March 12 - March 18, 2012
How do you determine if local water is suitable for use in UHP waterjetting, particularly in areas where potable water is scarce?
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John Triebe of Inspec Consulting on
March 17, 2012:
It is all well and good to specify that the water should be free of oil, chlorides and contaminants, but we do know that water contains dissolved salts and we will accept surface salts to various levels (30mg/m2 is a commonly used acceptance criteria). These salts can be measured, but what is an acceptable level for water washing or UHP water jetting? SSPC-SP 12 covers measurement of soluble salts at the surface (Appendix A) and sets levels for flash-rust (If a surface flash-rusts very quickly, it could be due to soluble salts remaining on the surface.) but does not (from my observation) set acceptable levels of salt for the process water. My experience is that local potable water (Brisbane, Australia) often reads 0.45 - 0.55mS/cm (450-550µS/cm) but has no measurable effect on soluble salt testing by Bresle DSP method. No one would agree that using seawater would be suitable for cleaning steel, yet many of our inland groundwater sources are quite salty and, unfortunately, must be used for processes (coal washing, etc) as the rainwater is too precious to use. So, how much is too much? Apps Lab has this to say: (from: www.appslabs.com.au): How is salinity measured? A quick way is to use a conductivity meter and read off the electrical conductivity. The idea being that a salty solution, because it is full of charged particles, will conduct electricity. Most conductivity meters give readings in micro Siemens per cm (µS/cm). So what's a micro Siemen? Well, most fresh drinking water will have less than 100 µS/cm conductivity eg Melbourne. However, in WA the statewide standard for drinking water is nearer 821 µS/cm (0.82mS/cm). Some slightly salty drainage water found on salt-affected farms in Victoria will be around 1800 µS/cm. In Western Australia salt drainage water conductivity could range from 8000 to 23000 µS/cm **. Very brackish water could be around 27000 µS/cm. By the way seawater has conductivity of around 54000 µS/cm (54mS/cm). [If you come across milli Siemens per cm (mS/cm) just remember that 1 mS/cm = 1000 µS/cm.] Now, here is where it gets tricky, and I start making some assumptions (not very scientific). We all know that surface salts are measured over a unit area (e.g. /m2), but conductivity of water is a volume so cannot be compared unless you know the spreading rate of the water left behind and its conductivity. If we do not want the water to contribute more than 10uS/cm to the Bresle test, and, assume that the water drying from the surface (depositing dissolved salts) was 10microns (1m2/cc), you would not want the process water to exceed 10,000µS/cm (10mS/cm). That sounds like a lot! However, if it is assumed that the spreading rate was only 0.1 m2/cc , the maximum permissible would be 1,000µS/cm (1.0mS/cm) and only 2 times that of our local tap water and hardly likely to give any comfort to our inland operators. Naturally, standards organisations, specifiers and coatings suppliers err on the side of caution. So what is an acceptable level of soluble salts/conductivity in process water for UHP Water Jetting? The ideal response, I believe, would be results from a multiple supplier test program of coated panels with ST Epoxy after water jetting with stepped levels of soluble salts to see if there was a clear jump in corrosion deterioration. Has this or something similar been done to date?
Lee Edelman of CW Technical Service on
March 14, 2012:
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The water supply needs to be checked for oil, chlorides and contaminants. If the water supply is free of oil, chlorides and contaminants, make sure the water supply has proper flow and pressure for the designated UHP system. Depending on the water supply, it would be a good idea to have a filtration system between the water supply and the UHP system. If the water supply is not free of oil, chlorides and contaminants, you will need to look at bringing in a potable water supply from another area. A clean water supply is very important when using the UHP system. Minimal contaminants can damage dynamic seals, which will result in costly down time repairing the UHP unit.
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