ALL CHEMICALS CARRIED AT SEA ARE HEATED BY HOT WATER AND NOT STEAM. THE TANK HEATING COILS AT EXIT POINT OF TANK , MUST CONTAIN HOT WATER NOT STEAM.
THE TEMPERATURE OF WATER IN THE COILS IS ADJUSTED BY THE RETURN VALVE FROM TANK . THE STEAM INLET VALVE TO TANK MUST BE 100% OPEN ALWAYS .
Heating is done to ----
Reduce viscosity. Viscosity is measured in centistokes. Water is one cst. As the temp rises viscosity reduces at the rate of 2% per degree.
Reduce pour point. Pour point is 3 degrees over temperature were liquid coagulates.
Reduce cloud point. Cloudpoint is the temp where cystallised solids forms inside liquid and settles on bulkheads.
Avoid crystallization. Some cargoes like Caustic Soda require heating to avoid crystallisation.
Avoid freezing of cargo. Some cargoes like Cyclohexane require heat to avoid freezing.
To avoid pre-wash at discharge port. Heating may also be required in order to comply with MARPOL regulation for prewash ( Phenol will have to be prewashed if dischg berth temp is below MP plus 10 deg.
To increase the water solubility. For example Phenol is soluble in water at a temp of 60 degrees.
To reduce unpumpables at tank sump.
To reduce delivery pressure of the ships centrifugal deepwell pumps.
To avoid claims from the charterers who want a particular voyage and discharge heat. Under heating and over heating will both result in claims from charterers.
To reduce clingage. Clingage does not include the tank bottom
For tank heating coil system the size of the main return line is smaller than that of the steam inlet line. Because the return line is supposed to contain water only, while the main inlet line is supposed to contain steam only. For the steam winches the case is opposite, the return line is bigger as it contains expanded steam.
From the engine room boiler steam arrives on deck through the main inlet steam pipe. Before it enters the deck it has to pass through a PCV or pressure control valve of 1 bar to 7 bar range. There will be a small bypass line around this valve for initial warming prior starting. This bypass must be operated for at least 45 minutes on a small chemical tanker, before the PCV is opened. Heating must be started slowly to reduce thermal stresses.
When not in use the steam coils should be full of fresh water---unless the cargo is water reactive in which case it must be blown dry and blanked.
For initial starting the PCV is put at 1 kg and then slowly increased. Open all drain cocks in the steam inlet line till water is ejected and steam comes out.
Only water should return to engine room through the return line –otherwise it is a huge waste of money. This is one of the energy conservation items . This water must be pure and should not contain any cargo. There is an inspection chamber ( called siphoning drum ) where the first one hour of return water must be monitored. On a chemical tanker you can view through a glass port to check for floating insoluble cargoes, get the odor from the vent to check for volatile soluble cargoes, and drain from the bottom to check for sediments or high SG insoluble chemicals.
When carrying heating cargoes the return siphoning drum content must be inspected daily check for any traces of cargo—if any are found then the heating coils must be drained individually to find out the culprit. This coil can then be blanked off. The drain valves and siphoning drum must be checked twice daily for ingress of cargo.
Heating of tank is regulated also by the number of active coils from inlet manifold. Temperature of return line has to be taken regularly to find out if you are over heating or under heating. Centre Tanks with double skin has to be heated less than wing tanks who have a cold interface with ballast.
IT IS IMPORTANT TO KNOW HOW TO STOP HEATING. OR YOU DESTROY THE HEATING SYSTEM. THE IDEA IS TO AVOID TRAPPING HIGH PRESSURE STEAM WHICH ON COOLING WILL CAUSE A HIGH VACUUM BUILD UP . THIS WILL CAUSE CORROSIVE / EXPLOSIVE/ TOXIC CHEMICALS TO BE SUCKED INTO THE COILS . First close the inlet valve . Then open drain cock in front of the return valve. Then close return valve.
One single leaking coil can contaminate other clean coils via the return manifold. Steam blow contents must always be checked for pH
If steam heating coils fail ---in an emergency ,use adjacent heat including live steam in ballast water around the tank. When no heating is done temp usually drops 1-2 deg C a day.
When you heat raise the temp to 5 to 6 deg a day---NOT MORE. Many cargoes perish due to indiscriminate overheating . Too high temp causes unacceptable VP or chemical/ physical change which can be permanent. When veg/ animal oils are overheated Carbon Monoxide is formed which can affect the sweeping party. Overheating luboils can cause oxidation of cargo and colour off spec. Overheating Molasses cause thermal decomposition and total destruction.
Make sure cargo is at the discharge heat four days prior arrival. To expedite the process, remove cold ballast interface.
The heating coils of tanks not required to be heated must be blanked on both entrance and exit and log entry made. In order to avoid confusion, heating coils should only be used when a cargo requires heating. Any other cargoes on board should have their coils blanked on both the inlet and return side. This is also of paramount importance if the cargo is inhibited, subject to polymerisation ( Styrene Monomer ), or has a violent reaction water ( Sulphuric acid ) etc.
Tanks which require heating must be pressure tested prior loading and log entry made. If the coils leak –you cannot load unless permanent repairs with proper stainless steel welding is done by chief engineer.
If a heated valve is tight allow it to cool. If you force it , you damage it.
Some ships have deck mounted heat exchangers –Framo pump discharges into a heater at about 60 bars.
The two methods for testing steam coils on board are:
Hydraulic testing using water
Steam pressure testing
It is possible to carry out pneumatic testing (air), but this method is considered unreliable and will NOT be considered suitable for meeting company standards, and is NOT to be used.
Hydraulic testing of heating coils is time consuming and cannot be carried out in all tanks on a regular basis. The hydraulic testing of heating coils must be carried out, by ship's engineers, once a year. or as dictated by the company. It is recommended that tank tests are staggered so they are not all due at the same time. Hydraulic testing must also be done by chief engineer after any permanent repairs have been made to the heating coils. The results of all hydraulic tests must be recorded in the log book and the planned maintenance system.
Steam testing of coils is to be carried out before the loading of every heated cargo as a policy.
Thermal oil heating coils are to be hydraulic tested, using oil, every two years. If defects occur, then the coils must be tested annually.The coils must also be tested after any repairs.
All coils must be hydraulically tested when they have been replaced following disassembly for coating work. etc. In a docking situation this should be done before leaving the yard. If this is impossible, then testing should be scheduled for as soon as it is practical.
Any cargo damage due to steam coil leak or ingress into steam coils is un acceptable to us, it is considered as personal failure.
For cold hydraulic testing use Graco pump and a 200 litre FW drum to pressurise to 12 kg. Do not forget to inspect the riser piping. Testing must be done coil by coil. Also check for loose nuts from steam coil clamps.
It is the more severe of the two test methods due to the fact that when using steam, heat expansion can cause small leaks to seal, and also higher pressure (1.5 times) than working pressure can be used.
When carrying out hydraulic testing, it is important that the tank is completely dry , and cold water ballast interface at the tank bottom is removed. This will make the detection of very small leaks much more easier—any water at the bottom means a leak.
Install blanks to provide the required test boundary. Connect the hydraulic test pump to the coils and install a suitable pressure gauge. Fill the coils with good fresh water. Build up the pressure in the coils to 1.5 times their maximum rated working pressure. Close the hydraulic pump valve to prevent pressure drop. Monitor the pressure of at least 30 minutes, without additional pumping. If a leak is indicated by a pressure drop, then the entire system must be checked until the leak is found.
After 30 minutes, the coils are to be thoroughly examined. The thorough examination must include a very close visual inspection and also feeling by hand under the coils in order to detect any pinhole leaks. Such pinhole leaks may not be indicated by a pressure drop on the manometer and can also be very hard to detect by the eye.
Repair and properly document all leaks detected. Repeat the test procedure after repair.
The testing of coils using steam pressure from the ship's boiler:
Misleading results can be obtained from steam testing due to steam condensation in the coils if the test is not properly conducted as described below. The maximum maximum boiler pressure is to be used . The coils to be tested should be exhausted to atmosphere until live steam is issuing from the return. Close the return against the steam. The steam pressure must be held for 30 minutes prior to inspecting.
Another 2 in one ( DUAL )method is with return valve closed, open the steam inlet at 7 kg . This will then give an easy hydraulic test on coil due to trapped condensate in line. Inspect the coil for water leaks. Following the above, return valve to be opened and steam allowed to flow through the coil via the steam trap. After 30 minutes have elapsed, coils to be inspected for leaks. By shining a torch light along the coil, escaping steam will be highlighted.
If a leak is detected and a section of piping has to be repaired, a proper hydraulic test should be carried out after the repair is completed as a policy. Install blanks to provide the required test boundary, ie isolate only the coil to be tested.
When carrying out visual inspections for leaks, look closely for signs of corrosion. Corrosion leaks may require further close examination of the coils to determine the amount of damage. Bad corrosion may mean the whole section may have to be cropped and renewed. All corrosion problems are to be reported to the chemical operator immediately.
Heating coils not in use must be steam pressure tested every 3 months and its results must be recorded. If heating is not to be done the coils must be blanked—and log entry made
If the Master is unable to comply with the charterers heating instructions he must inform the chemical operator well in advance. Make sure that there is enough bunkers on board to run the boiler.
Under no circumstances should cargo exceeding tank lining resistance tables temperatures be accepted for loading. Be aware of the maximum cargo temperature stipulated in the charter party.
It is the responsibility and duty of chief officer to take the temperature twice daily—the critical heating log must be signed by him daily without fail—signature must not be postponed to the next day. A signed copy must be hung in CCR bulkhead for Chief engineer to monitor.
The steam valves of wing tanks and forward tanks are usually opened more. The % of the return valve opened must be entered in the log column.
The Chief engineer must log down daily the bunkers used in boiler solely for heating cargo.
Heated cargo temps must be taken at 3 levels inside tanks. As the cargo is discharged , the steam heating into the tank must be reduced. Heated cargoes must be stripped immediately. IT MUST NOT BE POSTPONED.
There must be an action plan ( as part of the pre-load pre-dischg meeting ) to melt solidifying cargoes from inside the pipelines and to prevent valves from getting frozen in place. If steam hoses have to be used the crew must use PPE and heat resistant gloves.. Steam must be applied at the underside of
the pipes. It is more effective to use hot water on a burlap lagged pipes , especially at bends.
If the pump impeller is frozen steaming into empty DB via sounding pipe can be considered—but inform the chemical operator first.
Before discharging a high MP cargo consider recirculating the cargo before opening the tank delivery valve and the manifold valve.
Simply put, steam traps are automatic valves that release condensed steam (condensate) from a steam space while preventing the loss of live steam. They also remove air and non-condensables from the steam space.
Once steam has transferred Btus and becomes hot water, it is removed by the trap from the steam side as condensate and either returned to the boiler via condensate return lines. Continuous flow traps will continuously discharge condensate.
Normally there will be no need for level controller of tank cleaning heater if you do not excced 8 machines.
Steam traps on return line coils senses the difference between flow of water and steam. There are filters in front of each steam trap. Clean these filters and also steam traps regularly.
Steam traps must be overhauled regularly to ensure correct function of the steam trap and heating of the cargo. Twice a year and before loading high-heat cargo it is also recommended to open up the steam trap and blow coils through and remove any previous cargo which may have leaked into the coil thereby blocking the trap and narrow parts of coil on deck causing nil heating and subsequent slow discharge claim.
CAPT AJIT VADAKAYIL ( 28 YEARS IN COMMAND )