2 replies to this topic

[PMGreen]

Me again...
I´m just wondering if anyone out there has any practical experience with in-line measurement of viscosity.
I'm quoting a viscometer for fuel oil service and the technology that caught my attention is the oscillation principle (looks simple and compact instrument). But I'm not quite sure if this is the best choice, in terms of reliability and maintenance. Accuracy is not really an issue.

I hope someone could help. Thanks in advance!

[JMW]

Assuming heater control: you do not state if you have any particular manufacturer in mind: not all vibrating element sensors are alike and in some cases the differences can be significant.

My comments are mostly related to the 7829 ViscoMaster digital viscometer (with which I am most familiar) as this technology has become almost an industry standard for marine engine applications, and it is probably the first sensor to satisfactorily cross over to burner applications, though when it comes to black liquor (paper Industry) the Nametre was probably there first.

While not all such sensors are equivalent, they are mostly superior to the established technology and especially as regards maintenance, cleaning and re-calibration.

The 7829 was developed especially for this industry, for this application and, more importantly, for marine applications where the demands are more than for land based.
The prototype trials were very extensive and the only problem to emerge was asphaltene coating which can occur when the changeover between HFO and Distillate takes place.
This problem was solved by both the design of the installation and by a non-stick" coating so as together to make the sensor self-cleaning.

As for accuracy, as you suggest, it isn't generally critical and some sensors have been used satisfactorily that have been only 5%fsd but this is also often associated with a poor response time.
A fast response time means that the system can respond more effectively to temperature and quality transients which can mean a more stable quality at the engine and better efficiency.
The success of some vibrating sensors is such that they have successfully crossed over to burner applications where it was more usual to use temperature as a control parameter and to change the temperature set point in respect of; laboratory measurement of samples, and flame inspections and to also increase the excess oxygen.
There is an article by Huntsman about their use of the fork sensor in burner applications which is said to have been very cost effective. In trials required by the EPA, one power generator was expected to identify a suitable viscometer and install duty and stand by units at each burner.
The success of the trials with the vibrating element type were such that they were able to get by with just the duty viscometers.

Not all vibrating element viscometers are equal.

Not all need to be as fuel oil heater control does not need to be a demanding application and for vibrating element sensors, it si a benign application.
Accuracy is far from critical; some technologies have only been +/-5% fsd accurate and still been satisfactory.
While, low maintenance, lack of any skills requirements, a lack of a need to re-calibrate are all important parameters they may be less important in land based applications than in marine applications due to the availability of external support.
So, if you want an instrument that will be online virtually indefinitely, will not need cleaning, will not need re-calibration, will not give any problems and will operate this way year in and year out, you can have it.
Most vibrating element sensors will exhibit these properties to some degree or other and at least one will exhibit all these properties to the fullest degree. The contrast with the original technologies (mostly twin capillaries) is very marked.

I can suggest that self-cleaning may not be so essential for land based as it is for marine which allows a wider choice of sensor. Many applications will be well served by dynamic viscosity measurement only but for marine I would strongly recomend the 7829 because it also measures density and hence reports kinematic viscosity and other parameters such as Ignition Index.
I would suggest that if you visit the web site www.viscoanalyser.com (or the archive site www.viscoanalyser.co.uk) you will find extensive coverage of heavy fuel oil applications and many questions answered.
You might like to collect data on a range of such instruments and compare them. I would suggest you consider Nametre, Hydramotion, VAF and TDC as well as Mobrey's 7829.

PS note that most sensors measure only the dynamic viscosity. SO far the 7829 is probably unique in measuring both density and dynamic viscosity and so can report the true kinematic viscosity. A report for the US Navy suggests that perhaps they should test for dynamic viscosity only but why isn't made clear. The main point is that kinematic viscosity is density dependent and density can be very variable and is likely to be increasing due to the environmental influences on fuel oil production.

[PMGreen]

I really appreciate your advice, it has been very usefull.
In my case it is a land application, and mobrey it's one of the models I had in mind.

Thanx very much and regards