3 replies to this topic

[flangecoupling]

Hello,
I am working on utilization of heavy fuel oil storage tank for purified water storage.
The tank is manufactured by using mild steel sheets. One basic question which we came up with is that, whether in general, water is more corrosive than heavy fuel oil or vice versa?
I will be requiring your view points on this topic, as this is very critical before going in complete rating procedure.

Regards

[Art Montemayor]

Flange:

I believe you have your priorities on decision making a little mixed up. Of course carbon steel can be corroded by water - but the corrosion depends on the type, source, contamination, and blanketing (if used) of the stored water.

The corrosion contributed by crude oil may (and often is) be of a totally different nature. You have failed to fully identify your scope of work - which seems to be a favorite "trick" or omission of most engineers who write in to our Forums. By not telling us what - specifically, and in detail - you are going to be doing with this water, you make it difficult for yourself because you do not enable us to fully evaluate our responses.

For example: Is this "purified" water meant for human consumption? If so, then several major factors enter into play. How much iron can you tolerate in your drinking (and cooking) water? Local responsible authorities must have minimum standards for this. It is quite common to have carbon steel storage tanks for potable water. In fact, most of the residential water piping in the USA is carbon steel - according to one source. I know my house (& those in my neighborhood) was 100% furnished with carbon steel water piping. The main, elevated water supply tank is carbon steel, welded in a "golf ball" shape. So, carbon steel is not an unusual material of construction for drinking water - which has to be among the highest quality of water specified. It is a matter whether you want to put up with the inevitable corrosion rate that follows. I personally prefer not to put up with it in my house and I have replaced about 50% of the carbon steel piping with silver-soldered copper tubing.

Rather than the carbon steel corrosion rate, I would initially be concerned with how the tank is welded and whether it contains any possible combustible hydrocarbon gas pockets in any lapped welding seams on the tank. Cleaning this tank should be a priority as well as a subsequent application of an internal protective lining or coating.

[flangecoupling]

Dear Art,

Thanks alot for your reply. My apologise for the incomplete information.

The tank will be used for storing purified water from a reverse osmosis plant. This water will have app 20 ppm TDS.
The purified water from this tank will be used in boilers as boiler feed water. The tank capacity is of 900 metric tons.
Moreover the water quality is attached.

The ambient temperatures range is from 25-50 deg C. We have incorporated a team to completely decontaminate the original tank and then apply epoxy coating in it from inside. I was just wondering that if by any means this water becomes more corrosive than heavy fuel oil then corrosion rate will increase with time and eventually tank life will decrease than its desinged life.

Can you help me find any authentic literature of "the type of corrosion which occurs by heavy fuel oil?"?. Or your expert advise will be very helpful as always.

Attached Files

•  permeate RO BFW.xls   41KB   29 downloads

[Art Montemayor]

Flange,

Thank you for the detailed response. With basic data quality like this, you deserve a detailed response.

The tank’s useful life is directly tied to the protection you give it during its working time. From what you describe, you are already taking care of the high priorities – those being the adequate cleaning, preparation, and inspection of the tank. The degree of the quality internal epoxy coating and the degree of the quality installation of that same coating will tell - to a large degree - what length of life you will experience from this tank. Of equal importance is the process design that you are planning to incorporate with the change of service in this tank. If you value the length of tank life, you would be wise to apply nitrogen blanketing to this boiler feed water (BFW) tank. Now that you have revealed the application, we can get down to specifics.

I don’t know the quality of steam you plan to produce or the type and size of boiler you have, but to my point of view you are producing a BFW quality of water that requires further treatment – either after storage or within the boiler treatment scheme planned. The quality of the BFW in the tank doesn’t bother me as much as what the TDS will do to the boiler – and the product steam. If expertly applied and maintained, the epoxy internal coating will be sufficient to protect the tank – together with an inert blanketing system. I would not be concerned with tank life if we can rely on you applying your plan with quality engineering and supervision on quality of application.

You would also be wise to definitely plan on yearly or 1-1/2 yearly tank internal inspections. These, in my opinion are vital when an internal coating is depended upon for protection.

The water you produce and store under an inert blanket will not be corrosive – if maintained that way. In my opinion you should not bother yourself with crude oil corrosion of steel storage tanks. Concern yourself in protecting your tank from any internal corrosion. You cannot relate water corrosion to Crude Oil corrosion. There are crude oils and then there are crude oils. If you are talking about storing sweet and light Algerian crude, that is one thing. But if you are talking about storing Bachaquero crude, that is another completely different story. There are many corrosives attached to crude. Water, of itself, when pure is not corrosive. It becomes very corrosive when saturated with Oxygen and dissolved solids or other gases. If you produce a quality water devoid of oxygen and store it under an inert blanket, carbon steel construction should suffice – especially with a quality epoxy internal coating. I am sure that ICI has applied this design philosophy many times in the past.

As a matter of personal curiosity, how is it that you can specify 50 ^oC as an ambient condition? Humans (nor most animals) cannot survive at that temperature level. And how can a process plant exist in those conditions? Please elaborate on this.