Lately we have been experienced frequent trip of Furnace in DHDS, we get positive draft and zero oxygen where this causes furnace to trip. Root cause?
Answers
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24/11/2011
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Satish Angadi, Haldor topsoe, satish.angadi@gmail.com
Assuming all other operating parameters same and you have operated the same furnace over years : one of the possibility, remote(??), is that there could be a leak in furnace...
1) Do a visual and pyrometer temperature survey
2) exercise caution as flame may come out from inspection windows...
3) check tube-skin temperature profile.
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02/08/2011
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Jayaraj Jayam, Chennai Petroleum Corporation Limited, njayaraj@gmail.com
As it is specified in the question the cause for the trip was positive draft and zero oxygen, the reasons could be more than one.
1 Check the furnace inlet temperature for any variation. Sudden load change in furnace will lead to consumption of more fuel and result in more volume of product of combustion that is flue gas. ID fan rpm may not be sufficient to take away the flue gas and to maintain a negative draft.
2 Check for stack damper opening.
3 Check for condensate carry over in atomizing steam. Condensate carry over can cause sudden raise in arch pressure in the case of fuel oil firing.
Increase the rpm of ID fan to maintain higher draft than the requirement, and observe for the recurrence of the problem. If it solves, ID rpm could be the problem. Measure the rpm of ID fan with a tacho meter and compare it with DCS indication.
If combustion air preheat is low compare with the design or past experience, go for APH washing. Too much of scales/coke/debris in the APH duct will also cause restriction of flue gas flow.
Check the opening of flue gas inlet duct - shut of blind- at upstream of stack damper and check all dampers in the flue gas path up to stack damper.
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01/08/2011
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Assumptions:
1. No other service is sharing the breeching or stack.
2. The first symptom is loss of draft.
After checking for malfunctioning instrumentation components, I would consider the following steps and possibilities:
1. While the furnace is operating well, conduct a field check of the pressure profile from the firebox to the ID fan. This will establish the observed ∆P for each pipe bundle in the convection section and the ∆P of the air preheater.
2. Calculate the available draft, using the stack dimensions, fan data sheet, observed convection section ∆P, quantity and composition of the flue gas, etc.
Those steps will tell if the normal operation is too close to the available draft to expect reliable operation.
The convection section could be fouling between tubes (pipes), and could be cleaned to help the situation. If the fuel is gas, is the heating value constant? Is a hydrogen stream going in and out of the gas mix? If the fuel is oil, is it always the same oil, with constant heating value?
What is the instrumentation at the air preheater? Can a temperature controlled bypass be changing the flow and ∆P at the preheater, which in turn would change the ID fan load?
What dampers are in the system? Are they at the preheater only, or also in the stack? How are they controlled? Have those instrumentation components been checked for malfunction? Could the damper shafts be corroded and unreliable?
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