Abstrict A positive displacement flushable flow meter has sensing elements
that are rotated by and at a rate in accordance with a flow of fluid
through the flow meter. The flow meter is particularly adapted for
measuring the volume flow rate of coating material supplied to spray
coating apparatus, and to permit the flow meter to be quickly flushed
by means of alternate bursts of relatively high velocity air and
solvent, without overdriving the sensing elements and damaging the
flow meter, a bypass valve is provided. During a spray coating operation,
the bypass valve is closed and all of the coating material flows
past and rotates the sensing elements. During a flushing operation,
the bypass valve is opened to divert some of the relatively high
velocity flow of air and solvent around the sensing elements, while
allowing a controlled and limited amount of air and solvent to move
past and rotate the elements to clean them without causing them
to be overdriven.
Claims What is claimed is:
1. A positive displacement flow meter for measuring the volume
flow of liquid coating material delivered to spray coating apparatus
and for being flushed clean of coating material by fluidic flushing
media, said flow meter comprising a housing having inlet passage
means and outlet passage means; means for supplying either liquid
coating material at a first volume flow rate and under pressure
or fluidic flushing media at a second and greater volume flow rate
and under pressure to said inlet passage means; movable liquid flow
sensing means intermediate said inlet and outlet passage means for
flow therethrough of coating material and flushing media from said
inlet to said outlet passage means, said sensing means being moved
in response to and at a rate in accordance with the volume flow
rate of coating material or flushing media therethrough and generating
an indication of the volumetric flow of coating material flowing
therethrough; bypass valve means for selectively shunting a portion
of the fluid in said inlet passage means around said flow sensing
means to said outlet passage means; and means for operating said
bypass valve means so that no fluid is shunted around said flow
sensing means when coating material is supplied to said inlet passage
means so that a portion of the fluid is shunted around said flow
sensing means when flushing media is supplied to said inlet passage
means.
2. A positive displacement flow meter as in claim 1 wherein said
bypass valve means, when selectively shunting a portion of the fluid,
establishes a shunt path between said inlet and outlet passage means
so that only a portion of the flushing media supplied to said inlet
passage means flows through said flow sensing means to said outlet
passage means and the remainder of the flushing media flows through
said shunt path.
3. A positive displacement flow meter as in claim 2 wherein said
bypass valve means comprises a first channel in communication with
said inlet passage means, a second channel in communication with
said outlet passage means, and a valve member selectively movable
by said operating means between a first position establishing a
path and a second position interrupting said path between said first
and second channels.
4. A positive displacement flow meter as in claim 1 wherein said
movable flow sensing means comprises at least one flow sensing element
that is rotated by fluid flowing through said flow sensing means
and at a rate in accordance with the volumetric flow rate of the
fluid, and said bypass valve means, in shunting a portion of the
flushing media around said flow sensing means, permits only a limited
amount of the flushing media to pass through said flow sensing means
to clean said flow sensing means without causing said at least one
sensing element to be rotated at an excessive rate.
5. A positive displacement flow meter as in claim 4 wherein said
at least one sensing element comprises a pair of gears in meshing
engagement and having tooth spaces in which fluid is carried from
said inlet to said outlet passage means, and the rate of rotation
of said gears, when no fluid is shunted around said flow sensing
means, is in accordance with the volume flow rate of coating material
through said flow sensing means, and including means for detecting
the revolutions through which said gears are rotated to detect the
volume flow of coating material through said flow sensing means.
6. A positive displacement flow meter as in claim 5 wherein said
gears are rotated in response to a differential in the pressure
of fluid between said inlet and outlet passage means, and said bypass
valve means maintains a pressure differential of fluid when operated
to shunt a portion of the fluid in said inlet passage means around
said flow sensing means.
7. A method of operating a positive displacement flow meter having
at least one sensing element that is rotated in response to and
at a rate in accordance with the volume flow rate of fluid through
the flow meter, wherein the flow meter measures the volume flow
of liquid coating material supplied under pressure to a spray coating
apparatus, if flushed clean of coating material by fluidic flushing
media, and the at least one sensing element has a maximum rate of
rotation beyond which damage to the flow meter can occur, said method
comprising the steps of supplying liquid coating material under
pressure and at a first flow rate through the flow meter to the
spray coating apparatus; detecting the revolutions through which
the at least one sensing element is turned by the coating material
to measure the volume flow of the coating material through the flow
meter to the spray coating apparatus; upon completion of supplying
coating material, delivering fluidic flushing media under pressure
and at a second and greater flow rate through the flow meter to
clean the same of coating material; and, during performance of said
flushing media delivering step, shunting a portion of the flushing
media around the at least one sensing element so that the at least
one sensing element is exposed to only some of the flushing media
which is sufficient to clean the at least one sensing element of
coating material but is less than sufficient to rotate the at least
one sensing element faster than its maximum rate of rotation.
8. A method as in claim 7 wherein said shunting step comprises
establishing a bypass path around the at least one sensing element.
9. A method as in claim 7 wherein the at least one sensing element
is rotated in response to a differential in the pressure of fluid
thereacross, and said shunting step maintains a differential in
the pressure of flushing media across the at least one sensing element.
10. A positive displacement flow meter for measuring the volume
flow of liquid coating material delivered to spray coating apparatus
and for being flushed clean of coating material by fluidic flushing
media, said flow meter comprising a housing having an inlet and
an outlet; means for supplying either liquid coating material under
pressure or fluidic flushing media under pressure to said inlet;
movable fluid flow sensing means in said housing intermediate said
inlet and outlet for flow therethrough of coating material and flushing
media from said inlet to said outlet, said sensing means being moved
in response to and by an amount in accordance with the volume flow
of coating material or flushing media therethrough and generating
an indication of the volumetric flow of coating material flowing
therethrough; bypass valve means for selectively shunting a portion
of the fluid from said inlet around said flow sensing means to said
outlet; and means for operating said bypass valve means so that
no fluid is shunted around said flow sensing means when coating
material is supplied to said inlet and so that a portion of the
fluid is shunted around said flow sensing means when flushing media
is supplied to said inlet.
11. A method of supplying a measured volume of liquid coating material
to spray coating apparatus, comprising the steps of supplying liquid
coating material to the spray coating apparatus through a path including
a movable fluid flow sensor that is moved in response to and by
an amount in accordance with the volume flow of coating material
therethrough; measuring the volume flow of coating material through
the fluid flow sensor in accordance with the amount of movement
thereof; upon the measured volume flow of coating material reaching
a selected value, interrupting said supplying step and delivering
fluid flushing media through the path and the fluid flow sensor
to clean the same; and, during performance of said delivering step,
shunting a portion of the flushing media in the path around the
fluid flow sensor.
Description BACKGROUND OF THE INVENTION
The present invention relates to flow meters, and in particular
to a positive displacement flow meter for a spray coating system,
which may rapidly be flushed clean of coating material.
Color change systems for spray coating apparatus have particular
application in industrial operations where articles are to be spray
coated at a spray station or as they move along a production line.
Color change systems provide for a wide variety of colors to be
sprayed from a single spray gun. With many conventional systems,
a plurality of supply containers of coating material, each of a
different color and having a separate transfer pump or a source
of pressurization for the container, are connected with a manifold
of a color changer through valve controlled ports. An outlet from
the manifold is coupled to an inlet to a spray gun, and to spray
material of a particular color the manifold port valve associated
therewith is opened for flow of the material through the manifold
to the gun. After completion of spraying material of a particular
color, the manifold, gun and connecting line are cleaned with a
flushing media, which usually comprises alternate bursts of relatively
high velocity solvent and compressed air, to prepare the system
for spraying material of a different color.
In many spray coating applications, it is necessary that the quantities
of coating materials supplied to the spraying equipment be accurately
metered or measured to supply a specified amount of material to
be applied onto an article. For the purpose, positive displacement
flow meters are often used in line with the outlet from the color
changer. Such flow meters may have a pair of sensing elements in
the form of meshed gears that are rotated by and at a rate in accordance
with the volume flow rate of coating material through the flow meter.
The rate of rotation of one of the gears or the rotations made by
it is detected and used to provide an indication of the flow rate
and/or total volume flow of coating material to the spray apparatus.
Conventional positive displacement flow meters used in paint spraying
operations are prone to failure when cleaned with alternate bursts
of relatively high velocity air and solvent. The flow meters are
designed for specific flow ranges, and have limits on the internal
operating speeds of their gears or sensing elements. When such a
flow meter is subjected to a flow of relatively high velocity air
during flushing, the air causes overdriving or too high a speed
of rotation of the sensing elements, which can result in damage
to and failure of the flow meter. A second failure mode occurs as
a consequence of solvent being injected into the flow meter following
the air. When the liquid solvent hits the overdriven sensing elements,
the impact can cause severe stress and failure of the flow meter.
OBJECTS OF THE INVENTION
The primary object of the present invention is to provide a positive
displacement flow meter having at least one sensing element that
is rotated by and at a rate and by an amount in accordance with
the volume flow rate and the total volume flow of fluid through
the flow meter, which flow meter has a bypass valve that prevents
the sensing element from being overdriven in response to the flow
meter being flushed with alternate bursts of relatively high velocity
air and solvent.
SUMMARY OF THE INVENTION
In accordance with the present invention, a positive displacement
flow meter has at least one sensing element that is rotated by and
in accordance with a flow of fluid through the flow meter to generate
an indication of the volume flow of fluid. To permit the flow meter
to be quickly flushed clean by means of alternate applications of
relatively high velocity air and liquid solvent, without overdriving
the at least one sensing element, the flow meter includes a bypass
valve. The bypass valve is operable to establish a shunt path around
the at least one sensing element and between an inlet to and an
outlet from the flow meter, and when the flow meter is measuring
the volume flow of fluid, the bypass valve is closed. During a flushing
operation to clean the flow meter, the bypass valve is opened to
shunt some of the relatively high velocity air and solvent flow
around the at least one sensing element, while still allowing a
controlled and limited amount of air and solvent to pass by the
at least one sensing element to clean it without causing it to be
overdriven.
The foregoing and other objects, advantages and features of the
invention will become apparent upon a consideration of the following
detailed description, when taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional side elevation view of a positive displacement
flow meter, having a bypass valve arranged according to one embodiment
of the invention;
FIG. 2 is a view taken substantially along the lines 2--2 of FIG.
1 and shows sensing elements of the flow meter, and
FIG. 3 is a cross-sectional side elevation view of the flow meter,
showing the bypass valve arranged in accordance with another embodiment
of the invention.
DETAILED DESCRIPTION
Referring to FIGS. 1 and 2 there is indicated generally at 20
a positive displacement flow transmitter or flow meter of a type
with which the teachings of the invention may advantageously be
used. The flow meter includes a housing 22 having an inlet 24 and
an outlet 26. The housing also has a pair of semicircular openings
28 and 30 in which are received respective sensing elements or gears
32 and 34. The gears are mounted for rotation on respective shafts
36 and 38 in meshing engagement and are sized so that their teeth
closely seal with but slide across the walls of the semicircular
openings in which they are contained. Housing inlet passages 42
and 44 extend between the inlet 24 and an inlet chamber 46 to one
side of the point of meshing of the gears, and an outlet chamber
48 on the other side of the point of meshing connects through housing
outlet passages 50 and 52 to the outlet 26. A plate 54 is mounted
on and sealed with the housing 22 to enclose the gears and semicircular
openings. The housing and plate may conveniently be connected by
fasteners (not shown) extended through aligned passages in the same,
such as through passages 56 (FIG. 2) in the housing.
The flow meter 20 is adapted to measure the volume flow rate and
total volume flow of coating material supplied to spray coating
apparatus (not shown), so that a selected amount of material may
be applied onto an article. In operation of the flow meter, coating
material is supplied under pressure and at a relatively low velocity
flow rate to the housing inlet 24 and flows through the passages
42 and 44 to the inlet chamber 46. The coating material in the inlet
chamber flows into and fills each gear tooth space 58 of the gears
32 and 34 and the pressure of the material on teeth 60 of the gears
causes, with reference to FIG. 2 the gear 32 to rotate in a clockwise
direction and the gear 34 in a counter-clockwise direction. As the
gears rotate under the influence of the pressure of coating material
introduced into the inlet chamber, each gear tooth space is filled
with coating material as it passes through the inlet chamber. Then,
as the gears continue to rotate, the material is confined in the
tooth spaces by the surrounding walls of the openings 28 and 30
until it reaches the outlet chamber 48 where the gear teeth mesh
and force the material out of the spaces and through the passage
50 and 52 to the housing outlet 26. The volumetric capacity of the
tooth spaces 58 is known, so the revolutions through which the gears
are turned represent a measurement of the volumetric flow of coating
material through the flow meter.
To detect the revolutions through which the gears are turned, the
flow meter 20 has a gear revolution detector 62. The detector may
be a transducer which, with the housing 22 being of nonmagnetic
material and the gears 32 and 34 of magnetic material, includes
a magnetic pickup (not shown) extended into the housing and positioned
to sense movement therepast of outer ends of the teeth 60 on one
of the gears. The detector generates a signal each time the end
of a gear tooth moves past the pickup, whereby the total number
of signals generated represents the total volume flow of coating
material through the flow meter and the rate at which the signals
are generated represents the flow rate of material.
The positive displacement flow meter 20 is of generally conventional
structure and well suited for measuring the total volume flow and
flow rate of coating material to spray coating apparatus. A difficulty
arises, however, when it is used with color change equipment. Since
the coating material passes through the flow meter, to change from
spraying material of one color to spraying material of another,
the flow meter must first be thoroughly cleaned to prevent contamination
of coating material colors. The flow meter is connected between
an outlet from a color changer (not shown) and an inlet to spraying
apparatus (also not shown), and the flushing media normally comprises
air and solvent that are alternately injected through the color
changer and flow meter at a velocity that is relatively high with
respect to the velocity of flow of coating material through the
same. However, the flow meter is prone to failure when cleaned in
such a manner. The flow meter is designed for specific flow ranges,
and has limits on its internal operating speeds, i.e., the speeds
of rotation of the gears or sensing elements 32 and 34. When the
flow meter is subjected to relatively high velocity air during flushing,
the air causes overdriving or too high a speed of rotation of the
gears, which can result in damage to and failure of the flow meter.
A second failure mode then occurs when solvent is injected into
the flow meter following the air. When the liquid solvent hits the
rapidly rotating gears, the impact causes severe stress and can
result in damage to and failure of the flow meter.
To overcome the problems associated with flushing positive displacement
flow meters used in spray coating apparatus, and to permit such
flow meters to be quickly cleaned by a relatively high velocity
flow of air and solvent without sustaining damage, according to
the invention the flow meter 20 includes a bypass valve, indicated
generally at 64. During a spray coating operation, when the flow
meter is measuring the total volume flow and flow rate of coating
material, the bypass valve is closed. During flushing, however,
the bypass valve is opened to shunt some of the relatively high
velocity flow of air and solvent around the gears or sensing elements
32 and 34 while still allowing a controlled and limited amount
of air and solvent to pass by the gears to clean them. The controlled
amount is sufficient to ensure proper cleaning of the gears without
causing them to be too rapidly rotated and overdriven.
More particularly, the bypass valve 64 is operable to selectively
establish a path between a channel 66 in communication with the
housing inlet passage 42 and a channel 68 in communication with
the housing outlet passage 52 such that when the valve is opened
a bypass or shunt path is established for a flow of some of the
material introduced at the flow meter inlet 24 around the gears
32 and 34 and directly to the flow meter outlet 26. The bypass valve
includes a cylinder end closure 70 having a chamber 72 in communication
with the housing channels 66 and 68. A cylinder 74 is carried by
the end closure, and a piston 76 having a piston rod 78 is reciprocable
in the cylinder. The piston rod is slidable in and sealed with a
passage through a guide 80 and channels 82 in the guide accommodate
drain off of any material leaking into a space between a pair of
seals that seal the piston rod and guide. A tapered valve 84 is
at an end of the piston rod within the end closure opening 72 and
is movable against and away from a valve seat 86 formed in the housing
channel 68 to interrupt and establish a bypass or shunt path between
the housing inlet and outlet passages 42 and 52 and around the gears
32 and 34. To establish the bypass path, air under pressure is introduced
at an inlet 88 to the cylinder to move the piston and tapered valve
away from the seat against the urging of a spring 90 with an opening
92 in the cylinder accommodating escape of air from an opposite
side of the piston. To interrupt or close the bypass path, the pressure
of air is removed from the inlet 88 for movement of the tapered
valve against its seat by the spring. In place of or in addition
to the spring for closing the bypass path, air under pressure may
be applied to the piston through the opening 92.
In a contemplated use of the flow meter 20 its inlet 24 is connected
to an outlet from a color changer (not shown) and its outlet 26
is connected to an inlet to spray coating apparatus (also not shown).
During a spray coating operation, the bypass valve 64 is closed,
so that all of the relatively low velocity liquid coating material
flow supplied by the color changer to the spraying apparatus flows
around and past the gears or sensing elements 32 and 34 causing
them to rotate through a number of revolutions and at a rate that
are directly in accordance with the total volume flow and flow rate
of coating material to the spraying apparatus. The number of revolutions
through which and the rate at which the gears are turned is sensed
by the detector 62. When a selected volume flow of coating material
is detected, which is sufficient to coat a single article or a plurality
or articles, the supply of coating material to the spraying apparatus
is interrupted.
If the color of coating to be next supplied to the spraying apparatus
is different from the previously supplied color, the color changer,
spraying apparatus, flow meter 20 and connecting lines must first
be flushed clean of the previously supplied color of coating material
to prevent contamination of the next color of coating material to
be supplied. This is accomplished, as is conventional in the spray
coating art, by applying alternate bursts of relatively high velocity
flows of air and liquid solvent through the color changer and flow
meter to the spraying apparatus. Absent the bypass valve 64 the
flow meter could be damaged during flushing. The flow meter is designed
for specific flow ranges, and has limits on the operating speed
or rate of rotation of its gears or sensing elements 32 and 34.
If the gears are subjected to the full volume flow of relatively
high velocity air and solvent during flushing, the air can cause
overdriving or too high a rate or rotation of the gears, resulting
in damage to and failure of the flow meter. A second failure mode
can then occur when the solvent is injected into the flow meter
following the air. When the liquid solvent hits the overdriven or
rapidly rotating gears, the impact can cause severe stress and failure
of the flow meter.
Accordingly, to permit the flow meter 20 to be rapidly flushed
by relatively high velocity flows of air and solvent without the
potential for sustaining damage, during flushing the bypass valve
64 is opened to establish the shunt path through the channels 66
and 68 and divert some of the air and solvent around the gears 32
and 34 while still allowing a controlled and limited amount of
air and solvent to contact and move past the gears to clean them.
The shunt path presents some restriction or resistance to a flow
of air and solvent, so a positive pressure differential exists between
the inlet chamber 46 to and the outlet chamber 48 from the gears
and the gears are rotated and thoroughly cleaned by the air and
solvent. The restriction, however, is not so great as to cause overdriving
of the gears, but rather is such that the controlled amount of air
and solvent moving past the gears is sufficient to ensure proper
cleaning of the gears without causing them to be overdriven. At
the end of the flushing operation, the bypass valve is closed so
that the total volume flow and volume flow rate of coating material
next supplied to the coating apparatus is accurately measured.
FIG. 3 shows another arrangement of flow meter and bypass valve
contemplated by the invention. The flow meter 20' is substantially
the same as the one in FIG. 1 except that bypass channels 66' and
68' are formed in the plate 54 so in this case the gear revolution
detector 62 (not shown in FIG. 3) would be mounted on the side of
the housing 22 opposite from the plate. In this arrangement, the
bypass valve 64 is mounted on the plate 54 to interrupt and establish
communication between the inlet chamber 46 to and the outlet chamber
48 from the gears 32 and 34 opposite from the inlet passage 44 and
outlet passage 50. In operation, the flow meter and bypass valve
function in the same manner as described in respect of FIG. 1. When
the bypass valve is closed during a spray coating operation, all
of the relatively low velocity flow of coating material supplied
by the color changer flows past the gears, so that an accurate indication
is provided of the volume flow rate and total volume flow of coating
to the spraying apparatus. During flushing, the bypass valve is
opened to establish a shunt path around the gears, whereupon a controlled
amount of air and solvent is carried through the gear tooth spaces
58 to clean the gears without overdriving them. As compared with
the embodiment of FIG. 1 an advantage of this particular arrangement
is that, during flushing, the gear teeth 60 are exposed to the total
volume flow of flushing media at the inlet and outlet chambers 46
and 48.
While embodiments of the invention have been described in detail,
various modifications and other embodiments thereof may be devised
by one skilled in the art without departing from the spirit and
scope of the invention, as defined in the appended claims. |