THE CHALLENGES OF VISCOUS FLUIDS

Measuring flow in fluids with viscosities above 100 cP requires special consideration by those responsible for operating and maintaining the equipment.  In addition to heavy crude, other challenging high-viscosity products include heavy fuel oils, lubricating oils, bunker fuels, and grease components. In most cases, flow requires elevated temperatures and flow meters must be reliable in this environment.
Ultrasonic meters have emerged during the last 10 - 15 years as the choice for measurement that offers improved accuracy and cost / benefit ratios for high viscosity fluids. However, there are challenges associated with ultrasonic metering in such applications. In broad terms, these challenges are two-fold:
  1. Being able to transmit a good quality signal through the viscous fluid such that it can be detected by the receiving transducer
  2. Being able to integrate the multiple ultrasonic path measurements accurately and compensate for the variation in flow profiles that higher viscosity fluids can generate

SIGNAL QUALITY AND DETECTION, AND TRANSDUCER SELECTION

Transit time liquid ultrasonic meters are inferential type meters that derive the liquid flow rate by measuring the transit times of high-frequency sound pulses. Transit times are measured from sound pulses traveling diagonally across the pipe, downstream with the flow and upstream against the liquid flow. The difference in these transit times is related to the average liquid flow velocity along multiple acoustic paths. For the meter to work properly in high viscosity applications, the received signal should be of high quality in order for its transit time to be measured.
The first challenge in high viscosity applications is that the sound pulses get more readily absorbed by the fluid. This means the quality of the signal deteriorates (attenuates) and it becomes increasingly difficult to make the timing.
When selecting an ultrasonic meter for high viscosity applications, choose a meter with a transducer that can overcome the absorption levels and attenuation caused by higher viscosity fluids. Daniel’s high viscosity ultrasonic transducer together with superior digital signal processing of received signals allows the Model 3804 Liquid Ultrasonic Meter to operate in fluid viscosities in excess of 1,000cP in many applications.

HIGH VISCOSITY AND ITS IMPACT ON FLOW PROFILE

The second challenge in high viscosity applications relates to flow profile. As fluid flows inside a pipeline, it does not have the same velocity across the entire diameter. The flow is faster in the center of the pipe than it is at the pipe wall. The distribution of flow velocity is called the flow profile. The shape of the flow profile depends upon the following parameters:
For fully developed flow, there are two types of flow profile: laminar and turbulent. Figure 1 shows representations of these two profiles.
Figure 1. Laminar and Turbulent Flow Profiles*.
 
Typically laminar profiles have relatively high viscous fluid flows inside pipes with small pipe diameters at low mean velocities, whereas turbulent profiles have relatively low viscous fluid flows inside pipes with large pipe diameters at high mean velocities.
Flow regimes can be defined by using the non-dimensional parameter called Reynolds number which is calculated using

Reynolds Number Formula

Where Re = Reynolds Number
Density= Density
U = Fluid velocity
D = Pipe Diameter
Dynamic Viscosity= Dynamic Viscosity

When Re ≤ 2300, the flow is nominally laminar, and when Re ≥ 5000, the flow is nominally turbulent. The flow profile is considered in the transition region when Re is between 2300 and 5000 .
Daniel uses computational, analytical and empirical testing to fully characterize its Model 3804 Liquid Ultrasonic Flow Meter over an extremely wide range of Reynolds numbers, and consequently minimize viscosity effects on flow profile.

ADVANTAGES OF USING ULTRASONIC METERS IN HIGH VISCOSITY APPLICATIONS

COUNT ON DANIEL 3804 LIQUID ULTRASONIC METER TO HANDLE THE VISCOSITY

Daniel 3804 Liquid Ultrasonic Meter provides best-in-class performance for a wide variety of high viscosity applications including crude oil, fuel oil and refining applications. Daniel has conducted significant research in recent years on the topic of high viscosity ultrasonic metering. The two biggest challenges, viscosity and flow profile, are now well understood and solutions to their challenges have been developed. For further information, contact Daniel Measurement and Control at 1.888.flow.001.
* Illustrations powered by ANSYS®.