GB1604331A

GB1604331A - Centrifugal pump flowmeter

Info

Publication number: GB1604331A
Application number: GB2137178A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-05-23
Filing date: 1978-05-23
Publication date: 1981-12-09

Description

(54) CENTRIFUGAL PUMP FLOWMETER (71) I, WALTER MASNIK, a citizen of the United States of America, of 9, Penbroke Drive, Mendham, New Jersey 07945, United States of America, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by w which it is to be performed, to be particularly described in and by the following statement:- Mass-rate liquid flow meters of the recirculation type are disclosed in United States Patents 3,232,104; 3,232,105; and 3,662,599. In these patents a gear pump is used to recirculate a constant volume tlow q. Use of a gear pump is statisfactory 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 increase 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 tendency 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 been 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.

Centrifugal pump characteristics change with the viscosity of the liquid. Therefore, centrifugal type pumps have not, prior to the present invention, been used for creating the recirculating flow in mass-rate liquid flowmeters such as disclosed in the prior art patents identified above.

According to the present invention a mass flow meter adapted to measure the mass flow rate of an effectively incompressible liquid passing therethrough comprises inlet and outlet conduits having a flow which is to be measured, first and second branch conduits connecting said inlet and outlet conduits, first and second flow restrictors in said first branch circuit third and fourth flow restrictors,. in said second branch conduit, pumping means connecting said first and second branch conduits at points between the flow restrictors therein; said pumping means comprising a rotatable impeller for centrifugally pumping said liquid at a substantially constant volumetric flow rate which is either greater or less than the flow rate in said inlet and outlet conduits, two of said flow restrictors having the same flow characteristics, the other two of said four flow restrictors having the same flow characteristics, the impeller of said pumping means producing an undesirable variation in volumetric flow with changes in liquid viscosity, and means for compensating for said undesirable variation comprising either (a) at least two of said restrictors dimensioned to have the same flow coefficient but whose flow coefficient changes with viscosity in the direction to compensate for said variation in volumetric flow rate produced by the impeller of said pumping means due to changes in said viscosity, or (b) a fifth flow restrictor, for compensating for viscosity changes located in the flow line connecting the pump to a branch conduit at a point intermediate the restrictors in the branch conduit.

Preferably the pumping means is arranged to pump liquid at a volumetric flow rate which is greater that the flow rate in said inlet and outlet conduits, and said first and second restrictors have the same characteristics and said third and fourth restrictors have the same characteristics.

Alternatively the pumping means is arranged to pump liquid at a volumetric flow rate which is less than the flow rate in said inlet and outlet conduits, and said first and fourth restrictors have the same characteristics and said second and third restrictors have the same characteristics.

An embodiment of the invention is now described by reference to the accompanying drawings, in which Figure 1 is a graph showing the relationship between flow rate and pressure rise of a centrifugal ram pump which may be used in the flowmeter of the present invention as compared to the prior art centrifugal pumps and gear pumps.

Figure 2 is a schematic flow diagram of the centrifugal 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 centrifugal ram pump mass-rate flowmeter system of the present invention wherein the constant 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.

Figures is a graph of the relationship between the pressure rise AP and the flow rates q, for a centrifugal pump (including the ram pump illustrated in Fig. 8), 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 centrifugal 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 centrifugal ram pump used in the flowmeter of the present invention with the ram impeller in the pump housing.

Figure 9 is an elevational view of theimpeller shown in Figure 8.

Figure 10 is an elevational view of the impeller along the line 10--10 of Figure 9.

Figure 11 is a cross-sectional view, along the line 11-11, of the middle portion of the impeller shown in Figure 9.

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: qkW APst Equation (I) C2 A2 Equation (1) where qk CZ A2 is normally a constant and: AP1=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 it can be seen that if recirculating flow q were to vary then API 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 variation of flow rate q versus pressure rise AP for a gear pump, a conventional centrifugal pump and the centrifugal ram pump of the present invention. 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 increasing pressure rise across the pump.

It will be seen that the centrifugal ram pump used in the present invention, when operating in the region of low AP as shown in Figure 1, very closely approximates the constant volume flow characteristics of the gear pump. This low AP region is the selected region in which the ram pump operates in the mass-rate flowmeter of the present invention.

The construction of the centrifugal ram pump for use in the flowmeter of the present invention is shown in Figures 8, 9, 10 and 11. It comprises a housing 1 enclosing the impeller 2. The impeller is a solid disc having a multiplicity of flow passages 3, respectively connecting 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 inlet 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 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.

Thus, 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 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 pump will be seen to be substantially the same in constant flow characteristics as that of the conventional gear pump in the lower range of pressure rise across the pump. Because the ram pump pressure rise curve is very steep between point 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 viscosity of the liquid being pumped. This is also true, to some extent, of the centrifugal 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 AP1 different q's result with the different viscosities--that is, the recirculating flow q decreases with increasing viscosity. This decrease in q, however, 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 decreases with decreasing Reynolds number. Reynolds number, which relates liquid viscosity to liquid flow, is a dimensionless parameter whose equation is: sVD Reynolds No.= u where s=liquid density 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 flowmeter 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 tlow 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. Therefore, 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 viscosity compensation.

The flow equation for an orifice is

where q=volume flow C=orifice flow coefficient A=orifice area aP=pressure drop across the orifice 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 AP.

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 increasing 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 techniques 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 apparatus of the present invention, is described in the above referred to prior art patents and is illustrated in Figures 2 and 3 hereof.

The flow capacity of the passages 3 can be so proportioned (in cross-sectional area) relative to the volume of 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 the particular cavity has just passed the pump discharge port to the position wherein the cavity is initially opened to the pump discharge port.

A related mass flowmeter is described and claimed in my Application No. 32077/80 Serial No. 1,604,332 which is divided out of this Application.

WHAT I CLAIM IS: 1. A mass flowmeter adapted to measure the flow rate of an effectively incompressible liquid passing therethrough comprising inlet and outlet conduits having a flow which is to be measured, first and second branch conduits connecting said inlet and outlet conduits, first and second flow restrictors in said first branch conduit, third and fourth flow restrictors in said second branch conduit, pumping means connecting said first and second branch conduits at points between the flow restrictors therein; said pumping means comprising a rotatable impeller for centrifugally pumping said liquid at a substantially constant volumetric flow rate which is either greater or less than the flow rate in said inlet and outlet conduits, two of said flow restrictors having the same flow characteristics, the other two of said four flow restrictors having the same flow characteristics, the impeller of said pumping means producing an undesirable variation in volumetric flow with changes in liquid viscosity, and means for compensating for said undesirable variation comprising either (a) at least two of said restrictors dimensioned to have the same flow coefficient but whose flow coefficient changes with viscosity in the direction to compensate for said variation in volumetric ow rate produced by the impeller of said pumping means due to changes in said viscosity, or (b) a fifth flow restrictor, for compensating for viscosity changes, located in the flow line connecting the pump to a branch conduit at a point intermediate the restrictors in the branch conduit.

2. A mass flowmeter according to claim 1 and in which the pumping means is arranged to pump liquid at a volumetric flow rate which is greater than the flow rate in said inlet and outlet conduits, and said first and second restrictors have the same characteristics and said third and fourth restrictors have the same characteristics.

3. A mass flowmeter according to claim 1 and in which the pumping means is arranged to pump liquid at a volumetric flow rate which is less than the flow rate in said inlet and outlet conduits, and said first and fourth restrictors have the same characteristics and said second and third restrictors have the same characteristics.

4. A mass rate liquid flowmeter substantially as hereinbefore described and as illustrated in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Measurement of the signal indicative of mass flow, in the apparatus of the present invention, is described in the above referred to prior art patents and is illustrated in Figures 2 and 3 hereof. The flow capacity of the passages 3 can be so proportioned (in cross-sectional area) relative to the volume of 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 the particular cavity has just passed the pump discharge port to the position wherein the cavity is initially opened to the pump discharge port. A related mass flowmeter is described and claimed in my Application No. 32077/80 Serial No. 1,604,332 which is divided out of this Application. WHAT I CLAIM IS:

1. A mass flowmeter adapted to measure the flow rate of an effectively incompressible liquid passing therethrough comprising inlet and outlet conduits having a flow which is to be measured, first and second branch conduits connecting said inlet and outlet conduits, first and second flow restrictors in said first branch conduit, third and fourth flow restrictors in said second branch conduit, pumping means connecting said first and second branch conduits at points between the flow restrictors therein; said pumping means comprising a rotatable impeller for centrifugally pumping said liquid at a substantially constant volumetric flow rate which is either greater or less than the flow rate in said inlet and outlet conduits, two of said flow restrictors having the same flow characteristics, the other two of said four flow restrictors having the same flow characteristics, the impeller of said pumping means producing an undesirable variation in volumetric flow with changes in liquid viscosity, and means for compensating for said undesirable variation comprising either (a) at least two of said restrictors dimensioned to have the same flow coefficient but whose flow coefficient changes with viscosity in the direction to compensate for said variation in volumetric ow rate produced by the impeller of said pumping means due to changes in said viscosity, or (b) a fifth flow restrictor, for compensating for viscosity changes, located in the flow line connecting the pump to a branch conduit at a point intermediate the restrictors in the branch conduit.