The following specification defines the minimum requirements for design, fabrication, and inspection of fired process heaters (hereinafter referred to as heaters) given in API STD 560, Fired Heaters for General Refinery Services.

Fired Heater Design Specification


The following documents are considered part of this specification. Use the edition of each referenced document in effect on the date of the publication of this specification.

AMCA - Air Movement and Control Association

AMCA 850Application and Specification Guide for Flue Gas Isolation and Control Dampers

API - American Petroleum Institute

API STD 530 Calculation of Heater-Tube Thickness in Petroleum Refineries
API STD 560 Fired Heaters for General Refinery Services
API RP 551 Process Measurement Instrumentation

ASME–American Society of Mechanical Engineers

ASME B1.20.1 Pipe Threads, General Purpose (Inch)
ASME B31.1 Power Piping
ASME B31.3 Process Piping
ASME SEC I BPVC Section I - Rules for Construction of Power Boilers
ASME SEC II A BPVC Section II A - Materials Part A - Ferrous Material Specifications
ASME SEC II B BPVC Section II B - Materials Part B - Nonferrous Material Specifications
ASME SEC II C BPVC Section II C - Materials Part C - Specifications for Welding Rods, Electrodes, and Filler Metals
ASME SEC II D BPVC SECTION II Materials Part D - Properties Non-Interfiled
ASME SEC V BPVC Section V - Nondestructive Examination
ASME SEC VIII D1 BPVC Section VIII - Rules for Construction of Pressure Vessels - Division 1
ASME SEC IX BPVC Section IX - Qualification Standard for Welding and Brazing Procedures, Welders, Brazers, and Welding and Brazing Operators

ASTM – American Society of Testing and Materials

ASTM A 216/A 216M Standard Specification for Steel Castings, Carbon, Suitable for Fusion Welding, for High-Temperature Service
ASTM A 217/A 217M Standard Specification for Steel Castings, Martensitic Stainless and Alloy, for Pressure-Containing Parts, Suitable for High-Temperature Service
ASTM A 351/A 351M Standard Specification for Castings, Austenitic, Austenitic-Ferritic (Duplex), for Pressure-Containing Parts Used in USDOE-NE Standards
ASTM A 488/A 488M Standard Practice for Steel Castings, Welding, Qualifications of Procedures and Personnel
ASTM A 560/A 560M Standard Specification for Castings, Chromium-Nickel Alloy
ASTM C 680 Standard Practice for Determination of Heat Gain or Loss and the Surface Temperatures of Insulated Pipe and Equipment Systems by the Use of a Computer Program

In the event of a conflict among the above listed standards or between the standards and other instructions listed in the specifications and drawing, it should be the Suppliers responsibility to notify the Buyer in writing for a resolution of the conflict prior to starting work.

3.0 Heater Layout Design

The ratio of exposed radiant tube length to tube circle diameter for vertical cylindrical heaters, or vertical wall height to tube-to-tube centerline for cabin heaters, should be 1.2 minimum.

  • Vertical radiant section tubes greater than 45 ft (13.7 m) in length require approval by the Buyer.
  • Corbelling should be provided in the convection section per the configuration shown in figure below.

Figure 1 - Corbelling in the Convection Section

Corbelling should not be used on the bottom shield row of the convection section.

For natural draft operation with burners firing vertically or horizontally, minimum clearances should be provided per API STD 560, Table 12 with the following exceptions:

  • For oil firing, the burner centerline to centerline of tubes should be increased by 6 in. (150 mm) beyond the API requirement, and
  • For gas firing, the burner centerline to centerline of tubes should be increased by 3 in. (75 mm) beyond the API requirement.

All welded return bends should be located as follows:

  • Radiant section return bends should be located inside the heater unless otherwise specified by the Buyer.
  • Convection section return bends should be located in header boxes.

4.0 Tube Support and Tubesheet Design

The design temperature for tube supports, guides, and brackets should be equal to the maximum temperature of the flue gas in contact with the support at design operation of the heater.  Minimum design temperature of radiant section tube supports, guides, and brackets should be 1600°F (860°C).

Tube support, intermediate tubesheet, tube guide, and support bracket materials should meet design temperature ranges per API STD 560, Table 8, Section 6.3.1 with the exceptions shown in table 1 below.

Table 1:  Maximum Design Temperatures for Tube Support Materials

Flue Gas Temperature, °F (°C) Material
1500 (816) 18 Cr 8 Ni (A240, A167 304H)
1800 (982) Ni-Fe-Cr (B409 Alloy 800 HT)
1900 (1038) 25 Cr 20 Ni (A351 HK40)
2000 (1093) 25 Cr 35 Ni + Nb (HP45 Modified)21 Cr 11 Ni - 1.5 Si (A240 RA253-MA)
Above 2000 (1093) * Requires Buyer Approval

* Alternative proposals for tube support materials should be submitted to Purchaser for Buyer approval.  Meehanite HR and 25 Cr 12 Ni are not acceptable alternatives. 25Cr 12Ni is not recommended.  It can undergo sigma phase failure at 1200°F (649°C).

Bottom tube supports in compression for vertical radiant coil heaters should be buried in insulating floor refractory.  The minimum design temperature for the support should be the process coil outlet temperature.

Tube support and tubesheet maximum allowable stresses for the listed materials should be as per the table listed in article "Allowable stresses cast tubesheet and tube support materials" and the Allowable Stresses for Wrought Tubesheet and Tube Support Materials.

Tube support bearing surfaces should be smooth with rounded edges, 1/8 in. (3 mm) radius or greater. Maximum unguided length of vertical tubes fired on only one side should be 35 ft (10.7 m).

Convection section intermediate tube supports should be as follows:

  • If sootblowers are used, or space for addition of sootblowers is provided, any intermediate tube supports required between adjacent sootblowers (or spaces provided therefore) should be located equidistant between them.
  • The minimum thickness of cast and wrought tube support flanges and web should be 0.625 in. (16 mm) and 0.5 in. (13 mm) respectively.
  • If tube supports are to be refractory coated, sleeves should be attached to the tube holes to prevent the refractory from being damaged by the tubes.  Sleeves should have inside corners removed to prevent binding. The presence of sufficient vanadium and sodium can lead to corrosion of metal surfaces.  The ash produced during combustion can cause severe pitting.
  • Proposals to use multiple-piece, welded tube supports should be submitted for approval.

End tubesheets should be stiffened as necessary to withstand the frictional forces due to thermal growth of the coil, and the dead load stresses - both at tubesheet design temperature.  Tubesheets should be protected by a minimum of 5 in. (125 mm) of refractory suitably attached to the supports.

Supports for convection section extended surface tubes should be designed to avoid mechanical damage to the extended surface, and should permit easy insertion of the tubes.  Tube supports should be provided with a tube-bearing surface not less than 21/2 in. (63 mm) in width.

5.0 Convection Section Cleaning

Convection section cleaning requirements are as follows:

  • For fuel oils containing 0.01 percent (mass) or more ash, or heavier than 25° API, sootblowers should be provided throughout the convection section.
  • For fuel oils containing less than 0.01 percent (mass) ash, and lighter than 25° API, space for future sootblowers and inspection doors suitable for steam lancing should be provided throughout the convection section.
  • Where clean gas is the only fuel used, convection section cleaning facilities are not required.
  • For dirty gaseous fuels producing more than 5 ppm (mass) or 5 mg/kg particulates in flue gas, sootblowers should be provided throughout the convection section.

Maximum radial coverage of a sootblower or manual steam lance should be per the following:

  • 3.5 ft (1050 mm) or 4 rows, whichever is less, for fuel oils lighter than 10° API (1.0 relative density) and dirty gaseous fuels producing less than 50 ppm (mass) or 50 mg/kg particulates in flue gas.
  • 3 ft (900 mm) or 3 rows, whichever is less, for fuel oils 10° API and heavier; and, for dirty gaseous fuels producing more than 50 ppm (mass) or 50 mg/kg particulates in flue gas.

6.0 Structural Design

Piping or obstructions beneath bottom-fired heaters should not interfere with burner adjustment or normal exit paths from beneath the heater.

All heaters should have structural steel framing designed by the allowable stress method.  The heater casing should not be used to support structural loads, except for shear due to lateral loads.  The casing may be used to provide lateral stability to main structural members.
For vertical cylindrical heaters, alternative designs, where the casing is used as part of the main structure, may be proposed for approval by the Buyer.

Heater floor plates should be supported from structural beams which are a part of the heater structure, and attached to beams by 100 percent welding.  Floor plate thickness should be 1/4 in. (6 mm) minimum.
All tube penetrations through the heater casing or header boxes should be fitted with seals designed to accommodate any tube movement and provide 100 percent tight sealing against air leakage.
Lifting lugs should be provided for all removable header box sections, doors, and inspection panels.

7.0 Access and Observation Design

Access doors should be provided for each of the following:

  • Cabin or box heaters - a walk-through door, approximately 21/2 ft wide x 5 ft high (750 mm x 1500 mm), should be installed in the radiant section.  If the heater is compartmented, walk-through access should be provided for each of the compartments.
  • Vertical cylindrical heaters - an access door having a free area of 24 in. x 24 in. (600 mm x 600 mm) minimum, should be provided in the floor and in the arch.  Access provided through removal of a floor burner and associated piping should be approved by the Buyer.
  • Sootblower lanes - minimum one 18 in. x 18 in. (450 mm x 450 mm) clear opening per lane. These access doors can be used to clean all convection section tubes during turnarounds when required.
  • Flue gas and air ducting - manways 24 in. x 24 in. (600 mm x 600 mm) should be provided to give access to all duct interiors, dampers, flow devices, and other equipment.

All access doors should be gasketed and bolted.  Access doors weighing more than 150 lb (68 kg) should be provided with hinges or davits that cannot restrict sealing of the door. Observation doors should be as follows:

  • Doors should be located in the radiant section to provide good visibility of baffles, bridgewall, all burners; and all radiant and shield tubes, tube supports and guides.  Minimum view opening for each door should be 5 in. (125 mm) wide x 9 in. (225 mm) high.  View openings for each door should be covered with glass that is removable and replaceable onstream.
  • Radiant tube centerline spacing at observation doors should be three nominal tube diameters.
  • Air leakage through the doors, when closed, should be negligible.
  • Doors in floors should be 3 in. (75 mm) minimum diameter.  A glass covering is not required.

Convection section inspection doors should be provided as follows:

  • For convection sections that are 50 ft (15 m) or less in length, one set of inspection doors should be provided.  These doors should be located adjacent to an intermediate tubesheet, if provided, and should be vertically aligned to enable inspection of each convection section tube row.
  • For convection sections that are more than 50 ft (15 m) in length, two sets of inspection doors should be provided.  These doors should be located adjacent to intermediate tubesheets and should enable inspection of each tube row.

7.1 Platforms

Platforms should be provided as follows:

  • As a minimum, completely around the heater at floor level and extending to heater casing at observation door locations.
  • Minimum clear width for access to header boxes, sootblowers, wall burners, and decoking connections should be 4 ft (1200 mm).

7.2 Stairs and Ladders

Stairway access should be provided to platforms serving burners, burner controls, and sootblowers, unless otherwise specified by the Buyer. Access to other platforms should be by ladders, unless otherwise specified. Self-closing safety gates should be provided across ladder openings at each platform landing.

8.0 Steam Air Decoking

Additional thermal expansion provisions may be required for heaters which are to be steam and air decoked.  For such heaters, thermal expansion and flexibility calculations should take into account these factors as well as normal design and operating conditions.  Tube metal temperatures used for both radiant and convection section tubes and any external crossover piping during steam-air decoking should be as shown in table 4.

Table 4:  Tube Metal Temperatures during Steam-Air Decoking

Tube Material Temperature, [°F] Temperature, [°C]
Carbon Steel 1275 690
21/4 Cr 1325 718
5 Cr 1350 732
9 Cr 1375 746
18 Cr 8 Ni 1700 926
25 Cr 20 Ni 2050 1121

Supplier should also check thermal expansion capabilities of stack damper at decoking temperatures listed in table 4.

9.0 Stack, Breeching, and Ducting Design

9.1 Stack

One stack or stack take-off should be provided for each 40 ft (12 m) of exposed convection tube length.
Minimum draft available immediately below the convection section should be 0.1 in. H2O (0.025 kPa) when firing at heater design rates.

Top of stack should be a minimum of 10 ft (3 m) above any equipment which is located within 50 ft (15 m) horizontally of the stack.  Top of stack should also be at least 10 ft (3 m) higher than any working platform that is regularly used by operating or maintenance personnel (once per day or more) and is within a horizontal distance of 100 ft (31 m). Spacing is required to minimize exposure of personnel to flue gas at an elevated temperature and minimize exposure of personnel to flue gas prior to allowing for dispersion.

The minimum design height of the stack should be equal to 1.05 times the minimum calculated stack height for the draft requirements.  Stack draft calculations should be based on the specified summer design ambient temperature.

Guillotine type dampers or other approved means should be provided to isolate individual heaters when ducted to a common stack.  These dampers should be accessible for maintenance from a permanent platform.

9.2 Breeching and Flue Gas Ducting

Breeching and flue gas ducting should be lined with a minimum of 3 in. (75 mm) medium weight castable refractory.

If the heater is equipped with downstream flue gas heat recovery, the breeching and hot flue gas ducting should be insulated to produce a calculated cold face temperature of no more than 180°F (82°C) based on an ambient, still, air temperature of 80°F (27°C).  The cold flue gas ducting downstream of the heat recovery unit should be insulated to maintain the bulk flue gas temperature above its dew point. Top breeching surface should have a minimum of 20 degrees slope.

9.3 Ducting of Combustion Air

Plenum chambers around the burners, air ducts, and noise-muffling devices should be designed to permit access to burners, pilots, and to facilitate burner lightoff.  Designs should be submitted to Buyer for approval.

Combustion air ducting should be:

  • Above ground and meet thickness and reinforcing requirements for heater casing.
  • Designed to provide uniform flow distribution (±2 percent root mean square) to all burners.
  • Sized to limit the dynamic head in the distribution duct, at the point of maximum velocity, to not more than 10 percent of the burner pressure drop at design firing conditions.
  • Designed for maximum discharge pressure of the fan at minimum specified design temperature and maximum fan speed.
  • Insulated, if an air preheater is provided, to produce a calculated cold face temperature of 150°F (65°C) based on an ambient, still, air temperature of 80°F (27°C).

Discharge dampers should be provided for parallel fans discharging into a common duct.  Dampers should prevent backflow through any non-operating fan.  Discharge isolation facilities should also be provided on any fan that can be shut down for maintenance while the heater continues to operate.
Proposals to use automatic opening, drop-out doors in combustion air ducting should be approved by the Buyer.  When their use is approved, the doors should:

  • Open into an area where personnel have no access, or
  • Upon opening, have a shield to deflect any escaping hot air away from personnel.

10.0 Burners

Burner capacity - burners should be capable of operating at the specified excess air level and heat release rate without flame impingement.  Unless otherwise specified, burners should be sized with the heat release margins above the design rate shown in table 5.

Table 5:  Heat Release Margins for Burners

No. of Burners Heat Release Margin
5 or less6 or more 10 percent5 percent

For dual fuel burners where each fuel can supply design heat release, no margins should be applied to the fuel-side design capacities.  Air-side margins should be as specified in table 5.

11.0 Fabrication and Lifting

The annular space between tubes and end tubesheet should be packed with high temperature packing or rope.  For extended surface tubes, the space between studs or fins should be filled with refractory at the tubesheet location.

12.0 Inspection and Testing

Tube supports fabricated from plates should have welds inspected by dye penetrant inspection.  Acceptance criteria should be approved by the Buyer.

All tube supports and tubesheets should be measured after all required weld repair and/or heat treatment has been completed.  Maximum permitted deformations are as follows:

  • Bowing along length (or width)—this is the total convex or concave measurement taken from a straight edge placed along the entire length (or width) of the tube support or tubesheet.  This measurement cannot exceed 0.1 in. per ft (8 mm per m).
  • Twist—this is the rotation of the end centerlines relative to each other.  With one end vertical, the other end centerline cannot exceed 1/2 in. (13 mm) from vertical when measured at the support/guide locations.
  • Bowing along depth of tube supports—same limits as bowing along the length.