Quick depletion of anodes
Quick depletion of anodes
(OP)
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.
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.





RE: Quick depletion of anodes
= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm
RE: Quick depletion of anodes
What are the factors involving anode depletion?
Temperature, galvanic potential...?
RE: Quick depletion of anodes
It would be necessary to look at the design of the cathodic protection. To define the number and size of the anodes is necessary to account for a number of different factors and maybe during the design stage they had made wrong assumption on total surface to be protected, design life...
The material of your vessel is Carbon Steel? Is the surface exposed to seawater painted?
S.
http://www.corrosionist.com
RE: Quick depletion of anodes
The water varies from fresh to sea water up to approximately 90F in the summer with low flows.
RE: Quick depletion of anodes
If there is a lot of bare metal to be protected, DEMAND will be high and so will the dissolution rate.
If you need to insert resistance washers to reduce the dissolution rate of the anodes, then something is NOT being cathodically protected; I would never recommend this practice.
You need to look at what metal surfaces are in the "electrical circuit". Something is there causing a high demand that needs to be addressed.
RE: Quick depletion of anodes
I have seen resistance washers used when the space/geometry required that anodes be placed too close to some areas in effect causing 'over protection'.
Unless you can take measurements you don't know if you have any protection to give up.
Just remember as Mshimko says, the anodes don't do anything, the process is related strictly to demand and electrolyte condctivity.
= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm
RE: Quick depletion of anodes
The potential is usually measured from the protected metal to a half cell reference electrode placed in the water.
http://w
RE: Quick depletion of anodes
On building a corrosion meter:
http:/
Galvanic series in seawater:
ht
I'm trying to understand the voltage measurements that the first article talks about - and the current that EdStainless mentions.
I'm not clear what the voltage is indicating. If the case of the heat exchanger there is a steel casing and 90-10 copper-nickel tubes. If you put the silver anode into the seawater and connect the other lead to steel it should read 540 mV. If you connect it to the cu-ni tubes it should read 130 mV. If you connect it to the zinc anode it should read 880 mV. By substracting the numbers you can see the potential between steel and cu-ni or steel and zinc, etc.
Is this right so far? It doesn't seem to match up with the numbers in the first article.
I'm not sure how this helps. I guess if the voltage between the tubes and the steel is larger than the tubes and the zinc then the zincs are not preferentially corroding. How would the voltage tell me if I need more zinc?
RE: Quick depletion of anodes
(a) the max expected DEMAND, which dictates the effective surface area of the sacrificial anodes installed, and
(b) the expected service life of the anodes, which dictates the MASS of the anodes to be installed.
Both (a) and (b) above are in turn determined by the number of anodes (assuming you've already selected the anode type and dimensions); one of the above will be limiting, the other will not.
Regarding taking measurements in the field; you cannot measure the potential difference between electricall connected metals in the same system (such as the tubes and casing mentioned above). IOT measure this potential difference, you first need to electricall isolated the parts, then connect them via an appropriate meter, which is not typically practical in the field.
Bottom line is: you may be trying to dig into the science too deep at this point in time. If your anodes are being depleted too rapidly, you simply need to add more anodes, OR determine and address the cause of the high current demand.
RE: Quick depletion of anodes
I guess it's probably due to the temperature and flowrate but now I'm curious how these things work.
RE: Quick depletion of anodes
S.
http://www.corrosionist.com
RE: Quick depletion of anodes
The inside surface of the chiller is not painted.
Also, the bolts holding the anodes on are brass.
RE: Quick depletion of anodes
Your anode and the steel are bonded together, so you will read the same voltage wherever you place the lead. Per the article you linked, you should add zinc until the potential is 750 to 950 mV in order to protect the steel (the most active metal you need to protect).
Your galvanic series article used a different type of reference electrode, and shows the values for unprotected metals alone.
RE: Quick depletion of anodes
RE: Quick depletion of anodes
RE: Quick depletion of anodes
1. What is the number and dimensions of the anodes installed?
2. What is the total surface area of bare metal electrically connected to the anodes?
I suspect (but obviously don't know) that you simply have a larger surface area to protect than you realize.
RE: Quick depletion of anodes
Do you mean temperature change from day to night of the seawater inlet or across the chiller?
I don't think the seawater is aerated. We are getting cavitation on our pumps at times but there shouldn't be too much air in there. We have some trouble with marine growth in the sea inlet but it doesn't get as far as the chiller piping.
Mshimko,
You're right, it could be expected - it is just annoying to change them out so often and wanted to see if there might be another problem I should be looking into.
I'll try to get back to you on surface area, etc.
RE: Quick depletion of anodes
RE: Quick depletion of anodes
The chiller tubes (cu-ni) are 16mm diam. and 3400mm long. There are 120. Approximate surface area of 20.5 square meters.
There are 18 zinc anodes fitted to one end. They are 65mm diameter and 45mm long.
RE: Quick depletion of anodes
S.
http://www.corrosionist.com
RE: Quick depletion of anodes
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.
RE: Quick depletion of anodes
RE: Quick depletion of anodes
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.
RE: Quick depletion of anodes
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