Internal Furnace Pressure
Most atmosphere furnaces, whether they be batch or continuous, operate at internal pressures in the range of 0.01-0.10 inches of water column (2.5-25 Pa), with the upper value being a good targeted operating pressure for most furnaces. A very tight furnace system might reach as high as 0.20-0.30 inches of water column (50-75 Pa), but this is far less common in the heat-treat industry. To put this pressure range in perspective, an internal pressure of 0.10 inches of water column (2.5 Pa) is about equivalent to the pressure exerted on a flat table by 10 one-dollar bills stacked one atop another.
Common misconceptions are that increasing the flow alone will automatically increase the pressure or that simply adding more weights to an exhaust flapper will, in and of itself, force the furnace pressure higher. Pressure will always be lower in poorly sealed batch furnaces or in continuous furnaces since they are open on both ends.
Measuring Furnace Pressure
Use of a simple manometer (Fig. 1) is a fast, easy and inexpensive way to monitor furnace pressure. Digital displays, recorders and remote signal transmission simplify the task of accurate measurement and control. It is always valuable to monitor changes in pressure during normal operation (Fig. 2) and to determine the “normal” length of time a furnace is in a negative-pressure situation (e.g., after a load is discharged to the quench tank in an integral-quench furnace). Unusually long periods of negative pressure or changes in pressure are a good way to determine if furnace troubleshooting or maintenance is necessary.
Eliminating Furnace Leaks
We all know, or should know, that a furnace atmosphere must be present in sufficient volume and pressure so as to exclude air from entering the furnace atmosphere. Both parameters are important for safe operation, especially during door openings, load transfers and when the furnace is operating below 1400°F (760°C). In addition, if one is trying to precisely control the active furnace atmosphere to reach a specific carbon potential or dew point inside the furnace, changes in pressure can upset the delicate balance of the atmosphere.
Typically, leaks present below the furnace hearth will draw air in and those above the hearth will leak atmosphere out. Finding leaks above the hearth can often simply be done using a small torch to light off the combustible gas (observing all appropriate safety precautions!), but leaks below the hearth line are much more difficult to find. Techniques such as over-pressurizing the furnace (to force atmosphere out of these lower areas) or smoke-bomb testing are commonly used to locate them. Keep in mind a furnace leak that affects the atmosphere is typically considered a large leak, with a total area in the neighborhood of 0.75 in2 (480 mm2) or greater.
Leaky Radiant Tubes
Pressure changes that occur within a furnace are often due to leaky radiant tubes. If a tube ruptures, the combustion products will leak out and mix with the furnace atmosphere, creating a fluctuation in the internal atmosphere pressure in the furnace. If you suspect that you have a leaky tube, here are a few suggestions on how to find it from those who work on these problems every day. Remember that the burner, recuperator (if present) and all associated piping are very hot, and proper personal protective equipment must be used to avoid injury.
It is important to recognize that leaks in radiant tubes generally begin as a pinhole or small crack and gradually increase in size. Symptoms include an erratic (oscillating) carbon potential (i.e. oxygen-probe millivolt values) or a furnace dew point that slowly creeps up higher and higher. In either case, the atmosphere control system usually calls for more enriching-gas additions to hold the setpoint value. As the leak worsens, still more enriching gas is needed, until the point when you may not be able to hold the desired carbon-potential setpoint. Parts may even show evidence of decarburization despite large amounts of enriching gas in use.
Pinhole leaks are difficult to find with conventional techniques because any carbon dioxide (CO2) produced from combustion will react with endothermic gas present. Larger leaks are generally easier to spot but have been known to open and close with changing temperature, making the task far more difficult. In all cases, a recording system will assist in more quickly detecting upward or downward trends.
One method for leak testing is as follows:
1. Lower the furnace temperature to around 1500°F (815°C).
2. Stop the flow of all enrichment gases (e.g., hydrocarbon gas, air, ammonia).
3. Record the millivolt output from the oxygen probe as the combustion system cycles on and off at setpoint. It is always a good idea to record the combustion-system output percentages as well.
Note: The millivolt signal from the oxygen probe should be essentially steady state (i.e. not oscillating wildly).
4. Raise the furnace temperature to 1700°F (925°C) and record the millivolt signal as the furnace heats to its new setpoint with the combustion system on high fire.
Note: Under normal circumstances, as you raise temperature, the millivolt signal on the oxygen probe will steadily increase. If you have a leaky tube, you either won’t see an increase or there will be only a minimal increase in millivolt values.
5. When you reach the new higher setpoint, and as the combustion system cycles on and off, record the carbon-potential millivolt values and the combustion-system output percentages.
6. Turn the temperature setpoint back to 1500°F (815°C) so that the combustion-system output is zero and record the probe millivolt values as the temperature drops.
Note: The millivolt signal should steadily decrease. If it does not or if there is only a minimal decrease, a leak will be present.
7. Repeat Steps 1-6 again to confirm results.
Note: If the furnace is equipped with a plunge-cool option (where air can be blown through the tubes), heat back to a setpoint of 1700°F (925°C) and activate the plunge-cool feature. If the millivolt values fall rapidly, there is a leaky tube.
8. If you suspect a leaky tube, you may be able to use a digital CO2 analyzer to help confirm the leak. Watching the analyzer during firing cycles, there should be a trend of increasing CO2 levels with increased burner firing.
Once a leak has been confirmed, the difficult part of isolating the bad tube begins. Systematically test each burner tube. This should be done while the furnace is running and a stable atmosphere is present. Use the following testing procedure at each burner, one at a time:
1. Shut off the natural gas and air supply by closing the burner gas and air valves.
2. CAREFULLY loosen the exhaust elbow and insert a blanking plate to seal off the exhaust and retighten. If a recuperator is present, loosen the exhaust piping at its connection to the recuperator and remove the recuperator. Place a blank plate over the tube discharge leg and clamp in place to seal the tube.
3. With the burner radiant tube shut off and sealed, watch the CO2 analyzer and look for a cycling CO2 value with burner firing cycles. If the cycling disappears, that is the leaking tube. If there is more than one leaky tube, however, the CO2 content of the furnace will still be erratic. When this is the case, with one bad tube sealed and one or more still leaking, the CO2 content will not rise as fast during a burner firing cycle. Analyzing the data will help locate the leaky tube.
Note: Testing tubes for leaks with pressurized air outside of the furnace is not recommended by a number of OEM manufacturers since the tubes are not designed as pressure vessels and will not normally withstand pressures greater than 5 psig (35 kPa).
4. A similar test with an oxygen meter is to shut off the gas to a burner and allow the tube to purge. When the oxygen level reaches approximately 18-20%, shut off the air. Watch the oxygen meter, and if the level drops, this tube is suspect.
Recuperators can also be leak tested following the manufacturer’s recommendations. This often involves pressure testing. If the pressure is lost immediately, the recuperator is leaking. A small leak is often permissible. However, that recuperator should be marked for future testing or replacement.
As far as this writer is concerned, there is no excuse not to have internal-pressure-monitoring capability on every heat-treating furnace running protective atmosphere. The amount of information that can be gained far outweighs the cost of adding these types of sensors to our furnaces. IH
References available online
1. Baukal, Jr., Charles E., Industrial Burners Handbook, CRC Press, 2004, p. 483.
2. Mr. Kevin Peterson and Mr. Patrick Weymer, Beavermatic, private correspondence.
3. Mr. Jack Titus, AFC-Holcroft, private correspondence
4. Mr. Ralph Poor, Surface Combustion, private correspondence
This month we begin a podcast conversation called the IH Monthly Prescription with The Heat Treat Doctor. Every month, Dan Herring sits down with IH’s editor, Reed Miller, to talk technical. If you have a topic you would like them to discuss, drop us an e-mail at firstname.lastname@example.org. Find the podcast on our website. IH Monthly Prescription is sponsored by Praxair.