Viscosity measurement and control – even more vital in an age of increasing fuel options

In recent years the drive to decarbonise shipping has led to a drive to find alternative fuels and propulsion systems for ships.

by Adam Foster

Shipping’s decarbonisation has been the main driving force behind new regulations over recent years and inevitably that affects shipowners’ thinking when ordering new vessels. However, whilst the future may be a decarbonised one with ships running only on hydrogen and ammonia, the fact is that most ships in the world fleet are still reliant on oil fuels for their operation.

A new whitepaper released by Rivertrace examines the issue of viscosity control during fuel switch over and also introduces Rivertrace’s new SMART VISCO sensor which uses ultrasound as a more reliable and robust alternative to previous methods of measuring viscosity.

Even when it is considered that in 2022, alternatively-fuelled ships appeared to dominate the new orders, there is no reason to think that the dominance of oil will end anytime soon. A Clarksons Research report on 2022 new orders shows 61% of tonnage could run on alternative fuels. But that figure was achieved by tonnage and in terms of numbers that only accounts for 35% of the orders.

More to the point, last year’s orders were dominated by LNG carriers and large container ships where LNG is an obvious choice in the former and a trend in the latter. It was also a depressed year for the bulker and tanker sectors where oil burning engines dominate. What should not be forgotten as well is that almost all of the ships ordered have dual-fuel engines and can run on oil when situations demand. It could be argued that owners are hedging their bets on the future fuel mix rather than determining that oil has no place to play.

An engine for all options

The inventor of the engine, Rudolf Diesel, never initially intended his brainchild to run on oil fuels but it soon became clear that oil fuel and the Diesel engine were very compatible. So much so that more than a century after the marine diesel engine made its debut, it is still the propulsion system of choice for virtually every ship in operation today.

The biggest advantage that the engine has is its ability to operate on so many different types of fuel. As well as mineral oil fuels, be they residual heavy fuel oils or lighter distillates, a diesel engine can run on LNG, LPG, methanol, ethanol, biofuels, ammonia or hydrogen. The adaptation technology for the latter two fuel types is presently at an immature stage but prototype engines have been developed and it is anticipated that both fuels will become established options in the very near future. If the future of marine fuels is somewhat hazy, one thing that will not change is that ship operators need to keep costs low and for many if not most that means selecting the cheapest legal option.

When the 2020 rules on SOx became effective, only ships with scrubbers could continue to burn conventional heavy fuels with impunity but even they have been obliged to switch on occasion as washwater restrictions have been imposed at various places around the globe. For the rest it was a case of making use of new low sulphur fuels at sea and switching to MDO or MGO in ECAs and other controlled areas. Even before environmental regulations made switching fuels a desirable option, it was often the case that ships would switch to a lower viscosity fuel when manoeuvring in port because it made the engine more responsive to changes in load.

In the past switchovers would normally have only been necessary when going from a heavy or intermediate fuel oil to MDO/MGO but with modern blended and low and very low sulphur fuels, viscosity can vary between different bunkerings even though both fuel types meet the same ISO 8217 requirements at the supplied temperature.

Complicating factors around switching fuels

Different oil fuels have different combustion requirements both as regards temperature and viscosity. These two factors need to be well monitored and managed to avoid problems. The problems include potential for power loss, engine damage and fire or explosion. A highly experienced engineer may be able to manage the switching process manually when dealing with oil fuels but as the permutations multiply, then incorporating an automatic switching system into the fuel system is an obvious step. But even with an automated system, challenging and potentially dangerous situations do occur.

Three main factors needed to be controlled during fuel switch between heavy fuels and lighter distillates. These are viscosity, pressure and temperature. Highly viscous fuels such as HFO need to be heated to high temperatures to flow freely. The high temperatures present in the fuel lines can cause low-flashpoint distillates to ignite leading to loss of power and worse.

On the other hand, switching from a low viscosity distillate where fuel lines and injectors will be cool to a higher viscosity fuel that needs to be heated may cause interruptions in the fuel supply to the cylinders. The issue of loss of power during fuel switchovers is a well-known hazard and one that the USCG, P&I clubs, engine makers and fuel and lubricant suppliers have issued a number of safety alerts over.  In addition to these, time is also an important consideration. Because time is needed for the temperatures and viscosity to be adjusted, beginning the process too late can mean the process has not been completed when the ship enters an ECA or other emission controlled area.

The switchover process can be long-winded, and the various hazards need to be taken into account. In addition to the risk of fire, low-sulphur fuels may damage existing HFO pumps because of reduced fuel oil viscosity and lubricity leading to overheating and excessive wear. Fuel injection pumps can be similarly affected necessitating their replacement by special equipment such as tungsten-carbide-coated pumps.

Introducing ultrasound

A new whitepaper released by Rivertrace examines the issue of viscosity control during fuel switch over and also introduces Rivertrace’s new SMART VISCO sensor which uses ultrasound as a more reliable and robust alternative to previous methods of measuring viscosity. Due to the high frequency and speed of ultrasound, ambient vibration and flow speed have no impact on the measurements. The monitor incorporates an LCD display that visually indicates all parameters and can report data via analogue and digital outputs with volt-free relay contacts. The viscosity and temperature data can also be easily retrieved using RS485 connections or a USB stick.

In addition to viscosity measuring and connection to viscosity control systems, SMART VISCO, with its digital capabilities can be integrated with the RIVERTRACE CONNECTED service to collect and share the data with ship management to monitor and analyse trends in fuel performance at both a vessel and fleet level.