emissivity equation and its constants
f
1.2318
4.1 | Program deviation test on design
cases
The program was applied for two case studies that have
been already designed, and maximum TMT has been mentioned in their data sheets. This comparison between
results helps to identify the maximum deviation of the
program.
and B = GY + H
G
‐1E‐05
4.1.1 | Case Study 1: Pacol unit charge
heater
And C = IY + J
I
‐8E‐05
TABLE 5 Overall exchange factor equation in term of flue gas
emissivity and AR/αAcp
Overall exchange factor = AX2 + BX + C,where X is the flue
gas emissivity and Y = AR/αAcp
A = EY3 + FY2 + GY + H
F
G
H
R2
0.2822
−0.7842
−0.3077
1
J
K
L
R2
−0.2774
0.6635
1.1878
1
N
O
P
R2
0.0315
0.0494
−0.0363
1
E
−0.0361
3
2
In Pacol unit of the Egyptian Linear Alkyl Benzene
Company, natural gas is used as a fuel gas; it is fired with
15% excess air in a natural‐draft horizontal, box‐type
heater. Exit flue gas temperature from radiation section
is 785°C, and relative humidity is 100%. Inlet temperature
of the fuel gas at burner is 120°C and radiation loss to
refractory wall is 2.5% of total heat release. Fuel gas
composition is shown in Table 7.
Process fluid is a mixture of hydrogen and hydrocarbons in vapor phase; tube layout in radiation section is
total one row with outside diameter of 101.6 mm, and
tube spacing center to center of 260 mm. Radiation box
B = IY + JX + KX + L
I
0.037
3
2
C = MY + NY + OX + P
M
−0.0065
TABLE 6
Fuel gas (mol %)
Component
Derived equations for estimating Larson–Miller
parameter
Larson–Miller parameter = AXB where X = rupture stress
pressure (ksi)
2
Metal type
A
A335‐P5
44.768 −0.105 0.9987 0.9993 0.065
A335‐P9
43.762 −0.082 0.9986 0.9993 0.07
A312‐TYPE347H 41.97
TABLE 7 Fuel gas composition of Pacol unit charge heater
B
R
R
Error (%)
−0.102 0.9989 0.9994 0.055
Rich
Lean
Nitrogen (N2)
0.99
0.06
Carbon dioxide (CO2)
3.81
0.14
Methane (CH4)
77.03
97.91
Ethane (C2H6)
10.95
1.82
Propane (C3H8)
4.99
0.04
Isobutane (IC4H10)
0.78
0.02
Normal butane (NC4H10)
0.91
0.01
Isopentane (IC5H12)
0.22
—
0.16
—
Hydrocarbons heavier than hexane (C6 )
0.16
—
Total
100
100
Normal pentane (NC5H12)
+
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with 12.34‐m length, 4.76‐m width, and 12.45‐m height
contains 44 tubes in layout.
TMT for radiation section, as stated in data sheet, is
564°C. The estimated maximum TMT using the proposed
program is 568.45°C. It is noticed that the estimated value
corresponds well with the designed value with 0.432%
error. This consequently confirms the validity of the
considered program. More details are clarified in Pacol
unit charge heater data sheet available in Supporting
Information.
4.1.2 | Case Study 2: Hydrocracker feed
heater
Hydrocracker unit (HCU) feed heater installed in Egyptian
Refining Company is a natural‐draft vertical cylindrical‐
type heater. Fuel used for this heater is natural gas, and
air enters the furnace with an excess of 20%. Exit flue gas
temperature is 842°C, and relative humidity is 85%.
Temperature of fuel gas on burner inlet is 49°C, and
radiation loss to refractory is 2% of lower heating value
of fuel gas. More details for HCU fractionator feed heater
data sheet are available in Supporting Information.
Process fluid is a mixture of hydrogen, hydrocarbons,
hydrogen disulfide, and steam in a mixed phase. Due to
complexity of the given data, it will be difficult to determine heat absorbed in radiation section using process
fluid data. This is because the design inlet temperature
of process fluid at radiation section is unknown and its
composition has two phases. However, the program can
solve this problem if the fuel gas flow rate is known.
From material balance using excess air percentage and
fuel gas composition, flue gas/fuel gas ratio could be
calculated, which is 20.9. On the basis of flue gas rate
given in the heater data sheet (27,460 kg/hr), the calculated fuel gas flow rate is 1,313.8 kg/hr.
Tube layout in radiation section is total one row with
tube outside diameter of 219.1 mm and tube spacing
(center to center) of 406.4 mm. Heater radiation section is
cylindrical type with dimensions of 12.832‐m height and
6.31‐m diameter; it contains 40 tubes divided into four
passes.
Depending on the forgoing information, the estimated
TMT is 521.5°C, which deviates from the actual TMT of
513°C (as declared in the heater data sheet) by 1.67% in
radiation section.
4.2 | Program deviation test on case
studies have skin tube thermocouples
The investigated program has been applied for two other
case studies, which already have thermocouples that
ELSAID
ET AL.
measure TMT. The results of the program was compared
with real results of thermocouples to show the degree of
agreement between them.
4.2.1 | Case Study 3: Naphtha hydrotreater
charge heater
The furnace of this case study is naphtha hydrotreater
(NHT) charge heater installed in MIDOR Refinery Company (Egypt). For this heater, refinery fuel gas is fired in
a natural‐draft‐type vertical cylindrical‐type heater (all
radiant type) with one firing zone having vertical tubes
in radiation section. The flue gases are directed below
convection section of another heater. Figure F‐1 in
Supporting Information shows more details for NHT
charge heater.
Radiation section dimensions (height 11.315 m, diameter 4.65 m) contains 44 tubes divided into four passes;
each pass has six thermocouples used to measure skin
metal temperature. Tube layout in radiation section is
total one row of outside diameter 141.3 mm, tube spacing
center to center 254 mm, and overall tube length 10.02 m.
The process fluid, which is a mixture of naphtha and
recycle gas (hydrogen and light hydrocarbons) in gas
phase, is heated from 290°C to 315°C in four passes in
radiation section. Each pass has six thermocouples
(TT041, TT040, TT039, TT038, TT037, and TT036) used
to measure TMT.
The estimated TMT was achieved by using actual data
taken from the distributed control system (DCS) of the
considered heater as an input data for the proposed
program. The results of the program were compared with
the heater TMT actual data obtained at different tested
days.
As stated before, the heat absorbed in radiation section
can be calculated using two methods depending on the
availability of operating data for each method. As for
example, it is needed to know the composition of the process fluid to calculate heat absorbed in radiation section;
however, the only available compositions are for naphtha
and recycle gas before mixing. Thus, an external program
will be used to simulate the heater to determine fluid
composition entering the heater, and this will maximize
the error percent.
However, combustion data such as fuel gas flow rate,
excess air, and BWT can be collected from DCS. The heat
absorbed by radiation can be calculated through multiplying radiation section efficiency by total released heat
as in case study HCU fractionator feed heater.
The equation used to estimate the heat absorbed in
radiation section depending on the process fluid side will
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be used only if all the properties of the fluid are known,
especially its composition.
Operating parameters for NHT heater at May 10, 2016,
is obtained from DCS as addressed in Table 8. The radiation loss applied for the program of 2% of total heat
release was taken from unit data sheet, and the relative
humidity in combustion air and its temperature data
were extracted from weather online site (http://www.
weatheronline.co.uk/). The fuel gas composition is
depicted in Table B‐1 (see Supporting Information).
The present NHT heater has two thermocouples that
could measure BWT. The average of these two readings
will be used by the program to calculate the average
TMT. The readings of the six thermocouples for each pass
in radiation section at May 10, 2016, were obtained from
DCS as addressed in Table 9.
The estimated average TMT is 436.2°C, which is very
near to the average of the actual readings from the table
above, which is 419.09°C, with an error percentage of
4.08%.
It should be indicated that the calculated temperature
does not take into account the overheating in tube metal
like that takes place in pass 3# as thermocouple does.
But, it may give a very good approximation of whether
the heater operates safely or not according to its operating
conditions.
TABLE 8 Naphtha hydrotreater heater operating conditions at
May 10, 2016
Operating parameter
Value
bridge wall temperature (°C)
691.58
Excess air analyzer (%)
4.95
Relative humidity (%)
60.0
Fuel gas temperature (°C)
45.0
Air temperature, (°C)
25.0
Fuel gas flow rate, (kg/hr)
TABLE 9
381.7
For further confirmation of the program validity, the
operational TMTs were taken during several days started
from May 10, 2016 and ended at August 24, 2016. Then,
metal temperatures for the same conditions of the above
mentioned days were calculated via the program to be
compared with the average temperature obtained by the
thermocouples readings of the four passes. The operating
parameters for the heater in the test period are addressed
in Table B‐2, Supporting Information.
The program has taken into account the variation of
fuel gas composition shown in Table B‐1 (Supporting
Information) during the tested period while calculating
TMT.
By comparing the actual TMT values with the calculated ones in the tested period, as presented in Table B‐
3 in Supporting Information, it is noticed that the maximum deviation are margined between −5.93% and
4.437%. This consequently indicates a good correspondence between the actual and estimated values of TMT
all over the tested period. These results confirm the validity of the introduced program in calculating effectively
the TMTs.
4.2.2 | Case Study 4: Recycle gas heater
This case study considered the recycle gas heater
belonging to MIDOR refinery. The fuel gas for this
heater is fired in forced‐draft vertical box heater. This
heater is all radiant type with one firing zone and
vertical tubes in radiation section. The flue gases are
directed below convection section of another heater, as
described in the data sheet recycle gas heater (Supporting
Information).
The tubes are double fired from both sides, and radiation section box dimensions are 10.47‐m height, 15‐m
length, and 4.65‐m width. Additionally, radiation section
contains 140 tubes, divided into 10 passes; each pass has
Thermocouples readings for tube metal temperature (TMT) of naphtha hydrotreater heater
Thermocouple reading (°C)
TT041
TT040
TT039
TT038
TT037
TT036
Average
TMT per
pass
Pass
1#
383.49
378.41
403.81
406.35
398.73
419.05
398.31
Pass
2#
378.41
393.65
398.73
406.35
398.73
403.81
396.61
Pass
3#
411.43
467.3
523.17
477.46
411.43
401.27
448.68
Pass
4#
NA
426.67
454.6
446.98
413.97
421.59
432.76
Pass
No.
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ELSAID
three thermocouples used to measure TMT. The tubes
have outside diameter of 114.3 mm, tube spacing (center
to center) of 203.2 mm, and overall tube length of 9.75 m.
Data of July 30, 2016 at 6:00 p.m. have been taken to
calculate TMT as an example. Input data for the program
are shown in Table 10 below.
Radiation loss percentage is 3% of lower heating value,
and fuel gas temperature is controlled at 45°C. Relative
humidity in combustion air is found using the site
http://www.weatheronline.co.uk/.
Fuel gas composition analysis in mole percentage is
indicated in Table B‐4 in Supporting Information. On
the basis of the operating parameters mentioned above,
the maximum calculated TMT is 514.25°C. The reading
of thermocouples installed at the 10 passes inside radiation section at July 30, 2016, are stated in Table 11.
From the table above, average TMT would be
545.62°C; thus, error percentage would be −5.479%. For
further confirmation of the program validity, it is needed
to compare the actual thermocouple readings with the
estimated values for several operating days.
The operational data for recycle gas heater from July
30, 2016, to August 18, 2016 were used as input data for
the proposed program (Table B‐5, Supporting Information). Furthermore, it is required for the program to know
the fuel gas composition at the tested days. The fuel gas
composition for the tested days are addressed in Table
B‐4 (Supporting Information).
The average maximum TMT of the 10 passes measured by the thermocouples during the tested days was
compared with that predicted by the introduced
TABLE 10
Recycle gas heater operating conditions at July 30,
2016, at 6:00 p.m.
Operating parameter
Value
Bridge wall temperature (°C)
726.35
Excess air analyzer (%)
4.164
Relative humidity (%)
70
Fuel gas temperature (°C)
45
Air temperature (°C)
282
Fuel gas flow rate (kg/hr)
1,827.3
TABLE 11 Output data from skin tube thermocouples at recycle
gas heater at July 30, 2016
Maximum skin tube temperature in degrees Celsius
Pass 1#
Pass 2#
Pass 3#
Pass 4#
Pass 5#
534.51
543.88
556.39
553.26
559.51
Pass 6#
Pass 7#
Pass 8#
Pass 9#
Pass 10#
547.01
537.63
534.51
543.88
N.A
ET AL.
program, and the results are presented in Table B‐6 in
Supporting Information.The results show that the estimated values of TMT differ from the actual ones in
the range of −5.79% to 0.974%. On the basis of this deviation percentages, it is clear that there is a good correspondence between the actual and predicted TMT
values, and this in turn confirms the validity of the proposed program.
5 | C ON C L U S I ON
The main objective of the present study is to create a
computer‐based program that has the capability to provide accurately the most important fired heater parameters and calculations. These parameters and calculations
that can indicate the heater performance are as follows:
•
•
•
•
Heater thermal efficiency.
Metal tube temperature in radiation section.
Creep calculations to determine remaining tube life.
Economic study on fired heater.
The program has been applied for different tube layouts (vertical or U‐tube) and different radiation section
layouts (box or cylindrical type). The results of the four
case studies indicated that the maximum deviation
between the real and calculated data is lower than 6%.
This consequently confirms the validity of the proposed
program.
The program was developed to help both designers
and operators who are interested in fired heaters design
and operation, as described in the following points:
• Regarding fired heater design, the present program
can calculate the required average TMT without any
assumption related to corrosion or fouling nature of
process fluid, which will be important for tube metal
selection in radiation section. The program can also
perform an overall heat balance for the heater at the
design stage.
• Regarding heaters in operation equipped with tube
metal thermocouples, the program can estimate the
average TMT value, which can be compared with that
achieved by thermocouples; it will, in turn, determine
if the thermocouples works sufficiently or not.
• Regarding heaters in operation without tube metal
thermocouples, the program helps in calculating the
average TMT and compares the current TMT with
its design value. This will be helpful in evaluating
the heater safe operation and predicting the TMT
changes when studying the effect of operating conditions on its value.
ELSAID
ET AL.
11 of 11
ORCID
Abeer M. Shoaib
Ahmed A. Bhran
https://orcid.org/0000-0003-4572-3125
https://orcid.org/0000-0002-9027-3453
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portable image processing system. Experimental Thermal and
Fluid Science. 2011;35:416‐421.
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using optical tomographic and two‐color pyrometric techniques.
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flame temperature and its 3D distribution in a 660 MWe arch‐
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Additional supporting information may be found online
in the Supporting Information section at the end of the
article.
How to cite this article: Elsaid MR, Shoaib AM,
Bhran AA, Awad ME. Developing of a monitoring
system for evaluating and investigating the fired
heater performance. Asia‐Pac J Chem Eng. 2019;
e2314. https://doi.org/10.1002/apj.2314