Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF T~IE INVENTION
Mass-rate liquid flow meters of the recirculation type
are disclosed in United States Patents 3,232,1~4; 3,232,105;
and 3,662,599. In these patents a gear pump is used to recir-
culate a constant volume flow q. Use of a gear pump is satis-
factory for liquids having some lubricating quality, sufficient `~
to keep the gears of the pump from wearing. However, in many ''!:." .
applications the liquids being measured have either no lubricity
or are chemically corrosive or both. A typical liquid without
lubricity is water. Water has a corrosive effect on plain steel -~
gears. Other liquids that have a much more corrosive effect
are the many acids and bases that are used in the petrochemical ;
industry. If one were to use a steel gear pump for such liquids,
corrosion and wear of the gears would result. This would in-
crease the leakage across the gears and hence change the value
of q. One could use gears made out of stainless steel. However,
stainless gears present the problem of galling, i.e., the ten-
dency of the gear surfaces to stick or bind when they contact
during pump operation. Centrifugal pumps, which have no rubbing
surfaces exposed to the liquid flowing through the pump, have
previously heen considered unsuitable for use in mass-rate
flowmeters because the pumping capacity of centrifugal pumps
changes considerably with changes in the pressure differential
across the pump.
Thus, a centrifugal pump does not have a constant volu-
metric flow when its pressure rise is varied. Further, centri-
fugal pump characteristics change with the viscosity of the
li~uid. Therefore, centrifugal type pumps have not, prior to
~he present invention, been used for creating the recirculating
flow in mass-rate liquid flowmeters such as disclosed in the
prior art patents identified above.
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SUMM~RY OF THE INVENTION
According to the present invention, a new mass-rate
flowmeter is provided which obviates the above mentioned pro-
blems encountered with a gear pump and which enables the advan-
tages of a centrifugal type pump to be obtained. The centrifugal
pump, as pointed out above, can be made of materials that will
withstand corrosion, including corrosion by chemicals such as
acids, bases and other corrosive liquids. Because centrifugal
pumps having rotating impellers with no rubbing parts as in the
gear pump, there is no concern for wear.
The present invention also involves the provision of
a new centrifugal pump, herein referred to as a "ram" pump.
This new pump is characterized by the ability to operate, in the
lower ~P range (the lower range of pressure differentials be- `
tween the pump inlet and the pump outlet), with pumping charact-
eristics that are substantially the same as those of the pre-
viously used gear pumps.
There is another problem associated with the use of a
centrifugal pump in mass-rate flowmeters as disclosed in the
above identified prior art patents. With centrifugal pumps,
there are variations in output flow rate with changes in vis-
cosity of the liquid being pumped. According to the present
invention, this problem also can be solved. For example, a
fifth restrictor may be provided at the outlet of the ram pump,
having flow characteristics which compensate for the pump out-
put variations resulting from viscosity changes. An alternate
method is by designing the four restrictors in the branch
conduits to provide such compensation.
In accordance with a broad aspect, the invention re-
lates to: a centrifugal liquid pump having a substantiallyconstant vol~me liquid flow characteristic in the lower portion
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of the pump operating pressure range comprising a rotatable .
impeller, a casing surrounding and enclosing said impeller hav- ~ i
ing a single discharge port therein for receiving liquid pumped ~ : :
by said impeller, said impeller being in the form of a disc
having cut-away portions spaced circumferentially around its
periphery forming a plurality of circumferentially spaced ``~
cavities separated by lobes between said cavities, said lobes
having outer circumferentially curved surfaces forming a close
fit with the circumferentially curved inner wall surface of said
casing to form circumferentially extending seals between said
cavities, thereby preventing escape of liquid in each cavity ::
except through said discharge port during the time when the
respective cavity is in flow communication with said discharge
port, each said cavity being in flow communication with said .
discharge port in the pump casing during a portion only of -~
each revolution of the impeller, and at least one radially
extending flow passage in said impeller connecting the rear
end of each said cavity, with respect to the direction of
rotation of said impeller, to a liquid inlet located adjacent ,
the axis of rotation of said impeller, the arrangement of said
radially extending flow passages, said cavities, and said
discharge port being such that each said cavity is filled with
liquid forced outwardly through a radial flow passage by
centrifugal force exerted on the liquid as a result of rotation
of said impeller during that portion of each revolution of the ~.
impeller when said cavity is not in communication with said
discharge port, and that the liquid in said cavity is discharged ~ :
through said discharge port during that portion of each
revolution of said impeller when said cavity is in flow com-
munication with said discharge port.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a graph showing the relationship between
flow rate and pressure rise of the ram pump of the present inven-
tion as compared to the prior art centrifugal pumps and gear -
pumps. ~`
Figure 2 is a schematic flow diagram of the ram pump
mass-rate flowmeter system of the present invention, wherein the
constant volumetric recirculating flow q is less than the input
volumetric flow Q.
Figure 3 is a schematic diagram of the ram pump mass-
rate flowmeter system of the present invention wherein the con-
stant volumetric recirculating flow q is greater than the input
volumetric flow Q.
Figure 4 is a graph of the relationship between orifice
coefficient C for a sharp edge orifice and the Reynolds number.
Figure 5 is a graph of the relationship between the
pressure rise ~P and the flow rates q, for a centrifugal pump
(including the ram pump of the present invention), for liquids
having different viscosities.
Figure 6 is a graph of the relationship between orifice
coefficient C and the Reynolds number for a round edge orifice.
Figure 7 is a schematic diagram of the ram pump mass-
rate flowmeter system of the present invention, showing the
location of the ram pump and the fifth restrictor relative to
the branch conduits.
Figure 8 is a sectional view of the new ram pump of the
present invention with the ram impeller in the pump housing.
Figure 9 is an elevational view of the impeller shown
in Figure 8.
Figure 10 is an elevational view of the impeller along
the line 10 10 of Figure 9.
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Figure 11 is a cross-sectional view, along the line
11-11, of the middle p~rtion of the impeller shown in Figure -
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_TAILED DESCRIPTION OF A PREFERRED EMBODIMæNT OF THE INVENTION
Consider the flow equation of a Flo-Tron meter, such as
disclosed in United States Patents 3,232,104; 3,232, 105; and
3,662,599, and for the case shown in Figure 3:
~ Pl-4 ~ Equation tl)
where ~ is normally a constant and:
~Pl 4 = differential pressure output signal
q = volumetric recirculating flow
W = measured mass flow passing through the meter
c2 = orifice coefficient of meter
A2 = area of orifice
k = constant
From this equation is can be seen that if recirculating
flow q were to vary than ~Pl 4 would vary not only with mass
flow rate W, but also with recirculating flow q.
A centrifugal pump does not have a constant volumetric
flow when its pressure rise is varied. Figure 1 shows the vari-
ation of flow rate q versus pressure rise ~P for a gear pump,
a conventional centrifugal pump and the novel centrifugal pump
of the present invention, herein referred to as a ram pump.
The gear pump is a positive displacement pump and, therefore,
meets the requirements of delivering a constant volume flow,
regardless of pressure rise. On the other hand, a conventional
centrifugal pump has a changing (decreasing) flow with in-
creasing pressure rise across the pump.
It will be seen that the ram pump of the present inven-
` 30 tion, when operating in the region of low ~P as shown in Figure
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1 very closely approximates the constant volume flow character-
istic of the gear pump. This low ~P region is the selected -
region in which the ram pump operates in the mass-rate flowmeter
of the present invention.
The construction of the ram pump of the present inven-
tion is shown in Figures 8, 9, 10 and 11. It comprises a hous-
ing 1 enclosing the impeller 2. The impeller is a solid disc
having a multiplicity of flow passages 3, respectively connect-
ing the impeller inlet 4 to a multiplicity of cavities 6 spaced
around the periphery 5 of the impeller.
When the impeller rotates a pressure determined by the
centrifugal force on the liquid in passages 3 is generated in
transversely extending discharge cavities 6, formed by scalloped
portions in the periphery of the impeller. The impeller has a `-
close fit between its periphery 5 and the housing 6 to prevent
leakage and dissipation of the pressure of trapped liquid in the
cavities 6.
Rotation of the impeller causes liquid entering the in-
let 4 to flow radially outward through the passages 3 and into
the transversely or tangentially extending cavities 6. These
cavities are thus filled, as the impeller rotates, and when each
f' cavity, in turn, arrives at the rotational position wherein it
connects with the outlet 7, as shown in Figure 8, the liquid in
that cavity is positively displaced through the pump outlet
port 7 by the piston-like effect of the impeller face 8 forming
the rear wall of the cavity.
Thu~, although the pump is a centrifugal pump in that
centrifugal force causes the outward flow of liquid through
passages 3, and thereby pressurizes the liquid in the cavities
6, it additionally creates a "ram" pressure by the piston-like
effect created by the movement of cavity 6, and its rear wall - -~
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~, 7
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8, past the discharge port 7 of the pump housing. The resultant
pressure head in the outlet 7 is herein referred to as the ram
pressure.
Referring to Figure 1, the ram pressure created by the
novel pump of this invention will be seen to be substantially
the same in constant flow characteristics as that of the conven-
tional gear pump in the lower range of pressure rise across the
pump. Because the ram pump pressure rise curve is very steep
between points A and B of the region in which the ram pump would ;
be operating in the mass-rate flowmeter of the present invention,
it can successfully be utilized in the mass-rate flowmeter of ;
!' the present invention.
Centrifugal pump characteristics change with the visco- -~ -
sity of the liquid being pumped. This is also trl~e, to some
extent, of the ram pump. This is shown in Figure 5, where the ;
curves A, B, C are for liquids of different viscosities with C ~;
being the highest viscosity liquid. As shown on the curves,
for a constant pressure rise ~Pl different q's result with
different viscosities - that is, the recirculating flow q
decreases with increasing viscosity. This decrease in q, how-
ever, can be offset by having orifices in the meter bridge with
a decreasing coefficient.
Referring to Equation 1, if q and c2 both change in
the same proportion then their ratio q/C2 remains a constant.
~ Round edge orifices have such a characteristic as shown in
¦ Figure 6. One can see that the orifice flow coefficient de-
creases with decreasing Reynolds number. Reynolds number,
which relates liquid viscosity to liquid flow, is a dimension-
`~ less parameter whose equation is:
Reynolds No. = sVD
where s - liquid density
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V = liquid velocity
D = diameter of flow opening
u = viscosity
Thus, for increasing viscosity the Reynolds number decreases
and, as shown in Figure 6, this increasing viscosity brings
about a decrease in the flow coefficient C. Therefore, by
suitably matching the rounded orifice coefficients with the
pump characteristics, a mass flow meter can be provided capable
of operating over a very wide viscosity range with the output
signal linear and proportional to mass flow.
Another approach for compensating the decreasing pump
flow with increasing liquid viscosity is to place a fifth
orifice in the flow line connecting the discharge port of the
pump to a branch conduit at a point intermediate the restrictors
in the branch conduit, as shown in Figures 2, 3 and 7. The
fifth orifice is designed to have a flow coefficient that will
increase with increasing viscosity of the liquid.
An increasing flow coefficient means there is less
resistance to flow with increasing viscosity. ~herefore, by
proper matching of the flow coefficient of the fifth orifice
with the ram pump characteristic it is possible to maintain
a constant recirculating flow through the flowmeter regardless
of viscosity variation of the liquid. Figure 4 illustrates
a sharp edge orifice flow coefficient that can be used for
pump viscoslty compensation.
The flow equation for an orifice is
., q = CA
where q = volume flow
C = orifice flow coefficient
A = orifice area
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~P = pressure drop across the oriice
s = liquid density
From this equation one can readily see that raising or lowering
the value of C will raise or lower the value of q for a given
P.
When referring to "sharp edge" or "rounded edge"
orifices, the edges referred to are those on the side of the
orifice from where the liquid flows into the orifice. `~
The viscosity compensation techniques just described
are for the case of the pump having decreasing flow with in-
creasing viscosity. In the event the pump should have the
opposite effect, that is, increasing flow with increasing ;~
viscosity, similar compensation techniques can still be used
but with the use of sharp edge and round edge orifices reversed.
In other words, in the case of the fifth orifice the orifice
would be a rounded edge orifice and in the case where the
bridge orifices are used for compensation they would be sharp
edged orifices.
It is also possible to use a combination of both tech-
niques for viscosity compensation of the pump. That is, a
fifth orifice, as well as compensation type bridge orifices.
Further, in the case of the bridge orifices either a
pair of the orifices could be used having the identical correct
compensating flow coefficient, or all four orifices may have
identical flow coefficients for compensation.
Measurement of the signal indicative of mass flow,
in the appaxatus of the present invention, is described in the
above referred to prior art patents and is illustrated in Fig-
ures 2 and 3 hereof.
The flow capacity of the passages 3 can be so pro-
portioned (in cross-sectional area) relative to the volume of
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cavities 6 that a particular cavity will fill completely with :~
liquid pumped thereunto by the passage 3 during rotation of ~ .
the impeller from the position wherein the rear wall 8 of a .~.
particular cavity has just passed the pump discharge port to ~ ~
the position ~herein the cavity is initially opened to the ~ ~:
pump discharge port. ~
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