Chilingarian G.V. et al. Surface Operations in Petroleum Production, II
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15
VOLUMETRIC
FLOW
RATE MEASUREMENT
OF
LIQUIDS
Liquids are typically measured using a turbine meter or a positive displacement
meter. Turbine meters allow easy automation and, therefore, have been used for
many years, primarily for waterflood projects. Positive displacement meters have
traditionally been used for fluids having varying or high viscosities, such as crude
oil, but have not usually been adapted to automation.
Turbine meters can be used for both liquids and gases, but are used primarily for
liquids, with most applications in the waterflood area. These meters are not
necessarily ultra-accurate, about
1
%,
in different configurations, but ths is more
than adequate for waterflood applications. Actually, higher accuracy, which is not
required in most oilfield applications, is available from some of the manufacturers.
The turbine flow meter (Figs.
2-1
and
2-2)
typically works by converting the linear
velocity and momentum into a tangential thrust, or force, whch then rotates the
turbine or “propeller”.
The rotary vane type
of
positive displacement flowmeter is used extensively in
the oilfield on separators for both water and crude oil flow rates. Another major
usage is for waterflooding, which is also served by turbine flowmeters. The rotary
vane type of flowmeter works by trapping a volume of fluid in a rotating vane
segment made
up
by vanes,
or
other deflectors, that move in and out. The output
then typically drives a mechanical counter to give cumulative
(“
totalized”) and/or
instantaneous flow rates. This meter can also be coupled magnetically, or by other
means, to various systems for data display and automation purposes. Schematic
diagrams and photographs are shown on Fig.
2-3.
Other types of rotating positive displacement meters include the oval-gear meter
and the fluted rotor. These
also
trap definite volumes
of
fluid and transmit that
fluid through the flow meter as shown in Fig.
2-4.
Orifice meters can also be used for liquid flow rate measurements. Typically,
however, they are more difficult to adapt to automation purposes and have been
used more for gas flow measurements. Most of the discussion on the positive
displacement flow meters in this chapter involves the rotary vane type
of
meter.
Nutating disc and piston meters, however, also can be used for positive displace-
ment volumetric flow rate measurements. These are discussed later.
Turbine
meters
Turbine meters are typically used in waterflood operations and for measuring
low-viscosity oil flow. Inasmuch as ultra-accurate flow rates are not generally
required for waterflood usage, the nominal
f
1.0%
accuracy is satisfactory, espe-
cially because these turbine meters have
f0.05%
of flow rate repeatability. Better
quality and hgher priced meters are available from the same manufacturers
(0.25
or
0.5%
accuracy,
f
0.02%
repeatability). Rangeability is typically
10
:
1
(see Fig.
2-5),
although the range is often extended to higher and lower flow rates with different
linearity specifications. Typical upper temperature limits range from
100
O
F
to
16
Fig.
2-1.
Turbine flowmeter. (Courtesy
of
Halliburton Services.)
17
Fig.
2-2.
Cross-section
of
turbine flowmeter. (Courtesy
of
Halliburton Services.)
225
OF;
meters with an upper temperature limit
of
850
F
are available. Sizes range
from 3/8 in. to 24 in. with most
of
the economical waterflood meters being in the
smaller sizes. The 1-in. size is typically in the
6-60
gpm range, although some
companies quote linear flow ranges from
4
to 60 gpm, 5-50, 6-75, or 8-88 gpm.
The
24-in.
units handle up to 40,000 gpm in the normal range. Working pressures up
to
7500 psi are available in the smaller sizes, with 1440 psi (600#
ANSI)
being
available in the largest sizes. (See Table
2-1.)
Positive
displacement
meters
Positive displacement meters, used in
oil
production and transportation, range in
size from 1/2 in.
to
16 in. Flow rates from less than
1
gpm to almost
9000
gpm can
be measured. Units with temperature and pressure ratings up
to
450
OF
and 5000 psi
18
DRIVE
COUPLING
-CLOCKWISE
LOOKING
AT
END
OF
COUPLl
BRIDGE
BRIDGE
SEALS
ROTOR BLADE
Fig.
2-3.
Rotary vane type
of
positive displacement flowmeter. (Courtesy
of
171
Barton and Tokheim
Corporation.)
are available. The linearity, repeatability, and accuracy specifications depend on the
geometry
of
the meter, viscosity, and operating temperature and pressure. Lineari-
ties
of
k0.15%
of
maximum
flow
rate and repeatabilities of
f0.05%
are possible.
Accuracies
of
kO.25%
and
f0.5%
are available, although an accuracy of
0.1%
at a
rangeability
of
200
:
1
is quoted for one reciprocating piston meter.
19
Fig. 2-4. Oval-gear meter. (Courtesy
of
Brooks Instrument Division.)
Typical rotary vane meters range in size from
1
to
3
in. (see Table 2-11). The
rangeability
is
typically 10:
1
(6-60
gpm or
9-90
gpm), although meters with
rangeability greater than 16
:
1
(15-250 gpm) are manufactured. Maximum tempera-
tures are often in the 200-300
O
F
range.
The oval gear meters usually have a rangeability less than
10
:
1
for continuous
service, but over
80
:
1
for non-continuous applications. Their repeatability can be
*0.05%
or higher, with *0.25% accuracy and
+OS%
linearity.
TABLE 2-1
Sizes, linear flow ranges, and nominal calibration factors of various waterflood turbine meters (After
Halliburton Services)
~
Size Linear flow range
(in.) kpm)
Nominal calibration factor
(pulses/gal)
3/8
1
2
4
8
0.3-
3
5
-
50
40
-
400
100
-1200
350
-3500
20,000
870
55
29
3
N
0
Fig.
2-5.
Pressure drop curves
for
turbine flowmeters. (Courtesy
of
Halliburton Services.)
21
TABLE
2-11
Specifications
of
various rotary vane positive displacement meters (After
ITT
Barton Floco Flow Meters)
Size Flow rate range Pressure Temperature
1
6-
60
500/2500/500U
180
F
2
6-
60
500/2500/5000
180
OF
2”
15-250
500/2500
300
F
3
9-
90
500/2500/5000
180°F
3a
15-250
500/2500
300
O
F
a
Higher accuracy, flow rate, and temperature model.
(in.) (gpm) (Psi) (OF)
The piston meters can be
of
the oscillating (rotating) or reciprocating types.
Reciprocating piston units can be ultra-accurate, as indicated earlier, while still
having excellent rangeability.
3%
at a flow rate of only
3
gpm.
One 1-in. mutating disc (disk) is accurate to within
Readout devices
Positive displacement
(PD)
meters and turbine meters can usually produce pulses
that are directly proportional to flow rate. The signal is then converted to a totalized
and, often, an instantaneous flow rate. The
PD
meters can also use mechanical
drives with register counters. Orifice meters, however, require the conversion from a
pressure differential to a mechanical or electrical output. Also, the flow rate is
proportional
to the square root
of
the response, which complicates the data
reduction.
The blades in a turbine meter cut lines of magnetic force produced by the
magnetic pickup unit. The resulting electrical pulses are then transmitted to elec-
tronic instrumentation to indicate, totalize, record, and/or control flow rate. Analog
and digital instrumentation is available. Results are usually given in standard
engineering units, such as gallons or barrels.
Special options, which require additional measurement and processing devices,
can compensate for temperature and pressure effects or can compute the energy
change in Btu’s per unit volume of flowing hydrocarbons.
Most positive displacement meters use direct mechanical drives involving gears to
indicate totalized flow; however, some companies use magnetic couplings. There are
also two kinds of pulse systems:
(1)
In one system the rotor revolutions are counted
by means of magnetic fields, somewhat like a turbine meter.
(2)
The other method
involves a direct link to the output shaft that generates a pulse with a dry reed
switch, with a microswitch, or photoelectrically.
In the case of orifice meters, two pressures (upstream and downstream of orifice)
must be sensed and converted to flow rate. Mechanical devices involve the mercury
type and the dry bellows recorders. They typically record the static pressure
22
downstream of the orifice and the differential pressure across the orifice. Electrical
(electronic) devices include pressure transmitters (or transducers).
Orifice meters
Although orifice meters can be used for liquid measurements, they are primarily
used for gas measurements. The principles are discussed in detail in the following
section on the use of orifice meters for determining gas flow rates. Basically,
Bernoulli’s equation is used to convert pressure head to velocity head. Velocity can
then be determined at a known cross-section of the orifice, which in turn is
converted to a flow rate. In the case of gas flow, calculations become more
complicated because
of
compressibility effects.
VOLUMETRIC FLOW
RATE
OF
GASES
Orifice meters have traditionally been used for determining gas flow rates,
whereas turbine meters and positive displacement meters have been used for
determining liquid flow rates. Turbine meters and the positive displacement meters,
however, can also be used for gas flow measurements. The vortex shedding flow
meters have recently been used for both gases and liquids, but predominantly for
gas flow measurements.
Orifice
meters
Orifice meters convert a pressure head to a velocity head in order to obtain the
flow rate at a particular cross-section. Due to the compressibility of the gas, there is
a correction factor for the compressibility. Other correction factors are also pre-
sented here. Orifice meters are more difficult to adapt to automation and, therefore,
other devices, such as a turbine meter, have been used in their place.
Orifice meters have been the traditional device for measuring the flow rate of
gases, but are not used as much for liquid flow measurement. The orifice meter, in
fact, is still “one of the most important and widely used” flowmeters
(F
and
P
General Catalog, p.
1329),
even though technology has provided some new
sophisticated instrumentation. Advantages include its “simple, rugged, and reliable”
(Daniel Basic Fundamentals by Kendall, p.
6)
construction. Although they have
limited rangeability (typically
3
:
l),
orifice meters are relatively inexpensive and
have good accuracy for most gas metering applications.
A typical meter system consists of a concentric, square-edged orifice plate, a
fitting that holds the orifice plate and provides taps for differential pressure
measurement, and a pressure measuring-recording device. A meter tube provides an
orifice fitting, the orifice plate, and both the upstream and downstream piping in
one complete package.
23
ECCENTRIC
-
SEGMENT
A1
-
OUARTER-ROUND
Fig.
2-6.
Various types
of
orifice meter plates.
(Courtesy
of Daniel Industries, Inc.)
In addition to having the hole concentric (centered) in the orifice plate, the
orifice can be eccentric (off center)
or
segmental (part
of
a circle) as shown in Fig.
2-6.
The two latter types are often used when there are solids in the flow stream,
such as in dirty gases
or
slurries. For a square-edged orifice, a square, sharp edge
faces the flow, whereas the downstream edge is tapered (beveled). For high-velocity
fluids, a quadrant (quarter-circle) edge
or
conic (conical) entry orifice can be
selected. Both of these concentric orifice devices have a square edge downstream
with a rounded radius
or
a conical taper facing upstream.
The fitting that holds the orifice plate and provides pressure tap locations can be
as simple as orifice flanges (Fig.
2-7).
It can be as sophisticated as a double
chamber, however, where the orifice plate can be changed under pressure, without
interrupting the flow (see Fig.
2-8).
The intermediate fitting is shown in Fig.
2-9.
It
allows the orifice plate to be quickly changed without breaking open the pipe.
This
eliminates liquid spillage and pipe movement due
to
spreading flanges. Inasmuch as
the design of these single chamber devices does not allow the orifice plate
to
be
changed under pressure, a bypass line
or
pressure release (depressurizing) is neces-
sary.
Pressure taps can be of the flange, pipe, vena contracta, or corner design.
Pressure taps, however, are generally of the flange type, where the pressure is
24
Bar
I
Fig.
2-7.
Orifice plate holder. (“Simplex”, courtesy
of
Daniel Industries, Inc.)
measured one inch from the upstream face plate and one inch from the downstream
face of the orifice plate (see Fig. 2-10). The other common type is the pipe tap,
shown in Fig. 2-11. Pipe taps are located 2: pipe diameters
(ID’S)
upstream and
8
pipe diameters downstream (where the pressure recovery is maximum). This allows
a smaller meter run to be used for the same flow rate, but requires location
tolerances ten times those for flange taps. The vena contracta is the location of the
minimum cross-sectional area of flow, where the minimum pressure and maximum
velocity occur.
The pressure taps, shown in Fig. 2-12, are located one pipe diameter (ID)
upstream and at the vena contracta downstream. Although these taps give a greater
pressure drop
that results in better accuracy, the downstream location varies
between
0.9
and
0.3
pipe diameters for beta ratios (orifice diameter divided by the
pipe
ID)
ranging from 0.1 to
0.8
(Taylor Instruments “Flow Data”, p.
50).
They are,
therefore, usually used for fairly constant flow rates. Finally, corner taps (see Fig.
2-13) are located immediately adjacent to the upstream and downstream faces of the
orifice plate. They are used primarily in Europe.
The term orifice meter commonly designates a flow meter that uses an orifice
plate. It should be noted, however, that flow nozzles, flow tubes, and venturi meters
are also classified as obstruction or differential head (pressure) meters and, some-
times, referred to as “orifice meters”. According to Bernoulli’s equation, the square