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How to convert PPMw (by weight) to PPMv(Volume) for H2S & moisture(H2O) in gas mixture?
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Submitted Chris Haslego, Nov 21 2011 11:21 AM | Last updated Nov 21 2011 01:29 PM

Category:	Refining
Question:	Any advice on retrofitting a main crude heater with a natural draft fallback system?
Keywords:	natural,draft,fallback,retrofit
Answer:	BACKGROUND The current main crude heater includes only a problematic regenerative flue gas heat recovery system with a history of very frequent failure for the induced draft fan.RESPONSE A typical natural draft fallback system operates as follows:1. The furnace is provided with Drop Out Doors (DODs) at the furnace floor.2. The furnace has a fall back to Natural Draft interlock system.3. In case the furnace has to drop out of the Balanced Draft mode due to a Fan trip, the DODs open, the stack damper opens, and the running ID/FD fan trips. Fuel shutdown valves remain open. This brings the furnace on Natural Draft. The control room operator can manually adjust the stack damper opening to reflect the new mode of operation. In your case a retrofit should not be complicated. All you need to do is put in the DODs and examine the stack damper for further instrumentation. From there on, all you have to do is work in the logic in your interlock system.Your system will have to be completely rerated as your system was not originally designed for natural draft fall back operation. If the system is not properly designed you may end up with reduced capacity of your fired heater.
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