김상현
2008-09-10 11:57:49
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Heater tube 용접이 완료되고 모든 검사가 끝나면 module 별로 마지막으로 수압시험을 실시합니다. 그런데 이때 사용되는 물에서 염소이온 농도를 보통 30ppm 이하로 제한하고 있습니다. 왜 염소이온 농도를 제한하는지 그 이유를 정확히 알고 싶습니다. 선배님들의 조언 부탁드립니다.
Normally after hydrotest, all facilities will be dried according to the applicable procedure. Unfortunately the full dry may be very difficult unless applied the dew point control during dry after drain. It means some water or moisture will sustain inside of the facility. The remained water will be evaporated (the water is readily evaporated compared to chloride) on the local area or penetrate to the crevice of inside components (the chloride will more strongly penetrate to the crevice coated to water). As a result, the chloride concentration at the local area or crevice will be greatly increased (normally 2 to 10 times).
In order to prevent the corrosion damage by chloride concentration in the remained water, most company specifications as well as most code/standards have the limitation.
The chloride limitation is to prevent crevice corrosion (all metals) or pitting /stress corrosion cracking (mainly stainless steels or nickel alloys).
Therefore we can classify the limitation as below (as a typical).
- Stainless Steel or Nickel Alloy : max. 30 to 50 ppm in the hydrotest water (See Attachment)
- Carbon and Low Alloy Steel : max. 100 to 200 ppm in the hydrotest water
API 560 does not have the requirement for carbon steel or low alloy steel tubes. Please confirm the heater tube materials and your client requirements.
Some company standards require the limitation, as max 30 (or 50) ppm regardless of the metals
Meanwhile most standards say the chloride limitation for ASS (i,e., 304 SS) in the service is max. 200 ppm and max. 60 deg.C.
ChlorideLimitationofHydrotestWater.pdf
In order to prevent the corrosion damage by chloride concentration in the remained water, most company specifications as well as most code/standards have the limitation.
The chloride limitation is to prevent crevice corrosion (all metals) or pitting /stress corrosion cracking (mainly stainless steels or nickel alloys).
Therefore we can classify the limitation as below (as a typical).
- Stainless Steel or Nickel Alloy : max. 30 to 50 ppm in the hydrotest water (See Attachment)
- Carbon and Low Alloy Steel : max. 100 to 200 ppm in the hydrotest water
API 560 does not have the requirement for carbon steel or low alloy steel tubes. Please confirm the heater tube materials and your client requirements.
Some company standards require the limitation, as max 30 (or 50) ppm regardless of the metals
Meanwhile most standards say the chloride limitation for ASS (i,e., 304 SS) in the service is max. 200 ppm and max. 60 deg.C.
ChlorideLimitationofHydrotestWater.pdf
공지사항
염분은 Stainless Steel에 Stress Corrosion Cracking을 유발하고 Carbon Steel에는 Pitting등의 국부 부식을 유발하게 되며 드물게는 Stainless Steel에서 볼 수 있는 것과 유사한 Stress Corrosion Cracking을 조장하기도 합니다.
그래서 수압 시험을 볼때에 물에 녹아 있는 염분을 제한하는 것이고, 만약 그 기준치를 맞추지 못했을 경우에는 염분이 극히 작은 물로 Rinsing을 하거나 아예 염분이 많은 해수 등에 별도의 Corrosion Inhibitor를 투여하여 적용하지요
하지만 그 어떠한 경우에도 결국은 염분은 항상 국부적으로 농축되기 때문에 염분에 의한 국부 부식이 우려될 경우에는 적절한 Drain과 Water Condensing을 막을 수 있는 대안이 필요하게 됩니다.
각 Code & Standard별로 수압시험 시의 염분에 대한 규제가 약간씩 다르긴 합니다만 다들 기준을 제시하고 있지요.