Corrosion in Cooling Water Exchangers

[qaisarabbas]

We have numerous CW exchangers at site. The corrosion issues that we are continuously facing are related to tube-to-tubesheet seal welds which results in plugging a significant number of tubes at each hydrotest. Water chemistry is found fairly within recommended limits. We are planning following steps to cater the problem:

1. Sump cleaning of cooling tower to improve the water quality (w.r.t. suspended solids, mud, foreign materials)
2. Evaluating the need to install sacrificial anodes at channel heads or applying belzona coating on tubesheet with covering tube ends

Would appreciate if anybody shares his relevant experince and suggest some control measures. regards

[ashfaqanwer]

Installing sacrificial anodes would help you in protecting galvanic corrosion issues if present. For crevice and pitting corrosion, belzona coating would be a better option. Can you share some pictures of corroded tube-to-tubesheet seal welds?

Do consider using of corrosion coupon to estimate the extent of corrosion being caused by cooling water system. Coupon weight loss would be providing you a qualitative measure of corrosion rate. Alternatively, corrosion rate meters are also available which can help you directly measure the corrosion rate of a certain material in a certain environment. It would help you in deciding your way forward.

[sakib321]

would you please mention those corrosion rate meter??

their working and there details

[ashfaqanwer]

Instrument methods fall into two general categories: electrical resistance and linear polarization. With either technique, corrosion measurements are made quickly without removal of the sensing device. The electrical resistance method is based on measuring the increase in the electrical resistance of a test electrode as it becomes thinner due to corrosion. This method is desirable because the probes can be installed in both aqueous and nonaqueous streams.

However, the electrical resistance method also has its disadvantages: conductive deposits forming on the probe can create misleading results, temperature fluctuations must be compensated for, and pitting character-istics cannot be determined accurately.

The method based on linear polarization at low applied potentials provides instantaneous corrosion rate data that can be read directly from the instrument face in actual corrosion rate units (mils per year). Systems using two or three electrodes are available. This method offers the maximum in performance, simplicity, and reliability.

See the attached sample figure also.

[sakib321]

my one question about it is that how much time is required for this single test ?

[mechcolor]

qaisarabbas,
I need to understand your problem. You are plugging the tubes due to corrosion problem? Leakages are being found through tube or tube-to-tubesheet joints?

[ashfaqanwer]

Sakib,
For most of the equipment, the cycle time for estimating corrosion rate is of 60 seconds.