Water Flow Measurement

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Water Flow Measurement
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Double Regulating Valve
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Variable Orifice Double Regulating Valve
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Typical Fixed Orifice Double Regulating Valve
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Fixed Orifice for Flange Mounting
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Fixed Venturi Nozzle
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Accuracy in Balancing
Although it is common practice to refer to the
measurement of flow rate, the flow
characteristic actually measured is the pressure
differential across a calibrated device inserted
into the pipework system. The flow rate is
subsequently deduced from this pressure
differential reading using the manufacturers
calibration data for the flow measurement device.
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Accuracy in Balancing

• Accuracy in balancing, therefore, involves 2
• prime attributes -
• Accuracy of the flow measurement devices
• Measurement accuracy

• Instrument accuracy
• Setting

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1). Accuracy of the flow measurement device
This is the error between the theoretical true
flow rate, and that correctly computed from a
perfectly read and calibrated instrument
connected to a device installed strictly in
accordance with the manufacturers instructions
and good practice. This error is a property of
the installed device and cannot be controlled by
the commissioning engineer.
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2). Measurement Error
This is the error introduced by the commissioning
engineer in making a measurement. Two factors
are involved -
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a). Instrument Accuracy
The most accurate instrument is the mercury or
fluorocarbon manometer. Provided the tubes are
kept clean and the fluid passages do not contain
air bubbles, observations can be taken with
confidence. Used in ambient conditions, errors
resulting from temperature variations are
insignificant and instrument calibration is
unnecessary. Electronic instruments incorporating
automatic zeroing are inherently stable, but it
is recommended that the instruments calibration
be periodically checked. Some care may be
required in using such instruments for measuring
low differential pressure and, if necessary,
advice should be obtained from the
manufacturer. The commissioning engineer must
ensure that the instruments used are acceptable
within the designers specified tolerance range.
He must also take care to ensure that his
measurements are taken under appropriate
conditions and that the recorded value is the
most accurate determinable.
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b). Setting
The final flow rate setting will inevitably
deviate from the desired value, due to variations
in flow conditions. The error which is
permissible is limited by the designers
specified flow rate tolerance. The commissioning
engineer must achieve a setting which is within
the specified tolerance.
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Typical Accuracies for Flow Measurement Devices
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What range of water flow rates are measurable
using orifice type flow measurement devices?
British Standard BS73501990 Specification for
Double Regulating Globe Valves and Flow
Measurement Devices for Heating and Chilled Water
Systems indicated minimum measurable flow rates
for different sized orifice devices. These
correspond to pressure losses in medium grade
steel pipe of between 100Pa/m and 500Pa/m. In
response to industry demand, manufacturers have
provided flow measurement devices which are
capable of measurements down to 0.01 l/s in 15mm
pipes. Such flows generate pressure losses of
less than 10Pa/m in steel pipes and are therefore
outside the range covered by the British
Standard. The measurement of such tiny flow
rates can be problematic. At very low water
velocities flow measurements can be particularly
susceptible to errors resulting from dirt or air
blockages. For this reason the need to measure
such low flows should be avoided whenever
possible.
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• The flow rate measurement obtained from an
  orifice device may be
• affected by installation features such as
• Variations in pipework type, e.g. the use of
  copper, flexible or plastic pipes may mean that
  the diameter of the approach section is less than
  that of the steel pipe grades on which most
  manufacturers base their published flow
  coefficients.
• Male/female couplings or copper to male iron
  fittings screwed into the inlet side of screwed
  orifice devices, these fittings usually have
  reduced bore.
• Misplaced or undersized joint rings, burrs on the
  end of screwed pipes, intrusion of jointing
  compound, these may effectively reduce the inlet
  bore.
• Bends, tees, reducers in the pipework approach to
  the device, the effects of different combinations
  of fittings upstream of the device cannot be
  predicted.

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These effects, have been found to result in
measured flows which can occasionally vary by up
to ?20 from the true value. The largest errors
are normally associated with the measurement of
very low flow rates (corresponding to velocities
of less than 0.5 m/s). The errors are more
likely to be negative rather than positive, in
other words the true flow rate is likely to be
greater than the measured flow rate. The error
normally shows up when a summation of sub-branch
flow rates is different from the flow in the
branch serving them.
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Whilst ?20 is a worst case, in the majority of
situations measurement accuracies within ?10 are
achievable allowing for any instrument and
setting inaccuracies. This is particularly true
if the measured flow velocity is above 0.5m/s.
An assumed measurement accuracy of ?10 can
therefore be used as the basis for calculating
flow measurement tolerances.
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Constant Flow Device