Abstrict An apparatus and method for calibrating a flow meter are provided
which utilize a tank for maintaining the apparatus liquid full.
The tank has a freely movable barrier therein which divides the
interior of the tank into a first section and a second section wherein
the first section is in communication with a system which comprises
at least one prover and a meter in series. A biasing means, such
as a compressed gas in the second section of the tank, is also provided
for urging the barrier toward the first section of the tank. Employing
the tank according to the invention maintains a liquid filled system.
Claims That which is claimed is:
1. An apparatus comprising:
a first prover having an inlet and an outlet;
a flow meter having an inlet and an outlet;
first means for defining a flow path between said meter inlet and
said prover outlet;
second means for defining a flow path between said meter outlet
and said prover inlet;
a tank having a barrier positioned therein so as to divide the
interior of said tank into a first section and a second section,
said first section being in direct fluid communication with said
second flow path defining means, wherein said barrier is freely
movable within said tank so that the volumes of said respective
first and second sections change in response to movement of said
barrier; and
biasing means for exerting a force upon said barrier in a direction
generally toward said first section.
2. An apparatus as recited in claim 1 wherein said barrier comprises
a piston slideably mounted within said tank.
3. An apparatus as recited in claim 2 further comprising pump
means adapted to pump a continuous flow of liquid through said apparatus.
4. An apparatus as recited in claim 3 further comprising a second
prover, associated with said second flow path defining means, which
has an inlet in communication with said meter outlet and an outlet
in communication with said first prover inlet.
5. An apparatus as recited in claim 4 said apparatus including
a conduit means which extends between said first section of said
tank and said second flow path defining means so as to provide fluid
communication between said first section and said second flow path
defining means.
6. An apparatus comprising:
a first prover having an inlet and an outlet;
a flow meter having an inlet and an outlet;
first means for defining a flow path between said prover outlet
and said meter inlet;
second means for defining a flow path between said meter outlet
and said prover inlet;
a tank having a barrier positioned therein so as to divide the
interior of said tank into a first section and a second section,
said first section being in direct fluid communication with said
second flow path defining means, wherein said barrier is freely
movable within said tank so that the volumes of said respective
first and second sections change in response to movement of said
barrier; and
a source of pressurized gas having an outlet; and
a means for providing selective fluid communication between said
gas source outlet and said second section of said tank.
7. An apparatus as recited in claim 6 wherein said barrier comprises
a piston slideably mounted within said tank.
8. An apparatus as recited in claim 7 further comprising pump
means adapted to pump a continuous flow of liquid through said apparatus.
9. An apparatus as recited in claim 8 further comprising a second
prover, associated with said second flow path defining means, which
has an inlet in communication with said meter outlet and an outlet
in communication with said first prover inlet.
10. A method comprising:
(a) flowing a stream of liquid through a closed loop system comprising
a prover having an inlet and outlet and a flow meter having an inlet
and outlet, said system further comprising a first means for defining
a flow path between said prover outlet and said meter inlet and
a second means for defining a flow path between said meter outlet
and said prover inlet;
(b) providing direct fluid communication between a first interior
section of a tank and said second flow path defining means during
step (a) so as to allow said section to fill with liquid, wherein
said section is defined by the interior surface of said tank and
a movable barrier positioned therein; and
(c) controlling the position of said barrier during step (a) in
response to pressure exerted upon said barrier by the liquid so
as to maintain said system liquid filled.
11. A method as recited in claim 10 wherein said barrier divides
the interior of said tank into said first section and also a second
section, said second section containing a compressed gas therein.
12. A method as recited in claim 11 wherein said gas is nitrogen.
13. A method as recited in claim 12 wherein the pressure associated
with said compressed gas is generally above the vapor pressure of
said liquid.
14. A method as recited in claim 13 wherein said liquid is crude
oil.
15. A method as recited in claim 11 wherein said system further
comprises a second prover associated with second flow path defining
means, said stream of liquid being additionally passed through said
second prover.
16. A method as recited in claim 15 further comprising the following
steps which are carried out after step (c): determining the calibration
factor of said flow meter; and determining the actual volume of
said second prover utilizing said calibration factor.
Description BACKGROUND OF THE INVENTION
This invention relates to an apparatus and method for calibrating
a flow meter utilizing a prover.
In the use of flow meters to measure the quantity of fluid flowing
in a conduit, it is frequently desirable to determine the accuracy
of the meter in detecting the volume of fluid flowing through the
meter. A process for determining meter accuracy is commonly referred
to as meter calibration. A meter calibration process typically utilizes
a device known as a prover.
As used herein, the term prover refers to any of a number of devices
which include a conduit containing a solid member, such as a ball
or piston, movably positioned therein, and two detection points
along the conduit having suitable means, such as actuating switches,
associated therewith for detecting the solid member as it moves
past each detection point.
In calibrating a volumetric flow meter, a stream of fluid is passed
simultaneously through a prover and the flow meter. Liquid flowing
through the prover conduit moves the solid member from one detection
point to the other detection point. The output of the flow meter
is monitored in the time interval between actuating times for the
detection points. This output is correlated with the known volume
of the prover conduit between the detection points to yield an appropriate
calibration factor.
Some particular problems arise when calibrating a liquid flow meter.
More particularly, it is desirable that such a liquid calibration
system be liquid-full at all times during the calibration test in
view of the fact that a mixed phase of vapor and liquid causes the
meter and prover to give erratic and unpredictable readings. Most
prior calibration systems have utilized a large liquid supply tank
maintained at atmospheric pressure for helping maintain a single
liquid phase condition in the calibration system, and an associated
pump for pumping liquid from the tank for subsequent flow through
the meter and prover. Typically, the suction side of the pump is
connected to the supply tank so as to be maintained at atmospheric
pressure, and the discharge side of the pump discharges to the meter
and prover which are maintained at a higher pressure. The pressure
differential across the pump is usually substantial (i.e., 200 psi
or more). Since the power requirements for the pump are based on
the pressure differential across the pump, the large pressure differential
usually requires a pump with an undesirably high horsepower. Generally,
an increase in pump power means a resultant increase in heat introduced
to the system which contributes to temperature instability of the
system and consequent unreliable calibration results. Furthermore,
continual contact of ambient air with the liquid in the supply tank
causes aeration of the liquid and a two-phase condition which leads
to erroneous results as discussed above.
SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to provide an apparatus
and method for calibrating a flow meter which minimizes power requirements,
and wherein the apparatus is maintained liquid-full during operation.
It is also an object of the invention to provide a flow meter calibration
apparatus and method in which aeration of liquid is minimized.
The above objects are realized in an apparatus which comprises:
a prover having an inlet and outlet; a flow meter having an inlet
and outlet; first means for defining a flow path between the prover
outlet and the meter inlet; second means for defining a flow path
between the meter outlet and the prover inlet; and a tank having
a barrier positioned therein to divide the interior of the tank
into a first section and a second section, wherein the first section
is in direct fluid communication with the second flow path defining
means. The barrier is also freely movable within the tank so that
volumes of the respective first and second sections change in response
to movement of the barrier. The apparatus further comprises a biasing
means for exerting a force upon the barrier in a direction generally
toward the first section. The biasing means can take the form of
a compressed gas in the second section of the tank. The gas can
be supplied from a pressurized gas source having associated therewith
a means for providing selective fluid communication between the
gas source and the second section of the tank.
According to another aspect of the invention, there is provided
a method which comprises: flowing a stream of liquid through a closed
loop system comprising a prover, a flow meter, a first means for
defining a flow path between the prover outlet and the meter inlet,
and a second means for defining a flow path between the meter outlet
and prover inlet; providing direct fluid communication between an
interior section of a tank and the second flow path defining means
so as to allow the interior section to fill with liquid, wherein
the section is defined by the interior surface of the tank and a
movable barrier positioned therein; and controlling the position
of the barrier in response to pressure exerted thereupon by the
liquid so as to maintain the system liquid filled.
The apparatus and method of the invention require less power for
operation than prior calibration systems, thus providing improved
temperature stability and associated reliable calibration results.
In addition, the invention substantially avoids aeration of liquid
in the system to assist in avoiding a two phase condition.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of one embodiment of an apparatus
according to the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
A preferred embodiment of the invention will now be described with
reference to FIG. 1. The illustrated embodiment is applicable to
the calibration of any type of liquid flow meter, and can be used
with any liquid compatible with the flow meter. The apparatus of
FIG. 1 includes various elements, hereinafter described, effective
to accomplish calibration of a flow meter, and various lines or
conduits which, together with certain elements of the apparatus,
define flow paths between various other elements.
Referring to FIG. 1 the apparatus includes a prover 10 which is
preferably a ballistic piston type prover. A prover of this type
is commercially available from Brooks Instrument Division of Emerson
Electric Company of Statesboro, Ga. An additional prover 12 which
can be a conventional ball prover, is associated with line 14 in
a manner hereinafter discussed so that its inlet is in communication
with the outlet of meter 16 and its outlet is in communication with
the inlet of prover 10. For most applications, meter 16 is a volumetric
type meter for measuring the volume of liquid flowing therethrough.
Meter 16 can, for example, be of the type which produces a series
of electrical impulses, each impulse representing a certain volume
of liquid. Flow volume as measured by the meter is determined by
counting the number of impulses produced as liquid passes through
the meter. Such counting can be readily performed by electronic
counters which display a number representing the total number of
impulses received from the meter.
The apparatus shown in FIG. 1 also includes a tank 18 which has
a piston 20 slideably mounted therein so as to divide the interior
of the tank into two sections, namely sections 22 and 24. Piston
20 is a conventional piston comprising a cylindrical portion in
sealing engagement with the interior surface of tank 18 and a shaft
extending from one side of the cylindrical portion and through one
end of tank 18. The extent to which the shaft extends from tank
18 can be visually monitored by an operator to determine the position
of the piston within tank 18. Piston 20 is freely movable within
the tank so that the volumes of the respective sections 22 and 24
change in response to movement of the piston. It should be understood
that a suitably movable barrier other than the illustrated piston
may be used. One example is a flexible diaphragm connected at its
edges to the interior surface of tank 18. Such a diaphragm responds
effectively and quickly to pressure changes within the tank. However,
a diaphragm will not always be capable of accommodating the large
changes in volume of interior sections 22 and 24 which can be expected
in the illustrated system. Therefore, the illustrated piston is
generally preferred even though some energy losses can be expected
due to friction between the piston and the tank. One other alternative
arrangement could involve a small diaphragm mounted on the side
of piston 20 facing section 22. The diaphragm in such an arrangement
would assist in evening out pressure pulsations in the system due
to its quick response to pressure changes, and the piston would
accommodate the large section volume changes to be expected in the
illustrated system.
As shown, section 22 is in direct fluid communication with line
26 by means of a conduit or line 28 which extends between section
22 and line 26. Section 24 is adapted to receive a compressed gas
from gas source 30 via line 32. Valve 34 is positioned along line
32 so as to provide selective fluid communication between source
30 and section 24 of tank 18. Filling of section 24 with the compressed
gas serves to exert a force upon piston 20 in a direction generally
toward section 22. Thus compressed gas within section 24 serves
as a biasing means with respect to piston 20. It should be apparent
that other biasing means may be used, such as a spring.
The apparatus of FIG. 1 further comprises a strainer 36 for removing
various foreign materials from the liquid stream; straightening
vanes 38 which function to straighten the flow of liquid into the
meter so as to avoid an undesirable flow profile; a pump 40 which
is adapted to pump a continuous flow of liquid through the system;
and a four-way diverter valve 41 for directing liquid flow as will
become more apparent below. Of course, vent valves (not shown) can
be provided to selectively purge the system of gases.
The illustrated apparatus is a closed loop system whose various
important elements are connected as follows. A flow path defined
by lines 42 and 44 and also vanes 38 extends between the outlet
of prover 10 and the inlet of meter 16. Another flow path defined
by various lines and other elements of the apparatus extends between
the outlet of meter 16 and the inlet of prover 10.
The illustrated apparatus can be operated as follows. A flow of
a liquid, such as crude oil, is first established in the system
with the aid of a valve (not shown) along line 46. The liquid can
be supplied from a suitable source such as a tank (not shown) which
can be connected to line 46. Pump 40 is actuated so as to pump a
continuous stream of liquid through the closed loop system. Liquid
flows through strainer 36 prover 10 and into line 42. The liquid
then flows through line 42 vanes 38 meter 16 and into diverter
valve 41 via line 14. This stream is directed by diverter valve
41 into the inlet of prover 12 via line 51. Liquid then flows from
the outlet of prover 12 through line 52 and back into diverter
valve 41 and into line 50. The closed loop system can be drained
via line 53 using a valve along line 53.
Valve 34 is opened to allow gas from source 30 to enter section
24 of tank 18. The presently preferred gas is nitrogen due to its
ready availability and due to the fact that it is not flammable.
Flow of gas into section 24 is allowed to continue until a predetermined
pressure is reached. This pressure is dependent upon the liquid
employed, and is preferably above the vapor pressure associated
with that liquid. For example, for diesel fuel the initial pressure
within section 24 would be set at about 50 pounds psi. After the
desired pressure is reached, valve 34 is closed. In addition, piston
20 is preferably positioned within tank 18 to allow for any expected
changes in position due to temperature variations and consequent
expansion and contraction of the liquid. It should also be noted
that the pressure of liquid in section 22 and in the remainder of
the system is necessarily equivalent to that pressure in section
24.
Fluid communication between section 22 and line 26 is provided
during flow of liquid through the closed loop system. The position
of piston 20 tends to be automatically controlled in response to
pressure exerted upon the piston by liquid in section 22 so as to
maintain the system liquid filled. The combination of the compressed
gas in section 24 and slideable piston 20 also serves to maintain
a substantially constant pressure within the system so as to avoid
exceeding pressure limitations of various apparatus components,
and also tends to even out high frequency pressure pulsations which
can adversely affect performance of flow meter 16.
After a flow of liquid is established through the system and tank
18 is appropriately pressurized as discussed above, calibration
of meter 16 is accomplished as follows. It will be assumed for the
sake of discussion that meter 16 is of the type which produces electrical
impulses in response to volume of liquid flowing therethrough. The
piston or other movable member in prover 10 is allowed to be moved
by the liquid flow between two detection points within the prover.
Counting of impulses from meter 16 is initiated at the same time
that the movable member in prover 10 is detected at the first detection
point. This is typically done automatically by, for example, electrically
actuating an electronic counter (not shown) by means of a switch
at the first detection point in prover 10. Upon detection of the
movable member at the second detection point, counting of impulses
from meter 16 is stopped. The number of impulses between actuating
times for the two detection points is divided by the known volume
of the prover between detection points to yield a calibration factor.
The calibration factor is expressed as number of impulses per unit
volume. This calibration factor can be applied in the future operation
of the meter to obtain an accurate reading of the volume of liquid
flowing through the meter.
According to the illustrated embodiment, the correct volume of
prover 12 between detection points therein can be determined using
the calibration factor calculated above. This correct volume can
be determined by allowing a movable member such as a ball within
prover 12 to be passed between detection points therein and monitoring
the number of impulses produced by meter 16 in the time interval
between detection points, and then calculating the volume of prover
12 between its detection points using the previously determined
calibration factor. For example, if 100 impulses are counted and
the calibration factor is 20 impulses per unit volume, then the
correct volume corresponding to prover 12 would be five unit volumes.
After the procedure described above is completed, the movable members
in the provers can be returned to their initial positions by conventional
means to be ready for another test.
A determination of the volume corresponding to prover 12 by utilizing
prover 10 is desirable in the situation where prover 12 is used
frequently in the field and is subject to corrosion, various wall
deposits and deformation of the prover conduit which can change
the volume associated with prover 12. Prover 10 used only infrequently
to test the volume of prover 12 is assumed to have an accurate
known volume to enable accurate calibration of meter 16 and subsequent
accurate redetermination of the volume of prover 12. The volume
of prover 12 as determined above can then be used in routine calibration
operations. Periodic testing of the volume of prover 12 can be carried
out similarly.
Of course, diverter valve 41 can be set to pass a straight flow
of liquid therethrough so as to bypass prover 12 when only calibration
of meter 16 is desired using prover 10.
Several advantages of the present invention should be apparent
at this point. First, use of tank 18 and its associated piston and
compressed gas functions to effectively maintain the system liquid
filled so as to avoid a two-phase condition which can lead to inaccurate
calibration results. Use of tank 18 is particularly advantageous,
however, in that there is a much smaller pressure differential across
pump 40 than is possible with the previously noted systems utilizing
an atmospheric supply tank. The present invention can successfully
operate with a pressure differential as little as one-tenth that
of the pressure differential associated with the prior systems.
As previously noted, such a small pressure differential permits
the use of a smaller horsepower pump, thus saving energy and, more
importantly, enhancing the temperature stability and accuracy of
the system. Furthermore, since compressed gas in section 24 of tank
18 is separated from the liquid in section 22 by piston 20 aeration
of liquid in section 22 is avoided in contrast to atmospheric tanks.
Yet another advantage of the present invention lies in the position
of tank 18 with respect to the provers and the meter. Tank 18 is
positioned out of the flow path between the provers and meter. If
tank 18 were positioned in such a flow path, such as along line
42 or 44 temperature changes during a calibration test could cause
the volume of liquid in the tank to change so that the meter would
receive a different volume of liquid than that which flowed through
prover 10. This would destroy the accuracy of the calibration due
to the fact that it is essential in such a calibration that the
same volume of liquid flow through the prover and meter during the
test. The same erroneous result could arise if the tank were positioned
along line 14.
Obviously, many modifications and variations of the present invention
are possible in light of the above teachings. It is therefore to
be understood that within the scope of the appended claims the invention
may be practiced otherwise than as specifically described. |