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Heat Exchangers Failure - Fccu

tube rupture heat exchanger failure corrosion

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#1 eggzit1989

eggzit1989

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Posted 03 November 2015 - 12:31 PM

Dear all,

 

Please help me with information / suggestions / ideas regarding the following issue:

 

Unit: FCCU UOP side by side with bubbling bed regenerator

During FCCU operation all heat exchangers were verified for leakeages. The following heat exchanger was found to have leakeages:

- Debutanizer Condenser 1 out of 2 parallel units (LPG/Cooling water) - LPG was contaminanting cooling water ( presence of gas at cooling tower);

It should be noticed that this is a new heat exchanger that failed was brought in TAR 2014 (parallel with existing one);

Information regarding heat exchangers (condensers):

1. Both condensers have same heat transfer surface / number of tubes/ tube passes;

2. Tube material for both (as per UOP design) : admiralty brass (CuZn)

3. Velocities in condensers on tube side (CW): a) existing one HEX 1 ( v1= 0.6 m/s) and new HEX 2 (v2=0.4 m/s)

4. Existing condensers ( bundle fabrication 2003) & new condensers ( bundle fabrication 2014)

5. TEMA type: AES

 

During shutdown when the debutanizer column was steamed-out for isolating the new condenser (HEX 2) it was observed that also debutanizer reboilers had leakeages.

Therefore during shutdown debutanizer reboilers and condensers were pressure-tested and it was found:

a) existing condenser hex 1 (19 tubes were plugged);

B) new condenser hex 2 (14 tubes were plugged);

c) existing debutanizer reboiler REB1 (4 tubes were plugged);

d) existing debutanizer reboiler REB2 (11 tubes were plugged);

 

Information regarding reboilers:

Debutanizer reboilers:    tube side (HCO) & shell side (Stabilized Naphtha)

Fabrication year: 2003 for both reboilers;

TEMA type: AJS;

Velocity on tube side: 1 m/s;

HCO Temperature in: 295-300 degC;

Tube material: CS

 

Before start-up in September, FCCU running in steady-state untill Wet Gas compressor tripped on electrical issue. During FCCU start-up the flowrate of HCO to reboiler REB 1 started to swing. The pressure of debutanizer 10.5-10.7 barg and HCO pressure 9 barg. Samples were taken and the HCO contamination with naphtha was determined (aproxx. 30 m3/hr of naphtha were entering in HCO tube side). The contaminated HCO was returning to MF (main fractionator) and the flowrate from the reflux vessel (naphtha flow to primary absorber increased with 30-35 m3/hr) as crosscheck with lab analysis.

 

General data of FCCU

1. Feed composition aproximately same except a slightly increase in sulfur in front of FCC from 4500 ppm to 5000 ppm

2. No external streams processed in FCC (except DHT and KHT off gas) ~ 3000 Nm3/hr

3. The FCC catalyst - same except (dosage of sulfur reduction additive with 2 months before this issue happening)

4. No temperature or pressure rating were exceeded as indicated by online monitoring !

5. Samples were taken for TAN measurement but the values were very low even 0;

6. Samples of LPG were taken and ammonia of 70 ppm was determined; H2S levels average 700-800 ppm;

 

 

Questions:

1. On what basis UOP is selecting for LPG Section admiralty brass for tube material?

2. Have you experience failure of debutanizer reboilers (tube rupture)?

3. As mentioned REB1 "sick heat exchanger" is TEMA type: AJS and naptha flow is splitted in two pipes prior entering the debutanizer reboiler. One of this pipes  is cold indicating no flow going towards reboiler.

4. What causes might affect debutanizer condensers and reboilers? As both equipment are affected with different materials is quite odd but the cause might be from the product side !

Cooling water analysis were investigated and no variations of contaminants / pH were observed.

 

Thank you in advance!

 

Andrei,

FCCU Process Engineer


Edited by eggzit1989, 03 November 2015 - 12:34 PM.


#2 manojkaila

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Posted 07 November 2015 - 08:04 AM

Dear,

 

1 Pl check the sulfur additive. I have some bad experience about the bottom reduction catalyst. Which created the high vibration of our PRT expander. 

 

2 Is there any catalyst carryover in HCO system. May be it will create erosion of tubes. Because HCO flow & temp both is high in reboiler.

 

3 Do you have any caustic treatment system before debut column. We also face some caustic carry over in debut bottom & which give hard time for operation of debut column.

 

4 Regarding MOC of exchanger I can not tell much let me check if I found some data I will share.

 

Our is SHAW /HOFCC

 

Regards, 



#3 eggzit1989

eggzit1989

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Posted 11 November 2015 - 03:01 PM

Dear,

 

1. We asked the catalyst vendor and he told us (as expected) that the catalyst is just transforming the heavy sulfur compunds in more light compunds (tiofen/ benzotiphene) to ease the PTU severity and allow a increase in RON/MON for hydrotrreated gasoline.

2. We don't have regular analysis of BS&W for HCO, however for slurry no increase in ash content or BS&W was seen therefore we suspect that HCO is most likely the same.

3. No caustic treatment.

4. At the moment we are suspecting high sulfur and high temperature corrosion (sulfidation) for reboilers but this is just a possible cause because we don't know for sure only when we will pull out the bundle and take samples. So for the moment we assume this root-cause until we open the heat exchanger,

It should be stated that the lines to / from the debutanizer reboilers are A335 P5 ( Carbon steel with 5 % Cr) in the meanwhile the reboiler tube material is only Carbon Steel.

5. Regarding the new condenser it is more complicated, we assume also a corrosion phenomena (MIC development).

 

It is clearly something strange cause this happen already two times at an interval of just 1 month.

As action we will change the bundle material for debutanizer reboilers with carbon steel with 5% Cr and for condenser with 316SS. (the old condenser is also of 316SS and no major problem occured).

 

Regards






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