Cargo heating planning and optimization
In some ships, cargoes such as special oil products, special crude oils, heavy fuel oils, etc. may require heating in winter and cold climate regions. Some of this heat required can be supplied by exhaust gas economizer. However, in many cases an additional auxiliary boiler is needed to supply sufficient steam. Steam from exhaust gas is generally enough to heat the heavy fuel oil that is used on most ships; in port, however, steam from an auxiliary boiler may be needed. For cargo heating purposes and in order to reduce fuel consumption, reduce emissions and the heating costs, a voyage-specific cargo heating plan should be developed by the shipboard team with support from operation department at head office. For a proper plan, the following should be considered:
- Vessel tank configuration.
- Whether deck heater or tank heating coils are provided.
- Number of heating coils and surface areas.
- Cargo details including specific heat, pour point, cloud point, viscosity, and wax content.
- Weather en route including ambient air and sea water temperatures.
- Estimated heat loss and drop in temperatures.
- Recommended return condensate temperatures.
- Estimated daily heating hours and consumption.
Various parameters such as daily air/sea temperatures, weather, cargo temperatures at three levels, steam pressures, return condensate temperature, actual against estimated consumptions and temperatures are discussed between shipboard team and head office. The heating plan should be reviewed and revised appropriately throughout the voyage.
The optimum temperature to which cargo should be heated for carriage and discharge largely depends on the following factors:
- Pour point: It is the lowest temperature at which the liquid will pour or flow under prescribed conditions. It is a rough indication of the lowest temperature at which cargo is readily pump-able. General principle is to carry cargo at 10ºC above pour point temperature.
- Cloud point: It is the temperature at which dissolved solids are no longer completely soluble, precipitating as second phase and is synonymous with wax appearance temperature. Once separated, it requires temperature over 80ºC to dissolve the wax. Cargo temperature should not be allowed to fall below the cloud point.
- Wax content: High wax crude tends to deposit sludge, and therefore require to be maintained at a higher temperature to prevent wax fall out.
- Viscosity: High viscosity oils do not necessarily deposit sludge and may be carried at lower than the discharge temperatures. However, for discharge purposes, the heating will be done to reduce the viscosity to acceptable levels for cargo pumps.
- Ambient weather and sea conditions: This will also influence the cargo carriage and discharge temperatures as these impacts the level of heat transfer from cargo tanks or fuel tanks.
The cargo heating plan would need to take into account the above parameter. As part of cargo heating planning, relevant instructions will be developed. Heating instructions should be reviewed after loading cargo, based on charterer requirement. Permission to carry and discharge the cargo at optimum temperatures should be requested from charterer or cargo owner. The heating plan should be made soon after loading cargo and reviewed/updated on daily basis considering the various factors that affect the heating and customer requirements.
For best practice cargo heating planning, the following should be noted:
- Vessels should have a greater understanding of the voyage manager/charterer's heating instructions.
- Seek the receiver/charterer's permission for allowable range of cargo temperatures.
- Avoid heating during adverse weather period.
- Create and follow the proper cargo heating plan to verify the effectiveness of actual heating progress.
- Closely monitor and analyse cargo heating reports. Monitor heating daily to address deviations from the heating plan.
- Do not heat for short frequent periods and running boiler at low loads.
- Follow the recommended condensate temperature and optimum boiler settings for efficient cargo heating. Heating instructions, accompanying the heating plan, should further highlight these points.
- Maintain efficient and good communication between the vessel and the voyage manager/charterer about the plan and execution.
Cargo heating may also benefit from the use of effective insulation. For example, using lagging on heating coil water / condensate return pipes as well as steam, thermal oil and hot-water lines on deck area. This could be significant energy saving option as it has been observed that some ships lack insulation of branch lines and cargo tanks. It is important that the insulation material is of good quality. A poor quality of insulation material is likely to rot or lose its effectiveness (MariEMS 2017).