Mixing tanks are used for more than mixing, they are also used for venting and draining and for sample taking. They usefully help protect the system from excessive step changes in fuel temperature reaching the heaters and help ensure efiicnet change over during start-up or shutdown when MGO and HFo are switched.
Fuel velocity: many systems are designed with the high pressure pump rated for twice the maximum engine consumption rate. There is a desire to reduce this and there may be added benefits from doing so but one reason for these flows is to help prevent excessive temperature swings in the fuel arriving at the heater. More responsive viscometers and heaters can help reduce the circulation rate.
Temperature control is generally used in burner applications, though modern viscometers are successful in these applications too (fuel quality is the killer, fuel to engine is generally treated while to burners it is not and it is often this difference that accounted for viscometer failures due to contamination in burner circuits).
Most (all?) fuel modules for engines depend on viscometers.
Originally a variety of technologies were used but the twin capillary became the dominant technology for the best part of 30 or 40 years. Many of the instruments in service are of this type.
Modern replacements include the shuttle type from Cambridge and a variety of vibrational types.
Most of the vibrational types are only able to provide dynamic viscosity and some are not that accurate, which is OK for simple heater control.
Increasingly popular is the fork type digital viscometer which also measure the density and the pendulum type dynamic viscosity only from VAF. More functionality from the viscometers is valued and extends their role from simply controlling the heater to fuel quality checking.
I'll pass on the proximity of components beyond saying that if looked at purely from the heating control point of view, the viscometer needs to be very close to the heater and the heater module close to the engine to optimise the fuel heater control function and good insulation applied.
The days of 1 fuel supply system are probably about to disappear. The introduction of sulphur limits and the need to operate of different fuels in different sea areas is going to add complexity.
This becomes an especial problem when port operations are considered where even more strict sulphur limits apply and where the MEs are shutdown and the AEs continue. This is a serious problem and portside power facilities is one option to avoid having to complicate the fuel supply systems.
Anyone reading the draft Statutory Intruments may be concerned to discover that when prosecuted, a defence is to have taken "reasonable" care and excercised "due dilligence".
In other words, the legislation expects that having taken an approach which essentially depends on the "honour" system of accurate sample taking and record keeping, if there are problems, then the courts will sort it out.
The legislators do know what they will do if that is the case. The possibility (probability?) is that if (when) the currently proposed system of BDNs, samples and record keeping proves too much of a temptation to those who would falsify them, that X-Ray Fluorescence Sulphur analysers will be made mandatory in the engine fuel circuits.
If this heppens then it will also mean a denity meter in each of the fuel feeds and pulse transmitters on each of the fuel flow meters; they propose to derive the actual mass rate of flow of sulphur to the engines and to remotely (via satellite) monitor sulphur consumption from the moment a vessel enters a SEAC to the time it leaves.
Modern digital viscometers may provide an alternative through "fingerprinting" but this depends on how the industry and the legislators view this as an alternative.
For more information, try a web search for MARTOB and STEAM (the two main research projects undertaken by the EU) and visit the IMO site for information on Annex VI of MARPOL.
There are a variety of other reports and comments out there to be found and read but you will find discussion of fuel supply system changes in some of the MARTOB public reports.
