WO1995033981A1

WO1995033981A1 - Mass flow sensor

Info

Publication number: WO1995033981A1
Application number: PCT/GB1995/001366
Authority: WO
Inventors: James Wolryche Stansfeld; Eric Metcalf
Original assignee: Solartron Group Limited
Priority date: 1994-06-09
Filing date: 1995-06-09
Publication date: 1995-12-14
Also published as: GB9411571D0

Abstract

A thin-walled vibratable cylinder (11) is mounted coaxially inside a section of pipe (12) through which a fluid flows. The cylinder (11) is attached at its downstream end (13) to the inner periphery of the pipe (12) by a mounting arrangement (14) which allows fluid to flow over both the inner and outer surfaces of the cylinder. Harmonic vibrations are induced in the cylinder (11) by means of electromagnetic transducers (not shown). A differential pressure sensor is located to sense the difference in fluid pressure (ΔP1) between the point P1 on the inner periphery of the pipe (12), directly opposite the position of the central node, and the point P2 upstream of the cylinder (11). The pressure difference (ΔP1) fluctuates at the vibrational frequency of the cylinder with an amplitude which is proportional to the mass flow rate.

Description

MASS FLOW SENSOR

This invention relates to mass flow sensors which .operate in accordance with the Coriolis effect.

One known form of prior art Coriolis-type mass flow sensor is disclosed in United States Patent

No.4 949 583. According to the disclosure, an elastically deformable measuring tube is subjected to periodical deflections as the fluid flows therethrough. In a preferred arrangement, the measuring tube undergoes radial oscillations which are functions of both circumferential and axial postions. The fluid flowing through the vibrating measuring tube experiences Coriolis forces which produce a phase-displacement in the periodic deflection of the measuring tube wall . The magnitude of the phase displacement is measured to determine the mass flow using displacement sensors located upstream and downstream of the midpoint of the measuring tube.

Another known form of prior art Coriolis-type mass flow sensor is described in the present applicant's earlier U.K. patent application, GB 2 227 318A. In this case, a tuning fork structure is immersed in the fluid whose mass flow is to be sensed such that its tines extend perpendicularly to the direction of fluid flow. The whole tuning fork structure is elongated in the direction of fluid flow, so that fluid flows over and between the tines. The tines are excited to vibrate perpendicularly to the direction of fluid flow in the region of the midpoint of the structure, and resultant vibrations at the axial ends of the structure are sensed by two pick-up devices. A phase difference between the vibrations sensed by the two pick-up devices, which is caused by the Coriolis effect, is a function of the mass flow of the fluid.

According to a first aspect of the present invention, there is provided a mass flow sensor comprising: an elongate structure which, in use, is in contact with a fluid whose mass flow is to be sensed and is aligned substantially parallel to the direction of fluid flow; means for exciting resonant vibrations in the structure; and means for sensing fluctuations in the pressure of the fluid produced by the resultant Coriolis forces and generating a mass flow signal in dependence on the pressure fluctuations sensed.

The use of fluid pressure measurements may offer considerable advantages since it permits the vibrating structure to be designed independently of, and optimized in the absence of, the sensing elements. Fluid pressure measurements may be utilized in either of the aforementioned Coriolis-type mass flow meters, instead of the displacement sensors or pick up devices respectively. In the absence of fluid flow, the elongate structure vibrates to generate an oscillating pressure field. With the fluid flowing, Coriolis forces cause a change to this pressure field which is related to the fluid flow rate.

In one embodiment of the first aspect of the present invention, fluctuations in pressure sensed at one location provides the mass flow signal. More preferably, the difference in fluid pressure sensed at two locations provides the mass flow signal. In an alternative embodiment of the first aspect of the present invention the phase difference between the pressure of the fluid sensed at two locations spaced along the direction of fluid flow provides the mass flow signal.

Preferably the elongate structure is a tube immersed in the fluid. Whilst the tube may have^' a polygonal cross-section, a circular cross-section is especially preferred, with the tube being a right-circular cylinder.

In a preferred arrangement, the means for exciting the resonant vibrations causes a mode in which the circumferential harmonic has a higher number of nodes than the axial harmonic to increase the stability of vibration. According to a second aspect of the present invention there is provided a mass flow sensor comprising: a tube which, in use, is immersed in a fluid whose mass flow is to be sensed and is aligned such that the fluid flows parallel to the axis of tube,- means for exciting resonant vibrations in the tube; and means for sensing vibrations in the tube and producing a mass flow signal in dependence on sensed changes in the vibrations caused by fluid flowing past the tube; wherein guide means restrain the flow of fluid over the inner and outer surfaces of the tube.

The applicants have appreciated that constraining the fluid flowing over both surfaces of the tube increases sensitivity to mass flow and increases sensitivity to fluid viscosity changes. In consequence, this arrangement is best suited to measurement of fluids of low viscosity such as gases .

Preferably the guide means comprises an inner shroud and an outer shroud arranged inside and outside the tube respectively, the shrouds being radially spaced from and coaxial with the tube. In an especially preferred form, the inner shroud, the outer shroud and the tube are of circular cross-section. In this way, an inner and an outer annular flow path are created on the inside and the outside of the tube respectively. Fluid flowing over the vibrating tube is initially deflected radially outwards or inwards depending upon the cycle of the vibration. The presence of the inner and outer shroud inhibits radial fluid movement, encouraging fluid flow axially and tangentially instead of radially. This increases the sensitivity of the mass flow sensor to fluid loading effects .

The fluid flow rate through a mass flow sensor in accordance with the second aspect of the invention may be determined using any of the arrangements in accordance with the first aspect of the invention. For all these arrangements, the mass flow rate may be determined according to the following equation

D

Mass flow ra te = K -~ ( —- - --r-

P cs ^~ where K = Instrument Constant

P_c = Coriolis Pressure

P_cs = Excitation Pressure f₀ = Resonant Frequency at vaccum f = Resonant Frequency

Preferred embodiments of the invention in its various aspects will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 shows schematically, for explanatory purposes, an arrangement for generating Coriolis forces in a flowing fluid;

Figure 2 shows an enlarged cross-sectional view of the arrangement shown in Figure 1 along the line II-II;

Figure 3 shows schematically the location of pressure sensors in the mass flow arrangement shown in Figure 1, in accordance with a first embodiment of the first aspect of the invention;

Figure 4A and Figure 4B show schematically the location of pressure sensors according to a second embodiment of the first aspect of the invention;

Figure 5 shows schematically the location of differential pressure sensors according to a third embodiment of the first aspect of the invention; and

Figure 6 shows schematically an arrangement for restricting fluid flow in accordance with the second aspect of the invention.

The mass flow sensor of Figures 1 and 2, indicated generally at 10, comprises a thin walled vibratable cylinder 11 mounted coaxially inside a section of pipe 12 which is to convey a flowing fluid. The cylinder 11 is attached at its downstream end 13 to the inner periphery of the pipe 12 by a mounting arrangement 14 which enables fluid to flow over both the inner and outer surfaces of the cylinder. The cylinder 11 is formed of a ferromagnetic material and vibrations are induced in the cylinder by means of electromagnetic transducers 15 which are located outside the pipe 12. The transducers 15 are positioned so as to excite particular harmonic vibrations in the cylinder 11.

For descriptive convenience, the vibrating cylinder shown in Figures 1 and 2 is oscillating in what is commonly known as "mode 2 circumferential" and "mode 2 axial" harmonics. The circumferential mode number . specifies the number of fullwaves present in the circumferential direction and the axial mode number specifies the number of halfwaves present along the axial length of the vibrating cylinder. Thus, in this case, two fullwaves of equal amplitude are disposed around the periphery of the cylinder 11, for which there are four nodes 20 which are evenly spaced around the circumference. The envelope of the tube displacement due to vibration is shown by the dotted lines in Figure 2. In between the nodes, there exist antinodes 21 where diametrically opposite portions of the cylinder vibrate in phase. That is to say, diametrically opposite antinodes deflect alternately towards and away from each other. In addition to the circumferential oscillations, two further halfwaves are disposed along the axis of the cylinder, separated by a central node 22. The envelope of the two half waves is shown by the dotted lines in Figure 1. The two ends of the cylinder also act as nodes. There are two antinodes, located either side of the central node 22 between the ends of the vibrating cylinder 11, which oscillate in antiphase. The oscillation harmonics are controlled by the positions of the electromagnetic transducers 15 relative to the cylinder 11 and by the pattern in which they are activated. Clearly, other oscillation harmonics besides mode 2 circumferential and mode 2 axial are within the scope of the present invention.

In the absence of fluid flow, the cylinder 11 vibrates in a totally symmetrical manner and generates an oscillating but symmetrical pressure field. A fluid then flowing through the pipe 12 in the direction shown by arrow 23 over the vibrating cylinder experiences Coriolis forces as vibrational energy is absorbed at the upstream end 24 of the cylinder 11 and released at the downstream end 13. Coriolis forces cause a change in the symmetry of the pressure field by an amount which is related to the flow rate. Pressure sensors may be used to detect the change in symmetry of the pressure field in a number of ways . The pressure sensed at any point along the length of the cylinder is made up of three possible components. The first is the line pressure of the flowing fluid which drops slightly from the upstream end 24 of the cylinder 11 to the downstream end 13 due to frictional losses. The second component is variable and, as it stems from the cylinder vibrations, is maximum directly opposite an axial antinode. The third component is also variable but, as it is attributable to the Coriolis effect, is maximum directly opposite an axial node, where the flexure of the vibrating cylinder is greatest.

In Figure 3, a differential pressure sensor is located to sense the difference in fluid pressure between the point PI on the inner periphery of the pipe 12, directly opposite the position of the central node 22 and the point P2 upstream of the cylinder 11. The difference in pressure ΔP between the two points is calculated. As PI is directly opposite a node, the component of fluid pressure attributable to cylinder vibrations is zero. Therefore, in the absence of fluid flow, ΔP_X will be zero. However, when the fluid flows, ΔP_X will (to a large extent) be measuring only the component of fluid pressure attributable to the Coriolis effect. (The slight drop in pressure between PI and P2 due to frictional losses will also be measured, but can be accounted for) . The pressure difference ΔP_j^ will fluctuate at the vibrational frequency of the cylinder with an amplitude which is proportional to the mass flow rate.

The use of pressure sensors at PI and P2 enables the fluid pressure fluctuations attributable to the Coriolis effect to be analysed independently of the line pressure by determining ΔP_X . Obviously, if the pressure was only sensed at one location (PI) , the pressure fluctuations would have to be analysed on top of the line pressure. In the alternative arrangement shown in Figures 4A and 4B, the pressure is sensed at different circumferential locations around the position corresponding to the central node 22. Figure 4B is a cross sectional view of Figure 4A, through the central node 22. The circumferential positions PI, P3, P4 and P5 are in axial alignment with the antinodes 21 formed around the circumference of the cylinder 11. The pressure is sensed in at least two of the four antinode locations which are in antiphase and a pressure difference ΔP₂ determined. For example, the pressure may be sensed at PI and P3 or at P4 and P5. Alternatively, accuracy may be improved by sensing pressure at PI and P4 and determining the average and comparing this with the average pressure sensed at P3 and P5. As PI, P3 , P4 and P5 are all opposite the central node 22, the component of fluid pressure attributable to cylinder vibrations is zero. Therefore, in the absence of fluid flow, the pressure difference ΔP₂ will be zero. When the fluid flows, the pressure difference will fluctuate at the vibrational frequency of the cylinder, as in the previous example. However, the amplitude of the fluctuations will be about twice as great as that achieved with the arrangement shown in Figure 3. Furthermore, the pressure difference sensed will not include any pressure drop due to frictional losses . In addition to measuring the component of fluid pressure attributable to the Coriolis effect directly, it is possible to use fluid pressure sensors to sense a phase difference induced in the vibration harmonics of the cylinder by the Coriolis forces. The magnitude of the phase difference is proportional to the flow rate.

The arrangement shown in Figure 5 is an adaptation of that shown in Figure 3, utilizing two differential pressure sensors 30 and 31, each having one side connected to the line pressure at P2. PDCR 2100 series industrial differential pressure transducers manufactured by Druck would be suitable for this purpose. - The other side of the first differential pressure sensor 30 is located to sense fluid pressure at P6 on the inner periphery of the pipe 12, inbetween the axial node 22 and the upstream axial antinode 35. The other side of the second pressure sensor 31 is located inbetween the central node 22 and the downstream axial antinode 36 at P7. The points P2 , P6 and P7 are aligned parallel to the axis of the cylinder 11. P6 and P7 are symmetrically arranged either side of the central node 22, where both the vibration of the cylinder and the Coriolis effect contribute measurably to the pressure of the fluid.

When no fluid is flowing, oscillating pressure signals from both differential pressure sensors 30 and 31 are in a known phase relationship with each other.

However, with flow, the phase relationship between the two differential pressure signals changes due to the action of Coriolis forces. The magnitude of the shift in the phase relationship is proportional to the flow rate. The arrangement shown in Figures 4A and 4B may also be adapted to sense a phase difference induced in the vibration harmonics of the cylinder by the Coriolis forces . Two sets of the pressure sensors shown in Figures 4A and 4B are required. The first set is disposed upstream of the central node 22 in a position corresponding to P6 which is inbetween the axial antinode and the central node. The other set is symmetrically arranged on the downstream side of the central node 22 in a position corresponding to P7. In the absence of fluid flow, oscillating pressure signals from both sets of pressure sensors are in a known phase relationship with each other. However, with flow, the phase relationship between the pressure signals is modified by the action of Coriolis forces. The degree of change in the phase relationship is proportional, to the flow rate.

For a given axial flow velocity, the sensitivity of a vibrating cylinder mass flow sensor 10 is increased by restricting the fluid flow over both the inner and outer surfaces of the cylinder 11. The flow is restricted using the arrangement shown in Figure 6 where the vibratable cylinder 11 is sandwiched between the inner periphery of the pipe 12 and the outer periphery of a constraining cylinder 40. The pipe and constraining cylinder are relatively massive and rigid in comparison to the vibratable cylinder so that errors, resulting from induced vibrations, are not introduced into the sensing apparatus. The arrangement provides an inner and outer annular fluid flow path, indicated at 41 and 42 respectively, over the inner and outer surfaces of the cylinder 11 respectively. The constraining cylinder 40 and vibrating cylinder 11 are fixed at their downstream ends to the pipe 12 by a support body 43 which is streamlined to minimise any disruption to the fluid flow. The constraining cylinder 40, vibrating cylinder 11 and pipe 12 are coaxial, with the constraining cylinder 40 extending axially beyond the vibrating length of the vibrating cylinder 11.

The apparatus for determining the flow rate is not shown in Figure 6, as it is to be understood that any of the arrangements described with reference to Figures 3 to 5 would be suitable.

In use, the axial flow rates down the inner and outer annular portions 41 and 42 are similar. However, the inner radial spacing between the constraining cylinder 40 and the vibrating cylinder 11 and the outer radial spacing between the vibrating cylinder 11 and the pipe 12 need not be equal. In fact, the inner and outer radial spacings may be treated independently, as each makes a contribution to the increase in sensitivity. The approximate relationship for the factor, X, by which sensitivity is increased for annular flow on one side of the vibrating cylinder 11 is given by

Where: A is the inner diameter of the annular flow path B is the outer diameter of the annular flow path M is the circumferential vibration mode number.

The relationship ceases to apply when the fluid is constrained to such a degree that viscous drag becomes a significant factor. Thus for a 10% difference between the diameter of the pipe 12 and vibrating cylinder 11, vibrating with a mode 2 circumferential harmonic, the sensitivity is increased by a factor of about 5.3 compared with a cylinder with no constraint. If, in addition, an internal constraining cylinder is incorporated such that there is a 10% difference between the diameter of the vibrating cylinder 11 and the constraining cylinder 40, the overall sensitivity of the mass flow sensor 10 will be increased by a similar factor. Hence the total increase in sensitivity will be by a factor of about 10.6.

Claims

CLAIMS :

1. A mass flow sensor comprising: an elongate structure which, in use, is in contact with a fluid whose mass flow is to be sensed and is aligned substantially parallel to the direction of fluid flow; means for exciting resonant vibrations in the structure; and means for sensing fluctuations in the pressure of the fluid produced by Coriolis forces and generating a mass flow signal in dependence on the pressure fluctuations sensed.

2. A mass flow sensor in accordance with claim 1 in which the difference in fluid pressure sensed at two locations provides the mass flow signal.

3. A mass flow sensor according to claim 2 in which at least one location where fluid pressure is sensed is located radially outward from the position occupied by an axially central node.

4. A mass flow sensor according to claim 3 in which the two locations where fluid pressure is sensed are located radially outward from the position occupied by an axially central node.

5. A mass flow sensor in accordance with claim 1 in which the phase difference between fluid pressure sensed at two locations spaced along the direction of fluid flow provides the mass flow signal.

6. A mass flow sensor according to claim 5 in which the two locations are symmetrically arranged either side of an axially central node.

7. A mass flow sensor according to claim 6 in which each location is inbetween an axial antinode and the axially central node.

8. A mass flow sensor according to any preceding claim in which the elongate structure is a tube immersed in the fluid.

9. A mass flow sensor according to claim 8 in which the tube has a circular cross-section.

10. A mass flow sensor according to claim 8 or 9 in which both radial and axial harmonic vibrations are excited in the tube.

11. A mass flow sensor according to claim 10 in which the vibration is described by the circumferential wavelength being significantly shorter than the axial wavelength.

12. A mass flow sensor according to claim 11 in which the radial harmonic has four nodes and the axial harmonic has three nodes .

13. A mass flow sensor in accordance with any preceding claim in which the elongate structure is ferromagnetic, and the means for exciting the resonant vibrations includes an electromagnetic transducer.

14. A mass flow sensor in accordance with any of claims 1 to 13 in which an inner and/or an outer shroud is provided to constrain the flow of fluid over the surface of the vibratable structure.

15. A mass flow sensor comprising: a tube which, in use, is immersed in a fluid whose mass flow is to be sensed and is aligned such that the fluid flows parallel to the axis of the tube; means for exciting resonant vibrations in the tube; and means for sensing vibrations in the tube and producing a mass flow signal in dependence on sensed changes in the vibrations caused by fluid flowing past the tube; wherein guide means restrain the flow of fluid over the inner and outer surfaces of the tube.

16. A mass flow sensor according to claim 15 in which the guide means comprises an inner shroud and an outer shroud arranged inside and outside the tube respectively, the shroud being radially spaced from and coaxial with the tube.

17. A mass flow sensor according to claim 16 in which the inner and outer shrouds extend beyond the lateral extent of the tube.

18. A mass flow sensor according to claim 16 or 17 in which the inner shroud, the outer shroud and the tube are of circular cross-section.

19. A mass flow sensor according to any of claims 16 to 18 in which the outer shroud is a pipe through which all the fluid flows .