Residual fuel oil viscosity-temperature-pressure diagrams. 1

[21121956]

Hello everybody:

I am analysing some combustion problems in our Diesel engines. They burn Bunker C and the manufacturer´s manual indicates on a temperature-viscosity diagram the characteristics of the fuel at the moment of injection into the combustion chamber.

I have read a graphic (by Mobil Oil Company Ltd.) where does appear the average viscosity-temperature-pressure relationships for residual fuels. I have attempted to obtain the full paper on the web, but it does not work.

These diagrams differ a lot and I am a little bit confussed.

Can somebody help me indicating another source of information regarding with this subject?

Thanks a lot

Ricardo

 

[jmw]

Some good web sites to visit for information are

http://www.worldbunkering.com

and see the article on legal issues. This suggests that compliance with ISO 8217 is not necessarily sufficient and there are also articles on the problems associated with low sulphur fuels. See also the statement on low sulphur fuels by the Heavy Fuels working group on the CIMAC web site

http://www.cimac.com.

There are various other sites which define fuel quality issues such as

http://www.viswalab.com

and

http://www.IBIA.net.

Many of the test houses also have fuel quality issues discuessed on them such as DNV, FOBAs etc.

However, the engine manufacturer may be the best guide as they can provide detailed information on the impact of changing fuel qualities on their engines and the necessary steps for older engines on current fuels.
Caterpillar-MAK, for example, provide a fuel operation envelope that is defined in terms of the density at 15degC, the kinematic viscosity at 50degC (or 100degC) and the ignition index (CCAI as per BS MA 100).

Typical injection viscosities may range from 10-15cst i.e. lower than for burners. There is also a temperature limit above which damage may occur. This effectively limits the maximum fuel viscosity as the higher the viscosity the higher the temperature to which the fuel must be heated to obtain the correct operating temperature.
e.g. an engine with a maximum temperature of 150degC can probably use a 380cst (at 50C) fuel heated to 141degC to obtain an 11cst viscosity for injection.

Many oil companies provide a free fuel calculator as do some test houses. These not only allow fuel properties calculations for fuel blending but also allow the calculation of other properties such as injection temperature, specific energy etc.

Exxon Mobil Marine fuels provides just such a calculator as does Shell Marine Fuels and DNV PS. (these are usually free; some others offer similar programs for sale).

For a fuel viscosity temperature spreadsheet based on the ASTM D341 equation visit

http://www.viscoanalyser.com/xls.html

Notice that this spreadsheet is populated with typical fuel temperature viscosity data for ISO 8217 fuels.
Fuel quality is notoriously variable (13.7% of marine fuels sampled are found to be off-spec to ISO 8217 in one or more parameters; 6% off-spec for density i.e. too high; 7% off-spec for viscosity i.e. too high or too low) and each batch of fuel should be sampled and analysed. This laboratory analysis should provide the kinematic viscosity at both 50C and 100C. This data can then be used to find the correct temperature for injection.

Note: there are various other proprietary equations used for the fuel temperature viscosity relationship than the ASTM D341 equation but care needs to be used. I have encountered some operators using such equations and misapplying them.

Your post title indicates pressure but you do not elaborate. The pressure effect on density and viscosity is not normally an issue. The only pressure problems I have personally encountered are where the fuel contained excess water (to be limited to 0.5% in the new IS)8217 standard) which required the presure in the high pressure circuit to be increased significantly. This was where there was so much water the centrifuges couldn't manage it. This fuel was discontinued. Incidentally, another check is to monitor the fuel density at 98degC to ensure that the centrifuges are correctly set up.

At least one large diesel engine manufacturer exploits the functionality of the digital viscometer used to control the fuel heater to monitor and log the following data for evaluating fuel quality issues:
[ul]
[li]density at 15degC[/li]
[li]kinematic viscosity at 100degC[/li]
[li]viscosity at operating temperature[/li]
[li]operating temperature[/li]
[li]ignition index (CCAI)[/li]
[li]density at 98degC[/li]
[li]density at operating temperature[/li]
[/ul]

JMW

http://www.ViscoAnalyser.com

 

[21121956]

jmw:

Thanks for your reply.
With regard to the pressure of the fuel at the moment of the injection, please let me reproduce partially the explanation given on that document I am talking about:

"... this showed that the very high fuel injection pressure now common in many modern crosshead and trunk-piston Diesel engines increases the viscosity markedly. To compensate for this, thus ensuring a suitable viscosity at the injectors, it is now necessary to increase the preheat temperatures appreciably".

Always on the same document we can read: "This appendix reproduces a chart which enables a suitable fuel injection temperature to be determined, knowing the required injection viscosity, the injection pressure and the initial viscosity of the residual fuel, in this case based on kinematic viscosity in centistokes at 50ºC".

And more: "With increased fuel injection pressures now becoming common, the viscosity at the fuel pump would need to be appreciably lower and the preheat temperatures even higher assuming the heater capacity to be adequate".

Ricardo

 

[jmw]

In which case i am now very interested in the chart.. do you have a URl for it? I will need to look for this myself.

JMW

http://www.ViscoAnalyser.com

 

[21121956]

jmw:
I have been trying to send you the diagram to the electronic address you gave me, but it is not possible.

Can you give me another e-mail address?

Thanks

Ricardo

 

[jmw]

Hi Ricardo,
thanks.
Please use the "info at viscoanalyser dot com" address. (sorry not to put it into correct email address form, the site management don't like actual email addresses posted for lots of good reasons)

JMW

http://www.ViscoAnalyser.com

 

[jmw]

Ricardo,
I didn't receive anything. Try going to my website (URL below my signatiure) and using the links on the conact page.

JMW

http://www.ViscoAnalyser.com

 

[jmw]

Ricardo,
thanks for the chart but you'd better disregard what I said in my reply email.
I have now engaged brain and given it some more thought.

I suggested that for engines this should not normally be a problem as they measure the viscosity online and do not usually calculate the injection temperature.

However, it may not be this simple and you may need to ask the engine manufacturer some pertinent questions.

Fuel oil heater control is normally done using a viscometer in the high pressure circuit to the engine. This has been the standard for over 40 years.

What has changed recently has been, for some engines, the pressure in the high pressure circuit. I suggested that if the pressure increased and caused an increase in the viscosity that it wouldn't matter since the viscometer would report the higher viscosity and hence the heating would increase to maintain the optimum viscosity. Idiot. The comment in the article was that injector pressures have been increasing which is not the same thing.

The problem is a real one if the pressure at the injectors has increased and is significantly higher than in the high pressure fuel circuit to the engine where the viscometer is installed.

This means that if the injection pressure has increased significantly then if the original set point for heater control was 14cst then this viscosity would increase at the injectors to some value above 14cst e.g.20cst. However, if the injector pressure has increased such that this viscosity is now 30cst say, then the viscosity set point in the high pressure circuit should be reduced to reduce the injection pressure viscosity back to 20cst.

Thus the viscometer set point would also be reduced, but to what value if the pressure in the high pressure circuit is also significant and has also increased?

Here is where the chart is a bit simplistic. The temperature viscosity data presented is at atmospheric pressure. This is fine, if as shown, one computes the injection temperature based on the injection pressure and the optimum viscosity at the injectors.

However, if a viscometer is used to control heating it is necessary to know the pressure in the high pressure circuit also as it will record a higher viscosity than derived from the atmospheric temperature viscosity data i.e. it will be measuring a viscosity somewhere on the line between B and C.

The conventional place for the viscometer is in the high pressure circuit where pressures have been increased for some engines. The questions for the engine manufacturer are:
[ul]
[li]What is the pressure in the high pressure circuit?[/li]
[li]What is the injector pressure?[/li]
[li]Is the optimum viscosity reported as the optimum viscosity at the injectors or the optimum viscosity as measured by the viscometer?[/li]
[li]If injector pressures and or pressure in the high pressure circuit have changed, does the optimum viscosity reflect this? e.g. as the measured viscosity by a viscometer in the high pressure circuit.[/li][/ul]

In other words, if the manufacturer recomends 14cst as the optimum viscosity, are they refering to the injector viscosity? or are they refering to the viscosity measured in the high pressure circuit?

The problem posed is that both pressures could have changed and hence the questions to the engine manufacturers.

Incidentaly, I am supposing that for burner operations the problem is irrelevant since the pressures are much lower and it doesn't matter whether they use temperature as a set-point or viscosity, where they use viscometers for heater control... which isn't many since historically viscometers have not worked so well with burners as with engines. But I am ready to be corrected on this as well.

JMW

http://www.ViscoAnalyser.com

 

[jmw]

I contacted an engine manufacturer that uses the digital viscometer for heater control.

They advise:
"We are still pre-pressurizing the engine circuits by a 10bar pump, the circulating pumps have 5bar, so adjusted, that the resulting pressure in front of the injector pumps is around 5bar. But the injectors are bringing a pulsation (firing frequency) of approximately +/-10bar into the system."

They confirm that the viscometer works perfectly in this environment i.e. no significant pressure induced viscosity offsets.

Note that their comment regarding pressure pulstaion applies to the fuel supply to the injectors. They did not comment on the pressure after the injector pumps.


JMW

http://www.ViscoAnalyser.com