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Wall wash test and wash water analysis are two different methods for Tank Contamination Test. With the newly invented equipment “Spectrophotometer” we can analyze the water during Cargo tank washings. And the results record will be inspected by Cargo Surveyor (third party) prior to loading the next cargo. Nowadays, it is prevalent in chemical trades due to its very easiness, less time consumption, and no man entry into cargo tanks.
Prior to loading pure/high-grade clean chemicals, each tanks are inspected by a third party to confirm its cleanliness, which is widely known as WWT. Normally we seafarers do these tests during tank cleaning operations, so that we can estimate/ confirm tanks’ cleanliness before NORT.
As you are reading this article I assume you know how you will clean the tanks. So, I will not discuss Tank cleaning procedures today.
Let’s dig into WWT / Contamination test
Wall Wash test results depend on previous cargo miscibility/ dissolve capacity with water and tank coatings. This is not a single test. We do several tests to confirm tanks readiness, these are known as follows:
- Apha color test
- Hydrocarbons (HC) test
- Permanganate time test (PTT)
- Chlorides (CL) test
- pH–tests
- Suspended matter, Non Volatile Matter (NVM)
- Inhibitor residues
- Odour of tank
Preparation for the WWT
To do WWT, we must prepare the instruments/ crew entry into the tank and laboratory. The following are the instruments required for WWT:
| Equipment | Item |
|---|---|
| Nessler tube | For putting samples in for analysis. |
| Pipette | For adding reagents. |
| 500ml Sample bottle | For collecting wall wash sample |
| Washing bottle | For squirting pure methanol on the wall. |
| Funnel | To collect wall wash sample. |
| Reagent | Pure methanol (Use for WWT, making blank samples, and washing the equipment The highest quality of methanol should be used.) |
| Pure DI water | For Hydrocarbon test, Chloride test, and washing the equipment. Great care should be taken for the quality of the pure water, particularly for chloride contents in it as it sometimes shows high contents. |
| 20% Nitric Acid solution (HNO3) | As a reagent for Chloride test and for washing the equipment. It can also be used for removing dirt from the Nessler tube. |
| 2% Silver Nitrate solution (AgNO3) | As a reagent for Chloride test. |
| 0.02% Potassium Permanganate solution (KMnO4) | As a reagent for the Permanganate Time Test, the color of the reagent is purple. This reagent will be affected to deteriorate by the time passed and the temperature where the reagent is kept. Replacement at least every 3 months and kept in a refrigerator is recommended. |
| 10ppm Chloride solution | For making chloride standard sample. |
Now, let me talk about crew readiness. Initially, we must make sure the crew is entry into the tank with clean PPE (of course, make sure tank entry requirements are fulfilled), he must use a shoe cover, so that no dirt will fall into the tank. FW rinse area around tank entrance to remove salt from sticking to shoe cover.
The crew proceeding to collect samples must bring a 500ml Sample bottle, Lab grade pure methanol, and a funnel. For your own satisfaction, you may take samples from each bulkhead, corner, and even from pump stack pipings.
Sample collecting procedure:
- Squirt pure methanol by using a washing bottle on the wall at the highest practical point above the floor in a stream of about four inches wide.
- Allow the methanol to run down the wall for approximately 1m and begin collecting it with the funnel.
- Sampling from 2 places on each bulkhead.
- Sampling from the place which is near the bottom.
- Sampling from the blind area of the Butterworth machine.
- Pay attention not to pick any dirt and sweat into the sample.
- Do not take sample from wet parts.
- Do not repeat taking sample from the same place to gain more sample quantity.
But with the Surveyors, Wallwash is taken from 4 parts of all 4 bulkheads—but not from corners. WWT surveyor must not wash more than six feet in height.
Normally floor is not tested for contamination. If you require to do so, check surveyor is not wiping the filter paper. The standard procedure is to placing a filter paper on a spot wetted by Lab grade Methanol. For Lab test, put this paper inside a sample bottle and filled with Methanol (50cc).
Now, time to go for WWT in the Lab. Let’s dig in….
APHA COLOR TEST |
Initially, we do the Apha color test, it’s a visual assessment of the wall wash sample. Visual assessment is highly subjective due to background, lighting, personnel experience etc.
As per the shipper’s voyage order, we used to receive the Apha color result requirement. Normally ranges about Apha5, apha10, apha20 etc. We do the visual comparison with the standard. Standard Apha color is available on the shipping market.
Test Procedure: Pour standard Apha color in a separate Nessler tube (instead of comparing in the bottle), tank sample is also to be poured in a tube. Now adjust the background and lighting, and do Visual assessment. Background and ambient lighting environments are highly effective for color assessment.
HYDROCARBON (HC) TEST |
Immediately after the Apha color test, we used to prepare for the Hydrocarbon test. This is actually a Miscibility test. HC is insoluble in water. So, we mix with wall wash sample with water to check if there is any HC, it will not dissolve and will remain on top of the mixture. Before DI spray/ FW rinsing, we usually do this test, so that we make sure HC. After the DI spray, if HC fails, DI/ FW were of no use/wasted.
Procedures: Pour 25cc wall wash sample and 25cc DI water (Lab grade) and shake it vigorously for 2 mins and then keep for some time (if possible about 20 minutes). Now turn off the light, and make the room dark. Use pen light from the bottom of the sample.
Result: If there is no turbidity, means tanks passed, and if there is foam layer on top test has failed. The foam layer means you have very amount of HC, and might be re-circulated chemicals remain in the tank. To pass it must look like distilled water. If you shine a torch beam from the side, it must not look blue. No beam means HC free. Test result photos for reference.
Onboard Test Result
You may watch the video (live test onboard):
PERMANGANATE(KMNO4) TIME TEST (PTT) |
PTT is one of the other usual tests we used to do prior to loading clean chemicals. Immediately after HC, we do this test. PTT test detects all inhibited chemicals. To do PTT we need KMNO4 (Potassium permanganate) Solution, nowadays it is readily available in the tanker market with 1 year of validity. Keep the solutions in a dark area/brown bottle.
Procedure: Prepare a white color 15C water bath. Make a standard sample with 48cc lab methanol + 2cc (or 5 drops) KMNO4 solution. Now, mark the Nessler tube as per the Cargo tank number. Pour 48cc wall wash sample + 2cc (or 5 drops) KMNO4 Solution. Keep in the bath. Wait as per the shipper’s requirement. Normally 15 min to 30min.
Result: Initially, after mixing with KMNO4, color should be pinkish purple. If the color faded quickly and becomes straw yellow, means PTT fails. If multiple tanks, all tanks could be tested at a time, so that we can assume which tanks are much better/bad. Anyways, keep the sample inside the bath. Wait as per the shipper’s required time, if the color doesn’t change, means pass.
You may watch the video (live test onboard):
| CHLORIDE(CL) TEST |
The Chloride test is a common test on Chemical tankers, it is used to judge the presence of chlorides on bulkheads, etc. If you are intended to load high-grade clean chemicals like Methanol, or MEG, try to avoid any salty air (keep the tank dome crack open only and keep the barrier on the windward side). Normal sea air is with higher levels of chloride. Even if required tell the Master to alter course during ventilation and wall wash.
Chloride test in Lab: To do this clean the Nessler tube with pure methanol, and wear gloves. Mix 25cc wall wash sample, add 1/2 cc silver nitrate( strength 10%), add 2cc nitric acid ( strength 20%), and finally add pure methanol to make 50 cc total.
Shake and stand for 5 minutes. Compare milkiness with a standard comparator. More milkiness means higher level of chloride.
Actually, there is no firm mixture worldwide. It depends on your fleet/ charterer. Some owners ask to use DI water instead of pure methanol for the remaining volume. Remember, we use Nitric acid to dissolve the mixture quickly, so always fill with nitric acid after AGNO3 (Silver Nitrate) only.
You may watch the video (live test onboard):
Hope you are now confident about the subject topics. Some other tests like Odour, suspended matter, NVM etc are rarely used.
Thanks for your interest on Marine Legends.
Bon voyage.
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