6 replies to this topic

[Karyuu17]

Dear Engineers,

 

I'm having a hard time explaining the irratic flow through our stabilizer kettle reboiler bundle. Originally, the steam feed rate to the bundle was at 4500 lb/h at 100% open. Recently, with the replacement of the old steam traps with 2 new steam traps resulted in a reduction of the flow rate of steam through the bundle.

 

 

The strange things that is happening is that steam flow rate will slowly decrease over time quiet rapidly. As shown by this image above, we can increase the steam flow by shutting down the flow rate to the stabilizer for a duration, and restarting the process, but eventually it will start dropping by itself from 2400 lb/hr to 2000 lb/hr to 1500 lb/hr.

 

Ha anyone seen anything like this before?

[Bobby Strain]

Sounds to me that the traps are not passing condensate as they should and the condensate is accumulating in the heat exchanger. Bypass the traps to check for this problem. And check the pressure upstream and downstream of the traps. There could also be a problem on the process side of the exchanger, like not having adequate level. This will cause the steam flow to drop, too. So, troubleshoot both sides. Let us know what you find.

 

Bobby

Edited by Bobby Strain, 03 May 2014 - 11:28 AM.

[TechExpert]

Dear Karyuu

 

As rightly said by Bobby Stain, there can be reasons as:

 

1. Low level in the shell side of the kettle reboier, leading to less condensation of steam, hence less steam flow.

2. Faulty trap: less chances, as you decrease the load on reboiler, steam flow rate normalizes. or maybe Steam traps not adequate to pass full flow rate of condensate (4500 lb/h ). Steam trap PDS can be checked in this case.

3. You can check the condensate header pressure during the time when the steam flow rate decreases.

 

The effect of low steam flow rate on the column: means decrease in column bottom temperature and column overhead pressure. Are you experiencing this? If no, flow transmitted problem!!

 

More can be said after looking at the PFD and P&ID of the system.

 

Thanks

 

PKS

Edited by PKS, 04 May 2014 - 11:52 PM.

[Pilesar]

Does the exchanger have U tubes with bends in the vertical plane? If so, then the step-wise decrease in steam can be explained by rising condensate level on the tube side. As the condensate level rises, flow is abruptly blocked from entire tube rows at once.

[Karyuu17]

As noted by the suggestions above, I looking into if the steam traps were bottle necking the flow of steam by opening a drain valve before the steam traps. When the drain was opened about 300 lb/hr of extra steam rate was gained. The gain is a marginal improvement compared to the original flowrate. What was also noted was that the vented steam seems very "dry" and has not condensates much.

Looking further into the problem the entire bundle has been pulled out of the reboiler and a decent amount of scaling was found on the tubings:



The scaling could have been the problem. Since we are stabilizing liquids from natural gas processing, iron sulfide is most likely the major culprit.

Does anyone have any suggestions on treating this problem?
 

[rabindra]

As discussed above in previous posts, the steam flow to the reboiler can be attributed to 

 

1. Low heat transfer in the exchanger due to low level of the hydrocarbon in shell side, external scaling of the tubes.

2. Low level can also be attributed to the low input liquid to the reboiler may be due to chokage of draw off pan.

2. Steam blow by i.e. not giving enough time to condense the steam

 

In one of my units, the kettle reboiler malfunctioned due to the damage in the partition wall of the kettle reboiler. In that unit the column bottom level was increased to submerge all the tubes in hydrocarbon.

 

In another unit, the bottom most draw off pan was damaged resulting in low liquid inlet to the reboiler. In that case a drain line in the inlet of the hydrocarbon to the reboiler was opened and the reboiler was floated with the column bottom level and we some how managed to operate the unit till plant shut down. 

 

However scaling may also be the reason for poor reboiling, but in that case the steam flow would be lower than desired but would not fluctuate as happening in your case.

 

On the steam outlet side, do you have a condensate pot or a steam trap?

Edited by rabindra, 06 May 2014 - 10:50 AM.

[rabindra]

I read your post again. I think there is a chance of vapor lock on the reboiler outlet line resulting in low hydrocarbon flow. When you stop reboiling and the restart it is taking more steam. May be after startup you will find the reboiler running smooth.