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[QUOTE=Esam;176596]CONTROLLING FIRED HEATERS © Walter ********, P. Eng., 2000 May 20. walter(at)********(dot)ca

First published in Hydrocarbon Processing , April 1997.

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[IMG]http://www.********.ca/ce5_fh/5-1.jpg[/IMG]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 previous articles of this series^{1, 2, 3, 4}, the process aspects of controlling a piece of equipment were presented before dealing with protection and safety. This time the topics will be reversed: In the case of fired heaters, it must be safety first!
SAFETY. If fired heaters had not been invented and were being proposed for the first time, I 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. A million years, I'm told. 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 85⁵ and 86⁶ issued by the National Fire Protection Association. These have 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 easy 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 as defined in Chapter 3; but refinery off-gas with a high hydrogen content? Despite the disclaimers, the NFPA series is still an excellent guide to the instrumentation and control of any furnace.

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