Galvanic Corrosion potential on roof hatch of Crude Oil Tank...

[kmg1983]

Currently the Crude Oil Storage Tanks at our facility have roof hatches that allow for inspection of the internal floating roofs. The hatch lid is made of Carbon Steel (not sure which type), however, it is very heavy for operators to lift regularly when inspecting the internal floating roof. We are interested in replacing the steel lid with possibly an aluminum lid. Due to the fact that this would be a corrosive atmosphere, would there be an excessive amount of galvanic corrosion between the aluminum lid and the carbon steel tank itself?

 

[TVP]

There would be excessive galvanic corrosion unless suitable precautions were taken such as coating the steel tank interface, designing adequate drains to prevent accumulation of water, etc. Areas where the lid connects to the tank, such as at the hinge and around the periphery would need to be carefully evaluated.

 

[ash9144]

What about some type of plastic inspection hatch?

 

[gr2vessels]

kmg1983,
I believe that you are going the wrong way, contemplating the material change in an area of personnel safety. Is nothing wrong with the relatively heavy manway lid, it is only lacking the good old davit to help swing the lid. Have you tried that?
cheers,
gr2vessels

 

[metengr]

kmg1983,
I am going to ask the obvious, why don't you simply reduce the thickness of the hatches by making several new ones used for only inspection purposes (label them as such) to reduce weight and allow for easier entry? I would go the route of least resistance and use a thinner hatch plate, made of steel.

 

[mshimko]

Adding to (or if you prefer, "jumping on") KMG's question above, WHY is the current steel hatch so heavy" Is it large for equipment access?, is it thick in case people walk on it?

I'm obviously wondering IOT determine if it can be made thinner and lighter, and/or to propose an alternative material or design.

 

[kmg1983]

Well, its a completely steel hatch lid measuring 34 1/2" x 34 1/2". I'm not sure about it's thickness, maybe 1/4".

 

[kmg1983]

gr2vessels....what do you mean by "good ole davit"?

 

[gr2vessels]

Davit is a swivel type of bracket which is supporting the weight of the manway lid, during the period of manway access for service, inspection, repairs. Every vessel or tank fabricator has a number of Davit designs to suit any type or size of manway he's supplying.
cheers,
gr2vessels

 

[jay165]

How can galvanic corrosion take place when there is no electrolyte present? Is the hatch and steel lid submerged in water?

 

[kmg1983]

No, it is not submerged in water. However, a corrosive atmosphere is present, as it is a crude oil storage tank.

 

[jay165]

So it's submerged in crude???? I thought it was on the TOP of the tank.

 

[kmg1983]

No, it is not submerged in any fluid. It is located at the top of a crude oil storage tank, which is potentially a corrosive atmosphere.

 

[gr2vessels]

kmg1983,
It appears that you have limited understanding of the galvanic corrosion;- both of the "non-compatible" materials have to be submerged in an electrolyte, not simply wetted by some condensate, in order to produce material loss due to galvanic corrosion. Even so, you can isolate electrically the hatch cover and the manway flange by using special isolator sleeves on the bolts. The corrosive atmosphere is not sufficient to produce galvanic corrosion.
However, your concern with the weight of the steel manway cover is still valid.
gr2vessels

 

[unclesyd]

This is a hard way to get there but if you zoom in on the Powerhouse near the river in this picture you will see three carbon steel storage tanks with Al roofs. The two larger tanks had Al roofs installed in the 60's to alleviate a bad corrosion problem with the CS roofs. The small tank was built in the 70's with an Al roof. All roofs are bolted to a modified wind girder on the CS shell. These tanks store what we call domestic water and fire water that sometime has a lot of CO2. I looked at the larger tank in 1997 and there was no corrosion at the CS/Al interface.
I don't think that you would have a problem with Al vs CS in you case. I like the idea of lightening the CS but if you go with AL make sure it is capable of supporting several people.

http://www.mapquest.com/maps/map.ad...9999999&longitude=-87.253207000000003&dtype=h

PS:
I like inspecting the large tank as there is a 8'x8'x6'deep sump that people fall into, Panic, even though you tell them it is there before you enter the tank.