Issues associated with the use of low viscosity distillates
Admittedly there are issues that must be faced and addressed when considering long term use of distillate fuels in large marine and stationary plants. Here are a few areas for consideration and discussion.
1. Lubricity issues of distillates versus residual fuels.
2. Suitability of existing equipment for use with distillate fuels.
3. Operational safety when using distillates in large marine auxiliary and propulsion boilers.
Lubricity concerns when using distillate fuels in large marine and stationary engines runs in parallel with the resultant decrease in viscosity during the change over from residual fuel (HFO) to MGO/MDO. The system temperature of the fuel injection components is often in the range of 135 to 140 Deg. C. The change over process involves the gradual shift from HFO to distillate through integration in the plants mixing tube or tank and is done slowly to avoid thermal shock and resultant seizure of injection system components. The mass of such components further complicates this process as the tend to hold residual heat. This heat results in a reduction of the viscosity of the distillate often for extended periods after the full shift to distillate has been accomplished. Most injection system manufacturers mandate a minimum viscosity at the pump of 2 cst. This is to maintain the necessary liquid cushion between reciprocating and/or rotary components of the system thus avoid metal to metal contact. Many components are surface treated to reduce erosion and reduce the effects of reduced clearances. One must never forget the effect of "compressive swell" and resultant heat of the plunger of an injection pump just prior to the point of delivery. One method which has been used for more than 30 years is the fitting of a distillate cooler or in some cases today an actual refrigerated chiller in for very large unifuel plants.
Note : None of the above issues are insurmountable. A bit of system modification and again "prudent operations" by shipboard staff is all that is needed. The big issue that is often controlled "from the office" is maintaining optimum injection system component health through constant vigilance to expose internal wear, particularly of the injection pump plungers and bushings and prompt refurbishment when indicated. A hint to the shore side team ~ step back, take a moment to ponder and remember whose ticket is hanging in the rack aboard ship and the financial consequences of a failure to start due to worn components. Another point to ponder is the use of "pirate spares" when doing injection system maintenance. One will generally see that the use of OEM spare parts in all aspects of maintenance pays off in the long run.
Years ago there were issues involving some large four stroke auxiliary engines when operated on MGO. Excessive wear of the primary booster pumps was encountered. Investigation revealed the internal metallurgy of the booster pumps both helical gear and internal gear types was not suited for the lubricant properties of distillate fuels. The fix in this case was replacement of the booster pumps with ones designed for use with distillate MGO. The mind set of the period which continues today is operating auxiliaries on MDO (DMB/DMC) which generally had a higher viscosity and better lubrication properties. This is one area where the booster pumps used with marine auxiliary and propulsion boilers may require the installation of dedicated pumps for the distillate loop.
Boiler operational concerns include suitability of pumps and atomizers for use with distillate fuels. This should be confirmed with the actual boiler manufacturer. Major builders such as Aalborg and Mitsubishi have issued service letter or bulletins dealing with this. Realizing the fact that getting information from the myriad of licensees regarding upgrades and operations is difficult, head to the actual designer for assistance. This is also a good point when dealing with diesel engines and fuel handling systems such as separators, filters, etc.
One of the primary safety concerns of boiler operations on distillate fuels is the increased gassing inherent with distillates when compared to residuals. This is extremely critical during initial firing and flames outs. Looking into the service bulletins from some boiler builders will show that the installation of additional flame scanners and/or repositioning of these may be required. Reprogramming of purge cycles may be indicated as well.
Bottom line? When in doubt work with your OEM and keep the safety of crew, vessel, cargo and the environment in mind at all times. We are not going to beat the distillate regulations through waivers or exemptions. The regulatory agencies can put 2 and 2 together when seeing who continually applies for such exemptions. As engineers let's show what we can do to comply and keep the environment green as opposed to seeking waivers and exemptions. An analogy which can be applied to an operating marine engineer is John Deere's quote "I'll not put my name to anything that does not have in it all the best that's within me" aka ~ You sign the logbook!
Keep the lights burning and Safe Voyage!
Link to Aalborg Website with regards to MGO changeover. Many good service letters on their site.