THIS MONTH'S PUZZLER
Every 3–8 months we burn through an O2 serpentine coil made of chromium-lean nickel alloy. Data show the maximum O2 temperature exceeds 1,000°C, the maximum allowable by the alloy mill, for almost an hour on average during startups and shutdowns. Engineers in the past have advised us to change material or switch to a helical coil to address the problem. Our new engineering manager thinks that better control might extend the life of the coil. Currently, we use a cascade control system: a temperature controller handles fuel while a slave flow control loop regulates combustion air to maintain a 14% excess air ratio. Could better tuning or changes in the control system do the trick? Are there any other ideas worth exploring? Do we know enough about the root cause?
KEY IN ON COMBUSTION AIR
You can get better control of your temperature by controlling combustion air to meet the process requirements. Then adjust the fuel as a slave and ensure the controls do not let it exceed your maximum temperature of 1,000°C.
Rakesh Gupta, engineer,
URS Corp., Princeton, N.J.
USE MORE AIR
Improved control will help. At 14% excess (XS) air, your theoretical maximum flame temperature is about 2,000°C, assuming natural gas. So at a low load, some sections of the coil could get very hot. You have to turn down the temperature by starting up with 130% or more XS air and trim down as you increase load. Do you continuously monitor coil temperature? If this is your primary controlled variable and you cannot adjust XS air, the two loops could be fighting. At low load, maybe your controller is asking for 800°C but the coil is overheating due to the low XS air at startup.
John Purvis, senior process engineer
ATK Launch Systems, Magna, Utah
FIND THE CAUSE
Knowing the mode of failure will help in establishing exactly what is destroying the coils. Start at the bone yard. Cut samples from elbows, straight pipe and wyes, if gas flow is split. Take samples of the support holding the coil in place.
Next, inspect the base and top plates where the coil supports are anchored for signs of damage where the supports might have stretched and thus passed stress on to the coil. A hot coil has reduced mechanical strength. An acid dye penetration test can detect damage to the nickel alloy. An electron microscope can determine if the metal wall is sound.
Also, check whether the coil alloy is what it is supposed to be. What about the supports? In a similar coil failure, we found that forged coil hangers slumped because they were made of a chromium-rich alloy that cracked. In that case, though, coil measurements showed ballooning at the elbows caused a predictable failure.
If there's time, install skin thermocouples. Skin temperatures will be higher than the 1,000°C during startup.
Control changes might make a difference. Start by retuning the control loops. You may need separate sets of control parameters for startup, normal operation and shutdown. Consider electronic valves instead of pneumatic ones. Look at the temperature and airflow measurements: changing the location or type of temperature sensor or going with another type of air sensor might help.
Another option may be a change in the burner itself. Consider reducing the heat flux by installing an economizer or better mixing of the air and fuel at the burner.
Dirk Willard, senior engineer
Ambitech Engineering, Downers Grove, Ill.
An unknown organic film has built up in the tubes in our TEMA Class AEW shell- and-tube heat exchanger. An inexperienced production engineer decided to use dilute sulfuric acid to clean them. Not only didn't this work but we now are concerned about the exchanger's buna-nitrile gaskets. Should we be concerned? And can we remove the fouling more quickly than via hydroblasting for several hours?
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