Carburizing Process RX Gas Generator Endothermic Gas
Troubleshooting Tools
Recent innovations in fuel-injection and gas analysis equipment
have provided the modern heat
treatment professional with a number of useful tools to aid in
troubleshooting gas generator
problems. These tools can help to quickly identify any quality
related issue within the gas
generation process and provide the most direct course of action to
prevent or resolve any issue.
Endothermic Gas Quality Troubleshooting Tools
1. Air/Gas Ratio (Historical Data if possible)
2. Oxygen Probe Data
3. Dew Point Analyzer
4. Three Gas Analyzer
Air/Gas Ratio
The air/gas ratio is the leading indicator of any change inside the
endothermic gas generation
process. Typically, a higher air/gas ratio will produce endothermic
gas with a higher dew point.
However, the air/gas ratio can change considerably to maintain the
same dew point for a number of
reasons as detailed below. Therefore, it is important to understand
the normal ratio “range”
associated with a particular generator so that it is easier to
identify actual problems before they
become major issues. The air/gas ratio is simply a description of
the recipe being used in the gas
reaction. This ratio is typically displayed prominently when the
generator has a fuel-injection ratio
control system. However, if the generator utilizes a
carburetor/trim mixing package, the air/gas
ratio must be calculated by looking at the mechanical flow meters
on the generator and using the
following equation:
Air/Gas Ratio Calculation
(Air Flow + Trim Air Flow) / (Gas Flow + Trim Gas Flow) = Air/Gas
Ratio
Typical Air/Gas Ratio (40’F Dew Point)
Air / Natural Gas ............ 2.60 - 2.80
Air / Propane Gas ........... 7.60 - 7.80
If the dew point control system utilizes a trim gas solenoid that
“pulses” additional trim gas into the
reaction gas mixture using a time proportional scheme an additional
calculation will be required to find
the actual trim gas flow required in the Air/Gas Ratio equation
above:
Time Proportional Trim Gas Flow Calculation
(same calculation can be used for trim air if needed)
(% Output on DP Controller) x (Trim Flow when Solenoid Open) = Trim
Gas Flow Rate
It may seem obvious, but there are still a number of generators
manufactured today that do not
provide flow meters for all the gasses on the reaction mixing
system. It should be understood that
this is a serious disadvantage when trying to predict and
troubleshoot gas generation problems.
Further, a fuel injection system with true ratio control provides
the easiest and most accurate way to
document the actual air/gas ratio at any moment and does not
require any manual calculations.
In addition, modern controllers typically include a paperless chart
recorder that will log this ratio for
reference in generator troubleshooting.
Generator Startup after Weekend Shutdown or Longer
During the first 30 minutes of a generator startup the dew point
will remain high until all the
residual oxygen within the ceramic catalyst has been reacted and
the actual endothermic reaction
begins to occur. It is recommended that the operator resist the
urge to manually lower the air/gas
ratio much lower than the last known air/gas ratio to produced
endothermic gas at the desired dew
point. After approximately 30 minutes, the primary endothermic gas
reaction will begin and the
dew point will drop dramatically to a stable value and continue to
decrease slowly for the next 15-
20 minutes.
In the event the dew point does not fall to the desired dew point
after these events have occurred,
then an air/gas ratio adjustment should be made to bring the dew
point down manually. Once the
dew point has fallen below the desired setpoint, the dew point
controller should automatically begin
adjusting the air/gas ratio to maintain the desired endothermic gas
dew point.
Normal Reasons for Air/Gas Ratio Swing
The air/gas ratio can change significantly from month-to-month just
to produce endothermic gas of the same dew point quality. These
common ratio changes are
actually a normal response to
conditions that are not controlled of by the gas generation
process.
Reasons for Normal Air/Gas Ratio Swing
“Peak Shaving” in Natural Gas Supply........Large Influence (8 -
15%)
Ambient Air Dew Point Changes ................Small Influence (1 -
5%)
New Catalyst “Seasoning”...........................Small Influence
(1 - 5%)
The most obvious example of a “normal ratio swing” is between the
summer and winter months.
Over five years of data, it has been documented that a 10-12%
higher air/gas ratio is required to
produce endothermic gas of the same quality during the winter
months. The primary explanation
for this winter swing is due to “peak shaving” in the gas supply
and dryer than normal ambient air
conditions. Conversely, during the summer months, the ambient dew
point can be significantly
higher and therefore, a lower air/gas is needed to produce
endothermic gas with the same dew point.
Another normal ratio swing occurs when new catalyst is installed.
The initial air/gas ratio required
will be slightly lower than normal and gradually rise to a normal
operating range during the first 2
weeks of operation. During this time, the dew point of the
endothermic gas will remain constant.
The primary cause of this initial drift is presumed to be that
there is additional oxygen resident
within the inner cavities of the ceramic catalyst. This oxygen is
slowly reacting with the air-gas
mixture in the retort. Therefore, a small amount of additional gas
is required until the catalyst has
been completely “seasoned.”
It is important to note that during these “Normal” occurrences, the
endothermic gas quality is not
greatly affected. However, large “Peak Shaving” events can cause a
significant change to the
amount of Carbon Monoxide (%CO) in the endothermic gas. In this
case, the “CO Factor” might
need to be changed in the carbon controller at the furnace. Of
course, this should be done after a
proper shim test of the furnace atmosphere.
