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systematic uncertainty should be added to the uncertainty in the discharge coefficient to account for
swirl effects downstream of turns (3). Unorthodox installations require meter calibration to
determine flow coefficients or the use of special correction procedures (3, 7). The physical size
of a meter may become an important consideration in itself, since in large-diameter pipelines,
venturi and nozzle meters can take up a considerable length of pipe.
Pressure Loss
The unrecoverable overall pressure loss, Dp loss , associated with a flow meter depends on the b ratio
and flow rate (Fig. 10.7). These losses must be overcome by the system prime mover (e.g., a pump,
fan, or compressor driven by a motor) in addition to any other pipe system losses. In established
systems, flow meters must be chosen on the basis of the pressure loss that the prime mover can
accommodate and still maintain a desired system flow rate. The power, _
W, required to overcome any
loss in a system for flow rate Q is given by
_
W ¼ Q
Dp loss
h
ð10:18Þ
where h is the prime mover efficiency.
Costs
The actual cost of a meter depends on its initial capital cost, the costs of installing it (including
system down time), calibration costs, and the added capital and operating costs associated with flow
meter pressure losses to the system. Indirect costs may include product losses due to the meter flow
rate uncertainty.
Accuracy
The ability of an obstruction flow meter to accurately estimate the volume flow rate in a pipeline
depends on both the method used to calculate flow rate and factors inherent to the meter. If standard
tabulated values for meter coefficients are used, factors that contribute to the overall uncertainty of
the measurement enter through the following data-acquisition elemental errors: (1) actual b ratio
error and pipe eccentricity, (2) pressure and tap position errors, (3) temperature effects leading to
relative thermal expansion of components, and (4) actual upstream flow profile. These errors are in
addition to the errors inherent in the coefficients themselves and errors associated with the
estimation of the upstream fluid density (1). Direct in situ calibration of any flow meter installation
can assess these effects and their contributions to overall uncertainty. For example, the actual b ratio
value, pressure tap locations, and other installation effects would be included in a system calibration
and accommodated in the computed flow coefficient.
Turndown
The range over which a meter can be used is known as the meter turndown. The selection of a meter
should consider whether the system into which the meter is to be installed will be used at more than
one flow rate, and its effect on system performance over the entire anticipated flow rate range should
be included.
10.5 Pressure Differential Meters 443
13:4:39 Page 443
systematic uncertainty should be added to the uncertainty in the discharge coefficient to account for
swirl effects downstream of turns (3). Unorthodox installations require meter calibration to
determine flow coefficients or the use of special correction procedures (3, 7). The physical size
of a meter may become an important consideration in itself, since in large-diameter pipelines,
venturi and nozzle meters can take up a considerable length of pipe.
Pressure Loss
The unrecoverable overall pressure loss, Dp loss , associated with a flow meter depends on the b ratio
and flow rate (Fig. 10.7). These losses must be overcome by the system prime mover (e.g., a pump,
fan, or compressor driven by a motor) in addition to any other pipe system losses. In established
systems, flow meters must be chosen on the basis of the pressure loss that the prime mover can
accommodate and still maintain a desired system flow rate. The power, _
W, required to overcome any
loss in a system for flow rate Q is given by
_
W ¼ Q
Dp loss
h
ð10:18Þ
where h is the prime mover efficiency.
Costs
The actual cost of a meter depends on its initial capital cost, the costs of installing it (including
system down time), calibration costs, and the added capital and operating costs associated with flow
meter pressure losses to the system. Indirect costs may include product losses due to the meter flow
rate uncertainty.
Accuracy
The ability of an obstruction flow meter to accurately estimate the volume flow rate in a pipeline
depends on both the method used to calculate flow rate and factors inherent to the meter. If standard
tabulated values for meter coefficients are used, factors that contribute to the overall uncertainty of
the measurement enter through the following data-acquisition elemental errors: (1) actual b ratio
error and pipe eccentricity, (2) pressure and tap position errors, (3) temperature effects leading to
relative thermal expansion of components, and (4) actual upstream flow profile. These errors are in
addition to the errors inherent in the coefficients themselves and errors associated with the
estimation of the upstream fluid density (1). Direct in situ calibration of any flow meter installation
can assess these effects and their contributions to overall uncertainty. For example, the actual b ratio
value, pressure tap locations, and other installation effects would be included in a system calibration
and accommodated in the computed flow coefficient.
Turndown
The range over which a meter can be used is known as the meter turndown. The selection of a meter
should consider whether the system into which the meter is to be installed will be used at more than
one flow rate, and its effect on system performance over the entire anticipated flow rate range should
be included.
10.5 Pressure Differential Meters 443