Since NH3 is basically non-corrosive gas, the majority of piping corrosion takes place outside the pipe. The type of corrosion and loss of wall thickness frequently observed on ammonia refrigeration systems are as below.
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UT/NDT Corrosion Examination
UT/NDT NH3 piping/pressure vessels wall thickness examination.
- Galvanic or microcell corrosion
- Erosion corrosion
- Microbiologically induced corrosion.
- Mechanical damages
Below please see some examples reports and actual corrosion incidents.
Corrosion Under Insulation
Insulation on this liquid line has been breathing due to frequent on-off operation and relatively higher evaporating pressure. Type of corrosion is pitting. Client decided to remove all insulation and thoroughly inspect the piping.
Evapco Condenser Tube Pitting
Look at these pitting on Evapco condenser tubes. These pipes are hot dip zinc plated. This plant is located at coastal area and, therefore, is subject to salt rich environment. This is likely a case of polarity reversal in which zinc becomes noble and steel becomes less noble.
Numerous pitting depth were sampled and statistically analyzed for probability of exceeding depth limit as per B31G. Client decided to change the coil to stainless steel tubes.
A53 ERW pipe welding seam spread/crack
This is occurred on 8” LTRS. This pipe is A53 ERW. There was slitting at welding seam indicating inadequate fusion and corrosion slowly took place in the slit. Luckily whole section of heavily damaged/compromised insulation was all removed for examination. Actual depth of the slit could not be measured by our instrument, but it was no brainer !!
Pitting
Numerous pitting on untreated pipe of HSD or CD near evaporative condenser. Pitting depth were sampled and statistically analyzed for probability of the pipe wall thickness exceeding wall loss limit. Calculation showed 5 % probability of exceeding the limit. This client decided to change the all pipe and apply proper surface treatment.
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