페이지 정보작성자 최고관리자 댓글 0건 조회 136회 작성일 20-12-15 19:14
The introduction of the 0.50% IMO global sulphur cap on 1 January was always going to make 2020 a challenging year. When we entered this new era of environmental legislation, one of the big unknowns was the characteristics of new compliant fuels.
In very simple terms, an unstable fuel can cause sludge to form, which can block pipes, choke fuel filters and solidify in tanks. In extreme cases it can damage piston rings and fuel pumps.
Many VLSFO products are blends of different components. If there is a mixture of aromatic and paraffinic blend components, it increases the risk of instability.
Dealing with the consequences of an unstable fuel often requires the crew to work long hours over several days to keep the vessel moving, carrying out labour-intensive and dirty tasks such as cleaning centrifugal separators and removing blockages. This is likely to lead to delays in the vessel’s schedule. In extreme cases, there have been losses in propulsion, which is of course a big danger.
The usual test for stability is Total Sediment Potential(TSP). This is carried out by fuel-testing laboratories as part of the suite of tests for checking fuel quality against the parameters of ISO 8217. If the TSP is greater than 0.10%(by mass), then it is at higher risk of becoming unstable.
Since VLSFO products hit the market, fuels are failing on high sediment levels more frequently.
Short life span
There are also reported cases where the VLSFO product met ISO 8217 specification limits for TSP at time of bunkering and subsequently became unstable within a matter of weeks.
The shelf life of some fuels has decreased significantly, which couldn’t come at a worse time when some operators are placing their vessels into lay-up because of the economic impact of the COVID-19 pandemic.
*Michael Banning of fuel additive specialists Innospec told us: “Some blend components come from unstable refinery streams and can result in ‘distillate ageing’, a chemical process that can produce sediments, gums and polymers. On vessels, it is most commonly triggered by a rise in fuel temperature, such as heating during storage or when it is passed through a centrifugal separator. In general, a lower viscosity(below 100cSt) indicates a higher proportion of distillate components, leaving them more susceptible to distillate ageing.”
The ageing process can be prevented but not reversed by using additives. As always, it is very important that additives are used in consultation with the provider.
Confusing cold flow properties
VLSFOs with a high paraffinic content are more vulnerable to wax formation if the temperature drops below a certain point.
As we discussed in our article ‘New fuels and heat sensitive cargoes’ in Signals 119, the traditional ‘Pour Point +10°C’ rule of thumb for determining fuel storage temperature isn’t always reliable for VLSFOs. Depending on a fuel’s paraffinic content there is a risk that wax can still form at temperatures higher than this.
This is why the fuel testing company Veritas Petroleum Services(VPS) uses the wax appearance temperature(WAT) and wax disappearance temperature(WDT) tests to assist shipowners in identifying suitable storage temperatures of fuel.
*Steve Bee of VPS told us that the WAT of a VLSFO can be up to 22°C higher than its pour point. Furthermore, the WDT is currently on average 11°C higher than the WAT. With a wealth of test data at their disposal, VPS has found that around three quarters of VLSFOs tested had a WAT of 31–40°C, which emphasises the importance of storing the fuel at the right temperature to avoid waxing.
Early experiences suggest that shipping has adapted well to the
switch to the new very-low-sulphur fuel oil (VLSFO) products,
but the huge variability in their characteristics has brought issues.
Think before heating
Getting the right fuel temperatures for storage, treatment and burning is vital to prevent any operational problems on board.
VPS has provided some key points to remember when determining these temperatures for VLSFOs:
- VLSFOs that are very low in viscosity but have a high WAT and WDT need to be heated to ensure a stable flow and prevent wax formation. However, if heated too much, the stability will collapse quicker, potentially reducing the shelf life of the fuel.
- The problem is exacerbated if a low-viscosity fuel also has a high cat fine content. It becomes difficult to heat the fuel to the required levels for effective separation.
- Fuels on the verge of instability contain sediment which falls out of suspension when heated. This may mean the fuel temperature will need reducing in accordance with the centrifugal separator maker’s guidance.
- There is a risk that the fuel’s WAT/WDT is higher than the tested flashpoint.
Accelerated engine liner wear
We have seen an increase in reported engine liner wear, and this is supported by industry feedback and a recent VPS whitepaper. VPS suspects that recent engine damage on vessels burning VLSFOs may be related to the cylinder oil used.
The reduced sulphur content in fuels means fewer acids forming during combustion. This has led to a change to cylinder lubricating oils with a lower base number (BN), which is a measure of the oil’s ability to neutralise acids from this combustion process. If the BN is too high, it can result in calcium compounds being deposited on the top of the piston crown. These hard deposits are abrasive and can cause liner wear, scuffing or piston ring breakage.
The reduction from 70BN to 40BN cylinder oil may not be enough, so further reductions may be required in accordance with engine makers and cylinder oil providers.
Check your wear
The more you monitor your engine and how it is reacting to changes in fuels, cylinder oil and feed rate, the better chance you have of preventing damage.
Scrape-down analysis is a great way to measure liner wear in the early stages and ‘fine-tune’ cylinder oil feed rate.
The frequency of scavenge port inspections should be increased when new cylinder oils are being used or different fuels are being consumed.
VPS has recently issued several bunker alerts where flashpoints have been below 60°C, which is the minimum temperature required under SOLAS.
Some industry experts have questioned whether reduced car and aviation fuel usage during the COVID-19 pandemic has led to refineries pushing out more volatile blends made from this lower-than-normal priced fuel.
If your fuel has been tested with a flashpoint below 60°C, you must inform your classification society and flag state and be guided by their advice.
Thanks to the following for their help with this article:
•Michael Banning–Innospec Fuel Specialities LLC https://www.innospecinc.com/
•Steve Bee & Ian Crutchley–VPS Fuel Testing Services https://www.v-p-s.com/
■ Contact: https://www.nepia.com/topics/2020-vision/