Nitrogen+Syngas
7 August 2025
Problem No. 46: CO₂ cooler failure
CO2 coolers in the inter stages of CO2 compressors typically cool down the CO2 stream by means of cooling water. It is preferable for the cooling water through the tubes to have a minimum velocity to avoid fouling and corrosion issues. This is not a problem after the first and second stage when CO2 pressures are still relatively low. However, after the third stage the CO2 pressure is about 10 bar, too high (meaning too expensive) to have the CO2 on the shell side. Vendors specify coolers with the cooling water on the shell side and the CO2 through the tubes. This however introduces several failure mechanisms: As cooling water velocities cannot be guaranteed to be above a minimum value in the complete shell, fouling can occur. Also, chlorides accumulate (even when chloride levels are relatively low) and cause chloride stress corrosion cracking in austenitic stainless-steel tubes. In this type of cooler choosing duplex as the material of construction for the tubes is the best choice for a reliable heat exchanger.
Irfan Rashid, working in Operations at FFBL in Pakistan starts the discussion with the following question: Does anyone have experience of CO2 cooler failure where failure occurs after the third stage of the CO2 compressor?
Mark Brouwer of UreaKnowHow.com in the Netherlands replies: Please share your experiences with us as I think it will trigger more discussions.
Irfan continues: We have a Dresser Rand design CO2 compressor which takes CO2 at 0.34 kg/cm2g and compresses it to 145 kg/cm2g in four steps. We have a CO2 cooler after each compression stage. Recently we have been experiencing leakage problems in our CO2 cooler after the third stage. It receives CO2 at 76 kg/cm2g. Due to this problem, we replaced this exchanger but the new exchanger collapsed in a hydrotest, after just two years of service. I would like to know what types of problems can cause this type of failure.
Mark comes back with a question: Which materials of construction were applied?
Irfan replies: The tubes are stainless steel and the shell is carbon steel. In the old cooler the baffles were made of carbon steel but in the new cooler we specified stainless steel for the baffles to avoid any galvanic corrosion effect.
Ramchandra Nesari, a freelance consultant in India shares his valuable experiences: I have had experience of CO2 cooler failures in three urea plants. We had frequent failure of the third stage cooler in one stream but in the other two the failures used to occur in second stage cooler. You have provided some details about your CO2 cooler in the third stage. You mentioned that the tubes are of SS 304 whereas the shell and baffles supporting the tubes were of CS initially. You then replaced the baffles with SS material to avoid galvanic corrosion effect. By doing this, you have removed one of the major causes of tube failures. We had similar observations in our CO2 cooler failures. After replacing the baffle material with SS 304, we overcame the failures. So I would like to know whether you are still facing tube failures after replacing the baffle material. Please also provide details about the tube arrangement – whether it is a straight tube or U-tube bundle. Often, vibrations occur in U-tube bundles if the cooling water flow is above a certain limit.
Normally in the third stage cooler a temperature control valve is provided at the discharge of cooling water flow to limit the CO2 temperature above a critical temperature. So, if the failure is due to vibration problems, then the design of the baffle spacing needs to be looked into.
Koorosh Lieravizadeh from the process engineering department of Shiraz Petrochemical Complex in Iran shares his valuable experiences: We have also experienced some failures, not only of the tubes, but also on the shell of the third stage cooler of the CO2 compressor. The cooler type is a fixed tube sheet.
Faraham Jafarvand of the Engineering department of NEWJCM Turbomachinery manufacturing co. in China provides his expert opinion: Everybody may have their own ideas about the reasons for failure, but I think the following information is necessary to investigate the root cause of failure:
- Type of heat exchanger (TEMA class).
- Location of failure in the tubes: it would be very useful if you could provide a photo of the failed tubes as the crack direction and other features may indicate the type of failure. If a photo is not available please describe the crack feature according to your own observations (direction of crack, location of the cracks in the tube with respect to tube sheet or baffles or U bend in case of a U-tube, uniformity of failure in different tubes, surface morphology of failed parts, buckling of tubes if any…)
- Type of temperature control you are using to control the CO2 temperature: The CO2 condition in third stage cooler outlet is near the critical point and there is a probability of formation of CO2 condensate if the temperature falls below a certain level. So please provide some info about the temperature control method you are using.
Irfan provides further information: We carried out a detailed investigation of this failure and the probable cause is strain induced intergranular corrosion. Our exchanger is a U-tube type, and baffle design/spacing is not a problem. Harun Idrees, Technical Services of Fatima Energy Limited in Pakistan shares his experiences: At our plant, we have a brand new third stage intercooler with duplex tubes and after in 1.25 years of operation we have not faced any problems.
