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The dual Viscometer Method:

This method is based on the use of the ASTM D341 equation for temperature Vs viscosity:

Log10.log10(+0.7) = A-B.log10(T+273)

Where  is the kinematic viscosity (cst) at temperature TC.

To solve the equation the values of “A” and “B” must both be found as they usually vary significantly, even with small changes in quality.

The values of “A” and “B” can be found by substituting the viscosity of the fluid at two different temperatures.

Once “A” and “B” are known the equation can then be solved to find the viscosity at the reference temperature.

Key features:

• Two viscometers are used and separated by a heat exchanger
• Very accurate
• Unaffected by product quality changes
• Unaffected by Viscosity Index changes
• Continuous self-calibration of “A” & “B”
• Suitable for virtually all hydrocarbons
• Pipe side installation
• Fast response (typically less that 20 seconds control response)
• Ideal for closed loop control
• Low maintenance and virtually no operator intervention.
• Very tolerant of process temperature variation
• Precise temperature control is not essential
• Very flexible; provides viscosity at multiple reference temperatures including where the reference temperature(s) is itself a variable.

How it Works:

In this method, two viscometers are used, each measuring the viscosity at a different temperature by virtue of an intervening heat exchanger.

The values of “A” and “B” are determined in real time and at each calculation up-date using the live measurement of viscosity at the two different temperatures.

It is quite independent of any variation in fluid properties as there are no assumptions to be made. It is especially suitable for lubricating oils, for example, where the different Viscosity Index Values from one oil to another preclude using the equation or multi-curve methods.

This system provides accurate base viscosity determination with a typical control response of less than 20 seconds.

In process control the degree to which quality can be controlled depends on two factors, the accuracy of the measurement which in this case is comparable to the process capillary viscosity  analyser, and the speed of the control response. In this case the speed of response is much better than the capillary system for so long the oil industry standard and this means that quality can be more accurately controlled with significant savings in “give-away”.

Typical Applications:

• Refinery blending of bitumens and asphalts
• Refinery blending of heavy fuel oils
• Oilfield blending of crude oil and distillate for pipe line transport
• Lubricant blending
• Asphalt or bitumen blending

Benefits:

It can deliver a high level of confidence in the measurement with minimal operator or instrument engineer intervention.

Indeed, this method can be used where all that is known is that the fluids are hydrocarbons.

In refinery operations this system can be very powerful.

Of all the methods available this is the most universal in application.

Limitations:

There are very few limitations on this method.

However, although the most universally applicable solution, because it is more complex, it is not usual to consider it where other solutions are viable except where the cost to benefit ratio justifies its use.

It is generally the scale of operations in refineries and in large terminals or where high value refined products are to be measured (such as lubricants) that  provide the optimum cost to benefit ratios.

A dual viscometer using an electric heater to increase the fluid temperature to the second viscometer.

This system was designed for a crude oil cut-back system.

A schematic of the installation.