The PGA 3510 portable three/four gas furnace analyser for the carburising/nitriding process is one of SSi’s most popular products. This is largely because it displays and records data that is critical for verifying process quality and maintaining equipment. Below are top three ways SSi Europe engineers use the PGA:
1. Verifying carbon potential (carburising) and dissociation (nitriding).
The large majority of carburising processes are controlled with an in-situ oxygen probe (e.g. the SSi Gold Probe!), a carbon potential PID control loop that adjusts air and/or gas additions using a proportional valves you can see in the image. See a simplified diagram above. Theoretically, with stable carbon monoxide and carbon dioxide values, the carbon potential value on the controller/regulator is the carbon potential of the furnace atmosphere. In practice there are many potential reasons that a controller/regulator’s actual carbon potential value could be false or inaccurate. These include unstable or rich/lean carrier gas being produced by an endogas generator or nitrogen/methanol supply, furnace leaks, oxygen probe damage or degradation, contaminated reference air, improper installation, calibration status of the controller and many more. A verification is critical to ensuring that the carbon potential on the controller is the carbon potential of the atmosphere in the heat treatment furnace, and is a requirement of heat treatment standard CQI-9.
The same principle applies to gas nitriding where often the only method of atmosphere control is a periodic burette inspection which indicates the % of NH3 which has dissociated. The PGA can be used to verify the burette reading and also provides a Kn value. Where a diluent gas is used (e.g pre-cracked NH3 or nitrogen), burette readings are less useful but the PGA automatically calculates the Kn if the user enters gas flows. With a nitrocarburising atmosphere, the PGA can calculate both Kn and Kc.
2. Identifying furnace air ingress or radiant tube cracks.
Air ingress can be masked with high amounts of natural gas/propane additions. The control system variables might look healthy, but if there is air leak the furnace’s recovery times may be greatly extended, products may decarburise, and the additional gas needed to mask the air could produce graphite which can result in processing and furnace maintenance issues. To rule out furnace leak tightness, a simple comparison between the endothermic gas generator’s gas composition can be performed. With additions turned off, the gas value between the generator and furnace should be relatively similar (the furnace’s CO should be slightly lower than the endothermic gas generators). After noting the generator’s gas composition, turn the additions on and enter a carbon potential set point and observe the gas composition as they change. The CO will drop a little, the CO2 will also go down, and the CH4 will go up. Generally, if that does not happen then there is either air ingress, loss of positive pressure, or issues with carrier gas.
If the CO2 and CH4 values are unstable then a radiant tube leak is possible. The PGA’s chart can be compared with the furnace’s chart. If the fluctuations correlate with with the burner’s going into high fire/on then it is likely that the products of combustion are entering the furnace. On a continuous furnace, by running a stainless steel tube along the belt of the furnace, the general location of the leak can be identified.
3. Endothermic gas generator catalyst maintenance.
Common problems with endothermic gas generators we encounter are improperly conditioned catalyst and catalyst that is regenerated too infrequently (and where soot has been able to form in the retort). These issues can easily be identified and rectified with a three gas analyser. Catalyst can be conditioned using CO2 values, and can be regenerated when CH4 values rises above 3%. If soot has formed, it can be burned out with a lean ratio whilst monitoring CO2 values until a CO2 value of 0.25-0.50 can be stably controlled.
Get in touch to discuss any questions you have about carburising furnace or nitriding furnace control and atmosphere verification!
See our range of portable gas analysers here: www.supersystemseurope.com/product-category/analysers/portable-gas-analysers