Abstrict A variable orifice liquid flow meter comprises a body, a bore in
the boby defining a flow tube for liquid, a sharp edged orifice
in the bore, a piston having a head capable of blocking the orifice
and a stem by which the piston is supported for axial movement relative
to the orifice, resilient member urging the piston head into a position
to block the orifice, support for the piston mounted in the bore,
a plurality of fins on the support and extending radially thereof
to fix the support to the wall of the flow tube, the support having
a generally cylindrical portion formed with a blind bore in which
the stem of the piston is slidably mounted and which shields at
least a portion of the piston from liquid flow through the bore,
the end of the support adjacent to the blind end of the bore being
streamlined, a passage in the support permitting communication by
liquid in the flow tube with the portion of the piston in the blind
bore, and resilient member in the support arranged to urge the piston
to block the orifice.
Claims I claim:
1. A variable orifice flow meter comprising a body, an inlet in
the body, an outlet in the body, a bore in the body extending between
the inlet and the outlet and defining a straight flow passage for
liquid, a sharp edged orifice in the bore, a piston having a head
capable of blocking the orifice, support means for the piston mounted
in the bore, the support means having a blind bore in which the
piston is slidably mounted and resilient means in the support means
and arranged to urge the piston to block the orifice.
2. A variable orifice flow meter according to claim 1 wherein
the upstream end of the support means is streamlined.
3. A variable orifice flow meter according to claim 1 comprising
a passage in the support means permitting communication by liquid
in the flow passage with the portion of the piston in the blind
bore.
4. A variable orifice flow meter according to claim 1 wherein
the support means comprises a generally cylindrical portion housing
the blind bore, and wherein the support means is fixed to the wall
of the flow passage by means of fins extending radially from the
generally cylindrical portion.
5. A variable orifice flow meter according to claim 1 wherein
the end of the piston remote from its head is formed with a follower
which is slidable in the blind bore.
6. A variable orifice flow meter according to claim 5 wherein
a gland mounted in the blind bore adjacent to its open end is arranged
slidingly to support the piston.
7. A variable orifice flow meter according to claim 1 comprising
means for preventing axial rotation of the piston in the blind bore.
8. A variable orifice flow meter according to claim 1 comprising
a magnet fixed in the piston and arranged to drive a piston displacement
indicator.
9. A variable orifice liquid flow meter comprising a body, an inlet
in the body, an outlet in the body, a bore in the body extending
between the inlet and the outlet and defining a straight flow passage
for liquid, a sharp edged orifice in the bore, a piston having a
head capable of blocking the orifice and a stem by which the piston
is supported for axial movement relative to the orifice, resilient
means urging the piston head into a position to block the orifice,
support means for the piston mounted in the bore, a plurality of
fins on the support means and extending radially thereof to fix
the support means against movement in the flow passage, the support
means having a generally cylindrical portion formed with a blind
bore in which the stem of the piston is slidably mounted and which
shields at least a portion of the piston from liquid flow through
the bore, the upstream end of the support means being streamlined,
a passage in the support means permitting communication by liquid
in the flow tube with the portion of the piston in the blind bore,
and resilient means in the support means arranged to urge the piston
in a direction to block the orifice.
Description The invention relates to a variable orifice flow meter.
Variable orifice flow meters are intended as simple and low-cost
devices for measuring liquid flow. They comprise an orifice which
is blocked by a piston held in position by a spring, or in the case
of some vertical designs, by gravity. Liquid flow through the meter
displaces the piston against the spring, opening up the orifice,
the extent of displacement of the piston being directly proportional
to the liquid flow rate. In some designs, a flow tube in which the
piston is disposed is tapered and in others the piston is tapered
or suitably contoured. With many low-pressure meters, the piston
is contained in a glass tube so that the displacement of the piston
can be seen through the glass. A suitable scale can be calibrated
in the required units and fixed adjacent to the glass tube so that
the piston displacement can be measured directly. With meters intended
for higher pressures, glass is not suitable and the use of metal
or other opaque substances requires a different method of determining
the piston displacement. A method commonly used is to fix a magnet
to the piston with a follower external to the flow tube suitably
responsive to movement of the magnet and calibrated to show the
flow. This follower can be of a magnetic material such as steel,
or alternatively may itself be a linear or rotary magnet to which
an indicating pointer is fixed.
It is obviously necessary that the piston is concentric with the
orifice and is free to slide so that it can be easily moved by the
liquid flow against the resistance provided by the spring. The piston
is normally held centrally of the flow tube either by a central
guide bar or by means of fins on the piston which are a good fit
against the sides of the flow tube but with a small diametral clearance
to allow it to slide freely. Displacement of the piston is primarily
effected by the pressure drop across the orifice and to a lesser
extent, below the critical Reynolds Number, by viscous drag on the
surface area of the piston. Although a sharp edge orifice is inherently
free of viscosity effects, the viscous drag (which varies with the
viscosity of the liquid) will effect the piston displacement. In
addition, at higher flow rates above the upper critical Reynolds
number, where flow is turbulent, the pressure drop along the length
of the piston acts on the cross-sectional area of the piston and
provides an additional force tending to displace the piston. Under
turbulent conditions this pressure drop increases with the roughness
of the surface finish of the piston and the tube that is in contact
with the flow.
A substantial change in viscosity takes place with a relatively
small change of temperature in most mineral oils, and can give rise
to large reading errors. The change in viscosity affects the drag
and also changes the flow rate at which the liquid changes from
laminar to turbulent flow. For this reason, known flow meters of
the kind described above are usually calibrated for a liquid of
a given viscosity and at a specified density and temperature.
An object of the invention is to improve the accuracy of a variable
orifice flow meter used for measuring the flow of liquids by making
it less sensitive to changes in the viscosity of the liquid.
According to the invention there is provided a variable orifice
liquid flow meter comprising a body, a bore in the body defining
a flow tube for liquid, a sharp edged orifice in the bore, a piston
having a head capable of blocking the orifice and a stem by which
the piston is supported for axial movement relative to the orifice,
resilient means urging the piston head into a position to block
the orifice, and support means in which the stem of the piston is
axially slidably housed and shielded from liquid flow through the
bore.
From another aspect the invention is a variable orifice flow meter
comprising a body, a bore in the body defining a flow tube for liquid,
a sharp edged orifice in the bore, a piston having a head capable
of blocking the orifice, support means for the piston mounted in
the bore, the support means having a blind bore in which the piston
is slidably mounted, and resilient means in the support means and
arranged to urge the piston to block the orifice. Preferably the
end of the support means adjacent to the blind end of the bore is
streamlined.
Advantageously a passage is provided in the support means to permit
communication by liquid in the flow tube with the portion of the
piston in the blind bore. The support means may be fixed to the
wall of the flow tube by means of fins extending radially from a
generally cylindrical portion housing the blind bore.
Preferably the end of the piston remote from its head is formed
with a follower which is slidable in the blind bore, and preferably
a gland mounted in the blind bore adjacent to its open end is arranged
slidingly to support the piston. Means is preferably provided for
preventing axial rotation of the piston in the blind bore. A magnet
may be fixed in the piston and arranged to drive a piston displacement
indicator.
The invention is diagrammatically illustrated, by way of example,
in the accompanying drawings, in which:
FIG. 1 is a side view of a variable orifice liquid flow meter;
FIG. 2 is a cross-sectional end elevation on the line A--A of FIG.
1;
FIG. 3 is a cross-sectional side elevation on the line B--B of
FIG. 2; and
FIG. 4 is a scrap view of a detail of the device shown in FIG.
3.
In the drawings there is shown a variable orifice flow meter 37
comprising a body 3 formed as a generally rectangular solid metal
block having a bore 11 extending from an inlet 24 in one end face
of the body 3 to an outlet 26 in the opposite end face of the body.
The bore 11 defines a flow tube for liquid to be metered through
the body 3.
A sharp-edged orifice ring 9 is stationarily mounted in the bore
11 of the meter 3 and is thus positioned in the flow path of the
liquid. For zero flow conditions, the orifice ring is completely
blocked by the head 27 of a piston 7 disposed in the bore. The piston
7 is formed with a stem 29 which is axially slidably mounted centrally
in the bore 11 in the blind bore 36 of a support cylinder 2. The
support cylinder 2 comprises a central cylindrical hub 28 surrounding
the stem 29 of the piston and three radial blades or fins 30 which
engage the wall of the bore 11 to position the support cylinder
in the bore. The end 39 of the hub 28 adjacent to the blind end
of the bore 36 is streamlined. The support cylinder is held against
axial movement in the bore against a shoulder 31 formed in the bore
by means of a ring 4 threaded into the wall of the bore.
The stem 29 of the piston 7 which is formed with an axial bore
32 containing a longitudinal magnet 8 is held central in the support
cylinder 2 by a gland 6 fixed, e.g. by a screw-thread, in the support
cylinder and snugly surrounding the piston stem and by a piston
follower 10 secured to the free end 40 of the piston stem and slidable
in the cylinder 2. As shown in FIG. 4 the follower 10 is secured
to the piston stem 29 by means of a circlip 1. It will be understood
however that the follower could be fixed in other ways.
The piston is constantly urged into the support cylinder 2 by means
of a spring 5 surrounding the piston stem 29 and engaging the follower
10 and the gland 6 respectively. The free length of the spring 5
is so arranged that when contained in the support cylinder 2 it
exerts a pre-load on the piston 7 in excess of the piston's weight,
thus enabling the meter to be used in both the vertical and horizontal
positions. The piston is prevented from rotating in the cylinder
2 by a pin 25 mounted in the gland 6 and which engages in a longitudinal
slot 33 cut in the piston stem 29. This slot 33 also permits liquid
communication between the flow tube and the blind bore and ensures
that the liquid pressure acting on the piston stem is the same as
the pressure on the piston immediately down stream of the support
cylinder.
Liquid flow through the device pushes the piston head, which is
arranged to have a smooth surface finish, through the orifice ring
and opens an annular gap through which liquid can pass. If the flow
rate increases, the piston is forced further through the orifice
ring thus allowing a larger area through which the liquid can flow.
By arranging a suitable contour for the piston head, the displacement
of the piston can be made approximately linear with the increased
flow rate.
A cylindrical magnet 23 which is magnetised across its diameter
is fixedly mounted on a shaft 22 which is supported at one end by
a plain bearing in the body of the meter, the other end of the shaft
22 being supported by a deep-grooved ball bearing 38 mounted in
a carrier 21 mounted in the body and which allows free rotary motion
of the shaft 22 while providing axial support. The cylindrical magnet
23 is so positioned that movement of the longitudinal magnet 8 induces
it to rotate. The two magnets can be considered as a rack and pinion
device connected together by their magnetic fields rather than gear
teeth. For this reason, the longitudinal magnet 8 cannot be allowed
to rotate about its own axis since with many commerical magnets
the magnetic field is not evenly distributed and rotation could
give a similar effect to displacement. Connected to the end of the
shaft 22 by means of a screw 12 is the pointer 13 of a dial gauge
generally indicated at 20. The dial gauge 20 comprises a scale 14.
The pointer is shown at the mid-scale position for the sake of clarity
in FIG. 1 but with the orifice ring blocked by the piston, i.e.
zero flow conditions, the pointer would be resting against a stop
18 at the extreme left of the scale. The scale is calibrated in
suitable flow units and any non-linearity of the meter can be compensated
for by a corresponding non-linear scale. A window 17 which in this
case is made from clear polycarbonate, is used to protect the scale
and pointer. The window is held in position by a snap ring 16. O-rings
15 and 19 are fitted to the scale and pointer assembly of the dial
gauge, to absorb shock, to allow for some variation in tolerances
of the parts and to prevent the ingress of dirt and other contamination.
The bore 11 in body 3 has threaded ports 34 35 at its inlet 24
and outlet 26 respectively into which suitable adaptors may be screwed
for connecting the device to a pipeline. Apart from the magnets,
the flow meter is preferably mainly constructed from non-magnetic
materials.
Thus the variable orifice flow meter shown in the drawings is substantially
free of viscosity effects on its accuracy, this being achieved by
reducing and shielding a large part of the surface area of the piston
from the liquid flow, minimising the length of the piston in contact
with the liquid flow, balancing out the pressure differential along
most of the piston length and ensuring a smooth finish where it
is in contact with the liquid flow.
This invention is not restricted to the particular design shown
in the drawings. Thus the support cylinder could be placed down
stream of the orifice ring; alternatively, two support cylinders
could be used, one upstream and the other downstream of the orifice
ring. The meter could also be modified to measure bi-directional
flow by having a double-headed piston and by positioning the piston
follower 10 half way along the piston steam with a spring on either
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