WO1996035938A1

WO1996035938A1 - Slurry density measurement

Info

Publication number: WO1996035938A1
Application number: PCT/GB1996/001103
Authority: WO
Inventors: William Reginald Parker
Original assignee: Blackdown Consultants Limited
Priority date: 1995-05-10
Filing date: 1996-05-09
Publication date: 1996-11-14
Also published as: AU5655396A; GB9509420D0

Abstract

The measurement of the density of a slurry by means of a vibrational transducer (6) is preceded by the disruption of any space-filling structure which the slurry may have developed because of the aggregation of its suspended particles. Apparatus (1) for measuring the density includes means for destroying the space-filling structure before the measurement is made and these means may be passive, relying on relative movement between the means and the slurry to create the disturbance necessary to disrupt the structure in front of the transducer (6), or active, being driven to break up the structure in the field of measurement of the transducer (6). The passive means may comprise a screen (17) disposed in advance of the transducer (6) with respect to the direction of relative movement.

Description

SLURRY DENSITY MEASUREMENT

The present invention relates to the measurement of slurry density and is particularly concerned with the making of such measurements by means of a vibrational transducer.

In many aquatic environments, such as lakes, rivers and estuaries, sediment accumulates on the bottom as a soft slurry and it is often necessary to measure the density of the slurry in si tu for navigational, dredging or pollution-monitoring purposes. One way of making such measurements involves the use of a vibrational transducer, in which the frequency or amplitude of vibration of a vibrating element of the transducer is related to the density of the surrounding medium, that is, the slurry.

Vibrational transducers were developed for use in liquids but can also be used in suspensions and slurries, and it has been noted that particular types of vibrational transducer, in which the vibrating element comprises a blade, tuning fork or cylinder, are particularly suitable for density measurements in slurries. It has also been noted, however, that such transducers fail to give reliable results above a certain solids-content of the slurry. The solid particles of a slurry tend to coagulate or gather together into groups known as "aggregates" or "floes" and, where enough particles are juxtaposed, these aggregates or floes form a continuous space-filling structure. When this structure forms, the physical characteristics of the slurry change from predominantly liquid-like to increasingly solid-like as the structure develops. In these circumstances, a vibrational transducer is affected not only by the density of the slurry but also by its solid-like characteristics, particularly its shear modulus.

The object of the present invention is to overcome this disadvantage.

According to a first aspect of the present invention, there is provided a method of measuring, by means of a vibrational transducer, the density of a slurry the suspended particles of which have aggregated to form a space-filling structure, including the step of destroying the space-filling structure of the slurry before the density measurement is made.

In a second aspect, the present invention provides apparatus for measuring the density of a slurry, comprising a vibrational transducer which can be immersed in the slurry to effect a density measurement and means for destroying a space-filling structure formed by the suspended particles of the slurry before the measurement is made.

If there is relative movement between the transducer and the slurry the density of which is to be measured, whether the transducer is stationary relative to the flow of slurry or the transducer is movable through the slurry (which may be in a standing body or in motion itself), the space-filling structure of the slurry may be destroyed by passive means comprising a screen disposed in advance of the transducer, with respect to the direction of relative movement, and having a mesh size small enough to disrupt the structure by virtue of the disturbance caused by its relative movement through the slurry but large enough to avoid clogging by or deflection of the suspended particles of the slurry. In a preferred embodiment, the screen is at right angles to the direction of relative movement. The screen preferably has a mesh size in the range of from 2 mm to 50 mm, it having been found that a mesh size of less than 2 mm is prone to clogging, particularly by slurries of higher density, and a mesh size of greater than 50 mm is ineffective, particularly with slurries of lower density.

The screen may be part of a cage surrounding the transducer and may comprise a mesh, grating or grille. In one preferred embodiment, the transducer is surrounded laterally by a cage comprising a ring of spaced-apart, parallel bars concentric with the longitudinal axis of the apparatus, the transducer extending into the cage from one end thereof, and the screen comprises a grating of spaced-apart parallel rods disposed transverse the other end of the cage. In use, the bars of the cage may serve to disrupt the space-filling structure of slurry impinging on the apparatus in a lateral direction.

In another preferred embodiment, the apparatus includes a duct into one end of which the transducer projects, and the screen comprises a plurality of arrays of parallel rods at the other end of the duct, each array being at an angle, for example of 30°, 45°, 60° or 90°, to each other array.

In a variant of these two preferred embodiments, the screen may comprise a perforated sheet having holes from 2 mm to 50 mm wide and a blocking ratio (that is, the ratio of the holes to the solid part of the screen) of from 10% to 80%. The holes may be circular or polygonal.

In another embodiment, the passive means for destroying the space-filling structure of the slurry may comprise deflectors disposed in advance of the transducer and arranged to create sufficient turbulence in their lee to disrupt the structure of the slurry in front of the transducer.

Alternatively, and particularly if there is no relative movement between the transducer and the slurry, the space-filling structure of the slurry may be destroyed by active means which can be driven within or through the slurry to break up the structure in the field of measurement of the transducer. These active means may be rotary, for example an impeller, reciprocatory, for example a pair of reciprocating screens, or oscillatory, for example a paddle or oscillating screen.

The method and apparatus of the present invention are intended for use in the measurement of slurry densities substantially in the range of from 1.00 t/m³ to 1.50 t/m³. The exact quantity of suspended particles at the upper limit depends upon the density of the water, which varies with temperature and salinity. The suspended-particle content of the slurry may be as high as 500 g/litre (dry weight of particles per litre of slurry).

The present invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of apparatus for measuring the density of slurry according to a first embodiment of the invention;

Figure 2 is a partial longitudinal section on the line II-II of Figure 1 , Figure 3 is a partial longitudinal section of apparatus for measuring the density of slurry according to a second embodiment of the invention, and

Figure 4 is an end view on arrow IV of Figure 3.

In Figures 1^'and 2, apparatus for measuring the density of slurry comprises a probe, generally indicated 1, having a watertight cylindrical housing 2 which contains the electrical and electronic components of the probe 1 , and a cylindrical duct 3 which is of substantially the same diameter as the housing 2 and is spaced coaxially therefrom by lateral connecting members or brackets 4 so that a gap 5 is defined between the housing 2 and the duct 3. A vibrational transducer 6 projects from the housing 2 into the duct 3, through the gap 5. The end of the duct 3 opposite the housing 2 is occupied by a screen 7 comprising two arrays 7A, 7B of parallel rods at an angle of 90° to each other. The screen 7 has a mesh size in the range of from 10 mm to 30 mm.

In use, the probe 1 is passed through the slurry, the density of which is to be measured, in the direction of the arrow A of Figure 2 so that the slurry passes through the screen 7 to enter the duct 3 and leaves the duct 3 through the gap 5. The passage of the slurry through the screen 7 causes the breakdown or disruption of any space¬ filling structure which may have developed in the slurry because of the aggregation of its suspended particles and the transducer 6 is activated to measure the density of the disrupted slurry within the duct 3.

In the embodiment of Figures 3 and 4, the vibrational transducer 6 is surrounded by a cage 13 which -has a larger diameter than the housing 2 and comprises a ring of equiangularly-spaced parallel bars 14 concentric with the longitudinal axis of the probe 1. At one end, adjacent the housing 2 and the transducer 6, the bars 14 are attached to an annular support collar 15 which is fixed to the housing 2 and closes the respective end of the cage 13. At the other end, opposite the housing 2, the bars 14 of the cage 13 carry a support ring 16 in which a screen 17 is mounted at right angles to the direction of relative movement between the probe and the slurry in use (this direction being contrary to the arrow IV in Figure 3) .

The screen 17 comprises a grating of parallel rods 17A supported at opposite ends by two rods 17B which are at right angles to the rods 17A and lie on respective chords of the ring 16. The mesh size of the screen 17 (that is, the distance between the rods 17A) is in the range of from 10 mm to 30 mm. The bars 14 of the cage 13 may have a spacing in the same range so that they can serve to disrupt the space-filling structure of slurry impinging on the probe 1 in a lateral direction.

The probe 1 of Figures 3 and 4 operates in essentially the same manner as the probe of Figures 1 and 2, the slurry entering the cage 13 through the screen 17 and leaving through the gaps between the bars 14. As mentioned above, however, the use of a cage 13 having bars 14 with a spacing which falls within the preferred range of mesh sizes enables the probe of Figures 3 and 4 also to be used when there is lateral relative movement between the slurry and the probe.

The probes of both embodiments may be towed through the slurry or may be lowered into the slurry to log a vertical density profile.

The driving circuits for the transducer 6 and the circuits for processing the signals produced thereby, which may be housed within the housing 2 and/or at a remote location, are entirely conventional.

Claims

1. A method of measuring, by means of a vibrational transducer (6), the density of a slurry the suspended particles of which have aggregated to form a space¬ filling structure, characterised in that it includes the step of destroying the space-filling structure of the slurry before the density measurement is made.

2. A method according to Claim 1, characterised in that relative movement is allowed or created between the slurry and the transducer (6) and the space-filling structure of the slurry is destroyed by passive means (7; 17) disposed in advance of the transducer, with respect to the direction of relative movement, so as to disrupt the structure as a result of the relative movement between the means and the slurry.

3. Apparatus for measuring the density of a slurry, comprising a vibrational transducer (6) which can be immersed in the slurry to effect a density measurement and means (7; 17) for destroying a space-filling structure formed by the suspended particles of the slurry before the measurement is made.

4. Apparatus according to Claim 3, for use when there is relative movement between the transducer (6) and the slurry, characterised in that the means (7; 17) for destroying the space-filling structure are passive and are disposed in advance of the transducer (6), with respect to the direction of relative movement (A), so as to disrupt the space-filling structure of the slurry by virtue of the relative movement between the means and the slurry.

5. Apparatus according to Claim 4, Characterised in that the passive means comprise a screen (7; 17) having a mesh size small enough to disrupt the space-filling structure by virtue of the disturbance caused by its relative movement through the slurry and large enough to avoid clogging by or deflection of the suspended particles of the slurry.

6. Apparatus according to Claim 5, characterised in that the screen (7; 17) is at right angles to the direction of relative movement (A).

7. Apparatus according to Claim 5 or Claim 6, characterised in that the screen has a mesh size in the range of from 2 mm to 50 mm.

8. Apparatus according to Claim 5, Claim 6 or Claim 7, characterised in that the transducer (6) is surrounded laterally by a cage (13) comprising a ring of spaced- apart, parallel bars (14) concentric with the longitudinal axis of the apparatus (1), the transducer (6) extending into the cage (13) from one end thereof, and the screen (17) comprises a grating of spaced-apart parallel rods (17A) disposed transverse the other end of the cage.

9. Apparatus according to Claim 5, Claim 6 or Claim 7, characterised in that the apparatus (1) includes a duct (3) into one end of which the transducer (6) projects, and the screen (7) comprises a plurality of arrays (7A, 7B) of parallel rods at the other end of the duct (3), each array (7A, 7B) being at an angle to each other array.