Measurement of Fluid Forces in Mixing Apparatus and the Control of Mixing Apparatus in Response to Fluid Forces

The present invention relates to mixing systems, and particularly to methods and systems for measuring fluid forces acting on the impellers and shafts of mixing systems and for the control of mixing system in response to such fluid forces.

The invention is especially suitable for use in mixing systems which are susceptible to large fluid forces which may permanently bend or even break the impeller shaft of the mixing system, especially at draw-off or with impellers operating near the liquid surface.

Fluid forces are the main mechanical forces on the mixer shaft. They are generated at the impeller perpendicular to shaft axis and produce large bending movement (deflection) on the top of the shaft and on the drive mechanism (particularly the gear box of the drive mechanism which contains reducer gears and drives the shaft). In closed tanks having seals through which the shaft passes, these forces can damage the seal. Fluid forces are discussed in detail in Chapter 17 of the text, Fluid Mixing Technology by J. Oldsue, McGraw-Hill (1983). They're also described in article by R. J. Weetman and R. N. Salzman, Chemical Engineering Progess, pp. 71-75 (June 1981).

Fluid force measurements have required laboratory instrumentation such as strain gages or strain gage bridges which are connected to the impeller shaft. Electrical connections to such instrumentation must be brought out of the mixing system by way of slip rings. Optical systems which respond to light reflected from the shaft has also been used to measure the deflection of the shaft in response to the fluid forces. Since the fluid forces are greatest near the impeller which is submerged in the material being mixed, the optical devices must be located on the shaft well above the surface of the material being mixed. Since the deflection is small at such locations, the optical devices must be sensitive. The use of slip rings and sensitive optical devices has constrained the measurement of fluid forces to laboratory environments. The various instruments for measuring fluid forces mentioned above are discussed in greater detail in the above-referenced publications.

It has been discovered, unexpectedly, and in accordance with this invention that measurement of the movement of the drive mechanism, particularly the gear box which drives and supports the impeller shaft, has a direct correlation to fluid forces acting on the impeller. This discovery is unexpected in that the mixing system is a complex mechanical system having a complex frequency response, such that the deflection of the non-rotating portion of the system, namely the drive mechanism, would not be considered to correlate directly with the fluid forces. Moreover, prior measurements of fluid forces have depended upon instruments attached to and rotating with the impeller shaft. Even with such instruments the measurements have been difficult to make in that the runout of the shaft under no load conditions can be greater than the deflection of the shaft caused by the fluid forces. Accordingly, measuring fluid forces in response to the movement of drive mechanism which is supported over the tank and which does not rotate represents a significant improvement and simplification. The measurement is made well away from the material being mixed. The measurement can be made above the cover of a closed tank, such that contamination of the instrumentation from the material being mixed is precluded. The output of the measurement is useful in controlling the operation of the mixing system, as to prevent damage or unsafe conditions which can arise in the presence of excessive fluid forces. For example, the speed of the impeller can be reduced when excessive fluid forces are detected, thereby reducing the fluid forces and preventing any damage to the mixing system which might result if such forces become excessive.

Accordingly, it is a principal object of the present invention to provide improved mixing systems.

It is another object of the present invention to provide improved methods and systems for the measurement of fluid forces.

It is a further object of the present invention to provide improved methods and systems for the control of mixing systems in response to fluid forces.

Briefly described, a mixing system in accordance with the invention has a motor-driven drive mechanism coupled to a shaft having an impeller thereon. The shaft is disposed in a tank for mixing material therein. The system is controlled in response to fluid forces on the impeller, which deflect the shaft, by detecting the movement of the drive mechanism, which does not rotate, preferably by the use of a proximity gage disposed adjacent to the gear box of the drive mechanism. Signals are produced representing the fluid forces in response to the detected movement of the drive mechanism. The motor of the mixing system is controlled in response to these signals. When the fluid forces become excessive, the motor may be switched to operate at lower speed or even stopped. Accordingly, damage to the mixing system which might result from excessive fluid forces is precluded and safe operation of the system is assured.

The foregoing and other objects, features and advantages of the invention, as well as a presently preferred embodiment thereof and the best modes now known for practicing the invention, will become more apparent from reading the following description in connection with the accompanying drawings in which:

FIG. 1 is a sectional view showing a mixing system embodying the invention, the view being taken along the lines 1--1 in FIG. 2;

FIG. 2 is a fragmentary plan view of the apparatus shown in FIG. 1;

FIG. 3 is a block diagram of the electronics for controlling the motor of the drive mechanism of the mixing systems shown in Figs. 1 and 2 in response to signals corresponding to the fluid forces on the impeller of the mixing system;

FIG. 4 shows curves showing representative signals obtained from the proximity gages used in the system shown in FIGS. 1 and 2;

FIG. 5 shows other curves depicting the frequency response of the signals shown in FIG. 4.

Referring more particularly to FIGS. 1 and 2 of the drawings, there is shown a tank 10 having a liquid suspension 12 which is mixed by an impeller 14 mounted on an impeller shaft 16. The impeller is driven by a drive mechanism 18. The drive mechanism 18 is made up of a drive motor 20 and a gear reducer assembly in a gear box 22. The motor is mounted in a channel 24 attached by a front plate 26 to the gear box 22.

The tank may be a closed tank having a cover 28 connected to the walls of the tank, which may be either round or rectilinear, (rectangular or square). The cover 28 bears on gaskets 30 lining the upper ends of the walls of the tank 10. The tank may be approximately 5 to 70 feet in diameter or width. The tank is spanned by wide flanged I-beams 32 and 34 which provide a flexural support for the drive mechanism. The gear box 22 may be bolted to the beams by bolts 36. The channel 24, mounting the motor 20, may be connected to a bracket 38 which is also bolted to the beams 32 and 34 by bolts 40.

The mixer shaft 16 is cantilever supported from bearings in the gear box 22, and extends through an opening in the cover 28. The opening is closed by a seal assembly 42.

A proximity gage 44 is mounted in a fixed position adjacent to the exterior surface 46 of the gear box 22 on a support member 48. This proximity gage is preferably a magnetic flux density responsive gage of the type which is commercially available. A gage having a range of 100 mils (100 times 0.001 in.) is preferably used. The nose of such a gage may have a 5 millimeter diameter. This nose may be positioned approximately 50 mils from the exterior surface 46 of the gear box 22 so as to respond to the movement (displacement) of the gear box 22. The gear box is made of metal, preferably steel, so that the distance between the nose of the gage 44 and the surface 46 changes the flux density in the gap between the nose and the surface. The gage therefore provides an accurate response to the flux gap length and therefore of the movement of the gear box 22.

A rod 50 projects upwardly from the gear box 22 along the axis of the shaft 16 (in an axially direction opposite to the direction of the shaft 16). This rod is also preferably made of metal (steel). Another proximity gage 52 is mounted on a support member 54 adjacent to the top of the member 50. The member 50 serves to amplify the movement of the gear box. Therefore the proximity gage 52 may have a larger diameter nose than the gage 44 and a larger range, (for example one inch). The nose of the gage 52 may be spaced from the side of the rod 50 adjacent to which it is disposed by approximately 1/2 of the range. The spacings mentioned above are, of course, with the shaft non-rotating.

The beams 32 and 34 provide a flexural support allowing maximum pivotal movement (rocking) of the gear box about a flexure axis 56 (FIG. 2) which intersects the axis of the shaft 16 and is perpendicular thereto. The gages are perpendicular to this axis and therefore respond to the pivotal movement or deflection of the gear box together with the shaft 16. The deflection in one direction due to fluid forces operative upon the impeller shaft are illustrated by the dash lines for the shaft 16' and for the amplifying rod 50 at 50'. It will be noted that the deflection of the member 50 and the movement of the gear box 22 is 180 degrees out of phase with the deflection of the shaft.

The gage 44 provides and output signal PD1. The gage 52 provides an output signal PD2. These signals are used for control of the mixing system, by controlling the speed of the motor 20 to the use of the system of FIG. 3 in accordance with this preferred embodiment of the invention. Other uses of the signals will of course become apparent to those skilled in the art.

In order to verify that the movement of the gear box 22, even though it is non-rotating, correlates directly with the deflection of the shaft 16, a ring 58 is provided around the shaft 16 above the surface 60 of the material 12 in the tank 10. This ring 50 is machined to zero out the non-load runout of the shaft. Another proximity gage 62 is disposed adjacent to the ring 58. The ring 58 and the gage 62 are illustrated in lines consisting of long and short dashes to show that they are not used in operation of the system, but are presented herein merely to explain how the direct correlation is verified between the deflection of the shaft 16 in response to fluid forces on the impeller with the outputs from the gages which respond to the movement of the gear box 22.

Referring to FIG. 4, there is shown the curve representing the amplitude variation with time of the signal PD1 as well as amplitude variation with time of the signal PR from the gage 62. It will be noted that the signals are essentially similar in wave form. Because the mixing system is a complex mechanical system the dynamic response over the frequency range, up to for example the first critical speed of the shaft over which the signals may be taken, also shows how well they correlate. FIG. 5 illustrates the frequency response of the signals. It will be observed that they correlate closely. Most important at the frequency at maximum amplitude (where the fluid forces are the greatest which is shown at the frequency indicated by the cursor 64). The signals correllate very closely.

The curves of FIG. 4 and FIG. 5 are taken with a 180 degree phase shift so as to display them in in-phase relationship, notwithstanding that the deflection of the shaft 16 seen by the gage 62 is 180 degrees out of phase with the movement of the gear box 22, as sensed by the gage 44. The signals also have a different DC level off-set. Neglecting this off set, which can be accommodated readily in the amplifiers which amplify these signals from the gages, the direct correlation between the signals will be apparent from FIGS. 4 and 5.

Referring to FIG. 3 the signals PD1 and PD2 are provided to a switch 66. These signals may be suitably amplified and conditioned prior to being applied to the switch 66. One of the signals is selected by the switch, depending upon the amplitude range of the fluid forces. For example if the fluid forces are relatively low the signals PD2 are selected, otherwise signals PD1 are used.

Assuming the signal PD1 is used, it is applied to two level detectors, 68 and 70. These may be threshold circuits wherein the peak amplitude of the PD1 signals is compared to a fixed threshold. The threshold of the first detector 68 may be higher than that of the second detector 70. The signals which are applied to the detectors may be filtered to remove any high frequency artifacts or noise which might be mistaken as a signal representative of the fluid forces. If the signals exceed the threshold of the second detector 70, an output is provided which sets a latch circuit 72. Similarly, if a signal which exceeds the threshold of the first detector 68 occurs it sets another latch 74. If the latch 72 is set, an output is provided to the speed control circuit 76 of the motor to reduce the speed to 2/3 the operating speed during normal mixing. If the latch 72 is set, the speed control responds to the output of the latch 74 by stopping the motor. Accordingly, any excessive fluid forces result in a change in the speed of the motor to immediately reduce the fluid forces and prevent damage or any unsafe operation of the system. Normal speed operation may be resumed by applying a re-set input to the latches 72 & 74.

The electronics illustrated in FIG. 3 may also be implemented by a suitably programmed micro-processor which performs other control functions in the mixing system.

From the foregoing description it will be apparent that there has been provided an improved mixing system wherein fluid forces may be measured and used to control the operation of the system. Variations and modifications of the herein described system and the methods of measuring fluid forces and the control of the system and response thereto, within the scope of the invention, will undoubtedly suggest themselves to those skilled in the art. For example, a typical installation may use only one gage, either 44 or 52, and not both. Accordingly, the foregoing description should be taken as illustrative and not in a limiting sense.