Saturday, January 15, 2011

TOXIC GAS DETECTOR TUBES ON CHEMICAL TANKERS -- CAPT AJIT VADAKAYIL

KITAGAWA / DRAEGER TUBES


Toxic gas detector tubes should always be used when the cargo vapor presents a serious inhalation hazard.

Our ships are equipped with Draeger and Kitagawa toxic gas detectors. They give an accurate indication of vapor concentration IRRESPECTIVE OF OXYGEN CONTENT. 

However the presence of second gas may effect the readings and give inaccurate readings.

KITAGAWA:
    
If your ship is delievered in Japan , it is bound to have a Kitagawa set.  It has a temperature correction,, which Draeger does NOT have, and hence is superior .

You need to pull up the handle of pump only once.

There are 2 types of tubes:--
1)direct read off in PPM.
2)requires to be superimposed on a chart which gives concentration against a temperature table. This chart comes with the tubes. Only some tubes require temperature correction.

Break off the tube. Insert tip with red dot into pump inlet. Pull out the pump handle. To read off the
temperature chart synchronise the top and bottom lines of the tube on the top and bottom lines of the chart. Read off the colour change at the top level of powder in the tube.

DRAEGER:
      
You may have to give strokes from one to hundred.

First do a leak test. Insert an unopened tube and squeeze pump. After 15 minutes if the end of the stroke indicator has not appeared, it is leak proof. 

Press reset counter to zero. 

  
Point the pump down wards and break the tube at both ends. Insert the tube with the arrow pointing towards the pump. 

Check on the Draeger tube how many strokes are required and give only that many. Remove the tube and read off. Flush the pump with a few strokes of clean air.

Measuring range of tubes are different from 0.1 to 1000 ppm or even higher .  Select tubes of the range your require.

There are tubes with direct indicators. There are other tubes long term measurement upto 8 hours (TWA). There are tubes for inorganic chemicals like Hydrogen Sulphide, Ammonia, Hydrocyanic acid etc. Tubes are affected by humidity.

Adequate Drager tubes for the product to be carried must be ordered as soon as possible when cargo orders are received, if not already on board. 

Sufficient numbers of all tubes must be in date and ready for use as required.

When deciding upon a “sufficient” number, the following criteria should be borne in mind:-- The number of tanks being utilised to carry the product.

The size of the tanks, bearing in mind that two or three sampling points may be required to ensure a representative sample is taken.

The fact that each tank may have to be tested at least twice before a safe reading is obtained.

The Draeger log should be used to maintain an accurate and up to date list of the Draeger tubes and accessories on board.

Keep Draeger tubes in a cool place

Explosimeter give s total amount of all flammable gases. H2S gas  detector tube is required on board.  Some gases are so acutely toxic that no exceeding of TLV is acceptable. They have been assigned with the letter C for ceiling

Example of a Draeger Benzene tube instruction sheet:--

Benzene Application Range

Standard Measuring Range: 5 to 40 ppm
Number of Strokes n: 15 to 2
Time for Measurement: max. 3 min
Standard Deviation: ± 30 %
Colour Change: white —> red brown
Ambient Operating Conditions
Temperature: 0 to 40 °C
Absolute Humidity: max. 50 mg H2O / L
Reaction Principle
2 C6H6 + HCHO —> C6H5-CH2-C6H5 + H2O
C6H5-CH2-C6H5 + H2SO4 —> p-quinoid compound
Cross Sensivity
Other aromatics (toluene, xylene) are retained in the pre-layer causing a
reddish brown discoloration. If the toluene or xylene concentrations are
too high the entire pre-layer up to the indicating layer is discoloured making
a benzene measurement impossible. Petroleum hydrocarbons, alcohols
and esters do not affect the indication.


Example of a Draeger Phenol tube instruction sheet:--

Phenol Application Range
Standard Measuring Range: 1 to 20 ppm
Number of Strokes n: 20
Time for Measurement: app. 5 min
Standard Deviation: ± 10 to 15 %
Colour Change: yellow —> brown grey
Ambient Operating Conditions
Temperature: 10 to 30 °C
Absolute Humidity: 1 to 18 mg H2O / L
Reaction Principle
C6H5OH + Ce(SO4)2 + H2SO4 —> brown grey reaction product
Cross Sensivity
Cresols are alse indicated, but with different sensitivities. To determine
m-cresol multiply the indication by 0.8.
Benzene, toluene and other aromatics without the hydroxyl group
are not indicated.
Aliphatic hydrocarbons and alcohols are not indicated.
Additional Information
At a temperature of 0 °C the resulting indication must be multiplied
by 1.3, at a temperature of 40 °C it must be multiplied by 0.8.




The  Vervey’s  tankcleaning  guide  gives  the  VP/TLV  ratio.---anything  more  than  one  is  dangerous requiring the use of draeger tubes on deck. Epichlorohydrin is 17, Methanol is 0.46 (200/92)
   
-------CAPT AJIT VADAKAYIL ( 28 years in command )

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