Wall-Wash tests on Chemical Tankers
Wall Wash Test Procedures on Chemical Tankers
Preparation
-Reagent-
Pure Methanol (500 ml/bottle)
Pure Water (DI Water)
2% Silver Nitrate Solution (500 ml / bottle)
20% Nitric Acid Solution (500 ml / bottle)
10 ppm (0.01 mg/ml) Chloride Standard Solution (500 ml / bottle)
0.02% Potassium Permanganate solution (100 mg in 500 ml of DI Water) – to be stored
in a dark bottle in a dark bottle and cool temperature.
Concentrated Hydrochloric Acid (500 ml / bottle)
-Instrument-
Funnel x 1
Plastic Bottles (500 ml x 2 bottles for collecting wall wash samples)
Nessler Tube (100 ml x 6 nos.)
Pipette (5 ml x 4 nos. for Nitric Acid / Silver Nitrate/Chloride Solution / KmnO
4
A petridish or Shalle x 2 nos. (for testing Non-volatile matters)
Disposable Vinyl gloves (200 sheets / case)
Disposable Shoe covers
A.P.H.A. Color Standard (No. 3, 5, 8 & 10) for color test.
Black colored plate (for testing turbidity of Hydrocarbon / Chloride)
White colored Plate (for testing of turbidity of color)
Flash Light.
Wall Wash Sample Collection
Before entering into the tank for sampling, ensure that the tank has dried up completely
and has no humidity on the wall and gas freed.
Check for toxic gas /combustible gas / oxygen prior entry.
One man standby – in attendance
Carry the test kit for WWT in a closed bag or wooden box and keeping in mind not to
drop the same in the tank while going into and coming out of tank.
Put the shoe covers and wear the vinyl gloves.
Wash the equipment to be used for WW sample collection by methanol.
A sample of test methanol is sprayed on the bulkhead and collected in a sample bottle
by using half cut funnel.
Take sample from four parts of forward / aft / port / Stbd bulkheads and in the area of 1
meter in height x 30 cm in width at the position of 1.5 m from the bottom by methanol
wash on the wall.
The necessary quantity of sample from each tank is about 200 to 250 ml (50 ml for
color, 50 ml for Hydrocarbon, 50 ml for Chlorides & 50 ml for PTT).
Sample Analysis
Colour
This is represented by APHA from 1 to 30 where >APHA5 is the most stringent. The
number is determined in the lab by passing light through the sample. Most ships do not
have this equipment but if you hold the sample against a white sheet of paper and it
appears absolutely colorless you can be quite sure that the number is less than 5. For
this fill up sample about 50 ml and add 50 ml methanol into a 100 ml Nessler Tube.
Suspended matter
Switch of all light and hold torchlight at the bottom of the tube containing the sample.
Any suspended matter will be clearly visible.
Hydrocarbon Test
Hydrocarbons together with water will form a milky solution. To 50 ml of wall wash
methanol add 50 ml of DI water. The mixture is shaken and allowed to stand for 20
minutes. Initially turbidity will appear and the same will reduce in a few seconds.
Compare this to a nessler tube containing 50 ml of pure distilled water and 50 ml of
pure methanol (as a standard solution for comparison) on the black plate or
background. Switch off the lights and using a flashlight shine the beam first from one
side and then from other. Look into the tubes from the top for turbidity. If the sample
appears cloudy or not completely clear then hydrocarbons are too high and the tank
needs to be washed with methanol.
NOTE :- The proportion of methanol to distilled water for testing will depend on the
terminal where the product is to be loaded. Includes, where available, the required wall
wash specifications of terminals. If a terminal's required specification reads
"hydrocarbons pass (10/90)" this means that the surveyor conducting the wall-wash
test will use a mixture of 10ml of methanol to 90ml of distilled water. The responsible
officer conducting the test onboard his vessel may then use the same proportion of
methanol to distilled water, however the proportions of 30/70 will provide a more
accurate picture of the tank wall.
Chloride Test
Normally 10ppm standard chloride solutions are available on board:
To make a 2 ppm standard solution add the following:
Pure methanol 50 cc
10ppm standard chloride solution 8 cc
Distilled water 38 cc
20% nitric acid solution 2 cc
2% silver nitrate solution 2 cc
To make a 0.5 ppm standard solution add the following:
Pure methanol 50 cc
Distilled water 44 cc
20% nitric acid solution 2 cc
2% silver nitrate solution 2 cc
The principle of the test is that chloride together with a silver-nitrate solution gives a milky, cloudy solution.
In a clean, graduated Nessler-tube transfer 30ml of the methanol and top up to 100ml with certified distilled water, add 5 drops of nitric acid * and then 5 drops of silver-nitrate solution and mix the contents thoroughly.
* Before adding the silver nitrate, take a reading as well. If there is a reaction between the nitric acid and wallwash sample there is something else left on the coating which are not chlorides and will reject the tank also. Then add the silver nitrate and take a reading again. The difference in readings before and after is due to the actual chlorides. Prepare a standard solution of 30ml certified methanol and 70ml certified distilled water with 5 drops of silver-nitrate solution. In a dark room, and preferably against a dark background shine a penlight through the contents and compare against the standard solution. If turbidity in the test Nessler-tube is observed then chlorides are present.
Permanganate time test(PTT)
Make a standard solution with 0.1 gm of potassium permanganate in 500 ml of water.
This method serves as a means of detecting the presence of impurities in alcohols or ketones that reduce
potassium permanganate and is applicable to Methanol, Propanol, Butanol, Methyl Ethyl Ketone and Methyl Isobutyl Ketone.
Substances reacting with potassium permanganate in neutral solutions reduce it to manganese dioxide,
which colours the solution yellow. In the PTT the time required for the colour of the test solution to change to that of a standard solution is measured. The colour of the test solution changes from pink-orange to yellow-pink-orange. For shipboard testing the time for the colour to differ from a test solution measured against a standard solution of certified methanol is sufficient.
The equipment required for PTT are as
follows:-1 Cylinders, glass-stoppered, 50ml.
2 Constant temperature bath, capable of maintaining a temperature of 15.0 +/- 0.5 degrees C or 25.0 +/-
0.5 degrees C. If a constant temperature bath is not available then a bucket of water cooled with ice is an acceptable shipboard substitute.
3 Pipette, capable of delivering 2ml of solution.
4 Clock or stopwatch.
5 Potassium Permanganate Solution (0.100 g of KMnO4 per 500ml of water).
Fill a 50ml glass-stoppered cylinder beyond the mark with certified methanol and place it in the same bath.
When the samples have reached the bath temperature (about 5 minutes) add 2ml of potassium permanganate solution with a pipette, invert once to mix the contents and return to the bath.
Keep the samples in the dark, and check about every 10 minutes.
Determine the time from the addition of the KMnO4 until the colour differs (fades) from the standard of certified methanol.
Violet Light Violet Orange Color Light Yellow
When the test is finished wash the tubes with tap water and fill with concentrated hydrochloric acid. Use protective gloves and goggles for this and preferably do it in the open.
Non-Volatile Matter
NVM can only be checked in a lab. A small portion of the test sample is placed in a petridish or shalle and allowed to evaporate by applying heat to it. The weight of the dish when the entire methanol has evaporated will determine the quantity of NVM in sample. It is preferable to carry out this test with Acetone rather then methanol i.e. the sample should be taken using Acetone.
Acid Wash Test
Pour 7ml concentrated laboratory grade Sulphuric acid into a test tube. Add 21ml of the wallwash sample (toluene). Shake vigorously for 30 seconds. Place in the dark for 15 minutes. Look for colour (haze) in the Sulphuric acid layer. Compare with colour standard if one is available on board. When toluene is used as a test medium, acceptable haze/colour of the acid is 2 on the standard scale.
Wallwash test precautions and some causes of failure
Always use disposable plastic gloves when wall washing - the human hand contains more salt than normal requirements and is a common cause of chloride contamination.
Wear clean clothes and do not allow working gloves in the tank, avoid all contact of clothing with the tank, wear protective disposable shoe covers.
Use a clean bucket on a clean line for transferring test equipment into the tank.
Wash the funnel and sample bottles with certified methanol prior to collecting the sample.
Always test the distilled water and certified methanol prior to use for testing and always run a standard against the test sample.
The glue used for the seals on some types of sample bottles have been known to cause sample contamination.
When using acids for washing wear the proper protective equipment.
Take your time, sampling in a hurry or rushing through the test procedures cause failure.
If the sample passes all tests, be suspicious, and test again.
Never test a wet tank, the tank wall must be dry.
Allowing a tank to "rest" after prolonged washing (especially hot washing) can often turn a failed tank into a passed tank.
Keep a record of your wallwash results, especially if testing multiple tanks. Consulting this record over days of tank cleaning may provide a useful indication of which washing procedure is working best.
Removal of Hydrocarbon
If HC < 2 ppm Spray Methanol in the tank
If 2 ppm < HC < 3 ppm
Toluene Steaming (Only in inert atmosphere) for 3
hours & Methanol Spray
If 3 ppm < HC < 5 ppm Toluene Wash Steaming (within LEL) Methanol spray
If HC > 5 ppm Re-cleaning with detergent or solvent depending upon tank coating
Removal of Chloride
If Cl < 2 ppm Steaming or Spray DI Water and then Methanol in the tank If 2 ppm < Cl < 5 ppm Methanol Steaming (Within LEL)
If Cl > 5 ppm Hand wash by Fresh Water Steaming Methanol Spray
Removal of Oxide Residue (PTT)
If 30 min < PPT < 50 min Methanol Spray
If 15 min < PPT < 30 min Methanol Steaming Methanol Spray
If PPT < 15 min Re-tank cleaning with detergent or solvent
Removal of Color
No. 5 < APHA < No. 10
Treatment with Rust Remover in case of rust &
Methanol spray
No. 10 < APHA < No. 15 Treatment with Rust Remover in case of rust &Steaming & Methanol Spray
APHA > No. 15 Treatment with Rust Remover in case of rust &Hot water butterworth Methanol Spray
Removal of NVM Residue
Almost Nil NVM Methanol Spray
Normal Toluene wash Methanol wash