Securing seal performance for Tier III engines
The heat, pressure, humidity and corrosiveness of an engine can all be affected by NOx abatement - with a corresponding impact on seal performance
Omer Raoof, business line leader for oil and gas, Precision Polymer Engineering, looks at the challenges that Tier III NOx limits present for sealing systems in marine diesel engines.
Engineers are under increasing pressure to optimise their equipment for the delivery of greener operations. And in light of the latest limits on emissions from marine diesel engines - among them Tier III NOx - there are some potential challenges for sealing systems.
The key sealing challenge for newer engines relates to the fuel system and the ability of these engines to run on much cleaner fuels. The cleaner the fuel used, the more chemically aggressive the fuel is to elastomer seals within the engine. The chemical properties of the sealing material are therefore critically important – especially considering that manufacturers typically offer extended warranties to reflect the reliability of the engineering. An engine expected to run flawlessly for up to seven years should have sealing components that can keep pace. The seal technology required will typically migrate towards highly fluorinated seal materials, such as perfluoro-elastomers (or FFKMs).
If a seal manufacturer can give assurances of reliable long-term performance, potential savings are substantial. Typical dry dock costs for maintaining an engine on a vessel can be upwards of €100,000 per day. With seal changes within a fuel injection system taking approximately 10 days, an unreliable seal can prove incredibly costly. With the choice of a seal material with more reliable performance against chemical aggression and thermal extremes, the engine operator can expect a greater period of time between overhauls.
An additional challenge with sealing in newer engines relates to the greatly increased period of time spent running at warm to hot temperatures. While the thermal ceiling rarely presents a challenge for most high performance elastomers, long term exposure to mid-range temperatures can result in an increased rate of degradation over the lifespan of the seal. Choosing an FFKM is widely considered advantageous in newer engines.
Diesel engines may require additional works in order to comply with Tier III requirements, potentially involving the installation or retrofitting of system upgrades. An overview of some of the upgrades available to older engines, and sealing considerations to be taken into account, is presented below.
Low-pressure LNG engines
The development of marine engines able to use low-pressure liquefied natural gas (LNG) as fuel is set to be a popular means of compliance with Tier III standards. Engines of this type operate on a relatively high air-fuel ratio, with a pre-mixed charge of air and fuel ignited by pilot fuel. These low-pressure engines produce NOx emissions which are already below Tier III limits, saving the expense of implementing the exhaust gas treatments mentioned in this overview.
With engines of this type, the low-temperature capability of the chosen seal material is a key concern. An experienced sealing engineer can advise on the optimal choice of seal for applications demanding consistently flawless performance in thermally and chemically aggressive environments.
Selective catalytic reduction (SCR)
SCR is commonplace across many of the industry’s largest engine manufacturers and involves the injection of ammonia or urea into the exhaust gas, which is then passed through a catalyst unit at temperatures of up to 400°C. The ammonia or urea reacts with the NOx present in the exhaust gases, reducing NOx to harmless N2. Using retrofitted SCR technology in older engines can deliver a reduction in emissions of up to 90%, allowing older engines to comply with Tier III standards.
The pressure drop across the SCR unit requires a highly efficient turbocharger for the system. As the NOx to N2 reaction is only viable between 300°C and 400°C - and given the inherent temperature increases associated with turbocharger use - seal materials in these systems will need to demonstrate high thermal resistance.
Exhaust gas recirculation (EGR)
By recirculating exhaust gas through a scrubber unit after the turbocharger, engine operators can reduce NOx emissions by between 50% and 60%. Downsides related to the treatment include storage and disposal of cleaning water, which can become highly corrosive to seal materials after sustained use.
The choice of seal material with regard to EGR systems needs to demonstrate resistance to the degrading impact of corrosive media. Highly developed perfluoroelastomer materials, like Perlast, have demonstrated almost universal chemical resistance in the field. Steam resistance is also a key consideration, with many material grades offering excellent resistance to water vapour.
Miller-cycle valve timing has been used to improve engine efficiency since the 1950s. By leaving the intake valve open during part of the compression stroke, under a Miller cycle the engine is compressing against the pressure of the supercharger rather than the cylinder walls. In four-stroke engines, the expansion and cooling of intake air has a reducing effect on NOx production – particularly when coupled with direct water injection (DWI) or water-in-fuel emulsions.
NOx reduction through the Miller cycle method will require two turbochargers. With two turbochargers in the assembly, any elastomer seals will need to be able to resist the significant build-up of heat. Extreme thermal resistance is a typical characteristic of some leading edge FFKM grades, with demonstrable improvements to operational reliability and reduced overall cost of ownership. If using water-in-fuel emulsion to suppress the formation of NOx, the operator will also need to have confidence in their sealing system to perform well across a diverse band of chemistries.
Scavenge air moisturising
After passing through the turbocharger and compressor, air is at a high temperature. Seawater can be added to saturate the air and bring down the temperature. Typically, optimal levels of humidity are maintained by keeping scavenge air temperatures between around 60°C and 70°C. Engine operators can expect to reduce NOx levels by around 60% in this way.
Considerations for sealing choice include the high humidity and steam environment, which at certain levels has the potential to reverse the cure of many older seal technologies. In such harsh environments, specially engineered terpolymers or even tetrapolymers may provide a more reliable sealing solution.
Among green technologies in the shipping industry, CSNOx has been one of the most successful innovations. In this method, the scrubbing process uses electrolysis and seawater to convert atmospheric CO2, SOx and NOx into non-harmful substrates which may be discharged into the ocean. The CSNOx unit is compact and low maintenance, making it a popular solution for cutting emissions.
Some older seal technologies are prone to swelling in high concentrations of CO2. For a longer lasting sealing system, operators should be aware that material choices are available which are much more resistant to failure as a result of chemical swell.
Decisions taken in respect of engine modifications may significantly reduce NOx and SOx emissions, but operators need to be aware of the effect on sealing components. Changes to the operating environment of a seal operates can impact heavily on the suitability of a seal material.