Hot Water Tanks

Most hot water tanks have a bonded glass lining for corrosion protection as well as an anode. Stainless steel would be overkill.

BUt is the lining of any sort a must?

For carbon steel you need some form of corrosion protection. Depending on your water source there will be an amount of air dissolved in the water, atmospheric water sources tend to be at saturation limits. From chemistry, Henry's Law states that the solubility of a gas in a fluid is proportional to the pressure and inversely proportional to the temperature. What this means is that at higher temperatures the air which is 21 % oxygen will want to come out of solution. It is the oxygen in the air which will react with the iron in the presence of water to form rust, thus the requirement for corrosion protection. Note that there are other factors which will aggravate the situation. The first is heat, oxidation is an electrochemical process and chemical reaction rates are accelerated by heat. Second are the presence of other corrosive species in the water including chlorine ions and carbon dioxide which are aggressive ions known for pitting and crevice corrosion. Another consideration is that there are different types of corrosion that will take place. We are all familiar with general (surface) corrosion, however, in piping and storage tanks we need to be aware of other, accelerated forms of corrosion. Two forms of aggressive, localized corrosion are crevice corrosion and pitting corrosion. Localized corrosion can result in premature failure of a storage tank or piping due to pinholes or cracks. Also there is the danger of stress corrosion cracking and fatigue failures related to the thermal and pressure cycling of storage tank. To prevent corrosion, there are a number of techniques available which include water treatment, coatings and linings, and cathodic protection. Modern day water tanks generally include the latter two methods in their fight against corrosion. Even after that most hot water heaters, especially the gas fired type, will typically have an expected life of 5 to 15 years depending on their construction, the amount of use they see, and the composition of the water. Linings detach, spall and crack. Anodes become depleted and are not replaced. Flue gasses may condense resulting in acidic water eating the through the tank from the gas side, etc. However, comparatively speaking, hot water heater tanks are relatively inexpensive. If you want a longer lasting arrangement, then you must be prepared to spend money. You could use a boiler / hot water storage tank arrangement where the storage tank is not subjected to thermal cycling and localized heating deformation (especially prevalent in underfired arrangements). Yes you could use stainless steel tanks and they are available, at a cost. And, even at that, stainless steel will corrode in the presence of carbon dioxide and chlorine resulting in pitting and crevice corrosion. So, there are no guarantees with stainless steel, and it is a common myth that stainless steel will not corrode, but it will and can be costly.

Is it a must...? In a commercial or residential system I would say some sort of corrosion proof lining is a requirement if it is for a pressurized hot water system. If you are talking about something other than a normal pressurized hot water system, it would kind of depend on the contents the tank would be subject to, the pressure, tank wall thickness (pressure code rated) , local codes, and any number of other factors. A little more information would be usefull in answering your question.

Pls. note that tank is 100% pressurized to store hot water with 85 deg.C temperature which can go upto 90 deg.C and the size of the tank is for storing 30,000 Ltrs.

I'm not aware of any pressure vessel codes that would require a lining, let alone stainless (I say pressure vessel codes because you said "pressurized") - if this were an atmospheric storage tank, the applicable codes or regulations (and calculations) would be different.

That said, corrosion allowance should factor into your wall thickness calculations.

How much thickness you expect to lose will be significant - you got a flavor from the other posted replies, corrosion rate on water can vary - not all water is created (or controlled) equally.

I just love how these simple threads evolve from a run of the mill, generic question with very little detail, to a really great response from someone who shares their own experience in detail. My responses were based on my own experience in smaller scale potable hot water systems, where the use of tannin would not be permitted. Recirculated hot water systems in multi-residential buildings from a central plant have long been problematic in Vancouver due to pinhole leaks in copper piping. I have considered the use of dearators and also sodium silicate as an inhibitor. However, even though I know it would work, there is only one installation that I know of that uses sodium silicate for corrosion protection in a domestic hot water installation. That is a hospital, where their copper piping has been running almost leak free for over 40 years. Thanks for sharing, you have my vote. Cheers

Hi D-T,

Thank you for your kind response. I am interested in why tannin based water treatment is banned, we had the same issue some years ago with hydrazine but interestingly the 'nukes industry' got a dispensation to continue with it in their cooling circuits. The issue of pin holes in copper pipe work is a problem here also where it is most prevalent in systems using solder ring fittings and less of one in the 'Kuterlite' compression style systems.

Residual acidic flux deposits have been blamed along with swarf particles but I tend to subscribe to the electrolytic cell theory which of course needs an electrolyte to do its mischief. The pH of the circulating water is a crucial factor in whether this happens.

What type of distribution network do you have? The DH story in the UK is far from a satisfactory one and I suspect that a large part of the problem results from corrosion developing mostly as a result of excessive make-up rates introducing substantial levels of fresh oxygen into the system. Operators usually seem very reluctant to discuss this aspect of their activities.

Good chat,

Kind regards,

Massey.

Hi Massey,

This topic is quite dear to my heart and I spent years in the industry working on the problem and finding solutions related to our own problems. The history of the problems goes back quite far, but until the advent of the internet, it seems that nobody really new how prevalent the issues of pinhole leaks in piping were all over the world. My own research started back in the late 90's when I became a specialist in the repiping industry. Formal education as a metallurgical eng with 17 years experience as a mechanical eng in the HVAC & plumbing industry gave me (I believe) a good perspective of the problem and issues.

First I would like to start out saying that IMHO you are correct about this being an electrolytic reaction. I have walked through dozens of buildings all exhibiting the same failure symptoms (in Vancouver) where I witnessed pinhole leaks occurring with a regular frequency along copper pipes. In many instances it seems that the pinholes develop as "sacrificial" anodes protecting the rest of the pipe, where you would see a pinhole every 18 - 24 inches.

In the corrosion circuit, three things must be present for corrosion to occur: first a metal or carrier of electrons (the pipe), second a carrier or medium being the fluid or water, and thirdly aggressive ions such as oxygen, chlorine or carbon dioxide. Once you know these requirements then you can take appropriate measures to mediate the problem to break the circuit. One approach is to change the material from an electrically conducting metal to an insulating plastic. Second method is water treatment which can act in two ways (a) scavange out the aggressive ions and/or (b) deposit a thin film which will act as protective layer and block the electric circuit. The third approach is to install an insulating barrier between the electrolyte (water) and the carrier (pipe). All three approaches have their advantages and drawbacks or limitations.

This topic is quite lengthy so for now I will leave any further discussion of the corrosion aspects and remedies alone and focus on the other questions that you have posed. I cannot specifically say why the use of tannins are prohibited, but surmise it is related to toxicity. In Canada anything toxic which can find it's way into streams and kill fish and animals or make it's way into the water supply is a no-go.

As you suspect the introduction of cold make-up water is a likely contributor to your problems. Domestic water suppliers in North America (I suspect similarly in the UK) are very aggressive in their water treatments using both chlorination and carbon dioxide in their treatments to kill off biological contaminants such as giardia and chryptosporidium which are especially nasty, especially to populations at risk. These water treatments aggravate the corrosion problem in conjunction with the already dissolved oxygen in the cold water. Note the emphasis on "cold" water, because as I pointed out in an earlier post, heating the water will drive out the dissolved gasses and exacerbate corrosion. We do not have such issues in cold water piping due to the lower kinetics of the electrolytic reaction at lower temperatures, and lower exposure to oxygen. Thus in your corrosion protection regime it is essential to treat your water prior to introduction into the system. A suggested remedy would to pre-heat (boiling would be best) the make-up water, then dearate as the heated water will reject more of the dissolved gasses at the elevated temperature and finally condition with an inhibitor prior to introduction into the system.

Hope this helps.

Hi D-T,

Thanks for the added 'gen', you have an obvious depth of knowledge and understanding of the underlying mechanisms present in the corrosion processes. As you say there is 'no one size to fit all' scenario available as a quick fix.

My adopted make-up water pretreatment kit is based upon an ancient AlfaLaval hydro-kinetic deaerater which receives preheated raw water from a small PHE. The kit is of limited capacity and therefore feeds the main F & E tank via a small ballcock controlled supply. Once in the main tank I have arranged a nitrogen 'blanket' to avoid re-oxygenation. This is done by partly supplying a very low pressure supply of nitrogen which is maintained at a positive pressure (to atmosphere) of 2''WG in the tank; but also from dissolved nitrogen which emerges from the pressurised HTHW when spill occurs. The reason for this being that we use a pair of buffer vessels partially filled with water but pressurised with OFN to provide the transient resilience to small volumetric changes on the network. There are various arguments against using open surface pressurisation but so far my observations do not support any of them.

Having operated and developed the system over many years I have been able to compare a variety of water treatment regimes and their relative efficacies in keeping the DH network in good shape.

As I commented previously the history of DH in the UK is poor in general with a long list of failed and abandonned plant. Partly due to inappropriate design coupled with indifferent workmanship but always allied to poor site management, due mostly to ignorance of basic principles.

Good to chat , please keep in touch,

Best wishes,

Massey

Hi Niumec,

Before any suggestion concerning the heated water tank, please let us know how the tank is heated, the origin of the water, its pH, impurities, ..., and the rate of flow of the water through the tank. We all appreciate more explanations before commitment with suggestions, Gil.

It would be helpful in avoiding the formation of rust from the inside as otherwise it will be difficult to clean from inside.You can inquire of a special grade of rubber lining from inside which can withstand 95 Deg.C to surve the purpose.

Dear k-c,

You should understand that under benign water conditions that there will not be any rust formation; the bare metal surface remains clean and free from either scaling or corrosion. In my earlier reply I cited the internal surface condition of an unlined steel vessel which has been in continuous operation for 20 years. The water treatment provides the only protection needed.

Furthermore, you should consider the implications of the loss of integrity in any applied coating, because even pin holes can allow the entry of the media to the underlying raw metal surface with potentially damaging results. Once this occurs remedial action is difficult and unpredictable. In some cases there is evidence of the trapped water becoming even more aggressive with accelerated corrosive action progressing underneath the coating.

Personally I prefer to be able to see what is going on inside a pressure vessel and that view is shared by my Boiler Inspector as well.

Good luck.

Massey.

it depends on the type of water (chemistry) and treatment. Usually water tanks are internally lined and have a cathodic protection system installed.

http://water.me.vccs.edu/courses/ENV110/lesson10_3.htm

S

rust prevention