Quick depletion of anodes 5

[gcomyn]

We have an Air Conditioning Chiller condenser that is cooled by seawater. We operate in the middle east so seawater inlet temperatures are routinely 35C and so the flow is pretty high (200 m3/hr). The problem we are having is that the zinc anodes inside are depleting really quickly and we've had to increase the number and size of the anodes.

I was wondering if the cause of the rapid depletion was normal for the temperature and water flow or if there might be something else going on that we should be addressing.

 

[strider6]

I think you have a big problem of galvanic corrosion, Cu-Ni and Carbon Steel in seawater that increase the corrosion rate, also with a ratio anode area to cathode area unfavorable. 20 m2 to 6.5 m2.

S.

http://www.corrosionist.com

 

[mshimko]

My initial arithmetic may have had some arrors, as I don't seem to get the areas you indicate above.

Setting that aside, I offer two criteria for determining the amount of anodes needed for protecting bare metal (I didn't develop these, but they are "hidden" in other published criteria):

a. Accounting for ONLY the top surface of the anode, one M^2 of anode surface area will protect 50 M^2 of bare metal.
b. Accounting for the top surface AND the "sides" of the anodes, one M^2 of anode surface area will protect 33 M^2 of bare metal.

Next; it's difficult to determine the "effective" surface area of tubes when it comes to cathodic protection demand. The entire inner surface of the tube WILL NOT be cathodically protected as "current" simply won't travel that far down a resticted passage. I've seen mathematical models for estimating "how far", but don't think they are worth the effort. For relatively small diameter tube or piping, I recommend using a length of 10-diameters.

 

[gcomyn]

Mshimko: Just to confirm, the number in 'b' when considering top and sides of an anode is less than when considering only the top of an anode. Is that right?

 

[mshimko]

YES.

In "a" you use the surface area of the "top" only, and a larger "factor".

In "b" you use the surface area of the top AND sides, but with a smaller "factor".

The end result will not be that different for most anode shapes.

 

[chief]

Your flowrate is way too high and the water velocity is likely to be the main cause of your premature wasting of the anodes. If practical, control your sea water flow with the outlet valve of the condenser. You should be looking for a flow velocity of about 10 ft/sec.
It also appears that your condenser is undersize for the area of operation.
Usually marine installations have a standby condenser, if so, it would be advisable to use both in operation.
A field tip from a lazy ex-Marine Engineer. You may be able to clean your condensers in a hurry, by a quick back-flush. Depending on there being water pressure on the outlet and a convenient piping arrangement. Good luck.

Offshore Engineering&Design