Passivation of cargo tanks on chemical tanker

 

We as a constellation marine provides an extraordinary guidance and supervision on passivation Tank cleaning of a chemical tanker which is very efficient and economical.

There are several operations we had carried out to maximize the performance of passivation on chemical tanker. The new advanced technologies in tank cleaning have raised the standards in marine areas.

Passivation is one of the most important post-fabrication methods to enhance the inherent corrosion resistance of the stainless alloy from which the workpiece was produced. It is not a scale removal treatment, nor is it like a coat of paint.

The freshly machined or polished steel surface automatically acquires this oxide film from exposure to oxygen in atmosphere.

Practically however, contaminants such as shop dirt or particles of iron from cutting tools may be transferred to the surface of the stainless-steel parts during machining. If not removed, these foreign particles can reduce effectiveness of the original protective film.

The below mentioned two-step procedure can provide the best possible corrosion resistance:
1. Basic cleaning but it’s an overlooked procedure.
2. An acid bath, or passivating treatment.
On chemical tanker generally, passivation is done after discharging of wet phosphoric acid of fertilizer grade, hot caustic soda, Sulphuric acid and after discharging palm oil fatty acid, the prolonged washing with from hot salt water.

Passivation carried out by Nitric acid in an expensive affair as far as possible we should carry out passivation by Nitric acid only.

Citric acid passivation is a second-best option after nitric acid. The biggest disadvantage of Citric acid is that it’s not an oxidizing agent. Nitric acid is FOSFA banned. Citric acid is eco friendly it efficiently removes free iron from the surface of stainless steel. It can be used in lesser concentrations then nitric acid.

Initial passivation readings should be taken reach to conclusion that COT is active or passive. If COT is found to be active then order the amount of nitric acid, recirculation material and fresh water is required.

Pls find below recommended procedure but not limited to passivation done by Nitric acid.
Passivation procedure for Cargo tank capacity of 1000-1500 Cum.

A) Take 2.0 m3 of water in one tank, prepare all connection for recirculation’s. You can use tank cleaning machine cycle or calculate using hose flow through i.e fill up 200liter drum with hose & note the time taken to fill up 200liters, you can use the same calculation to achieve above qty or can use fixed machine cycle in prewash mode. Once FW is added in the tank, carry out leak test. FW to be heated to 40C or warm water to be added.
B) Always Add Acid to water (Tank Cleaning Machine Single pitch 1.5deg) 440 liters, (Nitric acid 68%)
purity) ensure PPE LEVEL 3 with acid resistant face hood (full face coverage) is worn, you can use Butterworth.
pocket or tank dome use acid resistant hose if using Wilden pumps. We are making the solution strength about 15%. While planning for passivation start with small tank & then go on bigger tank with spiking of 35litres nitric with 160litres of FW.
Presuming 4 tanks 4P/4S/6P/6S Nitric Acid Passivation 4P first tank, then spiking 4S (35 liter nitric+160 FW) followed with 6P (35 liters nitric+160 liters FW),
6S (35 liters +160 FW).
C) Test the recirculation connection prior adding acid to water by running the pump & repair/change gasket if found leaking by ensuring personnel are wearing PPE.
D) Start the cargo pump slowly, check for leak. If no leak increases the pump pressure to start recirculation, remove all personnel away from the location & monitor for leakage from catwalk. In case of any leakage, stop the pump, wash thoroughly with water prior reconnecting. Use 2 fix machine with 1.5 deg pitch & then undertake recirculation.
E) Towards last stage flush drop line, drains manifold, venting only by PV valve allow vapors to come out from PV valve only.
F) Recirculation timing to commence only when temp has attained, recirculate using 1.5deg pitch only.
Heating coil are shut once temperature of Solution is attained. Undertake recirculation for 4 hrs Once completed stop the pump close the connection of Octopus & transfer this solution into other set of tanks using fixed piping only. While transferring liquid ensure the solution fills the lines completely which can be achieved by keeping Drop valve of another tank closed.
When Solution is received on second tank. Carry out the same procedure by adding another 35 liters and 165 liters of FW in next pair of tanks
N) After connection to be prepared for FW rinsing for 40min or more, check FW rinsing until ph=7,
after 10 min of FW rinse (Cold) only start flushing lines pumps, drain. Always start pump after 10 min since it allows dilution below 10%.
O) Ventilation by Fixed gas free fan to commence on first tank, venting by PV valve, purge pipe or By pass of Vacuum valve.

Usually it takes time to shift hose from one tank to other due to mist of nitric vapors hence include time
gap of about 1hrs which occurs normally due arrangement/shifting. During FW rinse tank cleaning
machine to be in Prewash mode.

Personnel safety is very important & hence suggesting advising crew and carry out Tool box meeting, discussing hazards of nitric hazards exposure.
There should be ventilation for 2 days more the days of ventilation more will be passive layer build up after 1 day of ventilation use the hot air in the blower this will help.

Passivation reading is recommended to be carried out by stenchecker -II, passivation reading meter in Volts. Please follow the instructions as per manual and take average readings of all bulkheads and passivation reports should be prepared and sent to office for review.

By

Capt. Manish Kumar – Master mariner Principal Consultant (chemical and oil/products cargoes) Constellation marine services.

(Prime area of work – UAE ports, Fujairah, Abu dhabi, Ras Al Khaima, Mina Sagar, Sharjah, Ajman, Dubai, Hamriyah, Jabel Ali, Bahrain, Kuwait, Qatar (Mina Hamad) (mesaieed), Egypt (Alexandria) or fuel, Sohar, Muscat, Salalah).

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