Controlling
Fired Heaters
The purpose of a fired heater
is to add heat to a process fluid and it is important to discuss safety
first, says Walter Diredger.
Introduction
The purpose of a fired heater is very simple: To
add heat to a process fluid. Its representation on a process flow diagram
is also very simple. But, of course, fired heaters are among the most
complex pieces of process control equipment. Each furnace is, after
all, at least two pieces of equipment in one. Firstly, it is a special
variant of the shell and tube heat exchanger since its purpose is to
exchange heat. Secondly, it is a chemical reactor in which fuel and
air undergo extremely exothermic reactions to produce the required heat.
In the case of fired heaters, it is important to discuss safety first!
Safety
If fired heaters had not been invented and were
being proposed for the first time, one would probably say, “You’ve
got to be kidding. That thing will blow up in your face the first time
you throw a match in it.” However, at least a half a billion gas
fired heaters are in service around the world (according to the American
Gas Association). Most of them are operated by people with no technical
experience whatsoever; few heaters blow up. Still, the average domestic
water heater is not in the same league as a hydrogen reformer furnace.
The fact that accidents and disasters are as few as they are, is due
to the long experience the human race has in dealing with fire. For
the last century, this experience has been embodied in various codes
and standards that have been written into law and are en-forced by inspectors
around the world.
The Code
The most popular, or notorious, of these codes in
North America is NFPA 85 issued by the National Fire Protection Association.
It has been considerably updated in recent years, especially in terms
of clarity. Nevertheless, there is still the problem of interpretation.
The code is not at all eas y to read as it combines many facets of construction,
instrumentation and operation in a single document. Not only that, but
the code5 contains the following disclaimers: It is not possible for
these standards to encompass specific hardware applications, nor should
these be considered a “cookbook” for the design of safety
systems. and:
This standard applies to boilers with a fuel input of 12,500,000 Btu/
hr (3663 kW) or greater. This standard applies only to boiler-furnaces
using single burners firing:
a) Natural gas only as defined in Chapter 3.
b) Other gas with a BTU value and characteristics similar to natural
gas.
c) Fuel oil of No. 2
and:
Furnaces such as those of process heaters used in chemical and petroleum
manufacture, wherein steam generation is incidental to the operation
of a processing system, are not covered in this standard. What is an
engineer to use for a guide when the furnace is not a boiler, but a
feed heater; does not exceed 12½ million Btu/hr, but is only
four million; does not burn natural gas - but refinery off-gas with
a high hydrogen content? Despite the disclaimers, NFPA 85 is still an
excellent guide to the instrumentation and control of any furnace.
Fuel Gas Fired, Single Burner Furnaces
NFPA 85 deals with a variety of fuels, both oil
and gas. The discussion that follows restricts itself to fuel gas fired,
single burners. NFPA standards have been followed as much as possible
and have sometimes been exceeded by adding components and control functions
where the special requirements of process control make it advisable.
Figure 2, shows the in-line instruments typically installed on a burner
fuel gas train. Diamond symbols with an “I” in them refer
to I/O of the Burner Management System BMS).
cont....
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