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Discussion Forums - The Hendrix Group
HomeHomeDiscussionsDiscussionsOil Refinery Co...Oil Refinery Co...heat exchanges tubes failheat exchanges tubes fail
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6/7/2005 9:00 AM
 
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Hi again, I admit I will make some educated guesses and assumptions here. The use of 12 cr-1/2 Mo tubes in Preheater train suggests that the plant is designed to handle high S crude. Also your remark that column top dome Hastelloy lining had to be renewed indicates presence of high Cl- leading to acid dew point corrosion. Corrosion protection of high Cr-steels is given by the top oxide layer. These layers can be easiliy destroyed when high amount of Cl- is present in the system. Once the protective layer is broken, the corrosion rate will emulate that of CS and will be attacked by Sulfur / H2S or whatever corrodent is there. Lets say, corrosion in Crude is a bit complex issue and needs to be deliberated by proper experts in the field. Also please clarify some more points - like whether corrosion is restricted to this particular HE or not, design cl- limit ex desalter of unit, etc. and S %age.
 
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6/7/2005 9:00 AM
 
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Mark: I haven't seen in any of the posts on which side (tubeside or shellside) the exchanger tubes are failing on. Both the O.D. and I.D. environment can result in failure of pre-heat train exchanger tubes. David Hendrix The Hendrix Group Inc.
 
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6/7/2005 9:00 AM
 
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Hi Thanks chakara for your explanation and it was like what I am thinking amount The targets Cl at the over head is between 10-30 ppm and the S% never was measured but we know we have slupher in the light crude The desalted sometimes met the CL and salt targets and sometimes no. No the corrosion was not corrosion is restricted to this particular HE in all preheat trains also, in the overhead line before and overhead line fin fans (frequently had a corrosion failure) Borther Dehendrix Thanks to your involvement here and the corrosion was happened at the tube sides of the heat exchanger and the tube sheets. Really this problem is a corrosion challenge and we need to hear from many parts. Have a nice time
 
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6/8/2005 9:00 AM
 
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Ok, I did not ask the question (tube side/shell side), because most of these HEs have crude in tube side and Gasoil on shell side. Also the reference of F/H cover corrosion confirmed that. The use of 12 Cr tubes in Crude PT for low sulfur crude is a bit of puzzle to me. I have seen CS tubes for low sulfur crudes, 5 Cr-1/2 Mo for S<1.5% crudes etc. (Temp. -- 200-260 C). Anyway, corrosion in crude may occur due to various factors. The Cl- seems to be quite high. It appears that the brine is not properly separating from crude & you are getting high Cl in crude. The caustic injection can be done up to a limited amount, usually 10 PTB. There is another possibility that MgCl2/CaCl2 dissociating above 120 C and creating HCl in presence of moisture which is also corroding severely. This can be verified from simple check --whether severe corrosion exists below 120-130 C range or not. My suggestion would be few things - 1) Desalter --Check whether demulsifier agent or any other key factor like water addition has changed . (2) CPT fouling pattern - has it changed ? (3) Crude blending - has it changed ? BTW, what is the O/H remp in dist. column ? 'Hope the discussion helps someone. Chakra
 
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6/9/2005 9:00 AM
 
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Mark Mor From the explanation your exchanger tubes are maybe ANSI TP 410. In our unit we only use TP410 at the end of preheat train exchangers nº 7 and nº8. SS 410 surfaces form a passive layer during service life. If in areas of localized relatively high hydrogen sulphide and chloride concentrations, which happen in crevices and small pits, the passive layer is removed, you will have on those particular areas heavy corrosion by depassivation of those localized areas. For a good action of the Amine film the PH in the overhead drum should be kept as much as possible between 5.5 to 6. When chlorides level in the boot of the overhead drum is higher a proper water washing should be done. ChaKra suggestions are also important. Your unit have independent trains? Or your trains work in parallel? The reason for these questions is to know if your units have been periodic shut down for maintenance purposes and your exchangers have been kept long time with stagnant products. Regards
 
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