"suspected SS coil failure"

Question:

"We had a CS tank with a SS coil that was replaced recently. The outside of the coil saw tempered water and 200 psi steam. The inside of the coil saw regular water from our city water supply. It was thought that steam was leaking into the coil, causing a loud rumbling sound, and then discharging out the condensate line. We pursued finding the leak(s) so we could then analyze the actual failure mode (e.g., SCC). However, even with pressurizing the coil with 90 psi air (and using ultrasonic leak detection methods) and hydrostatically testing the coil with 400 psi using water, we have found no indication of a leak anywhere on the coils. I would think that the coil, if it failed, failed under tension. However, some think that it may have failed under compression from the 200 psig steam applied to the tank. But, I can't see the coil failing under compression and then not leaking under such a large tensile stress as we have provided with our testing. Both tests have been performed at ambient temperatures. We are going to apply 150 psi steam to the coil for a final test. Is it possible that the leaks only leak when the coil is heated up? Again, I would not think that possible without some evidence of a leak. If not, then we need to look at another reason for the steam rumbling through the coil and out the system's condensate line (which sounds like a big leak to me--too big to not find). The main symptom that brought the failure theory to someone's attention was a rumbling sound through the system and a discharge of steam out the stack to which this coil drains. All this would occur when we we stopped passing liquid through the coil and it emptied assuming the water drained to the stack by gravity. They noticed this discharge and assumed that the coil was cracked, allowing the 200 psig of external steam pressure to leak into the coil and discharge over 40 feet of 1-inch pipe out the 4-inch stack. I would think that the leak would need to be pretty big to have the effect they described. Even with the most recent test using steam through the coils at 150 psig, coil temp. at 350 deg F, we saw no signs of leakage. That tells me what they saw wasn't from this coil. Please give me one last reply, just to confirm that my reasoning about no leaks on the coil is reasonable."

Answer:

I am not certain that I can add anything to your description of the suspected coil leak problem. Even with chloride SCC cracks, which are notorious for being tight, discontinuous and contaminant filled, I would have thought that your leak detection methods would have found a leak site. You might try liquid penetrant inspection.

 

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