Wednesday, September 21, 2011

PARAFFIN WAX ON CHEMICAL TANKERS-- CAPT AJIT VADAKAYIL



                   PARAFFIN WAX



Adjacent tanks may have to be left empty in order to have the temperature of the bulkheads above the melting point .


 
UN/ 1223

Chris code / WPF

USCG group/ 31

FOSFA/  Approved

Water solubility/ Nil

SG/ 0.77 at 71C (at 20C / 0.88)

Colour/ Off white , light yellow

MP/ can vary from 50C to 60C

Non toxic and non irritating

MARPOL / Y

FP/ >20C

Flammable only at high temperatures

No odour --but cargo can pick up bad odour

Obtain cargo specs and heating instructions

Uses: candles, lubricants, floor polish, chewing gum base etc

BP/370c

FP/ 199c

Surface tension/ High

VH/ 5c

DH/ 85c

AH/ 90c

Nitrogen padding/ no

Stable

Coating/ check Paint/ lining resistance list for temperature restriction

 
Don’t rely on past practice ( of old timers ) , to find out if Paraffin wax requires a pre-wash ashore after discharge .

As you know since 1st Jan 2007—the Marpol prewash rules for high viscosity oils have changed. It is better to get the unloading temperature and the viscosity of Paraffin Wax at that temperature declared in writing in a proper signed and sealed document .
Ideally the viscocity in mPas at 20 deg C should be declared in the B/ lading —as this figure may vary or may be deliberately fudged.


Cleaning is more or less the same for all grades of paraffin wax, the only difference is the amount of residues remaining in tank - wax on the coils and under frames and the amount of white hard powder residue on bulkheads and floors.

Melting Points of  waxes differ greatly from different load ports and so the MP and heating instructions must be obtained in writing. You may do your own melting point test of a composite sample in the ships lab. Don’t allow steam inside the coils to scorch the cargo, form a insulation coating and lose heating capacity.



At the loadport remove all adjacent cold interface from SBT.

                                               

Once the steam coils are covered during loading, you should start slow heating in the tank .

                                                                         

The importance of repeatedly checking PV valve and lines during cold weather and bad weather for blockage by wax during the voyage cannot be over-emphasised. This may have to be checked physically by opening tank hatch and remove solidified wax from vent line.

                                                         

Before arrival at the discharging port, open the tank dome and one of the butterworth ports to check for solid cargo build up. Quite often after a freezing bad weather passage you may find that the huge orifice ( like tank dome or butterworth port ) is reduced to 20%. Run the framo pump to check if the impellers are free .


 
Check if the delivery lines are clear—it is common place to have them fully blocked. If it is not 100% blocked it must be understood that the hot cargo will scour and clear the lines 100% when it flows through. Use hot steaming water to melt wax internally inside valves by wrapping soaker rags –live steam is too local and unwieldy / thermally ineffective. Live steam is too local and takes too long to melt wax.


 
Samples at can be drawn from open tank domes—it is usual for the UTI sampler to get stuck in the extension tube.  Wax tanks should be discharged separately, one by one—don’t use the common collector. Steam tracing the common collector at manifold is also not the best way out.  Do not let shore personnel put pressure on the ship to rig more than one by one unless they supply separate shore hose. Make sure you make a protest letter to cover you up in case there are delays due to frozen lines.
                                                                                                    

During discharge, if have to stop pumping for any reason, steam line back to tank immediately to avoid blocked lines. In the pre-loading meeting find out if there are any stoppages due to shore reason and be stand by with steam hoses in hold weather.

Heating coils should be tuned down just before bottom coils are uncovered                                   
 

When ballasting avoid frozen wax on tank sides and bottom. Chief officer must personally ensure there is no cold interface.

 
Cleaning;

First steam the tank for one hour after removing adjacent ballast. Place extra cleaning machines in tank by hanging off at tank dome too. Let the hot sea water flow from tankcleaning line till the temp at tank inlet is 80C. Machine drops should be maximum 2.5 m, one hour each, temperature at outlet of tank cleaning heater should be 90oC which will give approximately 80oC inside tank at full pressure. Don’t compromise on both temp and pressure. 

If it is absolutely necessary to compromise on either temperature or pressure, then it is better to slightly reduce the pressure ( not too much ) and maintain the correct temperature. Keep a continious written record of the temp at tankcleaning inlet say every 20 minutes. This wont be wasted manhours as the wax has to be melted first before being washed away.

Clean with SW for 30 minutes hot, leave water in tank and keep steam coil heating on. Until discharging of the accumulated wash water keep live steam slowly into tank.


After the initial hot salt water wash graco inject an emulsifier plus surfactant and continue the same way as before with short drops.


In very cold weather only a chlorinated solvent ( Perchloroethylene or Trichloroethylene ) recirc can do the trick.


At the end of your tankcleaning –on tank man entry you may find hard white residue especially on heating coils –the amount depending on the wax--which has to be removed manually. Dry up the coils by passing steam and use scotch brite and wood hammers to shake it off. Keep aluminium ladders standby to remove any NVM traces from bulkheads


In stainless tanks, recirculate 40% metal brightener / 60% fresh water, approximately 3 cum at  45oC for 1.5 hours and then transfer to next tank. Finally rinse with hot fresh water for 30 min and steam tank.  This is the final polish up act if the next cargo is WWT with UV test.

Caustic recirc at 4% can prepare epoxy tanks for the UV test.  Zinc tanks hence are unsuitable for Paraffin wax as you can see the pH of the recirculation solution is incompatible.



CAPT AJIT VADAKAYIL
( 29 YEARS IN COMMAND )
..

2 comments:

  1. What can you do with the spent perchloroethylene after washing the tanks? And how do you get the small amount of the cleaning solvent out of the bottom of the tank sump?

    ReplyDelete
  2. hi hank,

    any cat Y tankcleaning chemical ( or even samples in bottles ) are disposed off as per Marpol stipulation for cat Y chemical cargoes. to see the rules--punch in MARPOL RULES FOR ANNEX 2 CHEMICALS - VADAKAYIL

    perchloroethylene is soluble in methanol ( sump ).

    nobody uses perchloroethylene anymore at sea. we have shifted to methylene chloride, which has got very little BP, and hence evaporates.

    i am posting on this today-- CHLORINATED SOLVENTS.

    capt ajit vadakayil
    ..

    ReplyDelete