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 3.3 Turndown Ratio. Turndown ratio, also known as rangability, is the ratio
of the maximum flow rate a meter can accurately measure to the minimum flow
that it can accurately measure.
3.4 viscosity. Viscosity, which influences meter selection, may be defined
as the ease with which a fluid flows when it is acted upon by an external
force. For example, water flows more rapidly from a container than heavy fuel
oil. Under this condition, water is said to have a lower viscosity than fuel
oil. An analogy for gases is somewhat more difficult to visualize, but it is
sufficient to know that the viscosity of gases is extremely small compared to
liquids. When a fluid flows through pipes, viscosity manifests itself as
pressure losses and distortion of the velocity profile.
4. REYNOLDS NUMBER. The Reynolds Number describes whether a flow will be
laminar or turbulent. The Reynolds Number is the ratio of inertia forces to
viscous forces. This dimensionless ratio is expressed as:
where:
R = Reynolds number
P = fluid density
V = average pipeline velocity
D = inside pipe diameter
= fluid viscosity
Fluid viscosity and specific gravity are obtained from reference tables. Such
tables are available from many sources. Engineering handbooks and
manufacturer's data are the most convenient sources.
4.1 Laminar and Turbulent Flow. The difference between a laminar and
turbulent flow is shown in Figure 3-1. Laminar flow is described as a smooth
flowing liquid. It flows in concentric layers around the center of the
stream. The velocity is highest at the center and decreases to the outer edge
of pipes. Laminar flow exists when the Reynolds number is approximately 2,000
or less. In applications with low velocity fluid flows or high viscosity
media, the flow is usually laminar. If the Reynolds number is greater than
approximately 3,000, the flow is assumed to be turbulent. Turbulent flow as
contrasted with laminar flow consists of a large number of erratic eddy
currents as shown in Figure 3-1. This results in a more uniform velocity
profile from the center of the flow to the pipe than is found in a laminar
flow; however, if the meter is not designed for this type of flow, its
accuracy may suffer. Most applications involve Reynolds numbers above 3,000
due to either high velocity or low viscosity of the fluid. When flowing
fluids have a Reynolds number between approximately 2,000 to 3,000, the flow
type is unpredictable. This is known as the transition range and flow range
may be either laminar, turbulent, or a combination. It should be recognized
that the point where the flow characteristics change is an approximate number
3-2
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