Abstrict A flow meter prover is disclosed which includes an outer fluid
housing having an inlet and an outlet, a measuring conduit coaxially
mounted within the outer housing and having first and second sets
of fluid apertures adjacent, respectively, the upstream and downstream
ends thereof, a fluid barrier mounted within the annular cavity
between the outer housing and the conduit, a controllable piston
mounted within the conduit, an actuating rod axially projecting
from the downstream side of the piston where the free end of the
rod extends through the downstream end of the outer housing, a bypass
valve connected between the inlet and outlet of the outer housing,
and first and second piston detection switches spaced apart along
the length of the measuring conduit. There are provisions for automatically
correcting for variations in the dimensions of the measuring conduit
due to variations in fluid temperature. The prover also includes
apparatus for continuously monitoring the integrity of the piston
seals, and may be operated with equal fluid pressure on both sides
of the piston.
Claims What is claimed is:
1. Apparatus for measuring a quantity of a flowing fluid, comprising:
an outer fluid housing having upstream and downstream ends, an
inlet communicating with the upstream end and an outlet communicating
with the downstream end;
a measuring conduit;
means for coaxially mounting the conduit within the outer housing,
forming an annular cavity between the conduit and the housing;
a fluid barrier;
means for mounting the barrier within the annular cavity in a manner
which divides the cavity into upstream and downstream sections;
a controllable piston slidably mounted within the conduit and having
upstream and downstream sides;
an actuating rod axially projecting from the downstream side of
the piston, where the free end of the rod extends through the downstream
end of the outer housing;
first fluid bypass means connected between the inlet and outlet
of the outer housing whereby the fluid can bypass the outer housing
when the bypass is open;
first and second piston detection means;
means for positioning the first and second detection means in a
spaced apart relationship where the distance between the first and
the second detection means corresponds to the length of the fluid
measuring portion of the conduit; and
second fluid bypass means including a first set of apertures provided
in the conduit adjacent the upstream end thereof, and a second set
of apertures provided in the conduit adjacent the downstream end
thereof, whereby the fluid can bypass the piston when the piston
is located at either the upstream or downstream limits of its travel
within the conduit, and where the measuring portion of the conduit
lies between the first and second sets of apertures; and where the
means for coaxially mounting the conduit within the outer housing
are located at positions of the conduit which exclude the fluid
measuring portion of the conduit, so that the entire inner and outer
surface of the fluid measuring portion of the conduit may be exposed
to the fluid.
2. The apparatus of claim 1 in which the means for coaxially mounting
the conduit include fastening the downstream end of the conduit
to the downstream end of the outer housing.
3. The apparatus of claim 2 which further includes a third set
of apertures provided in the conduit downstream of the second set
of apertures, whereby when the piston covers the third set of apertures
the fluid within the conduit smoothly decelerates the piston motion.
4. The apparatus of claim 2 in which the means for mounting the
conduit includes the fluid barrier, and where the means for mounting
the fluid barrier includes affixing one end of the barrier to the
inside surface of the outer housing upstream of the first set of
apertures, and sealably engaging the other end of the barrier to
the outer surface of the conduit in a manner which permits the free
expansion of the conduit in response to fluid temperature.
5. Apparatus for measuring a quantity of a flowing fluid, comprising:
an outer fluid housing having upstream and downstream ends, an
inlet communicating with the upstream end and an outlet communicating
with the downstream end;
a measuring conduit;
means for coaxially mounting the conduit within the outer housing,
forming an annular cavity between the conduit and the housing;
a fluid barrier;
means for mounting the barrier within the annular cavity in a manner
which divides the cavity into upstream and downstream sections;
a controllable piston slidably mounted within the conduit and having
upstream and downstream sides;
actuating rod axially projecting from the downstream side of the
piston, where the free end of the rod extends through the downstream
end of the outer housing;
first fluid bypass means connected between the inlet and outlet
of the outer housing whereby the fluid can bypass the outer housing
when the bypass is open;
first and second piston detection means;
means for positioning the first and second detection means in a
spaced apart relationship where the distance between the first and
the second detection means corresponds to the length of the fluid
measuring portion of the conduit; and
second fluid bypass means including a first set of apertures provided
in the conduit adjacent the upstream end thereof, and a second set
of apertures provided in the conduit adjacent the downstream end
thereof, whereby the fluid can bypass the piston when the piston
is located at either the upstream or downstream limits of its travel
within the conduit, and where the measuring portion of the conduit
lies between the first and second sets of apertures, and where the
means for positioning the first and second piston detection means
includes means responsive to the temperature of the fluid for altering
the spaced apart relationship between the detection means so that
the fluid measuring portion of the conduit represents a constant
displaced volume regardless of fluid temperature.
6. The apparatus of claim 5 in which the piston detection means
includes electrical switches, and the means for positioning the
detection means further includes mounting the electrical switches
along the length of and adjacent the outside surface of the conduit
and within the annular cavity between the conduit and the housing.
7. The apparatus of claim 6 in which the means responsive to the
temperature of the fluid includes bimetal temperature sensitive
elements to which the electrical switches are fastened, and which
elements move the switches in response to fluid temperature in a
manner which decreases the spacing between the switches as a function
of increases in fluid temperature.
8. Apparatus for measuring a quantity of a flowing fluid, comprising:
an outer fluid housing having upstream and downstream ends, an
inlet communicating with the upstream end and an outlet communicating
with the downstream end;
a measuring conduit;
means for coaxially mounting the conduit within the outer housing,
forming an annular cavity between the conduit and the housing;
a fluid barrier;
means for mounting the barrier within the annular cavity in a manner
which divides the cavity into upstream and downstream sections;
a controllable piston slidably mounted within the conduit and having
upstream and downstream sides;
an actuating rod axially projecting from the downstream side of
the piston, where the free end of the rod extends through the downstream
end of the outer housing;
first fluid bypass means connected between the inlet and outlet
of the outer housing whereby the fluid can bypass the outer housing
when the bypass is open;
first and second piston detection means;
means for positioning the first and second detection means in a
spaced apart relationship where the distance between the first and
the second detection means corresponds to the length of the fluid
measuring portion of the conduit; and
second fluid bypass means including a first set of apertures provided
in the conduit adjacent the upstream end thereof, and a second set
of apertures provided in the conduit adjacent the downstream end
thereof, whereby the fluid can bypass the piston when the piston
is located at either the upstream or downstream limits of its travel
within the conduit, and where the measuring portion of the conduit
lies between the first and second sets of apertures, and which further
includes means for applying an upstream directed force to the free
end of the actuating rod to move the piston from the downstream
end to the upstream end of the conduit, and to restrain the downstream
motion of the piston.
9. The apparatus of claim 8 in which the means for applying a force
to the free end of the actuating rod includes a pressure housing
surrounding a portion of the actuating rod external to the outer
fluid housing and means for applying a control fluid under pressure
to the interior of the pressure housing.
10. The apparatus of claim 9 in which the downstream motion of
the piston may be controlled by regulating the flow of control fluid
from the pressure housing.
11. The apparatus of claim 10 in which the flow of control fluid
from the pressure housing is regulated in response to the flowing
fluid pressure on the upstream side of the piston so that increasing
flowing fluid pressure causes an increase in the flow of control
fluid.
12. The apparatus of claim 10 in which the flow of control fluid
from the pressure housing is regulated in response to the differential
pressure of the flowing fluid between the upstream and downstream
sides of the piston so that an increase in the differential pressure
causes an increase in the flow of the control fluid.
13. Apparatus for measuring a quaantity of a flowing fluid, comprising:
an outer fluid housing having upstream and downstream ends, an
inlet communicating with the upstream end and an outlet communicating
with the downstream end;
a measuring conduit;
means for coaxially mounting the conduit within the outer housing,
forming an annular cavity between the conduit and the housing;
a fluid barrier;
means for mounting the barrier within the annular cavity in a manner
which divides the cavity into upstream and downstream sections;
a controllable piston slidably mounted within the conduit and having
upstream and downstream sides;
an actuating rod axially projecting from the downstream side of
the piston, where the free end of the rod extends through the downstream
end of the outer housing;
first fluid bypass means connected between the inlet and outlet
of the outer housing whereby the fluid can bypass the outer housing
when the bypass is open;
first and second piston detection means;
means for positioning the first and second detection means in a
spaced apart relationship where the distance between the first and
the second detection means corresponds to the length of the fluid
measuring portion of the conduit; and
second fluid bypass means including a first set of apertures provided
in the conduit adjacent the upstream end thereof, and a second set
of apertures provided in the conduit adjacent the downstream end
thereof, whereby the fluid can bypass the piston when the piston
is located at either the upstream or downstream limits of its travel
within the conduit, and where the measuring portion of the conduit
lies between the first and second sets of apertures; and in which
the piston includes two spaced apart seals each encircling the perimeter
of the piston, an annular cavity formed between the seals, a passage
connecting the cavity to one end of a flexible tube, and means for
connecting the other end of the flexible tube to the exterior of
the outer housing, whereby the integrity of the piston seals may
be continuously monitored from the output of the flexible tube while
the piston is stationary or in motion.
14. A method for determining the calibration factor of a flowmeter
comprising the steps of:
providing an outer fluid housing having an inlet and an outlet,
providing a measuring conduit coaxially mounted within the outer
housing and having first and second sets of fluid apertures adjacent,
respectively, the upstream and downstream ends thereof,
providing a fluid barrier mounted within the annular cavity between
the outer housing and the conduit,
providing a controllable piston mounted within the conduit,
providing an actuating rod axially projecting from the downstream
side of the piston where the free end of the rod extends through
the downstream end of the outer housing,
providing a bypass valve connected between the inlet and outlet
of the outer housing,
providing first and second piston detection points positioned in
a spaced apart relationship;
connecting the flowmeter in series within the outer fluid housing;
opening the bypass valve;
applying an upstream directed force to the free end of the actuating
rod, causing the piston to move to, and remain at the upstream end
of the conduit;
closing the bypass valve;
waiting an interval of time until the bypass valve is completely
seated and fluid flow has achieved a steady state condition;
releasing the force from the free end of the actuating rod, allowing
the piston to move downstream in synchronism with fluid flow;
measuring a time interval for the piston to travel the distance
between the first and second piston detection points; and
comparing the response of the flowmeter during the time interval
with the volume of the conduit defined by the distance between the
first and second detection points, so as to determine the calibration
factor of the flowmeter.
15. In apparatus for measuring a quantity of a flowing fluid by
means of a fluid barrier moving in a conduit and displacing a known
volume of fluid, and having first and second barrier detection means
spaced apart along the conduit so that the distance between the
first and second detection means defines the known volume of fluid,
the improvement comprising:
means for automatically altering the spacing between the first
and second detection means in response to the temperature of the
fluid to correct for the conduit dimensional changes due to fluid
temperature, so that the distance between the detection means represents
a constant displaced volume regardless of fluid temperature.
16. In apparatus for measuring a quantity of a flowing fluid by
means of a piston moving in a conduit and displacing a known volume
of fluid, the improvement comprising:
the piston including two seals each encircling the perimeter of
the piston, an annular cavity formed between the seals, a passage
connecting the cavity to one end of a flexible tube, and means for
connecting the other end of the flexible tube to the exterior of
the apparatus, whereby the integrity of the piston seals may be
continuously monitored from the output of the flexible tube while
the piston is stationary or in motion.
Description BACKGROUND OF THE INVENTION
This invention relates to flow meter provers, and more particularly
to an in-line flow meter prover having increased reliability and
accuracy and which may be constructed and operated with a minimum
of complexity.
In the use of flow meters to measure the quantity of fluid flowing
in a conduit, it frequently is desirable to determine the accuracy
of the meter while it is in service, without disrupting the flow
of the fluid being measured. A number of devices have been developed
and are in use for such purpose, and are known as in-line meter
provers.
Provers of the class to which this invention relates operate by
causing the fluid stream to pass simultaneously through the meter
and through a conduit containing a movable fluid barrier, typically
a ball which fits snugly into the conduit or a piston having a similar
fit. The barrier device is launched into the fluid stream at an
upstream position and travels with the fluid, passing two detection
points, and stopping at a downstream position. The barrier is then
returned by various means to the upstream position, from which it
may be launched again for a following test. The two detection points
are normally represented by electrical switches which are actuated
by passage of the barrier. A comparison of the volume of the prover
in the space between the two detection points with the measurement
by the meter of the same volume of fluid serves to determine the
meter accuracy.
The type of flow meter to which this class of provers is best applied
is one designed to produce a series of electrical impulses, each
impulse representing a certain volume of fluid. Flow quantity as
measured by the meter is determined by counting the total number
of impulses produced as the fluid passes through the meter. Such
counting is readily performed by electronic counters which display
a number representing the total number of impulses received from
the meter.
The purpose of proving is to calibrate the meter so that the quantity
of fluid represented by each impulse is precisely known. Additionally,
a precise measurement of rate of flow, i.e., volume per unit of
time, may be obtained from the prover by dividing the volume between
detection points by the time elapsed during the passage of the barrier
between the same points. This method of measuring flow rate enables
the prover to also be used to calibrate flow meters having an analog
output which is a function of flow rate.
With a flow meter designed to produce impulses, proving is performed
by connecting the flow meter impulses to a prover counter which
is an electronic counter having gating circuits activated by the
detector switches in the prover. The gating circuits cut off the
incoming impulses until the first detector switch is actuated. The
counter then starts and continues counting until the second switch
is activated, whereupon the counting is terminated.
The volume of fluid displaced between the two switches is a known
volume, having been precisely measured either by a displacement
test or by direct measurement of the conduit diameter and the linear
distance between the detection points. In the displacement test,
which is performed with the prover removed from the flowing stream
the volume displaced between the detection points is determined
by filling the prover with fluid, causing the barrier to move slowly
through the conduit, collecting the displaced fluid in a graduated
container, and noting the level in the container at each detection
point.
Following a proving test, a numerical factor defining the number
of meter impulses per unit of fluid volume is determined by dividing
the number of impulses produced during the proving test by the prover
volume. This factor is known as a calibration factor, or K factor,
and is expressed in terms of impulses per unit volume.
It has been found that the designs employed in the construction
of prior art provers may cause measurement errors which limit the
accuracy and repeatability achievable with these devices. For example,
in prior art provers the displaced volume of the prover will change
as the pressure of the fluid causes slight but significant changes
in the diameter of the prover conduit. Similarly, conduit dimensions,
and hence volume, will change as the temperature of the fluid causes
the conduit to expand or contract.
It is commonly accepted practice to calculate a compensation factor
to correct for prover volume changes due to the effect of temperature
and pressure on the conduit material. For example, in a typical
procedure the temperature and pressure of the fluid in the conduit
are measured during the proving run. Changes in the dimensions of
the conduit relative to a standard temperature and pressure are
then calculated, from which a corrected volume is computed.
Several assumptions are made in the above procedure which may result
in errors. First, it is assumed that the temperature of the conduit
is the same as the temperature of the fluid, which is not the case
if there is a difference in temperature between the fluid within
the conduit and the ambient air outside the conduit. Further, a
standard elastic modulus is assumed for the conduit whereas the
modulus actually varies due to variations in the composition of
the material from which the conduit is fabricated. Still further,
the effects on prover volume due to flanges or other stiffening
members at the ends of the conduit are difficult to analyze, and
are therefore generally ignored. Finally, the use of compensation
calculations is somewhat tedious, presenting opportunities for an
operator to make mathematical errors.
Howe U.S. Pat. No. 3273375 issued Sept. 20 1966 discloses
calibrating apparatus utilizing a double wall construction in which
the measuring conduit is enclosed in an outer housing. An object
of this type of construction is to reduce fluid pressure stress
on the conduit. Howe shows the measuring conduit supported by a
plurality of ribs spaced apart along the length of the measuring
conduit and structurally connecting the conduit to the outer housing.
In a conduit that is totally surrounded by fluid, stresses on the
conduit imposed by the fluid pressure are virtually eliminated.
In the Howe apparatus, however, large pressure differentials exist
in the area of the supporting ribs which can cause conduit dimensional
distortions. Accordingly, placement of conduit supporting structure
adjacent the measuring portion of the conduit contributes to measurement
errors as a result of fluid pressure imbalance.
Totally surrounding a conduit with fluid also has the effect of
equalizing the temperature of the conduit to the temperature of
the fluid. However, in the Howe apparatus described above, the measuring
fluid is precluded from contact with the outside surface of the
conduit in the area of the supporting ribs. Further, the ribs act
to transfer heat from the conduit to the outer housing. These conditions
cause temperature gradients which result in conduit dimensional
distortions and attendant measurement errors.
In existing provers the launching and return of the fluid barrier
device involves many difficult mechanical problems which are not
readily overcome. Mechanisms tend to be complex and the prover itself
is bulky and costly to construct. Some provers utilize complex reversing
valves to reverse the direction of the flow in the conduit and thereby
return the barrier to its original position. Other designs utilize
devices to retract the barrier and restrain it in the upstream position,
as well as to bypass the flow through the piston by means of a poppet
valve when the prover is not being used in a proving test. Provers
utilizing valves to reverse the direction of the flow are known
as bidirectional provers because proving tests may be made with
the barrier traveling in either direction. Provers utilizing devices
to retract and restrain the barrier are known as unidirectional
provers because the fluid and the barrier always travel in the same
direction in the conduit.
In the above described designs, there is an interval of time during
the operation of either the reversing valve or the poppet valve
when a portion of the fluid stream bypasses the barrier. Since the
barrier must move with the entire stream during the actual proving
of the meter, an additional length of conduit is normally provided
upstream of the first detection point to allow the valve to seat
properly and to shut off all bypass flow, thus assuring that the
barrier is traveling with the full stream flow when its reaches
the first detection point.
The Howe invention mentioned above is a bidirectional prover employing
a conduit, a free piston as a fluid barrier, and a four-port valve
to reverse the direction of fluid flow. Since no means are provided
for restraining piston movement during the operation of the valve,
a sufficient length of conduit is provided at both the upstream
and downstream ends to assure that the valve is closed and that
the piston is traveling with the full stream flow before reaching
the detection points. From the foregoing discussion it can also
be seen that the speed of operation of the valve is a critical factor
in the operation of the Howe apparatus. If the value does not close
quickly enough, a portion of the stream will bypass the piston.
Francisco U.S. Pat. No. 3492856 issued Feb. 3 1970 discloses
unidirectional flowmeter calibrating apparatus employing a piston
within a conduit, where the piston is restrained in the upstream
position by means of a complex motor, clutch and cable assembly
located upstream of the conduit. A poppet valve, held open by the
cable, provides a fluid passage through the piston when the apparatus
is not being used for flow measurements. Releasing the cable sets
the piston in motion and permits fluid pressure to close the poppet
valve. Because the valve operation and the piston restraint are
controlled by the same element, namely the cable, there are no means
for restraining piston movement during the operation of the valve.
Consequently, as in the case of Howe, additional conduit length
must be provided to assure that the piston is traveling at full
speed before reaching the first detection point. The response time
of the poppet valve is also a critical factor in the operation of
this apparatus.
Francisco U.S. Pat. No. 4152922 issued May 8 1979 also discloses
unidirectional flowmeter calibrating apparatus employing a measuring
piston within a measuring cylinder. The piston is restrained in
the upstream position by means of a second, retracting, piston mounted
within a retracting cylinder located on the upstream side of the
measuring cylinder. The retracting piston is connected to the upstream
side of the measuring piston. As in the earlier referenced Francisco
apparatus, a poppet valve is provided within the measuring piston
to form a fluid passage through the measuring piston when the apparatus
is not being used for flow measurements. In the upstream position
the retractng piston of the latter Francisco apparatus serves the
dual function of restraining the measuring piston and holding the
poppet valve open. Releasing the retracting piston sets the measuring
piston in motion and permits fluid pressure to close the poppet
valve. For the reasons stated above, this configuration suffers
from the same limitations as the earlier cited references, namely
the requirement for additional conduit length, and the critical
nature of the poppet valve response time.
Another problem associated with provers which employ movable fluid
barriers is caused by the friction developed by the seal between
the barrier and the conduit. For example, to reduce the chances
of leakage past the movable piston, prior art provers employ seal
rings between the piston and the conduit. Friction created by the
seals must be overcome to move the piston. As described below the
solution to this friction problem is further complicated by the
fact that it is desirable to have equal fluid pressure on both the
upstream and downstream sides of the piston to avoid affecting the
flow rate of the measured fluid. Equal fluid pressure across the
piston has the added advantage of preventing leakage around the
piston seals.
In those prior art provers employing free pistons, such as the
Howe apparatus referenced above, both the upstream and downstream
sides of the piston present the same surface area to the fluid pressure.
Thus, in this configuration equal fluid pressure on both sides of
the piston provides no net force to overcome seal friction. The
result is that the free piston will not move until the upstream
pressure is greater than the downstream pressure. The magnitude
of the friction may readily cause a change in the rate of flow through
the prover, introducing an undesirable effect in the total fluid
system. This effect of seal friction is especially undesirable where
the measured fluid is compressible, because an actual change in
fluid volume can occur, causing measurement errors.
The apparatus disclosed in the Francisco U.S. Pat. No. 3492856
discussed above suffers a similar problem. In fact, this problem
is aggravated by the inclusion of a cable assembly attached to the
upstream side of the piston. The cable and its drum assembly cause
additional drag on the downstream motion of the piston, requiring
an even larger pressure imbalance across the piston to overcome
both seal friction and cable drag.
In the apparatus disclosed in the Francisco U.S. Pat. No. 4152922
discussed above, a rod is connected between the upstream side of
the measuring piston and the retracting piston. The area of the
rod reduces the effective surface area, and thus the force of the
fluid, on the upstream side of the measuring piston, requiring even
greater upstream pressure to move the piston. The motion of the
measuring piston is further retarded by the friction caused by the
seals of the retracting piston. In an attempt to overcome these
retarding forces on the measuring piston, the Francisco apparatus
must apply pressure to the upstream side of the retracting piston
to aid the downstream motion of the measuring piston during a prover
test. Further, Francisco teaches that it is desirable to further
increase the pressure on the retracting piston beyond that necessary
to overcome system drag forces in order to maintain a higher pressure
on the downstream side of the measuring piston then on the upstream
side thereof. The necessity for this increased pressure is to ensure
that the poppet valve employed in the Francisco apparatus remains
closed and sealed. Thus the Francisco apparatus requires unequal
pressure across the measuring piston for its operation, with the
above described undesirable effects. Further, with high fluid pressure
in the measuring conduit, the piston rod is subjected to high compression
loads which can result in the rod failing as a column.
The Francisco U.S. Pat. No. 4152922 also discloses a modification
to the apparatus described above in which the retracting rod extends
completely through the cylinder from end to end. The object is to
reduce the force unbalance created by the area of a rod which is
located solely on the upstream side of the measuring piston. Evan
with this modification, which requires an additional seal further
increasing piston drag force, the Francisco apparatus still requires
unbalanced piston pressure to overcome all of the drag forces enumerated
above, and to ensure poppet valve closure.
In summary, prior art provers do not operate with equal fluid pressure
on both sides of the measuring piston, resulting in flow rate changes
and attendant measuring errors.
Means must also be included in fluid barrier type provers for stopping
the barrier at the end of its travel. Prior art provers employ mechanical
stops which result in an abrupt change in piston motion. For example
the Howe apparatus employs rods projecting from the center of both
ends of the outer housing to prevent the piston from exiting the
conduit. The rods are designed to directly contact the sides of
the piston, causing an abrupt stop. The Francisco U.S. Pat. No.
3492256 discloses a portion of the poppet valve associated with
the piston assembly which is designed to directly contact the downstream
end of the cylinder to operate the poppet valve. Apparatus disclosed
in both Francisco references employ direct contact between the piston
and the upstream end of the cylinder housing to stop the piston
motion on the return stroke.
The abrupt changes caused by the stopping mechanisms of the prior
art provers tend to produce undesirable perturbations in the fluid
system. They also produce high shock loads in the various mechanical
components of the system.
As described above, a variety of techniques are employed in prior
art unidirectional provers to return the fluid barrier to the upstream
position after a prover test. The Francisco apparatus disclosed
in U.S. Pat. No. 3492856 employs a cable and motor to retract
the piston, while in the Francisco apparatus disclosed in U.S. Pat.
No. 4152922 pressure is applied to the downstream side of the
retracting piston, forcing the measuring piston to its upstream
position. This latter prover configuration requires means for pressurizing
the upstream side of the retracting piston to assist the motion
of the measuring piston during a test, as well as means for pressurizing
the downstream side of the retracting piston to return the measuring
piston to its upstream position after a test. The retracting piston
and cylinder arrangement of Francisco is thus complex, requiring
high pressure piston seals and a plurality of valves and regulators
for proper operation.
Achieving accurate and repeatable measurements with fluid barrier
type provers requires that during the proving test there be no fluid
leakage past the barrier seals or past any bypass valve mechanisms.
In U.S. Pat. No. 4152922 Francisco discloses methods of testing
for seal leakage which require either shutting off fluid flow to
the entire system, or removing the prover from the system and performing
a separate bench test. In either case the flow of the fluid being
measured is disrupted and consequently these leak test methods cannot
be performed while the prover is in operation. Thus, there is no
way of assuring the integrity of the seals during an actual proving
test.
It is accordingly an object of the present invention to provide
a new and improved flow meter prover which requires no external
conpensation for pressure or temperature changes;
It is yet another object of the present invention to provide a
new and improved flow meter prover which may be operated with equal
fluid pressure on both sides of the fluid barrier;
It is yet another object of the present invention to provide a
new and improved flow meter prover in which the integrity of the
seals affecting prover accuracy are continuously monitored while
the prover is functioning.
SUMMARY OF THE INVENTION
The foregoing and other objects of the invention are accomplished
by a flow meter prover in which the fluid stream passes simultaneously
through a cylindrical conduit containing a movable fluid barrier
in the form of a piston, and through the flow meter under test.
The prover utilizes a double wall construction in which an inner
measuring conduit is enclosed in an outer housing in a manner which
permits the measuring portion of the conduit to be entirely surrounded
by the measured fluid. Thus, there are virtually no fluid pressure
induced stresses imposed upon the measuring portion of the conduit;
and compensation for changes in prover volume due to fluid pressure
is not required.
The above described double wall construction also assures that
the entire measuring portion of the conduit is at the same temperature
as the fluid, thus eliminating compensation errors due to heat transfer
through the outer housing. This arrangement enables the implementation
of automatic temperature compensation. This compensation is accomplished
by automatically varying the positions of the two detector switches
used to sense the position of the piston in the conduit. The detector
switches are mounted on bimetal elements which are arranged to sense
the temperature of the measured fluid. These bimetal elements vary
the position of the detector switches in response to fluid temperature
changes in an amount which exactly compensates for changes in the
conduit dimensions due to temperature.
In the operation of the prover the piston travels downstream in
the cylindrical conduit in synchronism with the fluid flow, and
is returned to its original upstream position by means of an actuating
rod attached to the downstream side of the piston. During the return
stroke a bypass valve external to the housing allows the flow to
continue without interruption. Upstream and downstream ports are
provided in the conduit to allow the flow to bypass the piston while
it is at rest in both the upstream and downstream positions. These
ports permit the flow to continue around the piston while the bypass
valve is in the process of closing (when the piston is in the upstream
position) and opening (when the piston is in the downstream position).
Prior to a proving test, the piston is held in the upstream position
by applying pressure to the rod. The bypass valve is then closed.
After the valve has fully sealed, pressure against the rod is released,
allowing the piston to move downstream. As soon as the piston covers
the conduit upstream ports, it is accelerated to full fluid velocity
within a very short distance, approximately the diameter of the
ports. At the downstream end of the conduit, the fluid is bypassed
immediately after the piston passes a first set of downstream ports
at which time the bypass valve is opened. A second set of downstream
ports is also provided which has the effect of smoothly decelerating
the piston to a stop without any abrupt mechanical shock.
By separating the flow bypass function, namely the bypass valve
and conduit ports, from the piston restraining function, namely
the rod, fluid equilibrium may be established in both the prover
and the flow meter prior to releasing the piston and starting the
test. This design permits nearly all of the conduit length to be
utilized for the actual proving function with only a short section
required at either end of the conduit for accelerating and decelerating
the piston. In addition, the response time of the bypass valve is
not a factor in the operation of the prover.
The placement of the restraining rod on the downstream side of
the piston reduces the effective downstream surface area of the
piston. The result is that with equal fluid pressure on both sides
of the piston, there is a net force aiding the downstream motion
of the piston and overcoming piston seal friction. Returning the
piston to the upstream side of the conduit simply requires the application
of pressure to the end of the rod, eliminating the need for a retracting
piston.
The prover also includes means for continuously monitoring the
integrity of both the bypass valve seals and the piston seals while
the prover is functioning. The bypass valve is of the double-back-and-bleed
type which incorporates a seal on each side of an intervening cavity.
Pressure in this cavity is monitored during the proving run to assure
that there is no leakage past either seal and, hence, no leakage
through the valve. Similarly, the piston also incorporates a double
seal and an intervening cavity which is connected by means of a
flexible tube through a port at one end of the prover housing. Pressure
between the piston seals can thus be monitored during the proving
run to ensure that there is no leakage past either piston seal and
hence no leakage past the piston.
These and other objects, features, and advantages of the invention
will become apparent from a reading of the specification when taken
in conjunction with the drawings in which like reference numerals
refer to like elements in the several figures.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a side elevation exterior view of the prover of the present
invention;
FIG. 2 is a partial cutaway view along section 2--2 of FIG. 1 showing
the interior construction of the prover of the present invention;
FIG. 3 is a schematic view of the prover of the present invention;
FIG, 4 is a detail view of the piston assembly portion of the prover
of the present invention;
FIG. 5 is a partial cutaway view along section 5--5 of FIG. 4 showing
the construction of the piston assembly;
FIG. 6 is a partial cutaway view of the detector switch assembly
portion of the prover of the present invention;
FIG. 7 is a schematic view of a pressure control system which may
be used with the prover of the present invention; and
FIG. 8 is a schematic view of another embodiment of a pressure
control system which may be used with the prover of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIGS. 1 and 2 there are shown, respectively, a side
elevation exterior view and a partial cutaway view of the prover
of the present invention. The prover includes a cylindrical outer
housing 12 which is supported on a horizontal surface by means of
struts 14. One end of the housing 12 defined as the upstream end,
is enclosed by a generally hemispherical end cap 16 which is affixed
to the housing 12 by means such as welding. The end cap 16 includes
a fluid inlet 18 and an inlet pipe 20 affixed thereto. A fluid outlet
22 is provided adjacent the opposite or downstream end of the housing
12 and an outlet pipe 24 is attached thereto, terminating in a
fluid outlet 25.
A flange 26 affixed to the downstream end of the housing 12 is
in turn fastened to a housing end plate 28 by means of bolts 30.
A gasket 32 is provided between the flange 26 and the end plate
28 to provide a liquid seal. Coaxially mounted within the housing
12 is a cylindrical measuring conduit 34 formed of a non-magnetic
material and open at both ends. The downstream end of the conduit
34 mates with an annular groove 36 provided within end plate 28.
The conduit 34 is fastened in this position by a plurality of clamps
38. This fastening means provides a fluid tight seal between the
downstream end of the conduit 34 and the end plate 28.
The conduit 34 is further supported near its upstream end by an
annular ring 40 and a flexible seal 42. The annular ring 40 is affixed
to the inside wall of the housing 12 and the seal 42 is positioned
between the ring 40 and the conduit 34 to provide radial support
for the upstream end of the conduit 34. In addition, the combination
of the ring 40 and the seal 42 acts to form a fluid barrier which
divides the space between the housing 12 and the conduit 34 into
an upstream chamber 44 and an annular downstream chamber 46. By
supporting the upstream end of the conduit 34 by means of a flexible
seal 42 the walls of the conduit 34 are free to expand when subjected
to elevated fluid temperatures. Three rows of circular ports are
provided in the wall of conduit 34. These are upstream ports 48
downstream ports 50 and deceleration ports 52.
A movable fluid barrier in the form of a piston 54 is slidably
mounted within the measuring conduit 34 with piston seals 56 and
58 providing a fluid seal between the piston 54 and the walls of
the conduit 34. One end of an actuating rod 60 is centrally fastened
to the downstream side of the piston 54. The other, and downstream,
end of the actuating rod 60 extends through an opening 62 in the
end plate 28. As described below, the volume of fluid displaced
by the downstream motion of the piston 54 within a measuring portion
of the conduit 34 is used to calibrate the flowmeter 100. The measuring
portion of the conduit 34 lies between the upstream and downstream
ports 48 and 50 respectively. As can be seen in FIG. 2 none of
the supporting means described above for the conduit 34 are located
along the measuring portion of the conduit 34. In the operation
of the prover 10 this support configuration permits the measuring
portion of the conduit 34 by completely surrounded by the measuring
fluid.
As shown in FIG. 2 the downstream end of the rod 60 is enclosed
within a cylindrical pressure housing 64. One end of pressure housing
64 is fastened to the outside surface of the end plate 28 by means
of a flange 66. An O-ring seal 68 is located between the flange
66 and the rod 60 to provide a fluid seal between the measuring
conduit 34 and the pressure housing 64. A compression spring 70
is axially mounted on rod 60 and acts between the flange 66 and
a shoulder 72 formed at the end of the rod 60. The downstream end
of the housing 64 is enclosed by means of an end plate 74. A second
compression spring 73 is provided, which is attached at one end
to a support bracket 75. The bracket 75 is in turn affixed to the
upstream end of the housing 12. The spring 73 acts between the support
75 and the upstream side of the piston 54 when the piston 54 is
in the upstream position.
FIG. 3 is a schematic diagram of the prover 10 of the present invention
showing the various hydraulic and electrical connections. Referring
to FIGS. 1 and 3 the interior of the pressure housing 64 communicates
with the outlet of a hydraulic pump 76 via tubing 78 and check valve
80. One end of the tubing 78 and a pressure switch 82 are both connected
to the end plate 74 which, via an internal passage, communicates
with the interior of the pressure housing 64. An hydraulic reservoir
84 is connected to the inlet of the pump 76. The reservoir 84 also
communicates with the interior of the pressure housing 64 via a
solenoid operated valve 86. Both the pressure switch 82 and the
solenoid valve 86 are electrically connected to a control box 88
via cables 90 and 92 respectively.
As shown in FIGS. 1 and 3 a motor operated bypass valve 94 is
connected between the inlet pipe 20 and the outlet pipe 24 by means
of T-pipes 96 and 98 respectively. A flow meter 100 which is the
meter under test, is mounted to the upstream end of the inlet pipe
20 in series with the prover 10. The arrows shown in FIGS. 1 and
3 indicate the directions of fluid flow in the operation of the
prover 10. Fluid enters the system at an inlet 102 upstream of the
flow meter 100. With bypass valve 94 in the closed position, the
fluid passes through the flow meter 100 and enters the housing 12
by means of inlet pipe 20. As described below, the fluid will exit
the housing 12 via the outlet pipe 24. When the bypass valve 94
is in the open position, the fluid flows from the inlet 102 through
the flow meter 100 through the bypass 94 via the T's 96 and 98
exiting from the outlet pipe 24 and bypassing the prover housing
12. The bypass valve 94 is electrically connected to the control
box 88 by means of a cable 104.
The bypass valve 94 is of the double-block-and-bleed type, well
known in the art. This type of valve, such as Model No. 6150-31-2236-TT-DBB
manufactured by Jamesbury Corporation, Worcester, Mass., employs
a valve seal on each side of an intervening cavity. A port is provided
in the body of the valve which communicates with this intervening
cavity. By continuously monitoring the pressure in the cavity via
the port, one can detect the presence of leakage past either valve
seal. As described below, this feature of the valve 94 is employed
to continuously monitor the integrity of the seals within the valve
94 while the prover is functioning. The valve 94 may be electrically
operated by employing a motorized actuator such as Model No. ERC250
also manufactured by Jamesbury Corporation.
The position of the piston 54 within the conduit 34 is sensed by
means of two magnetically operated reed switches 106 and 108. As
shown in FIGS. 2 and 3 the reed switches 106 and 108 are located
adjacent the outside surface of the measuring conduit 34 within
the downstream annular chamber 46. The switches 106 and 108 are
enclosed within a fluid tight tube 110 having a threaded flange
112 fastened to one end thereof. The tube 110 is fastened in place
by threading the flange 112 into a threaded hole 114 located in
the end plate 28 as shown in FIG. 2. Electrical connection to the
switches 106 and 108 is facilitated by means of a junction box 116
located exterior to the flange 112. The switches 106 and 108 are
electrically connected to a prover counter 118 by means of a cable
120 and the flow meter 100 under test is also electrically connected
to the prover counter 118 by means of a cable 122.
Referring now to FIGS. 4 and 5 there are shown detailed views
of the piston assembly 54. FIG. 4 is a view of the downstream face
of the piston 54 and FIG. 5 is a partial cutaway view of the piston
54 along section 5--5 of FIG. 4. The body of the piston 54 includes
a cylindrical disk 124 formed of steel or other suitable material.
A clearance hole 126 is provided in the center of the disk 124 to
receive the threaded end of the actuating rod 60. A seal 128 is
also provided to prevent leakage through the opening 126. The rod
60 is fastened to the downstream side of the disk 124 by a nut 130.
Recesses are provided around the circumference of the disk 124 to
reatain the piston seals 56 and 58. The seals 56 and 58 are arranged
to form a small annular cavity 132 around the periphery of the disk
124 between the seals 56 and 58.
As shown in FIG. 5 the annular cavity 132 communicates with one
end of a flexible tube 134 by means of a radially disposed passage
136 and an intersecting axially disposed passage 138 within the
disk 124. A fitting 140 is provided to connect the flexible tube
134 to the passage 138. As shown in FIGS. 2 and 3 the flexible
tube 134 is loosely wrapped along the length of the actuating rod
60 terminating in a fitting 142 which projects through the end
plate 28. During the operation of the prover 10 fluid leakage past
either of the seals 56 or 58 will result in fluid flow through the
tubing 134. By continuously monitoring the pressure in the tube
134 via the fitting 142 the integrity of the piston seals 56 and
58 may be continuously monitored during a prover test.
The piston 54 also includes means for providing a magnetic field
necessary to actuate the position-detecting reed switches 106 and
108 described above. As shown in FIGS. 4 and 5 a plurality of magnets
144 preferably in the form of ceramic disks, are equally spaced
around the circumference of, and on the downstream side of, the
disk 124. The magnets 144 are in turn sandwiched between two magnetic
pole pieces 146 and 148 in the form of thin annular disks of soft
steel. The magnets 144 are held in place by means of screws 150.
This arrangement of the magnets 144 which are preferably magnetized
axially, causes a magnet field 152 to be formed around the entire
perimeter of the pole pieces 146 and 148. While six magnets are
shown in FIG. 4 any number arranged to produce a reasonably uniform
magnetic field at the perimeter of the pole pieces 146 and 148 will
function equally well. As shown in FIG. 5 a wearing ring 154 is
also included as part of the piston 54. The wearing ring 154 is
supported by an annular support ring 156 which is in turn fastened
to the upstream side of the disk 124 by means such as screws 158.
The support ring 156 also serves as a retainer for the piston seal
56; and, in like manner the pole piece 148 serves as a retainer
for the piston seal 58.
Referring now to FIG. 6 there is shown a partial cutaway view
illustrating the details of the detector switch assembly of the
prover 10 of the present invention. The detector switch assembly
includes the cylindrical tube 110 which is formed of a non-magnetic
material and which has a closed end. The magnetically operated reed
switches 106 and 108 are enclosed in glass capsules and have leads
axially extending therefrom. Such glass encapsulated reed switches
are well known in the art. These switches 106 and 108 are slidably
mounted in recesses formed in supporting blocks 160 and 162 respectively.
The leads 164 and 166 of switch 106 and the leads 168 and 170 of
switch 108 project through clearance holes provided in the support
blocks 160 and 162 respectively. Bimetallic elements 172 and 174
preferably in the form of elongated strips, are mounted so that
one end of the elements 172 and 174 is fastened to one side of the
support blocks 160 and 162 respectively. The elements 172 and 174
are mounted in such a way that their unfastened ends are free to
move along an axis parallel to the axis of the tube 110.
Lead 166 of switch 106 is rigidly attached to the free end of the
bimetallic element 172 by use of high temperature epoxy or other
suitable means. Similarly, lead 170 of switch 108 is rigidly attached
to the free end of bimetallic element 174. The support blocks 160
and 162 are in turn rigidly mounted to a spacer bar 176 which extends
along the length of the tube 110. Spacer bar 176 serves to hold
the blocks 160 and 162 in a fixed spaced apart relationship. Electrical
connections are made to the switches 106 and 108 by means of thin
flexible wires 178 which are attached to leads 164 166 168 and
170 by means such as soldering. The wires 178 are routed along the
length of the spacer bar 176 terminating at the open end of the
tube 110 supporting the threaded flange 112.
As shown in FIG. 6 the assembly including the spacer bar 176 and
the support blocks 160 and 162 is inserted within the interior of
the tube 110. The tube 110 is in turn inserted within the annular
cavity 46 of the prover 10 and fastened in place by threading the
flange 112 into the threaded hole 114 in the end plate 28 as shown
in FIG. 2. When tube 110 is in place and prior to the installation
of the junction box 116 the spacer arm 176 is oriented by rotating
the exposed end 180 of bar 176. Spacer bar 176 is rotated so that
the reed switches 106 and 108 are adjacent the wall of the measuring
conduit 34. Spacer bar 176 is then locked into this position by
tightening a set screw 182 as shown in FIG. 6. This orientation
of spacer bar 176 assures maximum magnetic coupling between the
magnetically operated reed switches 106 and 108 and the magnetic
field 152 of the piston 54. After proper orientation of spacer bar
176 junction box 116 is threaded onto the exposed end of tube 110
and connections are made between the free ends of the conductors
178 and the cable 120 as shown in FIGS. 2 and 3.
As described below, the detector switch assembly is entirely immersed
in the measured fluid during a prover test. Accordingly, the temperature
of the bimetallic elements 172 and 174 will correspond at all times
to the temperature of the measured fluid. As the fluid temperature
increases, the bimetallic elements 172 and 174 deflect in a direction
which brings the two reed switches 106 and 108 closer together.
This axial motion of the switches 106 and 108 exactly compensates
for the increased dimensions of the measuring conduit 34 due to
elevated fluid temperature. Thus, the volume of fluid displaced
by the piston 54 between the two switches 106 and 108 remains constant
regardless of temperature. This action provides automatic temperature
compensation for the prover 10 without the need for calculations
by the operator.
The operating sequence of the prover 10 is as follows: In the normal
or rest position, the piston 54 is at the downstream end of its
stroke adjacent the end plate 28; and the electrically operated
bypass valve 94 is in the open position. Referring to FIGS. 2 and
3 it can be seen that the fluid will flow through the inlet 102
the flow meter 100 and the bypass valve 94 and exit from the outlet
25. A small amount of the fluid will also pass through the interior
of the prover 10 in a manner described below.
A prover run is initiated by starting the hydraulic pump 76 with
the solenoid operated valve 86 in a closed position. Pressurized
hydraulic fluid from the reservoir 84 enters the pressure housing
64 via the tube 78. This hydraulic pressure exerts a force against
the actuating rod 60 forcing it, together with the piston 54 to
move upstream within the conduit 34. At the completion of its upstream
stroke, actuating rod 60 compresses the springs 70 and 73; and the
actuating rod 60 and the piston 54 are brought to a smooth stop
when the springs 70 and 73 are fully compressed. This action avoids
any abrupt changes in stopping the motion of the piston 54 reducing
fluid perturbation in the system. When the piston 54 stops moving,
the hydraulic pressure within the housing 64 increases, actuating
the pressure switch 82 which indicates full upstream travel of the
piston. Closure of the pressure switch 82 is communicated to the
control box 88 via the cable 90 and relay logic within the control
box 88 initiates the closing of the bypass valve 94. When the bypass
valve 94 is fully closed flow ceases in the pipes 96 and 98 causing
the entire fluid stream to pass through the inlet pipe 20 and into
the upstream prover chamber 44. The ring 40 and the seal 42 prevent
fluid from directly entering the downstream chamber 46. The fluid
is thus caused to enter the conduit 34 via the upstream ports 48
filling the interior of the conduit 34. The fluid then exits the
conduit via the downstream ports 50 and the deceleration ports 52
fills the downstream annular cavity 46 and exits the housing 12
via the pipe 24 and the outlet 25.
After a sufficient time interval has passed to allow the bypass
94 to fully close and to allow the fluid stream sufficient time
to establish equilibrium in the system, the hydraulic pump 76 is
stopped and the valve 86 is opened. These actions cause the release
of hydraulic pressure in pressure housing 64 and allow hydraulic
fluid to return through valve 86 to the reservoir 84. Piston 54
will now begin moving in a downstream direction due both to the
force of the moving fluid and to the forces exerted by the springs
70 and 73. As the piston 54 moves downstream, it will first cover
the upstream ports 48. This prevents bypass flow around the piston
54 and causes it to move in full synchronism with the fluid flow.
It has been found that the spring 73 is required when the measured
flow is at low line pressure, to assist the initial downstream motion
of the piston 54 until the upstream ports 43 are covered.
The placement of the actuating rod 60 on the downstream side of
the piston 54 reduces the effective downstream surface area of the
piston 54. This effect creates a net force on piston 54 which aides
its downstream motion. Thus the piston 54 overcomes the friction
of the seals 56 and 58 while maintaining equal fluid pressure on
both sides of piston 54.
As the piston 54 passes the magnetically operated reed switch 106
the magnetic field 152 formed by the magnets 144 and the pole pieces
146 and 148 of the piston 54 extends through the nonmagnetic walls
of the conduits 34 and the tube 110 to actuate the switch 106. The
relatively close spacing of the pole pieces 146 and 148 as shown
in FIG. 5 causes the magnetic field 152 to be concentrated in a
limited area with a sharp gradient. This condition ensures closure
of the reed switches 106 and 108 at the same point on the piston
54 travel for each proving stroke.
The closure of the switch 106 provides a start signal to the prover
counter 118 via the cable 120. Near the end of its stroke, piston
54 actuates the detector switch 108 in the same manner described
above for the switch 106. The closure of the switch 108 sends a
second signal to the prover counter 118. The piston 54 continues
downstream past the ports 50. When the piston 54 finally covers
the deceleration ports 52 the piston 54 is smoothly decelerated
to a stop by the fluid which is trapped between the downstream side
of the piston 54 and the end plate 28. This fluid thus acts as a
hydraulic shock absorber preventing the piston 54 from a hard impact
upon stopping. The closure of the detector switch 108 may also be
used to initiate opening of the bypass valve 94 thus returning the
system to its original condition.
During the entire prover run, the integrity of the piston seals
56 and 58 may be continuously monitored by observing the pressure
within the tube 134 as described above. In a similar manner, the
integrity of the seals in the double-block-and-bleed bypass valve
94 may also be monitored during the test. The action of the bimetallic
elements 172 and 174 provide automatic temperature compensation
of the prover 10 during the prover test.
During the prover run, the flow meter 100 under test reproduces
a series of electrical impulses, each impulse representing a certain
volume of fluid. These flow meter impulses are communicated to the
prover counter 118 via the cable 122. Following a proving test,
a figure defining the number of meter impulses per unit of fluid
volume is determined by dividing the number of flow meter impulses
produced during the proving test by the prover volume. This figure
is known as a calibration factor or K factor and is expressed in
terms of impulses per unit volume. In the prover of the present
invention, a pulse interpolation technique is utilized within the
prover counter 118 which permits resolving the number of flow meter
impulses to a fraction of a single impulse. Such interpolation techniques
can provide a resolution of one part in ten thousand or better.
The pulse interpolation technique employed in the present invention
involves the measurement of two time intervals. The first interval
T.sub.o is the interval of time between the closures of the two
detecting switches 106 and 108. The second interval, T is the difference
in time between the first flow meter 100 impulse occuring after
closure of the first detector switch 106 and the time of occurrance
of the first flow meter impulse after the closure of the second
detector switch 108. The exact fractional number of impulses P.sub.o
occurring during the time T is obtained by multiplying the whole
number of counted impulses P by the ratio of T.sub.o to T:
The above computation is performed by a microprocessor within the
prover counter 118. The result is displayed on a lighted numerical
display. The actual calibration factor K may also be computed by
the microprocessor and is obtained by dividing the number of impulses
P.sub.o by the prover volume Q.sub.p :
The rate of flow q, is obtained by dividing the prover volume Q.sub.p
by the time interval T.sub.o :
Other measurement techniques, such as using a phase lock loop to
multiply the flow meter impulse frequency, may be employed to achieve
similar high resolution in the counting of the flow meter impulses.
Under certain operating conditions, it may be desirable to regulate
the level of fluid pressure on the upstream side of the piston 54.
Such conditions occur, for example, when the measured fluid is compressible
as in the case of a gas or a light liquid. A pressure control system
for performing this function is shown schematically in FIG. 7. A
pipe 184 is provided in which one end communicates with the upstream
chamber 44 of the prover 10 through the wall of the housing 12.
The other end of the pipe 184 is connected to a conventional mechanically
operated pressure regulator 186. Regulators of this type are well
known in the art and include a spring loaded diaphragm or a bellows
to actuate a poppet which in turn regulates the flow through the
regulator. The desired pressure level to be regulated is set by
means of an adjustable spring 188 within the regulator 186. As shown
in FIG. 7 the regulator 186 controls the flow between the solenoid
operated valve 86 and the hydraulic reservoir 84.
Operation of the pressure control system of FIG. 7 is as follows:
During the measuring stroke of the piston 54 hydraulic fluid inside
the pressure housing 64 is displaced by the downstream motion of
the actuating rod 60 causing fluid in the housing 64 to flow outward
through pipe 78 through the open valve 86 through the regulator
186 and into the reservoir 84. The pressure level on the upstream
side of the piston 54 is communicated to the pressure regulator
186 via the pipe 184. If the pressure in the pipe 184 is higher
than the pressure level set by the spring 188 the poppet within
the regulator 186 will move in an opening direction to permit a
higher flow of hydraulic fluid out of the housing 64. This action
reduces the back pressure on the actuating rod 60 thereby allowing
freer motion of piston 54 and resulting in lowering of the upstream
pressure on the piston 54. Conversely, if the pressure in the pipe
184 is less than the pressure setting of the spring 188 this will
result in a partial closing of the regulator 186 restricting the
flow of hydraulic fluid and thereby increasing the upstream pressure
on the piston 54.
An alternate embodiment of the pressure regulating system described
above is shown in FIG. 8. This system may be used to maintain a
constant pressure drop across the upstream and downstream sides
of the piston 54. As shown in FIG. 8 pipes 184 and 190 are provided
which communicate, respectively, with the upstream chamber 44 and
the downstream annular chamber 46 of the prover 10. The pipes 184
and 190 are in turn connected to a conventional differential pressure
transducer 192. The differential pressure transducer 192 senses
the difference in pressure in the pipes 184 and 190 corresponding
to the difference in pressure across the upstream and downstream
sides of the piston 54. The differential pressure transducer 192
provides an output signal which is proportional to this pressure
differential. The output signal of the transducer 192 is in turn
used to control a differential pressure controller 194. Controller
194 functions to compare the output signal from the transducer 192
with an internally set value. Any deviation between these two levels
will cause an output of the controller 194 which is used to operate
a control valve 196. Control valve 196 is used to regulate the flow
between the solenoid valve 86 and the hydraulic reservoir 84 in
a manner similar to the function of the regulator 186 shown in FIG.
7 and described above.
During a proving run, the pressure control system shown in FIG.
8 will regulate the flow of the hydraulic fluid from the pressure
housing 64 to maintain a constant pressure drop across the piston
54. As will be understood by those skilled in the art, a system
similar to that shown in FIG. 8 may be employed to regulate the
upstream pressure on the piston 54 in place of the mechanical pressure
regulator 186 of FIG. 7. Alternatively, a differential version of
the pressure regulator 186 may be employed in the system of FIG.
8 to regulate the pressure drop across the piston 54. Further, systems
similar to those shown in FIGS. 7 and 8 may also be employed to
regulate the downstream pressure on the piston 54.
While the invention is thus disclosed and the presently preferred
embodiment described in detail, it is not intended that the invention
be limited to the shown embodiment. Instead, many modifications
will occur to those skilled in the art which lie within the spirit
and scope of the invention. It is accordingly intended that the
invention be limited only by the appended claims.
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