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05/03/2021
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Pannalal Chakraborty, Indian Oil Corpn Ltd, chakrabortyp2@indianoil.in
We have gone through such issues. In our case, the top pressure was reduced from 1.3 kg/cm2 to 0.7 kg/cm2. The logic was that the furnace COT could be reduced by a few degrees and that would create a lot of savings. We had to abandon this scheme due to repetitive failures of the overhead air fin coolers.
With such a reduction in pressure, the volume of gases increases. The air fin coolers are designed for velocity limits. In the dry phase (within the inlet piping), the allowed velocity of gases can be very high - say a designer gives a velocity limit of 56 m/sec. However, within the air fin coolers, the velocity limit is much lower. The velocity limit is based on the percentage of liquid since liquid can shear off the protective FeS scale of the tubes at lower velocity. At full liquid condition, the velocity will be 2-3 m/sec. At the entry point of the air fin coolers, maximum velocity limit should be less than 18 m/sec. And yes, a deficiency in the overhead corrosion protection system can further worsen the situation.
I know a refinery where they faced an average leak free period of 22 days!
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27/02/2020
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Chris Claesen, Ecolab, cclaesen@ecolab.com
Is this a crude distillation unit atmospheric tower? The pressure change has probably changed the operational conditions and flows inside the tower and the OVHD and this can certainly lead to increased corrosion in certain locations. The water and salt dewpoint locations may have shifted. The tower and OVHD corrosion control program will have to be reviewed.
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23/02/2020
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Ganesh Maturu, Self, maturu.ganesh@gmail.com
Since you reduce the column pressure from 1.5 kg/cm2g to 0.5 kg/cm2g, vapor velocity increased almost 3 times the initial velocity (p1*v1=p2*v2 => v2 = 1.5*v1/0.5 = 3*v1). At high velocities, air-fin coolers located at crude column overhead might have exceeded erosion velocity and tubes leaked. When you isolated one air fin exchanger, velocity further increased to other fins due to isolation of leaked air fin exchanger. This causes worst situation for other exchangers and increases velocity further. Check the actual velocity at inlet of air-fin cooler and as per guidelines this may not cross 25-35 fps.
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23/02/2020
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Jon Isley, Fluor, jon.isley@fluor.com
This sounds like an exam question from a professor that likes riddles.
My guess is that the distillation column overhead temperature reduced to the point of being below the dew point of water, and acidic corrosion is occurring.
This is only a guess of course because I like riddles.
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22/02/2020
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keith bowers, B and B Consulting, kebowers47@gmail.com
Have any of the failed finfan tubes been removed and leak cause determined? It sounds like aggressive acids started coming overhead and corroded the air cooler tubes. A wet CO2/H2S/ light hydrocarbon environment sounds likely.
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22/02/2020
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Apoorv Gupta, IOCL Haldia, apoorv.work19@gmail.com
Reduction in column operating pressure from 1.5 kg/cm2g to 0.50 Kg/cm2g is good for distillation economics, However same has to be done after engineering adequacy.
Reduction in operating pressure will hamper the following:
1 Increased gas/liquid flow rates inside column trays, Same will lead to high dp across column as well as may cause flooding in column. please check before & after, dp. If flooding is there, overlaps between the products will widen, same will result in poor product quality.
2.0 Increased overhead condenser load will cause two things:
-One is overhead condenser can be limiting w.r.t heat sink. Same will result in high overhead receiver temperature , may cause flaring/overpressurization.
- Second is your apparent problem , increased vapor load in the overhead circuit will increase velocity inside AFC tubes leading to high rate of corrosion & errosion leading to tube failure. Please compare velocity inside AFC w.r.t permissible limits- 3.0-6.0 m/sec.
3.0 If column feed is coming as vapor/dual phase fuild, Inlet line adequacy also needs to be ascertained. As reduction in pressure will also hamper velocity inside that line. Vibration/erosion issue may happen.
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21/02/2020
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Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
Reducing operating pressure will also reduce operating temperature and increase velocity. It would appear you have either:
• Promoted an erosion/corrosion mechanism by increasing velocity, or
• Reached a low enough temperature to condense water or desublime a corrosive salt.
The exact mechanisms would depend on which service this column is operating in. Can you explain what function this column is performing and in what process unit?
Having said that, the speed of failure means the most likely mechanism is wet chloride or wet fluoride corrosion. Lowering temperature and pressure have opposite effects on condensation of water, but the different slopes of the saturation curves mean that reducing pressure could lead to violation of the dew point of the overhead vapour.
Reducing pressure to reduce energy cost and improve fractionation is a valid objective. Get in touch if you’d like some help finding the sweet spot where you can enjoy some of these benefits without accelerated corrosion.
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21/02/2020
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Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Since you provide no information about the system other than pressure all I can tell you is to have a metallurgical failure analysis done on the coolers around where they leaked then go from there. Decreasing pressure should not directly cause failures. Most likely you have corrosion going on. Could be something that was going on for a while and the timing of the failure was just a coincidence. Could be dropping pressure caused a drastic change in the corrosive environment.
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