Pressure vessel in carbon steel and fresh water

[cex10k]

Dear engineers

My company is issuing a RfQ for pressure vessels. Working fluid water 98% and 2% of mineral oil. Max. temperature 50°C, pressure up to 330 bar.

These pressure vessels work as accumulators, that means water level changes periodically inside the tank. The gas inside is air.

The pressure vessels should be made in carbon steel (no stainless steel can be used).

My question is: which is the corrosion rate of water for carbon steel? How thick should be the corrosion thickness for a working life of 50 years? Which are the important parameters of water to take into account for corrosion?

Thanks
Best regards
Michele

 

[MJCronin]

"The pressure vessels should be made in carbon steel" ...... no, they shouldn't

Do you understand that you do not have a binary choice between carbon steel or stainless steel ?

For an economical choice and a 50 year life, the correct choice for this vessel is "coated carbon steel" ..... Coated both internally and externally.

A quality epoxy based coating would be my choice ....

MJCronin
Sr. Process Engineer

 

[Chumpes]

hi
The free water into this vessel is in contact with the atmosphere, saturated with oxygen.
I consider corrosion rates of carbon steel becomes unacceptable (>0.1-0.2mm/y) when oxygen content goes above 100ppb.
in your situation, bare carbon steel will corrode fast.

I agree with MJCronin, carbon steel with internal polymer coating would be economical and will protect the carbon steel from corrosion by aerated water.

50 years of design life looks very long, why not 100 or 200years? (<- it is a joke)

 

[cex10k]

Thanks for your advices.

The internal coating was our first idea to protect the inside of the vessels, the problem is the high pressure of 330 bar (about 5000 psi). We should require a 600 mm (24") opening in order to allow the painting and the inspection. Such a large opening is not accepted by the customer. We will try to change his mind.

Do you know any alternative technique to paint the inside of a pressure vessel using an opening of 200 mm (8")? Is it possible to certify the quality of the painting by such an opening? The tank has a OD of 1800 mm (about 70") and length of 12.5 m (about 41').

Thanks again
Michele

 

[Chumpes]

what about internal cathodic protection? does it exist a painting/coating system able of handling 330bar?

 

[NBrink]

Your client wants a very large carbon steel vessel filled with compressed air and water at 5000 psi, in what sounds like cyclic service, and expects to NEVER perform an internal visual inspection in 50 years?

They are going to get someone killed.


Nathan Brink

 

[MJCronin]

I have doubts about being able to fabricate a single large vessel with this kind of diameter and pressure..

Anybody calculate the approximate wall thickness for CS ??

Would multiple smaller vessels be more appropriate for this service ?

Access manways can be useful to 18" diameter and smaller .... You do not have to use a 24" manway

In what kind of crazy service is this beast going to be used ?

MJCronin
Sr. Process Engineer

 

[cex10k]

These pressure vessel will be used as an hydraulic accumulator for a steel extrusion press.
Paradoxically the manufacturing of the pressure vessel is the smallest problem. It will be performed using forged rings welded together. The pressure vessel will be designed according to ASME BPVC sec VIII div.2 and U2 stamped. The thickness of the shell is about 150 mm (6") and the heads about 100 mm (4").

We have identified 3 possible solutions:
- To use distilled water with an addition of chemical anticorrosive. This fluid could preserve the carbon steel.
- To use nitrogen instead air, in order to avoid oxygen
- Create manholes and paint the inside of the vessels

All solutions shall come with a 3 mm of corrosion allowance.

What do you think?
Michele

 

[moltenmetal]

Why is the water being used "open loop", i.e. exposed to atmospheric oxygen?

Eliminate the oxygen and you eliminate the corrosion.

Your other alternatives are protection combined with a corrosion allowance. Coatings can result in greater localized corrosion at points of coating failure so there's that risk. Either option will require access to the interior of the vessel for periodic inspection and maintenance, which is a code requirement anyway.

 

[LittleInch]

First off you need to work out / describe what the fluid is doing and what the air is doing.

If, as I think, the water is inside a sealed system and just moves in and out and the air also a fixed volume which stays in the vessel for years on end without replenishment, then your oxygen supply will be rapidly used up and corrosion rates will fall very fast to effectively zero.

If you're dealing with water and air which is constantly being replenished then the corrosion rate will be significant.

Lining it is best, but using oxygen scavenger in the water (if its a sealed system) will reduce the corrosion rate even further as will Nitrogen as the gas.

A pressure vessel of that size and pressure is fairly unusual, but 150mm is probably about the limit. I has a 1.8m vessel at 110 barg that thick which was interesting to see made...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[cex10k]

Thanks for your answers. I'm getting a more clear view of the question.

The system works this way (also see attachment): the water from an open tank is pumped into the system. In the system water charge a water/air pressure vessel that is connected to other vessel on the air side. These vessels work as an hydraulic accumulator: when the required flow rate is larger than the available flow rate the water level in the water/air pressure vessel decrease. The air in the pressure vessel is always the same. Even when the system is off a minimum level of water is left in the water/air pressure vessel in order to avoid the leak of air.

However the water that enter in the water/air vessel is always different and this water can be in contact with air in the open tank.
Can this contact with open air bring new oxygen into the system?

Thanks again
Michele

 

[MJCronin]

More thoughts:

1) If the client does not want ANY manways/access to the interior of this tank, how does the client expect to get the tank inspected internally ? You gotta have something ???

2) Based on your diagram, I believe that the system can be redesigned to become a more-conventional closed system. If you eliminate the use of water and replace it with fireproof hydraulic fluid you will prevent a lot of long term corrosion problems.


MJCronin
Sr. Process Engineer

 

[EdStainless]

Our HP accumulators are about 24" diameter with one end fully removable. We have a bank of them.
The LP tank is one large internally coated tank.
Ours is a closed system with the water well degassed (heated and filled under by partial vacuum) and it has an inhibitor package added.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[cex10k]

UPDATE

Today I spoke with the suppler of the additive that the customer want to use in water.
He assured me that with 2% of its additive and using city water (potable water) no rust problems occur. Also the parts in the accumulator that are periodically wet and dry will not be affected by corrosion because the period between wet and dry is short (1 minute) and the lubrication effect is still working.

However he warned me about painting the inside of the pressure vessel because if the customer in the future will change the additive with another one it may be incompatible with the painting and this could create a problem.

Accordingly to this info the only problem now is not the liquid, but the air inside the accumulators. Does the water vapour in it could create corrosion to the carbon steel? Please consider that:
- With high pressure (330 bar, about 5000 psi) the boiling point of water increases, so we expect that the quantity of water vapour in the air will be small even at 50°C
- The air in the tank is always the same (until the vessel is opened, maybe once every 10 years), so the oxygen will end and the oxidation will decrease with time

Which will be the corrosion rate of carbon steel with such conditions?

I'll keep you informed
Michele

 

[cex10k]

SOLUTION

I found an article about the corrosion of carbon steel over long period of time due to humidity. The focus of the article is about waste container for the disposal of nuclear waste, but the data I found in it can be used also for our problem.

This article states that the corrosion rate of carbon steel in a 100% relative humidity atmosphere at 90°C is about 80 µm/year for an exposure time of 10 years (average corrosion rate reduces along with the exposure time because corrosion products generate a sort of barrier).

With these info (that are conservative because we work at about 50°C and the expected lifecycle of the vessels is 50 years, so the corrosion rate should be lower), we stated that with 4 mm of corrosion allowance (80µm/year*50years) our pressure vessel is protected against the corrosion of the humidity inside it.

The link of the article is the following:

https://www.osti.gov/scitech/servlets/purl/206639

I hope this info could be useful
Michele

 

[moltenmetal]

Assuming that the corrosion will be uniform is your first mistake.

This is a bad idea. There are better solutions to the problem.