Normally cargo heating uses steam as the media for heat exchange, but some ships use special hydraulic oil as a medium.
The voyage orders will clearly indicate if the cargo requires heating and the temperature to be maintained. Normally the heating clause in a charter party requires the cargo to be carried at loaded temperature but not higher than 135º F or 57.5º C. The cargo heating should be planned judiciously by taking into consideration the ambient temperature, the temperature to which the cargo should be heated or any other special instructions given by the charterer. If the ambient temperature is low and the difference between the ambient temperature and the cargo temperature is high then it is advisable to start heating immediately after sailing from the load port. On single hull tankers the wing tanks require more heating than the centre tanks
Can you understand why this would happen?
In case of the wing tanks, the heat loss is from the sides, top and bottom and in case of the centre tanks the heat loss is from the top and bottom; the side tanks with heated cargo allow no heat loss. In ships with a double bottom the heat loss from the bottom of the tank is reduced as the DB offers a kind of insulation. Normally the temperature is checked in the morning and a report sent to the owners / charterers. In temperature critical cargoes, the temperature may be checked twice a day as per instructions of the charterers. With the existing double hull tankers this problem does not exist.
Testing of heating lines
Before loading any cargo that requires heating, the heating lines should be pressure tested for integrity. This is done by pressurising the lines with fresh water / compressed air, to the max pressure that can be expected on the heating lines. If the pressure in a particular coil falls, then it is assumed that the coil has a leak and man entry has to be made into the tank to detect and rectify the leak.
The heating coils are normally made of a material called ‘Yucalbro’ which is an alloy of copper and aluminium. This being a soft alloy is liable to be damaged if it is subjected to physical pressure.
Welding cannot be done on this alloy and hence silver brazing is done. If there is a section of the pipe that is damaged or if the pipe is holed, then to avoid doing hot work in the tank, it is advisable to change that section of the pipe by cutting the damaged section and connecting the pipes again. The pipes are connected to each other using “Ermato” couplings. While bending the pipes used for heating coils they are filled with sand and then bent in the required shape. The pipes are cleaned thoroughly before putting them in place.
Each tank has a steam heating manifold or there may be a common manifold for a set of tanks. In this manifold, a line comes from the main steam line and it branches into lines for individual heating coils.
Normally, although the centre tanks are much bigger than the wing tanks, the number of heating coils installed in the wing and centre tanks is same. This is so because, as explained earlier, heat loss is more from the wing tanks than from the centre tanks and hence they require more heating.
Each steam manifold has a valve, which controls the steam to the coils in that manifold, and each heating coil has an individual valve to control the steam in that coil. A pressure gauge is provided after the stop valve of each manifold to indicate the steam pressure in that manifold.
Before commencing cargo heating, the deck steam is put on warm up and the forward bypass valve is opened, so that the steam main and return line are warmed up. Then the steam pressure is slowly increased and the individual valves of the heating coils are opened after opening the manifold valves on the steam manifolds for the tanks to be heated.
It is normal practise to keep the return line valves on the steam line always open unless specifically required to be closed. On the return line from the heating coil, a drain valve is provided. This should be opened to check if there is water coming from the line, which will confirm that the line is clear, and to check if there is any oil in the return line. This will indicate a leak in the heating coil.
The drain valve should be opened to check for leakages on a regular basis. It should be remembered that the water from the return line is taken to the drain well in the engine room and is then reused in the boiler. Hence, any oil in the water will be detrimental for the efficiency of the boiler.
The diameter of the heating coils in the tanks is bigger than the diameter of heating coils prior to the entry of the line into the tank. This ensures that there is more surface area available for the heat exchange to take place between the steam and the cargo. If water is found to be coming out of the drain valve on the return line, then it indicates that proper heat exchange is taking place in the tank.
It should be remembered that it is difficult to stop the fall of temperature once it reaches an accelerated rate. It is prudent to start cargo heating as soon as it is apparent that the temperature is going to drop.
In case of a long voyage, ships crew are sometimes tempted to allow the temperature to drop below the requirement of the charterer with the idea that it can be raised before arrival discharge port.
This is not advisable because:
- Once the temperature has started falling rapidly it is difficult to stop the fall in temperature and raise the temperature, and.
- The charterers may sell the cargo before reaching discharge port and at that time the new cargo owner will require the details of the temperature maintained and this can create a problem as he may then reject the cargo because of it not being maintained as per charter party requirement. Some cargoes like vacuum gas oil have a high pour point and they will solidify at ambient temperature.
- We know that in liquids, convection currents within the liquid transfer heat. Hence increasing the pressure of the steam will not expedite the process of heating the cargo appreciably. Also with some cargoes (like molasses) crystallisation may take place on the heating coils and this will serve as insulation and reduce the heat exchange process.
During discharge, it is advisable to keep heating the cargo until the level of the cargo has fallen to the level of heating coils. This prevents the cargo from losing its fluid state and helps in draining the tank. It also compensates the cooling by ballast water, which is taken in the DB tank below the cargo tanks.
Some ships carrying special cargoes have heaters mounted on deck and the cargo is pumped into the heaters on deck where the heat exchange takes place between heating coils and the cargo. The cargo is then returned to the forward end of the tank. These ships normally have deep well pumps in the tanks and hence the cargo from the tank is re-circulated without getting mixed with the cargo from another tank.