PHOSPHORIC ACID ON CHEMICAL TANKERS
Company policy requires ships SS tanks to be passivated after carriage of WET Phosphoric acid.
The corrosion to austenitic SS of SUS 316L and 316LN occurring due to carriage of wet process phosphoric acid is a very complex phenomenon. White Phosphoric acid (pure food grade) is not corrosive to stainless steel, however, the Wet PAC (fertilizer grade—green PAC and BK PAC ) is perhaps the most aggressive and difficult of all chemicals transported in a SS chemical tanker. The corrosion is affected by various impurities normally present in wet process acid. Only 317LN can be said to resist this acid without too much damage and that too if the loading temperatures are within limits..
Names: Green phosphoric acid, Bk phosphoric acid, Metaphosphoric acid; Orthophosphoric acid. MARPOL / Z
The corrosivity of crude phosphoric acid depends on the manufacturing process—wet or dry. In the wet process the Chlorides and Fluorides determine the corrosivity as they are corrosion accelerators.. The amount of Flourides and Chlorides differs from country to country where the raw stock comes from. Wet phosphoric acid must be constantly recirc during the voyage to prevents sediments from depositing.
Even if the ship is carring the same product again the tanks must be cleaned and desedimentised. The green phosphoric acid from
Ashdod does not hold any sediments and hence need not be recirc, however the rest of the worlds fertilizer grade has lot of abrasive sediments. Do not load Phosphoric acid ( wet, fertilizer grade ) in tanks without diffusers. Israel
Reducing acids and very high chloride solutions are unsuitable for carriage in SS tanks. During the production of wet Phosphoric acid, natural Phosphate rock is attacked and dissolved in Sulphuric acid. The Phosphate rock contains impurities that are carried through the process into the product acid. The surplus reducing Sulphuric acid and the impurities cause the corrosion in SS
H2SO4/ 4% ( corrosion accelerator )
SiO2/ 0.4% ( corrosion inhibitor )
Al2O3/ 1.2%( C inh )
Fe2O3/ 0.8% ( C inh )
In some cases the cargo acid may be loaded at temperatures grater than ambient. Depending upon the origin this can have an adverse effect on the stainless steel. It is important the Chemical Operator is consulted before loading any Phosphoric Acid that is over 40C. If the temperature of the acid is exceeding 50C the cargo will normally not be loaded.
Before loading consult the SS manufacturers graph for Flourides and Chlorides versus temperature –included in your SS resistance tables given by the yard. Aggressiveness of wet phosphoric acid increases with temperature . Higher temp requires a higher inhibitor content or lower accelerator content. An acid than can be safely carried at 35 deg C will cause corrosion at 45 deg C. Grades 316LN 317LN and 2205 (UNS S31803) --with high nitrogen content are suitable. 317LN can carry higher temps than 316LN. find out from shipper all specs incl temp and the number of days of carriage. When changing shore tanks or loading from wagons—be aware of temp differences.
ABCD of WET PAC corrosivity--
A)Conc P2O5 <54%
C)Conc CL<200 ppm
D)Temp 45 deg C.
Excess of any two of above items will damage the tank
Al and Ferric ions are corrosion inhibitors. Calcium , Magnesium and Sodium are secondary inhibitors. Insoluble silica forms the sediments. In case SS corrodes send the samples from each tank to a independent lab to find out the true specs. The conditions under the sediment layer will be more aggressive than the liquid itself. Prewash with FW –first and last with FW.
Clear, syrupy liquid
USCG / Group 1
Marpol cat/ Z
MP/5 (80% concentration). -- fertilizer grade
Solubility in water / complete
Phosphoric acid will not bum. It will react with many metals, giving off hydrogen gas that is highly flammable. If hydrogen is trapped in a confined space, it can form an explosive mixture with air
Non-irritating to eyes and throat.
Fairly severe skin irritant, usually causing pain and second degree bums after a few minutes contact
Minimum hazard. Includes most chemicals having threshold limits above 500 ppm.
Irritation of skin in contact with liquid. Burning of eyes.
Stable over a wide temperature range.
Very corrosive to ordinary ferrous metals and alloys particularly at temperatures above 82C
Wear rubber gloves, protective clothing. Have body shield available. Avoid contact with liquid
If leak on cargo transfer system, stop the transfer and empty the line system. Flush contaminated area with large amounts of water.
Pollution category D (old ) NEW--Z
Use Polypropelene hoses for the cargo transfer.Use acid spray shields to cover flanges on manifold .
Non inert gas backed secondary welds will ooze black tattoo and cause elephant skin on SS.
Tell shippers that chlorides and fluorides must be declared for your shipment . Even if chlorides and flourides are less that max specified by SS resistance tables-- PASSIVATE THE TANKS BEFORE LOADING IF TIME PERMITS. Don’t be too trusting or gullible with the shippers general toned down specs. The composition of WPA varies greatly and only a typical specification will be given. Remember an exact spec must be obtained for each shipment. As a general rule, a typical specification of the WPA should be requested from the shippers prior to each loading.
A graph showing the dangerous zone or chlorides and fluorides on X/Y axis is attached to this manual.
Wet PAC has very high abrasive sediments .
They will cement your Framo impeller if you do not recirc using diffuser nozzles daily .
Tsunami effect also can work where the cargo hits the ford bulkhead and washes back .
The sediment will exclude oxygen from the SS and cause those areas to become active.
The green PAC from
( Ashdod ) has nil sediments. But in rest of the world sediments is almost sure. Israel
High wear and tear on pumps cargo seals from acid containing sediments can be expected. Cargo pump seals integrity should carefully be examined prior to loading.
The SS tanks must be cleaned to W/W standards with absolutely no previous cargo residues.
The cargo pump cofferdam must be checked and dried out. Pumpseals checked and renewed if necessary.
Do not blow heating coils dry.
Keep the coils inside the cargo chock a block with fresh water and blank off the coils.
You need not heat this cargo.
No particular requirements. If loading through pump only, care must be taken not to exceed the maximum loading rate allowed, taking into account the heavy nature of the WPA can cause damage to the anti-rotation device on pump.
Too long recirc will wear out your seals. So ideal way is to recirc with octopus/ spider.
Shift sediment from tank bottom to top .
You cant do this in spray as additional chlorides of salt spray will destroy stainless steel.
Ensure the butterworth ports are checkbolted, at least 4 in case you are shipping seas on main deck.
To prevent pump blockage, it is necessary to blow the pumpstack with compressed air at least once a day. The pump should be run slowly, a few minutes to ensure that the impeller is operating freely.Ensure that the PV valve and ullage hatch are open during blowing.
Phosphoric acid (from
, Richards Bay ) require daily recirculation through diffuser installed on drop line. Morroco, Tunisia etc
Ensure that the pumpstack is blown with compressed air with PV valve open , to loosen cement at pump suction sump, before starting the framo impeller—lest you have a unwanted hydraulic breakdown.
Because of the high specific gravity of Phosphoric Acids, very high pump pressure may be experienced.
ENSURE THE PREWASH AFTER DISCHG IS WITH FRESH WATER.
Then use sea water till the pH is 7.
Immediately after that wash again with fresh water to remove all chlorides from tank.
This is crucial to avoid elephant skin.
ANY SEDIMENT AT THE BOTTOM OF TANK CAN ONLY BE REMOVED ,--
WITH MORE PURE PHOSPHORIC ACID LIKE
CRUDE OIL WASH.
IT WILL TAKE A LONG TIME TO
KEWMACHINE THE CEMENT OFF THE BOTTOM.
SO KEEP SOME GOOD CLEAN PAC IN 200 DRUMS FOR THIS MANUAL EFFORT.
Have a look at the first empty tank .
If the sediment is too much --it is usual to recirc at end of discharge of each tank ,
like CRUDE OIL WASH using tankcleaning machine and SS hoses.
Or you can do tsunami method like what is done for palm oil before we send people,
down for squeezing.
Acid/sea water mixture remaining in lines and stainless steel hoses will soon result in pittings.
After the tanks are thoroughly/finally cleaned, passivate the tanks with Nitric Acid as the Phosphoric destroys the passive oxide coating on the stainless.
PURE PHOSPHORIC ACID (white phosphoric acid ):
Since pure Phosphoric Acid is used as additives in food and Coca Cola ,beverages, a very high degree of tank cleanliness is necessary. Check with the Ship Operator if a wall wash is required. At
( Ashdod ) whre you load White Phosphoric acid, the tank inspection is stringent. After passing the initial tests and tank inspections , a small quantity of WPAC is loaded and recirculated and then transferred to the next tank. The final content is then degraded as GPAC/ BK PAC as fertilizer grade. Pure Phosphoric Acid 80% freezes at 5C and will therefore require heating in cold climate. Israel
After dischg white phosphoric acid ( Coca Cola grade ) immediately fill up the impeller sump with fresh water, before the cargo solidifies. The surveyor must be told of your intention to do this and he has to be available to issue tank dry certificate on the spot as soon as stripping is over. This cargo is fully soluble in water. Sometimes you will find that the valves are frozen with WPAC, and has to be steamed to free. Excess force will break the valve. Food grade phosphoric acid does not require recirculation. It does NOT corrode the SS tank or have inherent sediments.
-------CAPT AJIT VADAKAYIL ( 28 YEARS INCOMMAND )