Plant Layout & Fired Heaters
Plant Layout & Fired Heaters
Plant Layout & Fired Heaters
Table of Contents
1.
General
2.
Location
3.
Safety
4.
5.
Burner Piping
6.
7.
Instruments
1. General
1.1. Function
The primary function of a fired heater is to supply all heat required by the process in one form or another. A fired
heater utilizes gaseous or liquid fuels often produced as a by-product. The normal process function is raising the
process stream to its required temperature for distillation, catalytic reaction, etc. Sizes of heater vary
considerably, dependent upon the type of duty and throughput.
1.2 Types
There are two general basic designs or types of fired heaters:
Box type
Vertical type.
Such heaters may be fired vertically upward by panels, mounted in the heater floor or hearth, the heater
floor being elevated to provide headroom beneath. Alternatively, these heaters may also be fired
horizontally by burners mounted in the heater-end walls, in which case the heater floor is only elevated
above grade to provide air cooling convection to the heater foundations. This type of heater may contain
single or multiple radiation chambers discharging flue gases to a common convection section and stack.
Down-fired Heaters
In the down-fired heater, combustion gases generated in the radiant chamber pass downwards through a
refractory checker hearth into a collecting chamber beneath. From there the flue gases flow upward
through the convection section and then out to the stack. The down-fired heaters are basically intended
to fire on heavy residual fuels, where the flue gases are corrosive and may clog flue gas passages of
conventional heaters.
Convection sections are thus protected by removal of combustion solids and are usually provided with
inspection ports, soot blowing devices and tube facilities to keep the coils clean. Burners in down-fired
heaters are always mounted in the heater-end walls.
Vertical Heaters (See below figures 4,5 and 6) Vertical heaters are either cylindrical or rectangular. They
may have radiant section only or convection and radiant sections. The radiant section tubes will usually
be vertical, but some cylindrical heaters have helical coils. The convection section can be either vertical
of horizontal.
Types of Heater Firing
Heaters can be fired from any position, i.e. bottom, top, side or end.
By far the most common is bottom fired, mainly because it is more economical. The burner of a bottom
fired heater will be located 2.1-2.7 M above grade at a height which is suitable for an operator to work
underneath. Operating from under the heater is more dangerous than other types of firing, which is the
principle reason certain operating companies will not install a bottom fired heater.
Heaters are commonly light with an electric ignitor. some refineries use a propane torch while some still
light the burners with a rag soaked in spirits or kerosene.
Forced or Natural Draft
Consideration must be given at the layout stage to accommodate the additional equipment associated
with a forced draft heater. This will usually comprise an air inlet duct with silencer, forced draft fan and an
air preheater. The inlet duct may require a support structure.
2. Location
Heaters are always located at a safe distance 15 meters away from other hydrocarbon bearing equipment
and preferably upwind; however, on some process it is permitted for reactors to be within this distance to
prevent light volatile vapors from begin blown towards an open flame.
Space must be allowed for tube replacement for both horizontal and vertical heaters and this, together
with ample access for mobile equipment, should be considered on piping layouts and drawings. Ample
access is always needed for firefighting equipment with areas under or around heaters usually paved and
curbed.
No low points in the paving or grading are permitted as these provide excellent sports for trapping
hydrocarbon liquids which could be ignited by the open flames of the burners.
3. Safety
4.3 Flexibility
Because of the high temperature involved and length of pipe runs required to isolate the heaters, piping
flexibility must be examined carefully. Some heater manufacturers will permit a limited amount of tube
movement to take all or part of the piping expansion. This possibility should be investigated in conjunction
with the Stress Department, by the Piping Design Office during the study stage. It is generally necessary
to anchor the piping adjacent to the heater to remove stresses from the nozzles.
5. Burner Piping
See below figures 8 and 9.
Supply of fuel to individual burners is adjusted by individual valves. These should be so located that the
burners can be operated while observing the flame through peepholes or burner openings.
All burner leads for gas and steam (atomizing) must be taken from the top of the headers, and fuel gas
piping should be so arranged that there are no pockets in which condensate could collect. The fuel oil
header must have full circulation; under no condition shall it be adead-end line. Noncirculating branches
to burners should be as short as possible or insulated together with atomizing steam.
A ring header around the furnace mounted a short distance above the peepholes, having vertical leads
adjacent to the vertical doors to the burners, provides the greatest degree of visibility from the operators
point of view.
Atomizing steam to be used in conjunction with fuel oil shall be taken the main steam supply header at or
near the heater. Steam traps shall be provided to drain all low points in the atomizing steam system.
Separate leads to each burner shall be taken from the top of the atomizing steam subheader. Shutoff
valves shall be so located that they can be operated while observing the flames from the observations
ports.
the decoking header. Care must be taken to allow sufficient access and platforming when the
swing elbows are changed over. Coke is carried by this header to the drum or sump.
In some instances it may requested by the Process Department or Client that the decoking manifold is
connected to allow for reverse flow during the decoking.
7. Instruments
7.1 Stack
a. Damper: Mechanical or pneumatically operated to control the draft through the stack.
b. Draft gauge (P and I)
c. Flue temperature (TI)
d. Orsat (O2 CO CO2 analyzer).
Instruments b., c. and d. are used to access the correct combustion conditions. Steam is supplied for the
Orsat connection in the stack. Water is supplied to the O 2 analyzer. Platforming for the access to the
stack instruments is supplied with the heater.
Figure 7.
Figure 8.
Figure 10
Figure 11
DECOKING PROCEDURE
AFTER TUBES BECOME HEADED, STEAM IS INJECTED AT CONVECTION INLETS, VALVES 1,
2, 4 & 5 ARE CLOSED 3 IS OPEN.
TO START BURNING, STEAM FLOW IS REDUCED. AIR IS INTRODUCED BY OPENING VALVE 4.
FOR REVERSE FLOW, VALVES 2, 3, 4 & 6 ARE CLOSED, 1& 5 ARE OPEN. VALVE 2
IS OPENED ONLY IF REVERSE BURNING IS REQUIRED. WHILE PASS 1 IS BEING DECOKED,
STEAM IS INJECTED INTO PASS 2 TO KEEP TUBES COOL.