* 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.
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