Ambient Condition s6q3o

Frosio course handout: Section 07 Ambient conditions

AMBIENT CONDITIONS Humidity Air Temperature Substrate Temperature

Frosio 07 Ambient conditions

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Parameters influencing the corrosion speed. Atmospheric corrosion

x x x x

Humidity Temperature Concentration of salts Amount of air pollution, including acid rain, soot and dust particles


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Atmospheric corrosion rate depends on humidity Corrosion rate

0

20

40

60

80

100

Relative Humidity, % Frosio 07 Ambient conditions

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Atmospheric conditions. Requirement during blasting and painting

Surface temperature of the structure must be minimum 3 °C above the dew point of the surrounding atmosphere


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ISO 8502 - 4 Estimating probability of condensation • Cold liquid inside hull and warm air outside causes condensation • See how frames inside keeps the cold longer

B-24/2-4 4847-693


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Why Temperature & Humidity are important Condensation follows the contours of the double-bottom tank Ð

Ï Frozen overnight condensation, follows the frames of the ship


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Purpose of climatic evaluation

Evaluate the likelihood of the moisture in the air to form condensation on a substrate


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Moisture / Absolute Humidity • When water evaporates, it forms water vapour or moisture in the air • We may measure this moisture as gram water per cubic meter air (g/m3) • The maximum amount of moisture that can be contained in the air depends on the temperature of the air • Warm air can contain larger amounts of moisture than cold air • The maximum moisture that can be contained in air at the various temperatures is called the Absolute Humidity


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Absolute humidity DEFINITION: • The maximum content of water vapour in the air at a given temperature • The content will vary with temperature


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At 8o C, the maximum moisture content in the air is 8.28 g/m3


At 20o C, the maximum moisture content in the air is 17.31 g/m3

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Moisture / Relative Humidity • The Relative Humidity is a measure of how much of the Absolute Humidity (maximum moisture) that the air contains, expressed as percent. • If the air contains the maximum amount of moisture, it will have a Relative Humidity of 100% • If the air contain half of the maximum amount of moisture, it will have a Relative Humidity (RH) of 50% • If the air contain a quarter of the maximum amount of moisture, it will have a Relative Humidity of 25% • Question: Will air with RH=50% at 8o C have the same moisture content as air with RH=50% at 20o C?


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Relative humidity DEFINITION: • The ratio of – The amount of water vapour present in a given volume of air at a given temperature and

– The maximum amount of water vapour which this volume of air can hold at that temperature

• RH is expressed as a percentage • RH outdoors will vary


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Relative humidity, RH % R.H.: Water vapour in the air as percentage of the total amount water vapour the air can contain at the same temperature. Example Air at 20o C can contain 17.31 g. water/m3, but the actual content is measured to 15 g/m3. What is the R.H. ? R.H. =

15g x 100%

= 86.65%

17.31g


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Humidity / Dew Point • Air at 20o C and 73% RH contains 12.64 g/m3 water (12.64 as percentage of Absolute Humidity 17.31) • If the temperature drops to 16o C, it will still contain 12.64 g/m3, but the RH will increase to 92.69% (12.64 as percentage of Absolute Humidity 13.65) • If the temperature drops further to 12o C, what will happen? • At 12o C the maximum moisture content (Absolute Humidity) is 10.62 g/m3. This means the moisture content of 12.64 g/m3 is 2.02 g/m3 more than it can hold at this temperature • The consequence is that the excessive moisture will leave the air through condensation, ending up as dew on available surfaces or as rain or fog • The temperature at which a given moisture content will start to condensate is called Dew Point.


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Dew point DEFINITION: • The temperature at which the air becomes saturated with water vapour • At this temperature the RH = 100% and water will condense • The chance of condensation is high when the weather is changeable


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Dew Point / Substrate Temperature • On an early morning inspection we measure the air temperature to be 8oC and the RH to be 83% • Due to cold over-night temperature, the steel temperature of a structure to be painted was measure to only 4o C • Would this cause any problem? • At 8oC and RH 83%, the moisture content is 6.87 g/m3 • Although the general air temperature is 8o C, the air next to the steel surface will be cooled by the steel and perhaps only be 4o C as well • At 4o C the max moisture content is 6.36 g/m3 • This means that although we may not see it clearly, there will be condensation on the steel • This is why Steel Temp. must be min. 3o C above the Dew Point!


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Main rule for pre-treatment and paint application: Temperature of substrate should be at a temperature of min. 3oC above dew point of the air in the vicinity


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Measuring ambient conditions • Electronic multi-function gauge • Electronic humidity gauge • Sling Psychrometer (Sling Hygrometer, “Dry and Wet bulb”) • (Hair hygrometer, not suitable!) • Dew point calculator • Steel thermometer


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Electronic humidity gauge • Needs frequent calibration. • Dependent on batteries • Multi-function models will measure air temperature, surface temperature, relative humidity, and calculate dew point and 'T (*) (*)

difference between dew point and surface temperature)


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Sling Psycrometer A dry and a wet thermometer put together in one unit. • The difference in the measured temperatures indicate the amount of humidity: • Large difference = low RH% • Small or none = high RH%


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ISO 8502-4 Measuring the relative humidity • Photo: Sling Psychrometer for measuring: - Dry temperature - Wet temperature • Measure the temperature in the vicinity. • Calculate the relative humidity • Use together with dew point calculator


Cd-4932-88

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ISO 8502-4 Dew point calculator • For calculation of dew point • To be used together with surface temperature thermometer and sling psychrometer. • Use this frequently during pre-treatment, application and drying of the paint. • Recommendation: Every 6 hours and when weather conditions are changing


Cd-4932-89

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ISO 8502 - 4 Humidity Guidance on the estimation of the probability of condensation prior to paint application 1. 2. 3. 4.

Air temperature % Relative humidity Steel temperature Dew point

Steel temperature min. 3 oC above the dew point


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thermometer • Electronic instrument for measuring the steel temperature • Other types of thermometers are also available. E.g. magnetic thermometers


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IX - diagram for determination of relative humidity (%RH)

Example: Measured values Air dry temp.: Wet bulb temp.:

14.0 °C 10.5 °C

Read Relative humidity: 65 % Dew point: 7 °C Air-water content: 6.3 g/kg


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Using the IX diagram • Draw a horizontal line at the appropriate dry bulb temperature • Draw a 45o line at the appropriate wet bulb temperature • At the intersection you will determine the RH • Draw a vertical line from the intersection point all the way to the 100% line • This is the dew point


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Ambient conditions, general Painting should not be started when: • The relative humidity exceeds 85% (unless the paint manufacturer consider it safe) • The substrate temperature is less than 3o C above the dew point


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Ways to control the climatic conditions

• Dehumidifier, reduces the actual humidity in the surroundings • Heather, heat the objects (surfaces) prior to application in order to reduce the danger for condensation. • Forced ventilation is a must in order to secure a proper evaporation of the solvents from the paint film.


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Ambient Temperature and Steel Temperature Is the temperature important ? YES ! • Ambient temperature will influence: – shelf life (storage) – potlife (two-pack paints: time between mixing and no longer useable) – induction time (prereaction before application) – viscosity/sprayability

• Steel temperature will affect: – probability for condensation – solvent evaporation – speed of cure (drying time) – degree of cure – recoating interval – service life of the coating

– Steel Temperature


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Ventilation. Good practice

Solvent vapours are heavier than air

Suction from lowest points in enclosed areas


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Ventilation. Bad practice Using an inlet fan

Using an exhaust fan

Air flow Solvent vapour

Air flow Solvent vapour

Inlet fan, air through pipe Solvent vapours

Air out

Air in


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Control of relative humidity and temperature • Check the atmospheric conditions before the work starts • If required: Install proper ventilation • Check the atmospheric conditions while the work is ongoing and during drying / curing

Paint technology: Humidity_control1


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Temperature and humidity of air used for drying. • • • • •

Supply of heated air immediately after application may lead to skin drying and entrapped solvents Cold air will keep the film open longer and ensure proper evaporation Avoid high air temperature (especially epoxy) High humidity will slow down the drying time Exhaust from heating equipment using propane or paraffin oil contain water and Carbon dioxide and may create Amine sweating


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Blisters inside a tank due to poor ventilation. Entrapped solvents • Blisters sometime contain water • Rust will not be formed inside intact blisters, due to the high pH • As soon as the blisters break corrosion will start • Use cathodic protection to back up the paint system.


Cd-4934-68

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