Ambient Condition
-
Upload
longlong3003 -
Category
Documents
-
view
154 -
download
2
Transcript of Ambient Condition
-
Frosio course handout: Section 07 Ambient conditions
1
Frosio 07 Ambient conditions slide # 1
AMBIENT
CONDITIONS
Humidity
Air Temperature
Substrate Temperature
Frosio 07 Ambient conditions slide # 2
Parameters influencing the corrosion speed. Atmospheric corrosion
x Humidityx Temperaturex Concentration of saltsx Amount of air pollution,
including acid rain, soot and dust particles
-
Frosio course handout: Section 07 Ambient conditions
2
Frosio 07 Ambient conditions slide # 3
Atmospheric corrosion rate depends on humidity
Relative Humidity, %
Corrosion rate
0 20 40 60 80 100
Frosio 07 Ambient conditions slide # 4
Surface temperature of the
structure must be minimum
3 C above the dew point
of the surrounding atmosphere
Atmospheric conditions.Atmospheric conditions.
Requirement during blasting and paintingRequirement during blasting and painting
-
Frosio course handout: Section 07 Ambient conditions
3
Frosio 07 Ambient conditions slide # 5
B-24/2-4
4847-693
ISO 8502 - 4Estimating probability of condensation
Cold liquid inside hull and warm air outside causes condensation
See how frames inside keeps the cold longer
Frosio 07 Ambient conditions slide # 6
Why Temperature & Humidity are important
Frozen overnight condensation, follows
the frames of the ship
Condensation follows the contours
of the double-bottom tank
-
Frosio course handout: Section 07 Ambient conditions
4
Frosio 07 Ambient conditions slide # 7
Purpose of climatic evaluation
Evaluate the likelihood of the moisture in the air to form
condensation on a substrate
Frosio 07 Ambient conditions slide # 8
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
-
Frosio course handout: Section 07 Ambient conditions
5
Frosio 07 Ambient conditions slide # 9
Absolute humidity
DEFINITION:
The maximum content of water vapour in the air at a given temperature
The content will vary with temperature
Frosio 07 Ambient conditions slide # 10
At 8o C, the
maximum moisture
content in the air is
8.28 g/m3At 20o C, the
maximum moisture
content in the air is
17.31 g/m3
-
Frosio course handout: Section 07 Ambient conditions
6
Frosio 07 Ambient conditions slide # 11
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?
Frosio 07 Ambient conditions slide # 12
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
-
Frosio course handout: Section 07 Ambient conditions
7
Frosio 07 Ambient conditions slide # 13
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. ?
15g x 100%
17.31g
R.H. = = 86.65%
Frosio 07 Ambient conditions slide # 14
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.
-
Frosio course handout: Section 07 Ambient conditions
8
Frosio 07 Ambient conditions slide # 15
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
Frosio 07 Ambient conditions slide # 16
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!
-
Frosio course handout: Section 07 Ambient conditions
9
Frosio 07 Ambient conditions slide # 17
Temperature of substrate should be at a temperature of
min. 3oC
above dew point of the air in the vicinity
Main rule for pre-treatment and paint application:
Frosio 07 Ambient conditions slide # 18
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
-
Frosio course handout: Section 07 Ambient conditions
10
Frosio 07 Ambient conditions slide # 19
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)
Electronic humidity gauge
Frosio 07 Ambient conditions slide # 20
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%
Sling Psycrometer
-
Frosio course handout: Section 07 Ambient conditions
11
Frosio 07 Ambient conditions slide # 21
ISO 8502-4Measuring 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
Frosio 07 Ambient conditions slide # 22
ISO 8502-4Dew 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
-
Frosio course handout: Section 07 Ambient conditions
12
Frosio 07 Ambient conditions slide # 23
1. Air temperature
2. % Relative humidity
3. Steel temperature
4. Dew point
ISO 8502 ISO 8502 -- 44
HumidityHumidity
Guidance on the estimation of the probability
of condensation prior to paint application
Steel temperature min. 3 oC above the dew point
Frosio 07 Ambient conditions slide # 24
Contact thermometer
Electronic instrument for measuring the steel temperature
Other types of thermometers are also available. E.g. magnetic thermometers
-
Frosio course handout: Section 07 Ambient conditions
13
Frosio 07 Ambient conditions slide # 25
IX - diagram for determination of relative humidity (%RH)
Example:Measured valuesAir dry temp.: 14.0 CWet bulb temp.: 10.5 C
ReadRelative humidity: 65 %Dew point: 7 CAir-water content: 6.3 g/kg
Frosio 07 Ambient conditions slide # 26
-
Frosio course handout: Section 07 Ambient conditions
14
Frosio 07 Ambient conditions slide # 27
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
Frosio 07 Ambient conditions slide # 28
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
-
Frosio course handout: Section 07 Ambient conditions
15
Frosio 07 Ambient conditions slide # 29
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.
Frosio 07 Ambient conditions slide # 30
Ambient temperature will influence: shelf life (storage)
potlife (two-pack paints: time between mixing and no longer useable)
induction time (pre-reaction before application)
viscosity/sprayability
Steel Temperature
Ambient Temperature and Steel Temperature
Is the temperature important ? YES !
Steel temperature will affect: probability for
condensation
solvent evaporation
speed of cure (drying time)
degree of cure
recoating interval
service life of the coating
-
Frosio course handout: Section 07 Ambient conditions
16
Frosio 07 Ambient conditions slide # 31
Ventilation. Good practice
Solvent vapours
are heavier
than air
Suction from
lowest points
in enclosed areas
Frosio 07 Ambient conditions slide # 32
Ventilation. Bad practice
Solvent vapour Solvent vapour
Using an inlet fan Using an exhaust fan
Air flow Air flow
Air out
Solvent
vapoursAir in
Inlet fan, air through pipe
-
Frosio course handout: Section 07 Ambient conditions
17
Frosio 07 Ambient conditions slide # 33
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
Frosio 07 Ambient conditions slide # 34
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
-
Frosio course handout: Section 07 Ambient conditions
18
Frosio 07 Ambient conditions slide # 35
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