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[purvamehta_5]

Hello All-

 

I am currently working on designing a flue gas system for Fluid Catalytic Cracking Unit on the Regenerator side. The typical setup is Regen effluent Flue Gas comes out of the regenerator at 1300F. It passes through Waste Heat Boiler (WHB) generating 600 psig (high pressure steam), by recovering heat from the effluent flue gas. The flue gas then pass through a series of pressure let down device (double disc slide valve, followed by an Orifice chamber (bunch of orifice plates lined in a duct)), before going through 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 device) is used to control regenerator pressure. This valve is also degined with a min. cut out and/or a mechanical stop, that prevents the slide valve from going completely close and the reason being this serves as a path of relief in case slide valve goes slam shut, causing source of overpressure on the regenerator. In this case the regenerator is designed for 38 psig.

 

Scenario: One of the scenarios to be considered credible under the current setup is if 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 create an overpressure. As mentioned above the min. opening in the slide valve, through orifice chamber, through wet gas scrubber will be 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).

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 calc needs to be performed) to determine the flow.

 

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

 

Sorry for making this so ellaborative.

 

Greatly appreciate any suggestion/respones.

 

Thanks

Edited by purvamehta_5, 25 June 2020 - 01:25 PM.