Cooling tower motor corrosion

[gdn1984]

I have been asked to look at adding cathodic protection to some cooling tower motors. Approximately one year ago the cooling tower structure was replaced with stainless steel. The motors mount directly to the SS. Since then the corrosion of the motor housing and the bare part of the motor shaft has been rapid. I am making inquiries about whether appropriate corrosion inhibitor is being used - presumably this is the case. The motors are run via VSD, but aside from that I do not know details of the electrical installation.

The photos show a motor back from refurbishment. This motor is not more than three years old. Apparently a better paint system has been used for the refurbishment. The cooling fins on the housing have corroded to be "comb-like" - I am not familiar with this. The corrosion on the bare part of the shaft is visible - the keyed part of the shaft should be a straight shaft. Below the shoulder, the material loss will be the same as immediately above. A coating has been applied to try and protect it.

My thoughts are:
- galvanic corrosion between motor housing and SS structure could be reduced by attaching an anode to the motor housing
- in addition, the motor shaft has electrolytic corrosion due to a shaft voltage caused by the VSD.
- A review of the chemical dosing should be done to ensure the use of corrosion inhibitors is adequate

Am I on the right track? Thank you

Note: Two photos are of a freshly overhauled motor. One is of a motor recently installed following the same refurbishment process.

 

[TugboatEng]

Cathodic protection only works if the parts being protected are immersed in an electrolyte. You'd be better off specifying a motor rated for the application (stainless steel shaft and appropriate coatings). Adding fiberglass (Garolite G-10 or similar) isolators would help with corrosion where the motor and frame meet. Gasket the connection with some EPTFE tape, too.

 

[Dhurjati Sen]

You can only put a superior paint. Rest of the things like environment (wet /dry), draft loss of the cooling water, biological corrosion, cooling water treatment programmes, etc are all ouside your scope.

DHURJATI SEN
Kolkata, India

 

[gdn1984]

Thank you for your replies.

TugboatEng, I had wondered whether isolating the housing from the stainless frame could be an option.

Thanks also for the clarification on the cathodic protection. I had thought that a generally very damp environment would be sufficient - so good to not go mistakenly down that path!

The motors come from a reputable company - and the application would have been known. It is odd they didn't spec them more appropriately for the application.

 

[gtaw]

Those accordion fins in the bottom photograph, are they cooling fins for the electric motor? How effective are they once they have that heavy paint applied?

Best regards - Al

 

[gdn1984]

gtaw, interesting and good point. I don't know how the new paint system compares (type, thickness etc) to the old one that didn't work so well. Though the company that did the refurbishment of the motors is one of the better known ones in the motor manufacturing industry. Hopefully they know what they are doing and selected the paint system with consideration to the service conditions (corrosive but probably not pushing the motor to its thermal limits).

 

[TugboatEng]

GTAW, the motor is a laminated plate construction so all of those serrations may appear to be for cooling but are just a feature of the design.

https://www.baldor.com/cooling-tower-motors

 

[gtaw]

Got it. It isn't to increase the effectiveness of the cooling.

Thanks - Al

Best regards - Al

 

[gdn1984]

Thank you for clearing up the reason for the laminations. The good thing about this job is that I know I won't get bored with the same things day in, day out.

This paper was useful: Direct Drive Cooling Tower Motor Paper: Link

In this case it does like the laminated fins do provide for cooling too.
The paper reinforced the need for a good paint system. I would think that once the laminations are exposed, all that potential extra surface area doesn't help with corrosion rates.

It'll be interesting to see how the new coating performs.