This section describes the general arrangement of various type of furnaces and discusses how operation and maintenance affect furnace elevation, how to plan all necessary access and platform requirements, associated equipment and stack design.
Following figure shows a typical arrangement of a circular furnace.
Setting the Furnace Elevation
- The location of burners and the possible need for air preheating ducts are two primary factors to be considered when the elevation of the furnace is established.
- If as shown below, the burners are floor mounted, the elevation should be set high enough to allow an operator to walk freely under the furnace or ducting but not necessarily under all the burner piping.
- The preferred location for the burner operating valves is on the firing platform at the peep doors, not beneath the furnace.
When wall-mounted burners are used, the elevation of the furnace can be lower because there are no major maintenance or operational requirements beneath the furnace to be addressed.
- Access for operation and maintenance is a major consideration in the development of a furnace layout.
- Access requirements are reviewed for tube removal, soot lowers, damper control, header boxes, burners and burner management skids and control panels.
- Because furnace tubes must be periodically replaced, clear areas are provided directly in line with the tubes.
- The removal mechanism may be trolley beam located on the stack of a circular furnace, a crane, or a cherry picker.
- If only one or two tubes require removal, a tube extractor, similar to the type used for extracting shell and tube exchanger bundles may be used.
- As seen below, radiant tube removal in a circular furnace is accomplished by pulling the tubes through an access door located in the top of the radiant section.
- This is performed by using a hoist attached to the circular trolley beam on top of the stack.
Once the tubes are lifted out of the radiant compartment, they are pulled away from the furnace and lowered to grade.
- The convection section tubes are removed in a manner similar to the one shown for the box type furnace below.
- Here again the access area must allow the tubes to be pulled clear of the furnace, permitting the crane to swing away and lower the tubes to grade.
- Soot blowers have different access requirements, depending on the type used.
- One type is fixed to the convection section wall and occupies a relatively small area, unlike the retractable type, which is furnished with a horizontal support beam that enables the mechanism to insert the lance during operation.
- Dampers, located in the stack, are usually regulated by a device located at grade, as depicted below.
Cables shrung through guides fixed to the damper and adjustment lever, or control handle, may be locked into various positions as desired.
- Air actuated operators are less commonly used for damper adjustments, which are routinely made during start-up when the proper pressure balance within the furnace must be obtained.
- Header boxes as shown below are enclosed around the fittings that connect two tubes.
- Header boxes are sealed from the inside of the furnace by an insulated tube sheet and because this area is a possible source of fire, a snuffing steam connection is provided for fire suppression.
- A drain connection is also provided for leak detection.
- Clear access must be provided around the header box to permit inspection and tube cleaning or replacement.
- Burner access is required for ignition on start-up, air flow control through register adjustment or control handles, and oil-gun maintenance.
- Access is also required to allow routine operator inspection for any sign of leaks, particularly when flexible hoses are used.
- Burner management systems are vendor-supplied packages consisting of all primary valving and instrumentation necessary to start up, operate and shut down all burners of a furnace under regular or emergency conditions.
- A typical burner management skid is shown below.
Burner management control panel
- Platform requirements for furnaces are no different than any other large piece of equipment that needs operator and maintenance attention.
- Because each heater is designed to meet specific process needs, may fire with different fuels and require air preheating or soot blowers and is subject to client needs, each design should be approached as a unique layout, following basic guidelines.
- A circular furnace platform layout is shown below.
- The firing platform, which is usually a full 360 circle, can be reached by a variety of stairway configurations and ladders, as illustrated on the lower plan in this drawing.
- Each scheme shown may be set to accommodate the layout as required.
Stairway variations shown here are:
- Stairway A – Perpendicular, which can be set to either side of the landing.
- Stairway B – Radial.
- Stairway C – Tangential.
- Access to all other platforms on this type of furnace is by ladder, indicated on the exhibit as follows :
- Ladders D and E – Limited to 30-ft (9150 mm) maximum ladder run per OSHA. An intermediate platform is required.
- Ladder F: Enables plant personnel to gain access to both sides of the upper furnace by the convection section platform.
- Additional platform requirements – for example for access to soot blowers or interconnecting platforms between groups of similar heaters as shown below.
- Catwalks are planned for the firing platform and top of the radiant section for operator convenience.
Pyrolysis furnace shown below has both wall and floor-mounted burners.
- The wall mounted burner piping can be operated from the firing platform and viewed from the peep door on the end wall.
- The floor-mounted burners can be operated from grade at the edge of the furnace or from an optional lower firing platform.
- Additional platforming is required for access to draft gauges, peep doors and the convection section.
Because the primary operator level is at the burners, stairways are used for access and ladders are provided for all other levels.
- A reformer furnace shown below has a pethouse and roof-mounted burners.
- In addition to platforms and a stairway for regular maintenance and operation, ladders are provided in case of an emergency.
- Other platforms are for access to peep doors, draft gauges and piping as required.
- A soot blower mechanically cleans soot build-up on convection section tubes in fuel-oil-fired heaters.
- A soot blower essentially consists of a lance (i.e. pipe) with nozzles specifically located for the tube section to be cleaned and a connection for a blowing medium, which is steam or air.
- There are two types of blowers: fixed and retractable.
- A fixed soot blower shown below is mounted directly on the convection section wall and is hard piped as shown.
A rectractable soot blower, shown below, allows the lance to be removed from the convection section during operation.
- Some of the principal components are the support channel, the gear-driven carriage, the poppet valve (used to control the flow of the cleaning medium), and the lance with nozzles.
- Following figure shows the soot blower in operation.
- As the lance enters the heater, the blowing medium cuts a path through the deposits until lance reaches its apex.
- The lance then reverses rotation and is indexed so that on the retraction path it cleans surfaces not covered in on insertion.
- The reversed rotation and indexing allow the soot blower to peel and strip all deposits efficiently and with less chance of heater tube erosion.
An induced draft fan may be required for furnace operation, as shown on top of the convection section below.
- Platforms designed specifically for each fan configuration should be adequate for maintenance and operation of the drive and fan mechanism.
- Following figure shows a typical arrangement of a single-zone pyrolysis furnace with an integral waste heat system, which requires a steam drum and a heatexchanger (often called a transfer line exchanger).
- Because the preheater and ducting on an average sized combustion air preheating system are somewhat large, the plant layout designer should pay extra attention to the amount of space that is finally used for this equipment.
- After enough room is allotted for maintenance and operation of the forced and induced draft fans and blowers, the distance from the outer edge of the preheater to the centreline of the furnace should be kept to a minimum as shown below.
Whenever furnaces are arranged in groups on a plot plan, the plant layout designer should consider using a common stack as shown below.
Economic constraints usually determine the feasibility of this design feature; all furnace configurations can use this breeching concept.