SABP-F-001

Best Practice SABP-F-001

7 February 2007

Process Heaters Optimizations Document Responsibility: CSD/M&CED/PEU

Saudi Aramco DeskTop Standards Table of Contents

1

Introduction.......... Introduction........................ ........................... ............................ ............................. .......................... ........................ ............ 2 1.1 Scope................... Scope.................................. ............................. ........................... .......................... .......................... ............... .. 2 1.2 Conflicts with Mandatory Standards................. Standards....... ...................... ....................... .............. ... 2

2

Intended Users.................... Users.................................. ........................... ........................... ............................ ....................... ......... 2

3

References............. References........................... ............................ ............................ ........................... .......................... ...................... ......... 2 3.1 Saudi Aramco References................. References...... ..................... ..................... ...................... ................... ........ 2 3.2 Industry Codes and Standards.................. Standards....... ...................... ...................... ..................... .......... 2

4

Equipment Introduction......... Introduction....................... ............................ ........................... .......................... ...................... ......... 3 4.1 Purpose and Use of Process Heaters................ Heaters..... ..................... ..................... .............. ... 3 4.2 Process Heater Types.................... Types.......... ..................... ...................... ...................... ...................... ........... 3

5

Process Heater Operating Variables Monitoring and Control.............. Control..... ......... 6

6

Thermal efficiency............. efficiency........................... ............................ ........................... .......................... ........................ ........... 21 6.1 Calculating Thermal Efficiency Using the Input/Output Method................................................ 22 6.2 Calculating Thermal Efficiency Using the Heat Loss Method.................................................... 23 6.3 Thermal Efficiency Efficiency Improvement............ Improvement.......................... ............................ ................... ..... 23 6.4 Reduce Excess Air..................... Air.................................... ............................. .......................... ................. ..... 24 6.5 Reduce Stack Temperature................ Temperature..... ..................... .................... ..................... ................. ...... 25 6.6 Reduce Other Losses................... Losses......... ..................... ...................... ..................... ..................... ........... 26 6.7 Effects of Firing Rate on Thermal Efficiency............... Efficiency.... ...................... ............. .. 26

Previous Issue: New Next Planned Update: 7 February 2011 Revised paragraphs are indicated in the right margin Primary contact: Ibrahim H. Al-Hamam Al-Hamam on 966-3-873-7722 Copyright©Saudi Aramco 2007 2007. All rights reserved.

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Document Responsibility: Consulting Services Dept. Issue Date: 7 February 2007 Next Planned Update: 7 February 2011

1

SABP-F-001 Process Heaters Optimizations

Introduction 1.1

Scope This document covers the best practices that are specifically applied to optimize the performance and efficiency of natural draft process heaters in general refinery services. The objective of this document is to provide guide lines to monitor and control essential process and fire side parameters in order to achieve performance and efficiency optimization. The document will also give guidelines to fine tune process heaters in order to achieve the objective.

1.2

Conflicts with Mandatory Standards In the event of a conflict between this Best Practice and other Mandatory Saudi Aramco Engineering Requirements, the Mandatory Saudi Aramco Engineering Requirements shall govern.

2

Intended Users The intended users for this SABP are all process engineers and energy conservation teams working in operating facilities and design engineers who are involved in or working on Saudi Aramco project.

3

References This Best Practice is based on the latest edition of the references below, unless otherwise noted. 3.1

Saudi Aramco References Saudi Aramco Engineering Standard SAES-F-001

Design Criteria of Fired Heaters

Saudi Aramco Materials System Specification 23-SAMSS-29

3.2

Manufacture of Fired Heaters

Industry Codes and Standards American Petroleum Institute API STD 530

Calculation of Heater Tube Thickness in Petroleum Refineries

API PUBL 535

Burners for Fired Heaters in General Gene ral Refinery Services Page 2 of 26


4


API RP 556

Instrumentation and Control Systems for Fired Heaters and Steam Generators

API STD 560

Fired Heaters for General Refinery Service

Equipment Introduction 4.1

Purpose and Use of Process Heaters

To optimize the operation and performance of process heaters in refinery services it is very important to discuss the purpose and use of process heaters. The purpose of a process heater is to supply heat to a process fluid by burning fuel. The process fluid might be in the form of gas, liquid or two phases (liquid and gas). Process heaters in refinery services are used mainly to vaporize liquid hydrocarbon or to increase temperature for hydrocarbon liquid or gas. The principle uses of process heaters are in distillation for charge heaters and reboilers and for reaction temperature control such as hydrotreaters and reformers. Process heaters are used when there is a large heat duty and when the process fluid outlet temperature is usually over 500ºF that is difficult to achieve with steam. 4.2

Process Heater Types

Process heaters are sometimes referred to as furnaces and the terms are used interchangeably. 4.2.1

Direct Fired/Fire Tube All furnaces and heaters are classified in one of two categories; directfired heaters (flame outside tubes) or fire tube heaters (flame inside tubes). Because most furnaces and heaters in a refinery are direct fired, the following discussion will be limited to direct fired equipment.

4.2.2

Cylindrical/Cabin 4.2.2.1

There are two basic types of direct-fired furnaces, cylindrical and cabin. Within each type there are many different configurations. The furnaces can have have different coil arrangements: horizontal, vertical, or helical. Also, the furnace can be all-radiant (no convection section) or have a convection section. Several configurations for the vertical cylindrical and cabin type furnaces are shown in Figure 3 and 4.

4.2.2.2

The all-radiant cylindrical furnace is the simplest and least expensive. Typically, an all-radiant furnace operates with about a 60% efficiency and a stack temperature of about Page 3 of 26



1200ºF. Adding a convection section to an all-radiant vertical cylindrical furnace increases the overall furnace efficiency to about 80% and drops the stack temperature to about 750ºF. Of course, the convection section significantly increases the furnace cost.

Figure 1. Example of Vertical Cylindrical Direct Fired Furnaces

4.2.3

Figure 2. Example of Cabin Direct Fired Furnace

Advantages and Disadvantages Some of the advantages for the two types of direct-fired furnaces are identified below: 4.2.3.1

Advantages of Cylindrical Furnace a)

Require the smallest plot area for a given duty.

b)

The cost is usually 10 to 15% lower in the larger sizes.

c)

Can accommodate more parallel passes in the process coil.

d)

For large duties, a cylindrical heater has a taller firebox and more natural draft at the burner.

e)

The flue gas velocity is usually higher in the convection section, hence, the flue gas film coefficient is higher.

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4.2.3.2

4.2.3.3

4.2.4


f)

Few expensive tube supports or guides are required in the convection section.

g)

Fewer soot blowers are required in the convection section. (Soot blowers are not needed for gaseous fuel.)

h)

If coil drainage is a problem (vertical tubes), a helical coil may be used when there is only one pass

Advantages of Cabin Furnace a)

The process coil can always be drained (Horizontal Tubes).

b)

Two-phase flow problems are less severe (slug flow can generally be avoided). (Horizontal Tubes)

c)

Cabins can accommodate side-firing or end-firing burners instead of only vertically upward firing. This permits the floor of the heater to be closer to the ground. (Some burner manufacturers prefer to fire liquid fuels horizontally.)

The choice of vertical or cabin process heater will depend on the application and size of the heater. Cabin heaters are sometimes called box heaters. The most common Saudi Aramco heaters are cylindrical. Certain applications such as reformer heaters are usually cabin heaters (arbor design). Cabin heaters usually have a larger heat duty than cylindrical heaters.

Cabin/Box Heaters - Tubes and Headers 4.2.4.1

Cabin/box heaters are very much same. The major difference is the shape of the breeching. The cabin heater has a sloped roof as shown in 3C. The radiant section tubes are usually mounted horizontally in a cabin/box heater as shown in 3C and 3F. More than one coil is usually mounted in the radiant section to minimize pressure drop on the process side.

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Source: API STD 560, Fired Heaters for for General Refinery Services.

Figure 3. Typical Furnace Types

5

Process Heater Operating Variables Monitoring and Control 5.1

Typical Process Flow Diagram (PFD)

Figure 4 is a typical PFD for process heaters. Furnace controls and safety devices vary considerably depending on the furnace service. Figure 1 shows an example of control system for a direct-fired heater. It should not be considered complete, but only representative of the type of instrumentation that should be carefully considered in designing a control system for a furnace service. Page 6 of 26



Figure 4. Example of Direct Fired Heater Controls/Safety Controls/Safety Devices

5.2

Heater Charge Flow

The heater charge flow is the flow rate of the process fluid. This flow rate is very important since it cools the metal of the heater tubes. The heater charge rate is alarmed when it is below a minimum for that heater. A flow rate below the minimum rate will result in tube overheating which could result in coke formation, deformation of tubes, and possibly, rupture of tubes. Page 7 of 26


5.3


Pass-Flow Balancing

The flow through each parallel tube pass should be maintained at the same rate. This is referred to as pass-flow balancing. In cases where pass-flow balancing is critical, each pass of a heater has a flow indicator and at least a manual control valve. The flow through each pass must be equal for proper control of tube metal temperatures. An imbalance in flow can result in coking and tube overheating. A reduced flow will result in less heat being removed from the tubes in that pass. Since the heat flux on all tube passes is similar assuming uniform heat distribution, the tube pass with the reduced flow will have a higher fluid temperature because there is less fluid to absorb the same amount of heat. A higher fluid temperature will result in a higher metal temperature. A higher fluid temperature can also cause coking that will further increase the metal temperature. The outlet of each coil or pass has a thermocouple to measure the temperature. The coil outlet temperatures must be within at most 10ºF for pass-flow balancing. If the flows indicate pass-flows are balanced but the temperatures do not, then the flow meters and thermocouples should be checked to resolve this discrepancy. Otherwise, investigation for the possibility of coke formation should be conducted. 5.4

Tube-Skin Temperature - Tube Metal Temperatures

As the temperature of the tube metal increases the allowable stress decreases, which in turn reduces the internal pressure handling capability of the tube. Metal temperatures can also affect the thermal stability of the process fluid in the tube resulting in hydrocarbon coking at high temperatures. For these reasons the tube metal temperature is often monitored using tube-skin thermocouples. Additionally, high tube temperatures can also provide an additional indication of low flow or ineffective pass-flow balancing or coke formation. 5.5

Variables Affecting Tube-Metal Temperature

The tube-metal temperature is a function of the heat transfer resistances. As the heat transfer resistances increases, the tube-metal temperature will increase. Figure 5 summarizes the resistances to heat transfer. Anything that affects these resistances will affect the metal temperature. In the radiant section the resistance on the outside of the tubes is low since almost all the heat is transferred by radiation.

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Figure 5. Heat Transfer Resistance

5.5.1

The following changes in operation are the usual causes of tube - metal temperature changes: 5.5.1.1

A change in firing rate: the change in the firing rate will change the radiant section heat flux (Btu/hr ft²), which will change the metal temperature. However, if the change in firing rate is balanced with the change in the process flow, the impact is negligible.

5.5.1.2

A change in flame pattern: the flame pattern can result in localized overheating especially with high number of burners. Therefore, care should be taken that the firing rate must be distributed evenly among all the burners.

5.5.1.3

A change in heater outlet temperature (change in firing rate (heat flux) and temperature of fluid in tubes). Page 9 of 26


5.6

5.7


5.5.1.4

A change in process fluid rate which will change both the heat flux and the inside tube heat transfer resistance.

5.5.1.5

A change in fouling resistance inside tube: the change in fouling resistance inside the tube is usually due to deposits such as coke.

Variables Affecting Coke Deposits

5.6.1

Coking is a polymerization reaction and is controlled by time at a temperature. An equal amount of coking can occur with a relatively low temperature (750 - 800ºF) if given long residence time and with a relatively high temperature (900ºF) and a short residence time.

5.6.2

Certain chemical compounds have a greater tendency to coke than others. Materials with heavy hydrocarbons are more likely to coke. The heavy hydrocarbons crack easily at 750ºF forming coke.

5.6.3

High tube-metal temperatures will cause coking even at the relatively low residence time in a heater tube. A heater is always more susceptible to coking when operating at low velocities because the residence time is longer. Steam (often called velocity steam) is injected to the heater coil to maintain high velocity and a low residence time in order to minimize coke formation.

5.6.4

Once coking has started it increases the tube-metal temperature because the cooling process fluid is insulated from the tube by the coke. Increased tube - metal temperatures due to coking can result in tube failure. Coke deposits can be observed by the change in tube color due to the higher metal temperature.

Heater Outlet Temperature

5.7.1

In cases where flow balance is critical or where coke formation is more likely, the heater outlet of each coil is monitored to verify that the flow through the coils is balanced or evenly divided.

5.7.2

The heater outlet temperature of the combined coil outlets is controlled at a desired value for the process by controlling the firing rate. The control of this variable needs to be tuned to avoid over firing which can result in tube overheating. In dual firing (combination of fuel oil & gas), the firing rate can be controlled by controlling either fuel gas or fuel oil flow, but not both. Usually, the gas flow is controlled and oil is fired under manual flow control.

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5.8


Heater Charge Temperature

The heater charge temperature is monitored because it can have a major effect on the heat duty required to raise the process fluid to the heater outlet temperature. Changes in heat duty affect firing and heat flux, which then affect tube-metal temperature. Changes in the heater charge temperature also will change the temperature of the fluid in the tubes. The metal temperature will be affected by both the firing rate and the fluid temperature in the tubes. 5.9

Stack Temperature

The stack temperature is useful in evaluating the overall performance of the heater and in keeping the temperature within the design. Stack temperature is an indication of the amount of waste energy going up the stack. The primary control of this variable is proper design of the convection section in the heater. A leaking or ruptured tube can result in a sudden increase in this temperature because the process fluid is burning and the flow is uncontrolled. Gradual increase in stack temperature is an indication of coke formation on the process side or fouling on the outside heat transfer surfaces in the convection section which will increase the firing rate and lower the thermal efficiency. Sever coke build up or external fouling coke lead to sever over firing and eventfully tube rapture. 5.10

Fuel Variables

5.10.1 The fuel variables for process heaters include fuel flow, fuel gas pressure, fuel oil pressure, fuel oil temperature, and the steam/fuel oil differential pressure. 5.10.1.1 Fuel Flow 5.10.1.1.1 5.10.1.1. 1

Fuel flow is controlled to meet heater outlet temperature requirements. Fuel flow can change due to changes in the heater outlet temperature and changes in the heating value of the fuel. Fuel flow should not be a function of fuel supply pressure. Supply pressure to the control valve should have an independent control.

5.10.1.1.2 5.10.1.1. 2

Fuel flow will be shut off by the emergency isolation block valve. Shutdown events will close this valve. One such event is a low low heater charge flow which if continued would result in tube failure because of tub overheating.

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5.10.1.1.3 5.10.1.1. 3

On heaters with the ability to burn both gas and oil fuels, the temperature control of the heater outlet will control one rate but not both. When both fuels are fired the oil rate is usually manually controlled by the number of oil burners in service and the gas rate is controlled by the heater outlet temperature.

5.10.1.1.4 5.10.1.1. 4

Changes in the heater charge temperature also will change the temperature of the fluid in the tubes. The metal temperature will be affected by both the firing rate and the fluid temperature in the tubes.

5.1.10.2 Fuel Pressure Fuel pressure is usually measured and controlled. Too low fuel gas pressure can result in an unstable flame and possible flame failure. Too high gas pressure can result in flame lift-off and possible flame failure. Flame failure can result in fuel gas accumulating in the firebox and being ignited by hot refractory with explosive force. 5.1.10.3 Fuel Oil Temperature Fuel Oil Temperature is usually measured and controlled. The oil temperature required is dependent on the grade of oil being used and viscosity requirements of the burners. Too low a temperature and high viscosity can cause a poor flame or inefficient combustion (incomplete combustion) and low efficiency. Too high a temperature can cause carbonization of the burner tip and will result in plugging of burners tips. 5.1.10.4 Steam/Fuel Oil Differential Pressure 5.1.10.4.1

Typically, fuel-oil-fired fuel-oil-fir ed heaters are provided with steam atomization or steam-assisted systems. When steam atomization is used, it is usually necessary to control differential pressure between the steam and the fuel. Typical differential pressure is in the range of 20-30 PSI in order to maintain efficient atomization Steam-assisted burners require a constant steam pressure.

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5.11


5.1.10.4.2 5.1.10.4. 2

For good atomization and combustion of liquid fuels, the steam must be perfectly dry. If there is moisture in the steam, this moisture will flash when it mixes with the oil, causing erratic oil flow. The atomizing steam should be superheated about 50ºF.

5.1.10.4.3 5.1.10.4. 3

Mechanical atomization can be used when steam is not available. The kinetic energy in the oil itself is used for atomization by releasing the oil through the tip under very high pressure. Mechanical atomization is usually used only in large burners or with very clean fuels, since the very small orifices required in smaller burners can become plugged by small dirt or coke particles in the fuel.

Firebox Draft

5.11.1 The firebox draft is a critical variable in a natural draft furnace because it controls the available pressure drop of air across the burner. A minimum pressure drop across the burner is required for proper mixing of air and fuel. Changes in the draft can also change the amount of combustion air to the burner. Low draft can result in smoking and long flames. High draft can result in a low thermal efficiency because too much air is flowing through the furnace. 5.11.2 Process heaters are designed to operate with a draft (negative pressure) in the firebox. 5.11.3 Figure 6 shows a typical draft profile for a natural draft process heater.

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Figure 6. Furnace Natural Draft Profile Profile

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5.12


Flue Gas Oxygen

5.12.1 Flue gas oxygen is the amount of oxygen in the flue gas. To establish and maintain the efficiency and safety of the combustion process, excess oxygen is an important measurement. The fuel feed is continually adjusted to meet the varying load requirements and maintains a correct proportion of air to bum the amount of fuel being fed at any time. To obtain a good measurement relating to the quality of combustion, samples should be taken as near as possible to the point where combustion is completed, normally at the exit of the radiant section. This will minimize the impact of air leakage on the measurement. To obtain the percent of oxygen in the flue gas, an oxygen analyzer is used. Flue gas oxygen can be measured by an analyzer or by taking a sample to the laboratory. 5.13

Excess Air/Oxygen

5.13.1 Burners are not 100% efficient and need some excess air to operate properly. Figure 7 and Figure 8 show that a burner has an optimum range for excess air. The optimum will be different for different burners. Forced draft burners usually have lower optimum range than natural draft burners. Oil burners normally have higher optimum range than gas burners. 5.13.2 The optimum operation for excess air is dependent on the fuel and the burner design. Fuel oils require a higher optimum excess air for proper operation. Each burner design will have a different optimum excess air. Forced draft burners have a lower optimum excess air than natural draft burners. Staged burners such as low NOX burners have a lower optimum excess air than normal burners. The optimum excess air for any firebox can be determined by reducing the air ratio until the CO content of the flue gas starts to rise. Then increase the air ratio until the flue gas CO returns to normal. Since the optimum is an air ratio, the firebox will have to be rebalanced when the fuel rate changes significantly as in a load change. Uneven burner loading can lead to tube overheating. 5.13.3 The air-to-fuel ratio is critical to proper burner operation. Too low an air-to-fuel ratio will result in long flames and can result in an unstable flame. Too high an air-to-fuel ratio will result in short flames but can result in an unstable flame. Too high an air-to-fuel ratio is an uneconomic operation.

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5.13.4 The fuel-to-air ratio in a natural draft furnace is controlled by the furnace draft and burner air registers. The furnace draft also provides some kinetic energy for mixing in a natural draft furnace. Inadequate draft will result in a low air-to-fuel ratio, long flames, and an unstable flame. Too high a draft will result in a high air-to-fuel ratio, short flames, and could result in an unstable flame. Too high a draft is an uneconomic operation and will result in increased air leakage into the heater.

Figure 7. Optimum Excess Air for a Fired Heater

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Figure 8. Typical Combustibles Emission from Fired Heaters

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5.13.5 Air Leaks High excess air can be caused by air leaks into the furnace. Figure 9 shows likely sources of furnace air leaks.

Figure 9. Furnace Air Leaks

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5.13.6 Operating Guidelines Both the stack damper and the burner air registers are used to control both the excess air and the furnace draft. The operating guidelines in Table 1 and the flow chart below can be used to judge which control should be used.

Low Draft

High Draft

Low Excess Air (O 2)

Open Dam per

Open Burn er Air

High Excess Air (O 2)

Close Burn er Air

Close Dam per

Table 1. Operating Guidelines

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Source: API PUBL 535, Burners for Fired Heaters in General Refinery Services

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5.14


Flame Characteristics and Patterns

5.14.1 Flame characteristics are primarily determined by the burner design and the fuel-to-air ratio. The burner design determines how well the fuel and air are mixed. 5.14.2 The better the mixing the quicker the combustion reaction can occur and the shorter the flames will be. The burner design also determines the shape and stability of the flame during various operating conditions. 5.14.3 The flame pattern can be adjusted by adjusting the primary and secondary air registers. 5.15

Over-Firing

Over-firing the burner (overdesign fuel rate) will result in longer flames and unstable flames. Flames can tend to lift off the burner with the possibility of flame failure. Under-firing a burner can also result in flame instability and the possibility of flame failure. Burners should be operated within the design limits of the turndown ratio. 5.16

Burners Distribution

All burners should normally be operating in a firebox. At low loads, it may be necessary to shut off some burners in order to have stable flames. For reduced load operation, the number of burners operating in a firebox should be set such that the load for each burner is approximately equal and about midway in the turndown ratio for the burners. The operating burners should provide an even flame distribution over the firebox. Uneven loading of burners and/or uneven distribution of burners can result in overheating the tubes.

6

Process Heaters Thermal efficiency Thermal efficiency is defined as the percentage of the absorbed energy to the total energy input. Calculation of thermal efficiency is based on an energy balance around the process heater. Factors that increase the losses will decrease the thermal efficiency. For example, operating with too much excess air reduces the thermal efficiency by increasing the stack heat loss because the excess air is heated from ambient to stack gas temperature. The thermal efficiency, for which a process heater is designed, is an economic evaluation involving the cost of fuel and the cost of equipment to reduce the losses. Examples of economic analyses include:

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(1)

the amount amount of insulation or refractory used to reduce heat losses to the atmosphere,

(2)

the amount of heat transfer surface provided in the radiant and convection sections to reduce the stack temperature,

(3)

use of a preheater to reduce the stack gas temperature, and

(4)

types of burners used which determines minimum excess air requirement.

6.1

Calculating Thermal Efficiency Using the Input/Output Method

6.1.1

The thermal efficiency can be calculated using either the higher heating value (HHV) or the lower heating value (LHV). The LHV is a better measure of achievable thermal efficiency since the latent heat of vaporization of the water in the flue gas cannot be recovered. The HHV efficiency is several percentage points lower than the LHV efficiency. It is common practice in the furnace industry to use the LHV in calculations while the boiler industry uses the HHV efficiency.

Example

A heater is heating a 20º API oil from 410ºF to 620ºF for use as a hot oil heating media for reboil heat to distillation columns in the plant. The flow of oil is 150,000 Bbl/D. The oil is not vaporized in the heater and its specific heat is 0.40 Btu/lb-ºF. The heater is firing 895 Bbl/D of 5º API fuel oil with a lower heating value of 17,000 Btu/lb. Calculate the heater efficiency. Solution:

Heat Absorbed, Q a Specific Gravity = SG =

1415 . ⎛ ⎞ ⎛ 141.5 ⎞ ⎜ ⎟ = ⎜ ⎟ ⎝ 1315 ⎝ 131 . + ° API ⎠ 131.5 .5 + 20 ⎠

Mass rate = m = 150,000 X 0.9 34 X

350 24

= 0.934

= 2,043,125 lb/hr

Qa = m Cp ∆t = 2,043,125 X 0.40 X (620 - 410) = 171.6 X 106 Btu/hr Heat released, Q r SG =

1415 . ⎛ ⎞ ⎛ 141.5 ⎞ ⎜ ⎟ = ⎜ ⎟ ⎝ 1315 ⎝ 13 . + ° API ⎠ 131. 1.5 5 + 5 ⎠

Mass rate = m = 895 X 1.0366 X

= 1.0366

350 = 13,530 lb/hr 24

Qr = 13,530 X 17,000 = 230 X 106 Btu/hr

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Heater Efficiency, E E =

⎛ Q a ⎞ 171.6 ⎜⎜ ⎟⎟ = Q ⎝ r ⎠ 230

6.2

= 74.6%

Calculating Thermal Efficiency Using the Heat Loss Method

The heat loss method calculation is used when the heat absorbed cannot be readily calculated such as most process heaters. The heat absorbed can be calculated by subtracting the heat losses from the heat fired. In process heater the primary heat loss is that lost to the stack gas. The heat loss in the stack is a function of the stack temperature, the amount of excess air and the carbon and hydrogen ratio in the fuel. API RP 530 specifies a detailed procedure for calculating the thermal efficiency. This procedure is long and requires an analysis of the fuel composition. The API RP 530 procedure is a detailed heat balance on the combustion side of the furnace to determine the amount of heat lost up the stack. 6.3

Thermal Efficiency Improvement

6.3.1

The thermal efficiency of a heater can be improved primarily by reducing the heat loss out of the stack. Increasing heat transfer surfaces or adding a new heat recovery can reduce stack temperature. An example of adding heat recovery is a waste steam generator as is done at Ras Tanura at the crude unit and reformer heaters.

6.3.2

Another way of reducing heat loss out of the stack is to add an air preheater. An air preheater exchanges hot flue gas with cold air to the burners improving the heat recovery in the furnace and thereby increasing the thermal efficiency. Lower limits on flue gas temperature and upper limits on air to the burners that must be met in design of air preheaters.

6.3.3

Another means of reducing the heat loss out of the stack is reducing the excess air. Each burner design has a minimum excess air for a design fuel. The excess air should operate near the minimum for the burner design. Consideration can be given to replacing burners with a more efficient burner that will permit operation at a lower excess air.

6.3.4

Example below in table 1 illustrates the increase in efficiency and the fuel savings that can be attained with a reduction of the stack temperature of 50ºF and a reduction of 10% in excess air.

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To Increase Efficiency:

1. Lower stack temperature. a. Add more surface to convection. b. Add more surface to convection convection section and preheat another process process stream. stream. A 50ºF reduction in stack temperature would increase efficiency from 84.5% to 85.9%. c.

The 50 °F reduction in stack temperature reduces the fuel consumption 1.62%.

2. Reduce percent excess air. a. A reduction of excess air from 20% to 10% increases efficiency from 84.5% to 85.4%. b. The reduction of excess air from 20% to 10% reduces the fuel consumption by 1.04%. As shown by table 2, the improvements improvements are all of the same order of magnitude. magnitude. Which one (or all) is used depends depends on the specific furnace under consideration.

Case

Base

Lower Stack Temp.

Heat absorbed, MBtu/hr 310.13

31

310.13

55

600

20

10

85.

85.40

1.6

1.04

0.13

Stack temperature, ºF 600

Reduce Excess Air

0

Excess air, percent 20 Furnace efficiency, percent 84.5 Fuel savings, percent

91

Base 2

Table 2. Furnace Fuel Savings

6.4

Reduce Excess Air

All the air that enters a furnace is ultimately discharged to the atmosphere at the stack temperature, and the energy it contains is lost. The primary objective of efficient furnace operations is to minimize airflow beyond that required for good combustion. The air required for combustion should enter only through the burners. The following steps can be taken to reduce excess air: a)

Seal air leaks. This is particularly important in furnaces, which operate with a draft (negative pressure) throughout the furnace. These furnaces are

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more susceptible to air infiltration. Figure 6 above shows typical sources of air leaks into a furnace.

6.5

b)

Fire all burners at the same rate (close off idle burners).

c)

Control furnace draft to its optimum level. Typical draft range is 0.05" to 0.1" H2O at the bridge wall.

d)

Determine excess air targets for each furnace through a series of plant tests. These targets are the minimum excess air rates that are necessary for good combustion. Since no two furnaces are exactly the same, there can be different targets for each furnace in the plant.

e)

Add improved combustion control systems. 1.

Automatic draft control and process heaters

2.

Use closed loop oxygen and/or CO trim control.

f).

Replace oversized burners. It is difficult to operate oversized burners efficiently at the high turndown rates desired without excessive excess air.

g).

Use high-capacity, high-intensity, or axial flow forced-draft burners for improved, low excess air combustion.

h)

Use low NOX burners for reduced emissions and low excess air.

Reduce Stack Temperature

6.5.1

Fouling of the convection section tubes is the primary cause of stack temperatures exceeding design. The extent of fouling can be determined by visual inspection of the tubes or by observing an increase in stack temperature over time. A 40ºF increase in stack temperature typically represents a loss of 1% in thermal efficiency.

6.5.2

Fouling can be reduced by operating sootblowers in furnaces. Sootblowers should be provided for all furnaces where heavy liquid fuels are fired. Units without sootblowers should be periodically cleaned during turnarounds. Fuel oil additives can be used to reduce deposits.

6.5.3

Reducing the stack temperature of a furnace that is operating satisfactorily satisfactoril y usually requires the addition of heat transfer surface. The following are means of reducing stack temperature: a)

Add heat transfer surface in convection section of process heaters.

b)

Add waste heat boiler to convection section.

c)

Add combustion air preheaters. Air preheaters can transfer heat from the flue gas leaving the stack, to the air used for combustion. Depending upon the flue gas temperature, the incoming air can be Page 25 of 26



heated to a maximum of 65ºC (150ºF) as specified by SAES-F-001. The flue gas temperature should be kept above about 300ºF to prevent corrosion of the heat exchanger due to sulfuric acid. 6.6

Reduce Other Losses

Although less important than excess air and stack temperature, heat losses out of the heater can be reduced by improving the refractory design and material selection such as using refractory ceramic fibre which has very low insulating values and would minimize heat losses. 6.7

Effects of Firing Rate on Thermal Efficiency

As the firing rate is increased the loss to the stack increases primarily because the heat transfer area is fixed. The increase in heat loss is not necessarily proportional to the increase in firing rate. Increased loss will reduce thermal efficiency. Similarly, a decrease in firing will slightly improve thermal efficiency. At very low firing rates the heat losses to the atmosphere become significant and the thermal efficiency may decrease. Over firing a process heater will reduce thermal efficiency.

7 February 2007

Revision Summary New Saudi Aramco Best Practice.

Page 26 of 26

SABP-F-001

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