GB2126358A - Apparatus and methods for monitoring inertia - Google Patents

Abstract

Inertia of a body e.g. a solids- liquid separating centrifuge (16) is monitored while in use so that extent of cake build-up can be inferred. Means is provided for maintaining constant rotational speed of the centrifuge bowl and then changing (reducing) the speed sharply. inertia of the centrifuge and hence cake build-up is deduced from a measurement of deceleration and power. The centrifuge is driven by an air turbine (21). Air is cut off to initiate deceleration. Turbine power is proportional to inlet pressure and measured on gauge 33. Deceleration is determined using a speed sensor 25 and an analogue differentiator. <IMAGE>

Description

SPECIFICATION Apparatus and methods for monitoring inertia This invention relates to apparatus and methods for monitoring inertia, particularly but not exclusively the inertia of solids-liquids separating centrifuges.

During use of a solids-liquid-separating centrifuge the centrifuge bowl requires to be changed because of build-up of solid (cake) on the walls of the centrifuge bowl. It is necessary to estimate in some way the degree of loading due to the cake build-up. If the estimate of the loading in incorrect and high then the complex procedure of bowl changing is performed prematurely. If the estimate is incorrect and low then the separating efficiency is reduced and the bowl walls may be over-stressed.

One approach, which could be called the weighing approach, is to stop the centrifuge, drain it, disconnect it from its inflow and overflow lines and weigh it. This has the disadvantage of being a rather complex procedure and additionally introduces the problem of cake-slump, that is, with the centrifuge stopped, the cake, which is normally distributed in a balanced manner on the walls of the centrifuge bowl, falls from the walls and takes up an unbalanced configuration which introduces problems of bearing stress should the centrifuge need to be restarted.

Complexities arising from the problems referred to above are greatly increased when either the solids or the liquid being processed in the centrifuge is toxic or radioactive.

According to one aspect of the invention apparatus for monitoring inertia comprises a rotatable body, means for rotating the rotatable body, means operable on the rotating means for effecting a sharp change in the rate of rotation of the rotatable body, and means for comparing the rate of change of the rate of rotation with the power output of the rotating means and deriving therefrom a representaton of the inertia of the rotatable body.

The operable means may be arranged to effect a reduction in the rate of rotation.

The operable means may comprise means for maintaining the rate of rotation constant and then to effect said sharp change.

The means for maintaining the rate of rotation constant may comprise a feedback controller responsive to the rate of rotation and arranged to vary the power input to the means for rotating the rotatable body.

The means for rotating may comprise an electric motor, and the operable means comprise a fast-acting trip to cut off rapidly the power supply to the motor.

There may be an electronic analogue device or a digital differentiator arranged to act on a rate of rotation signal to measure maximum deceleration.

The rotatable body may be the bowl of a solids-liquid separating centrifuge.

According to another aspect of the invention a method of monitoring inertia of a rotating body comprises effecting a sharp change in the rate of rotation of the body, comparing the rate of change of the rate of rotation and the power output of means rotating the body, and deriving therefrom a representation of the inertia of the rotating body.

The invention may be performed in various ways and one specific embodiment will now be described, by way of example only, in association with a particular centrifuge system drive by an air turbine, with reference to the accompanying drawings, in which: Figure 1 shows in sectional elevation a known centrifuge and one to which the present invention can be applied, Figure 2 shows a schematic diagram of the centrifuge of Figure 1 in use with an inertia monitoring system, Figure 3 shows controls of the inertia monitor, and Figure 4 is a graph of corrected deceleration as ordinate and measured deceleration as abscissa, with data points generated by a digital computer simulation of the system.

Referring in general to the drawings, a solids-liquid separating centrifuge is driven by an electric motor.

Inertia of the centrifuge, and by inference the mass, can be deduced from a measurement of deceleration following the cutting-off of power to the motor.

Circumstances can arise where a deceleration signal is not dominantly representative of inertia and other factors such as speed, power spent in changing inertia, power spent in lifting solids, power spent in pumping liquor and power expended against friction can have a masking and misleading effect.

However, provided that the power trip is initiated from a steady centrifuge speed, it is possible to compensate for the secondary factors. At a steady centrifuge speed, the sum of all the powers expended is equal to the power supplied. Provided that a means exists to measure or infer the power supplied (PT), then it is possible to deduce the change in moment of inertia (Jc) due to the presence of a solids-cake from the equation.

PT JC=42NcDm Js Where No is the steady operating rotational speed, N0Drn is the maximum deceleration immediately after the centrifuge drive power has been tripped and J5 is the moment of inertia of the rotating components of the centrifuge including its drive when empty of solids cake, but containing a residuum of liquid.

The preferred means of measuring Dm is to use an electronic analogue or digital differentiator acting on a speed signal. However, using such an electronic device to convert the speed signal into a deceleration measurement necessitates the use of an electronic analogue or digital filter, which has the effect of introducing a small error into the measurement. It is possible to reduce this error through the use of a correction curve of corrected versus measured deceleration. The preferred means of generating such a correction curve is to use a digital computer dynamic model to simulate the centrifuge system and to observe the calculated effects of various liquor flows on the simulated expected deceleration.

Reference is directed to Figure 1, which is not described in great detail as it shows a known type of centrifuge, the centrifuge comprising a vessel 10 with inflow 11, overflow 12 and clarified liquor outflow 13.

Inside the vessel 10, there is a steel basket 14 with ports 15, and a bowl 16 with an entry cone 17 and a weir 18. The bowl 16 is driven by a quill shaft 19 from an air turbine 21 (see Figure 2). The bowl 16 is removed by dismantling the centrifuge. In use, cake builds up on the inside wall of the bowl 16 and it is this which has to be assessed for mass.

Reference is now directed to Figure 2, in which the centrifuge is shown with the bowl 16 dipping into a solids/liquid feed liquor 20 and driven by an air turbine 21 which has an atmospheric exhaust 22 and a driving air inlet 23.

The turbine 21 includes an electromagnetic transducer 24 which gives two pulses per revolution of the turbine. The transducer 24 supplies speed information to a centrifuge speed indicator 25 and an inertia monitor 26. The speed indicator 25 is used to provide speed control to the air turbine 21 via a controller 27 which accepts a voltage input from the speed indicator 25 and provides a proportional - plus - integral current output to a current/pressure transducer 28 supplying an air pressure signal to a valve 29, which controls the flow of air to the turbine 21, thus creating a proportional - plus - integral feedback loop.

Main air supply is taken at point 30 and passes through a pressure reducing valve 21 and a fast-acting trip valve 32. The valve 32 is controlled from the inertia monitor 26. A turbine air inlet pressure meter 33 is provided. The trip valve 32 is used to shut off air supply to the turbine 21 to initiate a deceleration transient.

Inlet air pressure is displayed on meter 33. The inertia monitor 26 includes a 'start' button for closing the valve 32 and a timer controlled by a potentiometer for setting the period of time the valve 32 is closed. An electronic analogue differentiator is provided to convert the rotational speed signal into a deceleration signal. There is also a digital voltmeter for detecting the highest indicated deceleration of the turbine during a deceleration transient.

Reference is now directed to Figure 3. A timer 40 has an adjustable timing control 41. When a start button 42 is pressed, a pulse of duration ten seconds maximum is generated and this pulse closes the trip valve 32 for a defined interval. A frequency-to-voltage converter 44 converts pulses from the transducer 24, which appear at a frequency proportional to speed, to a voltage proportional to speed. Signal conditioning is effected by a gain control 45 and back-off control 46. A differentiator 47 employs analogue computing techniques and has a transfer function, H(s), of the form: = T0s H(s) (1 + Tis) (1 + T2s) Where T is a gain term, adjustable at 48, T1 and T2 are smoothing time constants, adjustable at 49 and 50 respectively, and s is the Laplace operator.

The differentiator 47 computes the first differential coefficient of rotational speed. The terms T1 and T2 are introduced to reduce the effect of small disturbances or noise in the turbine speed signal. During the test period following depression of the start button 42, the differentiator 47 rises to a peak, then subsequently decays. This peak is measured by a peak detector 51. Peak detector 51, which has the capability to store the peak values of a voltage during a transient, can output to an instrument 52 for recording purposes. By means of a reset button 53, the stored value can be cancelled and the differentiator 47 primed for the next test. A voltage-to-frequency converter 54 enables pulses to be produced at precisely defined frequencies for calibration and test purposes.

Analysis shows that for an air turbine, the turbine power supplied depends only on inlet air pressure to the turbine and instantaneous speed, provided that the downstream air pressure and ambient air temperature are constant. Maintaining speed constant at design value through the action of the feedback control system allows power to be characterised by the inlet air pressure. The preferred means of eliminating the effects of small transient variations from steady speed and steady air pressure is to filter the air pressure signals using an electronic analogue or digital averaging filter.

Calibration of power against inlet air pressure is achieved by the following procedure. The centrifuge is set in operation at its designed speed with a low flow of solids-free liquor passing through it. Air pressure is measured on the meter 33 and recorded. The monitor 26 now has its start button 42 depressed and this causes valve 32 to provide a sharp cut-off. The maximum deceleration is measured. After a short time, typically ten seconds, the monitor 26 causes the valve 32 to re-open, and the design speed is re-established.

The procedure is repeated, with typically five trials being performed, and an average measured deceleration being found. The deceleration correction curve shown in Figure 4 is generated from a digital computer dynamic simulation study and is used to find the correct deceleration, Drn, from the average measured deceleration. The turbine power PT iS then found from PT = 4 X2 NO J5 Dm Turbine power PT iS piotted against the measured inlet air pressure. The liquor flow is increased, and the procedure repeated. Typically ten or more power/pressure data points are found, so as to cover the whole of the expected range of power operation of the turbine.

In order to assess cake build-up in the bowl, the turbine speed is held constant at the predetermined speed No so that steady conditions are maintained and in this position, power supply to the turbine is equal to the sum of the powers dissipated. On the basis that the downstream pressure at the turbine and the ambient air temperature are constant, pressure measurement at meter 33 represents turbine power by using the calibration curve of power versus air pressure at the design speed NO derived by the process described above. The monitor 26 has its start button 42 depressed and this causes valve 32 to provide a sharp cut off. A measurement of the maximum deceleration is then made using the decelerometer. After a short time, the trip valve is opened automatically by the monitor, and a steady-state design speed re-established.

Typically, five deceleration trials are carried out and the measured values are averaged. The correction curve of Figure 4 is used to produce an improved estimate Dm of the average deceleration. The change in moment of inertia JC due to the presence of solids cake can then be calculated as J ~ PT J C 4qr2 PT 41T2 N0 Dm The actual cake mass can be derived from this change in moment of inertia by consideration of the cake constituents, but, for many purposes, the change in moment of inertia can be the figure of interest, and when this has reached a predetermined level, the centrifuge can then be regarded as fully loaded.

Calculations associated with determining change in inertia, cake build-up and other parameters are conveniently carried out by a microprocessor which can also provide control and information signals for powering operating devices (valves or the like) and data displays, respectively.

The inertia monitoring system can also be applied to other rotating bodies, for example turbines and drive shafts. Such bodies may have an electrical drive instead of a pneumatic drive, in which case electrical sensors and controllers would be substituted for the pneumatic devices mentioned above.

Claims (10)

1. Apparatus for monitoring inertia comprising a rotatable body, means for rotating the rotatable body, means operable on the rotating means for effecting a sharp change in the rate of rotation of the rotatable body, and means for comparing the rate of change of the rate of rotation with the power output of the rotating means and deriving therefrom a representation of the inertia of the rotatable body.

2. Apparatus as claimed in claim 1, in which the operable means is arranged to effect a reduction in the rate of rotation.

3. Apparatus as claimed in claim 1, in which the operable means comprises means for maintaining the rate of rotation constant and then to effect said sharp change.

4. Apparatus as claimed in claim 3, in which the means for maintaining the rate of rotation constant comprises a feedback controller responsive to the rate of rotation and arranged to vary the power input to the means for rotating the rotatable body.

5. Apparatus as claimed in any preceding claim, in which the means for rotating comprises an electric motor, and the operable means comprises a fast-acting trip to cut off rapidly the power supply to the motor.

6. Apparatus as claimed in claim 5, comprising an electronic analogue device or a digital differentiator arranged to act on a rate of rotation signal to measure maximum deceleration.

7. Apparatus as claimed in any preceding claim, in which the rotatable body is the bowl of a solids-liquid separating centrifuge.

8. A method of monitoring inertia of a rotating body comprising effecting a sharp change in the rate of rotation of the body, comparing the rate of change of the rate of rotation with the power output of means rotating the body, and deriving therefrom a representation of the inertia of the rotating body.

9. Apparatus for monitoring inertia substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

10. A method of monitoring inertia as claimed in claim 8 and substantially as hereinbefore described.