Super System T4409 User Manual

T4409
Issue 2

AN OVERVIEW OF ENDO­THERMIC GENERATORS.

by Theodore P. Berry Technical Data

INTRODUCTION

Endothermic atmosphere, commonly called endo, carrier gas or Rx ® gas, is synthesized in the
catalytic retort(s) of endo generators. This gas, combined with an additive gas such as natural gas
or propane, or air, is used in heat treating furnaces to modify the surface chemistry of work
positioned in the furnace. Other carrier gases such as exot hermic gas, dissociated ammonia and
other nitrogen-based atmospheres are found in many heat treating facilities, but endo is most
common. Because endo can have a profound impact on the quality of processing, it is important
to establish a preventive maintenance program for the generator, and then follow the program
religiously.

The most common source of endo is the reaction product of air and natural gas in ratios between
about 2.5 to 1 to 5.5 to 1. Since the reaction is not spontaneous below ratios of 6 to 1, it is
necessary to supply heat to the generator--- hence the term endothermic, meaning heat absorbing.
Typical endogas generators produce an atmosphere of approximately 20% carbon monoxide,
40% hydrogen and 40% nitrogen, trace amounts of carbon dioxid e and other gases originating in
the natural gas or created by the reaction. The application of heat is not sufficient to create the
desired products rapidly, so the reacting gases must be exposed to a catalytic agent to accelerate
the reaction.

Principal components of an endogas generator are:

a heating chamber to supply heat by combustion or electric heating elements,
one or more cylindrical retorts (usually vertical) in the heating chamber with
numerous small, porous ceramic pieces, impregnated with nickel as a catalyst for the

reaction. Also included as part of the generator is

a cooling heat exchanger to rapidly cool the reaction products to a temperature that will

not allow the reaction to proceed further. One of the most critical parts of the system is

the control system that maintains the reaction temperature and adjusts the gas/air ratio

to provide the desired dew point.

* Rx is a registered trade-mark of Surface Combustion

S U P E R S Y S T E M S & T E C H N I C A L D A T A S H E E T

Printed in U.S.A. SSi Super Systems Inc CINCINNATI, OH 45215

The endothermic generator creates an
atmosphere to provide a positive
pressure in a heat treating furnace,
and a platform on which a carburizing
or decarburizing environment can be
formulated, by addition of enriching
gas or dilution air. Generator
maintenance scheduling, operation,
and control are discussed.

T4409

Note: The maintenance schedule suggested in the

following paragraphs has been culled from numerous

"industry standards" as well as over twenty years of

application experience of SSI staff working with the

equipment. Because of the make and design of your

equipment, and your routine maintenance procedures,

Issue 2

ENDOTHERMIC GENERATORS

Technical Data

MAINTENANCE SCHEDULE

not all of these recommendations may apply.

Daily:

Visually examine all instrumentation to assure that operation is normal...without

incident. Determine that control outputs are within the expected range of operation.

Check temperature of water discharged from heat exchanger.

Weekly:

Regenerate (burn out) carbon in generator using air (preferred method), exothermic gas,

or lea n endothermic gas.

After regeneration and readjustment of generator to proper dew point, check the gas

analysis (including CH4 content) with an infrared analyzer if possible.

Clean the air filter.

Monthly:

Clean air-gas mixing valve (carburetor) thoroughly.
Check calibration of gas analysis and control equipment such as automatic dew point

controllers, manual dew point indicators, CO/CO2 analyzers and oxygen probe carbon
potential control systems. The primary standard for carbon analyzers is shim stock tests.

Alnor or equivalent dew point testing gives a reasonable indication of correct operation.

Inspect thermocouples and protection tubes and replace every 3 to 4 months.
Check natural gas pressure after the regulator to maintenance balance to the carburetor.
Verify correct operation of over temperature controls.

Semi- annually:

Replace heat exchanger with standby, clean and refurbish for next service.
Inspect catalyst in retort and fill to proper level or replace.
Inspect and clean all burners.
Clean endo delivery lines to furnaces.
Inspect cooling water thermostats, solenoids.
Perform complete instrument calibration and service, including safet y controls.
Have oxygen probe (carbon sensor) refurbished, inspected and certified.

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T4409
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ENDOTHERMIC GENERATORS

Technical Data

Annually:

Check compressor blades.
Check motor and compressor bearings.

Additional Troubleshooting Considerations

Problems at the furnace can alter an otherwise correct endo atmosphere due to air or water
incursion. Therefore both the atmosphere manifold and the furnace should be checked carefully,
when a problem is encountered, before attempting any corrective changes in the generator
atmosphere control system.

Air Maintenance

One of the major sources of difficulty, and hence a nagging maintenance problem, is poor quality
air. Contaminants in the air can include dust, fumes from acid cleaning tanks and oil quenching
systems. Airborne dust has been linked to failure of pumps and flowmeters, and poor electrical
contact in relays. Combustible vapors can cause a carbon sensor to read low, resulting in over
carburization.

In order to maintain a good air quality, it is necessary to establish a routine for cleaning filters. In
some cases it may be necessary to install ductwork to bring outside, contaminant free air to the
equipment. Dust-tight electrical housings are sometimes necessary to eliminate dust and fume
problems.

Cooler Maintenance

The design objective of coolers is to cool the generated atmosphere as quickly as possible to
below about 2600F in order to stabilize the composition before being delivered to the furnace.

Two problems impact on operation of the water-cooled heat exchanger. First, dissolved and
suspended solids in the water will deposit and coat the tube walls, thereby reducing the heat
transfer rate. Further, dissolved oxygen in the water can promote oxidation of the tubes, resulting
in premature failure.

Water-cooled heat exchangers should be cleaned and pressure tested for leaks during catalyst
replacement. Air-cooled exchangers should be blown out with air or rodded as necessary also
during catalyst replacement.

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T4409
Issue 2

ENDOTHERMIC GENERATORS

Technical Data

Gas Supply

For the successful operation of any gas generator, an adequate supply of gas of the proper
composition is required at all times.

Dirt, scale and water vapor can cause plugging and failure of regulators and control valves.
Flowmeters are especially susceptible to dirt build-up on the float and walls. Gummy deposits
from a poor gas mixture can also cause sticking of valve parts and carburetors.

One of the most troublesome problems is the inconsistent analysis of the supply fuel. Propane -air
additives to natural gas supplies ('peak shaving') alter effective generator ratios. Unsaturated
hydrocarbons such as ethylene and propylene break down quickly into oily soot or coke.
Unfortunately, there is little choice when using utility-supplied natural gas. In some instances,
propane is selected to avoid these problems, but is not always an economical solution.

Sulfur, both naturally occurring and as additive mercaptans, can cause poisoning of the nickel
catalyst and an ultimate failure of the generator to crack gas properly.

It is therefore essential that the gas supply system be kept under close observation, and that all
critical components of the system, such as those previously mentioned, be inspected and cleaned
as required on a programmed basis. It may be advisable to inform your gas/utility supplier of
your special concerns should they, at their discretion opt to alter your feedstock supply by
"dosing" or "spiking" (i.e. peak shaving) your gas. W e find, in many instances, that the gas utility
companies will not advise you of this practice and are reluctant to discuss it.

Maintenance of the Combustion System

The maintenance of the combustion system of a gas fired endothermic generator is not different
from other combustion systems; that is, it should be kept clean and adjusted. Most such systems
are relatively simple.

Electrical Maintenance

A reliable, uniform supply of regulated power is necessary to operate relays and solenoid valves.

Electrical problems are usually easy to spot, since maintenance personnel are well trained and
have sufficient test equipment and wiring diagrams for trouble shooting.

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