Demin water closed loop cooling and heating medium systems - need for corrosion inhibitor?
Demin water closed loop cooling and heating medium systems - need for corrosion inhibitor?
(OP)
Hi all,
I am currently working on a FEED phase study for a large, offshore gas production plant. The current design includes three closed loop cooling and heating medium systems, briefly described below:
- Main cooling medium system, normal operating temperatures 34 to 55 °C
- Turbine inlet air cooling medium system, normal operating temperatures 4 to 16 °C
- Heating medium system, normal operating temperatures 140 to 177 °C (pressurized water, not steam)
An amine based acid gas removal system will also be included, which requires continuous high quality demineralized water make-up with the following specifications:
- Max oxygen content: 10 ppmw (ppm by weight)
- Max chloride content: 2 ppmw
- Max calcium (Ca2+) content: 50 ppmw
The current design is based on using this demineralized water as make-up and first fill of the closed loop heating medium and cooling medium systems described above, even though this is not strictly required. The reason for this is that generating two different qualities of water adds more complexity and increases cost.
All closed loop systems are based on carbon steel piping and equipment, and are fitted with nitrogen blanketed expansion tanks. Our current design includes for the batch dosing of corrosion inhibitor, scale inhibitor, pH controller and oxygen scavenger (exact number of chemicals still to be decided) into those closed loop systems, in order to minimize corrosion. The turbine inlet air cooling medium system includes glycol (MEG) for antifreeze protection, while the other systems use only water (except for the corrosion prevention chemicals).
However, one of the engineers from our client is arguing that no chemicals should be needed unless we can calculate the yearly corrosion rate and prove that this is needed. The design plant life time is 30 years. I have been trying to explain that to my knowledge, there are no models that can accurately predict the corrosion rate and that all designs I have seen for other projects add corrosion inhibitor and other chemicals to their closed loop cooling and heating medium systems, but with litte success so far.
The oxygen content of the make-up demineralized water is low, and so is the chloride concentration. But in the closed loop systems, oxygen may enter during maintenance of equipment when flages are opened. Chlorides could potentially enter into the cooling medium system in case of a pin-hole leak in one of the seawater plate heat exchangers. Therefore I am still arguing that it is too risky to design without the provision for corrosion prevention chemicals and that if serious corrosion occurred it could have huge economical consequences. So my question is:
- Is there any reliable and accepted method for calculating the corrosion rate in these systems?
- Would it be possible to achieve a 30 year life time without adding corrosion prevention chemicals?
I am currently working on a FEED phase study for a large, offshore gas production plant. The current design includes three closed loop cooling and heating medium systems, briefly described below:
- Main cooling medium system, normal operating temperatures 34 to 55 °C
- Turbine inlet air cooling medium system, normal operating temperatures 4 to 16 °C
- Heating medium system, normal operating temperatures 140 to 177 °C (pressurized water, not steam)
An amine based acid gas removal system will also be included, which requires continuous high quality demineralized water make-up with the following specifications:
- Max oxygen content: 10 ppmw (ppm by weight)
- Max chloride content: 2 ppmw
- Max calcium (Ca2+) content: 50 ppmw
The current design is based on using this demineralized water as make-up and first fill of the closed loop heating medium and cooling medium systems described above, even though this is not strictly required. The reason for this is that generating two different qualities of water adds more complexity and increases cost.
All closed loop systems are based on carbon steel piping and equipment, and are fitted with nitrogen blanketed expansion tanks. Our current design includes for the batch dosing of corrosion inhibitor, scale inhibitor, pH controller and oxygen scavenger (exact number of chemicals still to be decided) into those closed loop systems, in order to minimize corrosion. The turbine inlet air cooling medium system includes glycol (MEG) for antifreeze protection, while the other systems use only water (except for the corrosion prevention chemicals).
However, one of the engineers from our client is arguing that no chemicals should be needed unless we can calculate the yearly corrosion rate and prove that this is needed. The design plant life time is 30 years. I have been trying to explain that to my knowledge, there are no models that can accurately predict the corrosion rate and that all designs I have seen for other projects add corrosion inhibitor and other chemicals to their closed loop cooling and heating medium systems, but with litte success so far.
The oxygen content of the make-up demineralized water is low, and so is the chloride concentration. But in the closed loop systems, oxygen may enter during maintenance of equipment when flages are opened. Chlorides could potentially enter into the cooling medium system in case of a pin-hole leak in one of the seawater plate heat exchangers. Therefore I am still arguing that it is too risky to design without the provision for corrosion prevention chemicals and that if serious corrosion occurred it could have huge economical consequences. So my question is:
- Is there any reliable and accepted method for calculating the corrosion rate in these systems?
- Would it be possible to achieve a 30 year life time without adding corrosion prevention chemicals?





RE: Demin water closed loop cooling and heating medium systems - need for corrosion inhibitor?
I'd stay clear of printed circuit heat exchangers for any process heat exchange service despite the sales talk and the weight advantages.
Getting to 177degC for heating apps with hot water will require pumping pressures that will maintain the water in liquid phase at up to TAHH (185degC or so ?) on the hot water medium WHRUs exit.
RE: Demin water closed loop cooling and heating medium systems - need for corrosion inhibitor?
2. How lucky does the end user feel? If they are set on not using chemicals from the word go, perhaps offer them a monitoring strategy with the equipment capability to add such chemicals if the monitoring indicates a problem in subsequent operation?
Steve Jones
Corrosion Management Consultant
http://www.linkedin.com/pub/8/83b/b04
All answers are personal opinions only and are in no way connected with any employer.
RE: Demin water closed loop cooling and heating medium systems - need for corrosion inhibitor?
You would have to keep the oxygen very low (1ppm or less) and keep the water conductivity low.
If the water is conductive you will get corrosion of some form.
I have seen systems that are continuously polished through ion exchange, but that is more costly than chemical treatment.
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P.E. Metallurgy, Plymouth Tube
RE: Demin water closed loop cooling and heating medium systems - need for corrosion inhibitor?
Including for corrosion prevention chemicals is what I consider good engineering practice, and a very cheap "insurance" with low CAPEX impact compared to the overall cost of the plant. Since the required dosing concentrations of these chemicals are low, OPEX would be small as well. The main cooling medium system has a design duty of around 400 MW and a total flow rate of roughly 18 000 m3/h, that gives you some idea of the dimensions.
I will have to continue persuading our client to include for chemical dosing from day one, and to accept the fact that some design decisions are based on experience and good engineering practice, and not necessarily detailed calculations.
We will maintain the expansion tank at a pressure that is sufficient to prevent boiling inside the WHRU tubes, based on the maximum film temperature calculated by the WHRU supplier, plus a margin. So if the calculated film temperature is 200 °C, we will maintain the expansion tank at a pressure that corresponds to the water saturation pressure at for example 210 °C. The pressure in the expansion tank is maintained via nitrogen.
In addition to adding chemicals, we will also recommend corrosion monitoring coupons to be installed at strategic locations, so that the actual degree of corrosion within the system can be assessed. We need to convince our client to include a chemical injection system, so that these chemicals can be added when necessary. Since it is a closed loop system it would not be continuous injection, but rather batch dosing based on laboratory measured concentrations of chemicals or testing kits provided by the chemical suppliers.
RE: Demin water closed loop cooling and heating medium systems - need for corrosion inhibitor?
Stainless steel is out of the question due to cost, as our client is very specific when it comes to one requirement: lowest possible CAPEX. Of course we could exclude chemical dosing to reduce CAPEX even further, but we need to engineer a system that will actually work for the intended life time of 30 years...
Your points regarding oxygen concentration and conductivity are very interesting though, thank you!
RE: Demin water closed loop cooling and heating medium systems - need for corrosion inhibitor?
From a process safety perspective, would you need to add MEG to the cooling water closed loop ? - if you had a pinhole leak on a process HX in this loop with low temps created on the CW side, you could freeze the path to the CW side PSV??
RE: Demin water closed loop cooling and heating medium systems - need for corrosion inhibitor?
The pressure downstream the pumps (and hence in the WHRUs) is determined by the expansion tank operating pressure, plus the differential head of the pumps. We could take this pump differential pressure into account when setting the expansion tank operating pressure, but the advantage of keeping the pressure sufficient already in the expansion tank is that we prevent boiling also if the pump differential pressure is lower than normal, or if the pumps trip. Yes, this requires a somewhat higher design pressure of the system, but the difference is not dramatic and we are in any case in the 300# piping region.
Regarding the pinhole leak and potential freezing issues, it is unlikely that this would be a problem since the PSV nozzles are typically located some distance away from the tube sheets, which are the most likely locations of pin hole leaks.
RE: Demin water closed loop cooling and heating medium systems - need for corrosion inhibitor?
If the tube rupture scenario relief load is accomodated at this drum, then freezing of this relief path would occur? Or is this drum design pressure high enough to keep the lowest predicted tube rupture relief stream temp at above 0degC ?
RE: Demin water closed loop cooling and heating medium systems - need for corrosion inhibitor?
RE: Demin water closed loop cooling and heating medium systems - need for corrosion inhibitor?
My next question is: what type of chemicals would be required? Would we for example need both corrosion inhibitor and oxygen scavenger, or is it enough with only one of them?
The demineralized make-up water contains only 10 ppmw oxygen, and the expansion vessel of each closed loop system is blanketed with 99.9 % nitrogen. So the amount of oxygen scavenger needed to remove 10 ppmw to say 1 ppmw should be relatively small.
RE: Demin water closed loop cooling and heating medium systems - need for corrosion inhibitor?
Steve Jones
Corrosion Management Consultant
http://www.linkedin.com/pub/8/83b/b04
All answers are personal opinions only and are in no way connected with any employer.
RE: Demin water closed loop cooling and heating medium systems - need for corrosion inhibitor?
RE: Demin water closed loop cooling and heating medium systems - need for corrosion inhibitor?
RE: Demin water closed loop cooling and heating medium systems - need for corrosion inhibitor?