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I am currently working on designing a flue gas system for a Fluid Catalytic Cracking unit on the regenerator side. The typical set-up is regen effluent flue gas comes out of the regenerator at 1300F. It passes through the Waste Heat Boiler, generating 600 psig (high pressure steam) by recovering heat from the effluent flue gas. The flue gas then passes through a series of pressure let down devices (double disc slide valve, followed by an orifice chamber (bunch of orifice plates lined in a duct)), before going through the wet gas scrubber (caustic wash for any catalyst carryover) into the stack and vent to the atmosphere.

Normally the slide valve (one of the pressure let down devices) is used to control regenerator pressure. This valve is also designed with a minimum cut out and/or a mechanical stop, that prevents the slide valve from going completely closed, the reason being this serves as a path of relief in case the slide valve goes slam shut, causing a source of overpressure on the regenerator. In this case the regenerator is designed for 38 psig.



One of the scenarios to be considered, credible under the current setup, is if the tube in the WHB ruptures, high pressure BFW on the shell side of the WHB can pass through the tube (process gas side), which is open to the regenerator and creates an overpressure. As mentioned above the minimum opening in the slide valve, through the orifice chamber, through the wet gas scrubber, will be a relieving path for this fluid.



Question: I am trying to calculate the amount of relief that will be generated through the tube. Since BFW on the shell side is at saturated conditions, there will be flashing (two-phase flow) passing through the tube. As the relieving flow exits the tube, it should be under sonic conditions (choked flow).

I need to determine how much flashing will occur across the tube as it exits the tube, and secondly the amount of flow that will exit the end of the tube (probably a two-phase restriction orifice calculation needs to be performed) to determine the flow.



I would greatly appreciate if someone can guide me as to how to perform this calculation as I have not done much two-phase through tube (at sonic conditions) and two-phase RO calculation.



 
Answers
05/07/2020 A: Prince George, Bharat Petroleum Corporation Limited, prince.george@gmail.com
As the intention of a waste heat boiler is to utilize the available heat in the flue gas into steam why should the same be brought into the critical path of flue gas, before the double disc valve?
RR system pressure balance is where focus should be given rather than recovering the waste heat. If you rearrange the flue circuit where the pressure let down devices viz double disc valve and orifice chamber is placed upstream of the waste heat recovery system the issue of tube rupture of WHB leading to pressure imbalance in RR system can be avoided.
You can also provide an interlock for tripping the plant at low WHB steam drum level.
Again you can have the process gas (flue gas) in the shell side and BFW in the tube side of the WHB. Steam generation will be due to thermo syphoning effect and for this we should have BFW in the tube side with a steam drum at the top.