Page 111 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT
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& Well suited for cryogenic applications involving temperature should be maintained at a factor
LNG, liquid helium, and so on. of at least three times the value of DP across the
& With normal flow measuring devices, equipment and meter.
components are affected by extreme cold conditions. & A logical way to minimize flashing is to limit DP by
Moving mechanical parts, wetted seals, and other specifying a meter that is larger than that might
parts can cease to function, fail prematurely or give be used in comparable noncryogenic service.
rise to other problems. Advanced digital signal processing makes it possible
& A complication arises because the cost of keeping to filter out the noise associated with a large meter at a
cryogenic fluids cold enough to remain in the liquid high turndown and still achieves a highly accurate
state goes up as the temperature goes down. Common result.
practice is to keep cryogenic fluids at temperatures
only slightly below their boiling temperatures. As a 4.1.7 Positive Displacement Flow Meters
consequence, as the fluid flows through a pipeline or
past an obstacle like valve, DP, even small, can cause . What are positive displacement flow meters?
flashing to take place. Small or large pockets of gas & Operation of positive displacement meters consists
form in the liquid, which makes flow measurement of separating liquids into accurately measured incre-
erroneous. ments and moving them on. Each segment is counted
. Summarize the plus and minus points of Coriolis flow by a connecting register. Because every increment
meters. represents a discrete volume, positive-displacement
units are popular for automatic batching and account-
& Plus points:
ing applications. Positive displacement meters are
➢ Higher accuracy than most flow meters ( 0.1%
good for measuring the flows of viscous liquids or for
for mass flow).
use where a simple mechanical meter system is
➢ Can be used in a wide range of liquid flow
needed.
conditions.
. What are the characteristic features and applications of
➢ Measures the mass flow rate directly, which elim-
positive displacement flow meters?
inates the need to compensate for changing tem-
& Positive displacement meters provide high accuracy
perature, viscosity and pressure conditions.
( 0.1% of actual flow rate in some cases) and good
➢ Ideally suited to products that are accounted for on
repeatability (as high as 0.05% of reading). Accuracy
a mass basis, such as LPG, NGL, ethylene, and
is not affected by pulsating flow unless it entrains air
CO 2 .
or gas in the fluid.
➢ Capable of measuring hot (e.g., molten sulfur and & PD meters do not require a power supply for their
liquid toffee) and cold (e.g., cryogenic helium and
operation and do not require straight upstream and
liquid nitrogen) fluid flow.
downstream pipe runs for their installation.
➢ Low maintenance, because there are no parts that
& PD meters are available in sizes from 6 to 300 mm
wear with time.
(one-fourth in to 12 in.) and can operate with turn-
➢ The meters can be significantly over ranged with- down ratios as high as 100:1 although ranges of 15:1
out causing damage to the sensor. or lower are common.
➢ Low DP. & PD meters operate with small clearances between
➢ Suitable for bidirectional flow. their precision-machined parts.
& Minus points: & As fluid viscosity increases, slippage between the
➢ High initial setup cost. flow meter components is reduced, and metering
accuracy is therefore increased. Although slippage
➢ Clogging may occur and difficult to clean.
through the meter decreases with increased fluid
➢ Larger in overall size compared to other flow
viscosity (i.e., accuracy increases), pressure drop
meters.
through the meter also increases.
➢ Limited line size availability.
& The maximum (and minimum) flow capacity of the
. How does flashing is avoided or minimized in cryogenic
flow meter is decreased as viscosity increases. The
applications of Coriolis flow meters?
higher the viscosity is, the lesser the slippage and the
& A good rule of thumb for avoiding or minimizing lower the measurable flow rate are.
flashing is that the difference between the discharge & As viscosity decreases, the low flow performance of
pressure and the liquid vapor pressure at the fluid the meter deteriorates.