Desulfurization has reduced the lubricating properties of fuel, often leading to pump failure. Here, Philip Sulzer, technical manager KRAL GmbH, explains how the surface hardness of its screw pump components was increased to handle the low sulfur fuels now used in today’s shipping.
When the International Maritime Organization’s (IMO) new regulations for sulfur content of marine fuels came into effect on 1 January 2020 (also known as IMO 2020), industry experts expected to see a significant change for the shipping industry. The entire marine fuel supply chain, from refining, through distribution, bunkering, handling and storage on board the ship, through to final use in marine diesel engines or in boilers for propulsion, is now affected.
The new regulations stipulate that only marine fuels with a sulfur content of less than 0.5% may be used on ships worldwide – even on the high seas outside of environmental protection zones near the coast. An exception is made only for ships equipped with an exhaust gas cleaning system (EGCS) also known as scrubber. However, it is likely that in many ports of the world fuels with higher sulfur content will not be available any more in the future.
Investment decisions Due to this development, ship owners and ship operators have to make important investment decisions for their ships already in operation. While in new vessels the operation with low sulfur fuels is already considered in the selection of components such as fuel pumps, many decision makers are still cautious when it comes to retrofit existing ships. What risks exist and which solutions are available to minimize the risk when using existing fuel pumps.
The main technical challenge is the fact that low sulfur fuels have a low viscosity and have reduced lubricating properties due to desulfurization. However, the screw pumps which are important for conveying fuel in the ship need some lubricity of the pumped medium to reduce friction, especially at high pressures. Too low lubrication threatens a failure of the fuel pump and thus repair costs and downtime.
The most important and most critical application are the pumps in the booster module, i.e. the feeder pumps and circulation pumps. Particularly high pressures must be generated by the feeder pumps. From about 8 bar it becomes critical for conventional screw pumps when they need to convey low sulfur fuels. Special strains occur when the pump is started or operated at high temperatures.
Technically, situations might occur in which it comes to solid friction (friction of metal against metal) instead of liquid friction (friction of metal against the liquid). Usually this transition proceeds smoothly and an intermediate state is established. Technically spoken: It comes to mixed friction.
The mixed friction conditions within the system between screw and pumped liquid as well as enclosing pump housing lead after a short period of time to predominantly abrasive and in the worst case to adhesive wear. This manifests itself mainly in the wear forms scratches, grooves and scoring and in the formation of crests up to material transfer.
When the adhesive wear has reached a certain extent, the pump drive can no longer move the screw against the surface of the pump housing. The pump is blocking. Abrasive wear, however, leads to material removal mainly in the screw bores. This increases the internal backflow and the pump can no longer maintain the required system pressure. The pump is running, but flow rate significantly decreases.
Sufficient experience Kral GmbH, Lustenau (Austria), is a manufacturer of screw pumps and flowmeters for many applications, including marine engineering and oil & gas. In applications such as power plant technology, the company has many years of experience in conveying low-viscosity fuels.
The experience of Kral is based on products for the conveying of desulfurized fuels and fuel oils such as extra light fuel oil (HEL), diesel fuels (EN 590) and marine fuels according to ISO 8217, even at temperatures above 40°C. Based on the experience gained from international projects, the company also assures users in the shipping industry a reliable operation in the pumping of low sulfur fuels.
This innovation advantage is based on comprehensive technical tests carried out by KRAL. The aim was to increase the surface hardness of the pump components. During the tests, it had to be ensured that the selected process could be used in the series production of pump components. Due to the high process temperatures, certain hardening processes can lead to distortion on the components, which is impossible to correct and render the components unusable.
The selected process had to support the precise production process at Kral with very tight manufacturing tolerances for screw pump components, so that the Kral pumps also achieved the usual optimum efficiency.
Coating advantage A binding coating is applied to the metal screws used in Kral pumps. Through this coating the screws receive a strongly hardened surface with much less friction. The friction coefficient of the coated screws is reduced by a factor of 10. At the same time the screw surface achieves a very high degree of hardness.
With these values, Kral screw pumps are well prepared for the new challenge of conveying low sulfur fuels in international shipping. Ship owners and ship operators can make the necessary investments in their existing ships with peace of mind.
