10 replies to this topic

[Eric Nguyen]

Dear all!

 

My RFCC shutdown during 2 day and still keep PRU in hot circulation. At C3/C4 splitter has 2 vertical reboiler using LP steam heating . It is type of thermosiphon reboiler and we had a problem at there. During keeping C3/C4 spliter with hot total reflux to have good condition waiting for receiving LPG feed but condensate pot draining from reboiler alway full eventhough steam inlet flowrate is a haft compared to normal operation. Why condensate can't flow to condensate header in this case?. when we received LPG feed, condensate pot level was slowly decreasing together with steam inlet flowrate increasing! Could anyone in forum explain what was happened in my thermosiphon reboiler and give ideas how to solve condensate pot level in that case?

 

Thanks so much

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[paulhorth]

Hoang,

When you run the reboiler at reduced duty (turndown), the steam pressure in the reboiler will be lower than normal. Lower pressure makes it more difficult to discharge the condensate through the LCV and back up into the condensate header on the piperack. The condensate flowrate will be reduced, but there is still the  static head and the minimum DP across the control valve to overcome.

 

The reason that the steam pressure is lower is because the  duty of the reboiler can only fall if the temperature difference falls - and for steam, that requires a lower pressure.

 

Paul

[Eric Nguyen]

Actually, my condensate pot level was full. We tried to reduce level by fully open control valve and bypass but unsucessful.I also thought like mr Paul but could you give me the way to drop condensate level. Could i use bypass LP steam inlet to condensate pot to pressure up pot to push condensate to header or continue to increase inlet steam flowrate to pressure up reboiler. If we do that splitter bottom temperature will raise up!

[Bobby Strain]

Just leave it alone. Nothing is wrong. The pot is full because the exchanger is holding condensate to cover tubes.The pressure is just enough with the flooded exchanger to push the condensate out. Q =UAdT.

 

Bobby

[Eric Nguyen]

Actually, condensate flow to header is slower and colder so caused seriously hammering at LCV downstream to header. I couldn't leave it alone as you said...

[Killer Queen]

Dear Hoang,

 

Did you mean you have to shutdown the RFCCU for 2 days because of this iussue?

[Eric Nguyen]

Dear sir

 

We shutdowned RFCC by another reason! The MAB pressure control valve was malfuntion and hunting so it caused MAB surging.The C3/C4 splitter reboiler was only minor but i don't have answer until now...!

 

Best regards

[Bobby Strain]

Sparge some steam into the pot condensate as a temporary measure.

 

Bobby

[Babu Prasad]

Dear Hoang,

 

When there is no LPG feed to splitter, column bottom will be enriched with C4 and there will be less steam consumption. Hence you cannot expect normal thermo siphon effect in the reboiler..

 

Due to less/unsteady steam flow, pressure in the condensate drum will be less or equal when compared to condensate header. Same time condensate LCV will be remain open in auto mode to drop level but due to less pressure in pot condensate from header will back into the pot and level will be full which will create hammering in the pot. 

 

 

Try following

1. Once you decided for shutdown, keep reflux drum high level and tight shut C3 product outlet. Keep maximum reflux until unit ready to take fresh feed. Do not loose C3 anyway even thro flare.
2. Keep your steam FCV on manual keep constant opening which will allow constant pressure. No need to maintain your tray temp like normal.
3. Open 2" globe valve which is connecting steam header & condensate pot which will maintain condensate pot pressure and push the condensate to header. ( actually this valve will be used during higher feed rate to push high condensate flow)
4. If steam consumption is very less, better drain condensate outside rather than putting into header. .

[Eric Nguyen]

Dear Babu

 

''Due to less/unsteady steam flow, pressure in the condensate drum will be less or equal when compared to condensate header. Same time condensate LCV will be remain open in auto mode to drop level but due to less pressure in pot condensate from header will back into the pot and level will be full which will create hammering in the pot''

I agree with your answer about less/unteady steam flow cause less/unteady pressure in the condensate pot and pot was full because it was not enough pressure to push condensate to header But in my case, steam flow is steady at 9000Kg/h and normal about 15000Kg/h I don't know steam amount was less or enough.If steam flow was less caused by less thermosiphone phenomenon in reboiler but i think condensate pot pressure should be enough to push condensate to header?

 

''Open 2" globe valve which is connecting steam header & condensate pot which will maintain condensate pot pressure and push the condensate to header. ( actually this valve will be used during higher feed rate to push high condensate flow)''

 

I agree with your answer but i tried to open 20% this balacing valve as you said and kept 2 hours but didn't see any change. If we opened this valve fully is there any backpressure in condensate pot and cause reduced steam inlet flowrate and any hammering. Have you used this valve fully in your case?

 

Best regards

Edited by Hoang Tri, 20 September 2013 - 10:23 PM.

[Babu Prasad]

Dear Babu

 

''Due to less/unsteady steam flow, pressure in the condensate drum will be less or equal when compared to condensate header. Same time condensate LCV will be remain open in auto mode to drop level but due to less pressure in pot condensate from header will back into the pot and level will be full which will create hammering in the pot''

I agree with your answer about less/unteady steam flow cause less/unteady pressure in the condensate pot and pot was full because it was not enough pressure to push condensate to header But in my case, steam flow is steady at 9000Kg/h and normal about 15000Kg/h I don't know steam amount was less or enough.If steam flow was less caused by less thermosiphone phenomenon in reboiler but i think condensate pot pressure should be enough to push condensate to header?

 

 

Steam flow may be half but it is shared by two reboiler , so one reboiler may be  getting more load other with low load. Possible cut one reboiler (E-2103A) from service and maintain only one until normal feed restored.

 

 

''Open 2" globe valve which is connecting steam header & condensate pot which will maintain condensate pot pressure and push the condensate to header. ( actually this valve will be used during higher feed rate to push high condensate flow)''

 

I agree with your answer but i tried to open 20% this balacing valve as you said and kept 2 hours but didn't see any change. If we opened this valve fully is there any backpressure in condensate pot and cause reduced steam inlet flowrate and any hammering. Have you used this valve fully in your case?

 

Actually this valve useful to maintain pressure in the pot due to full condensing of steam  during full load of plant . It suppose not to make back pressure to steam.condensate will flow to header as long as level maintained in the pot.  your problem is less load to reboiler,  if good composition of HC maintained in the column bottom  with help of good reflux  you can maintained steady steam flow . Try isolating one reboiler , it may improve .

 

 

Edited by Babu Prasad, 21 September 2013 - 04:06 AM.