GB2110553A - Pressurized filtration system - Google Patents

Abstract

The system is equipped to provide electrical signals representative of volume V and the rate of flow Q through or into the filter of the system, concentration of the material Cs being filtered, density of the particles of the material Dp, and elapsed time te as a filtration cycle proceeds. Of the signals, the signals V, Cs, Dp and Vf (volume of the filter) are combined to obtain a signal CSa, representative of the percentage of cake solids then existing in the filter. Next, and upon sensing that the pressure in the system has reached a selected value, the signals te and CSa are fed to a computer in order to derive therefrom an output indicative of the performance of the system.

Description

SPECIFICATION Pressurised filtration system This invention relates to pressurised filtration systems, used for the separation of solids from liquids, and particularly to a system wherein parameters of operation are generated indicative of the operation of the system.

There persists, and there has persisted for a long time, a major problem for operators of pressurised filtration systems of the character described, and that is that the operators simply have no way to determine actual conditions in such a system, while it is in operation, to the degree necessary to utilize such determination to predict or to control conditions which will exist in the future.Thus, for example, if the liquid-solids composition should change, as in concentration or de-waterability, this will not be known, and termination of a filtration cycle based on a normal cycle period of time or a normal terminal filtrate flow may result in the discharge of unacceptably wet filter cake with the frequently attendant timeconsuming task of washing the filter, or, termination of a filtration cycle based on a normal filtration time, or a normal terminal filtrate flow may result in plugging of the core or feedhole of the filter, which can result, among other things, in catastrophic failure of filter plates.If, on the other hand, there existed in the system means of indicating critical parameters of operation, particularly changes in these parameters, accurate prediction of the length of filtration cycles and terminal filtrate flows under varying conditions would become feasible, and, in many instances, adjustments in the feed rates of conditioning chemicals or materials to the slurry being filtered could be made to restore conditions to the desired level of operation.

Historic methods of monitoring the progress of a filtration cycle comprise the discrete observation of pressure, elapsed time, and filtrate flow.

Unfortunately, these are useful only to the point that conditions remain constant from one filtration cycle to another. Discrete observations are totally misleading and useless as a basis for making process decisions where changes occur, for example, changes in concentration, permeability of the forming cake, or changes in response to the feed of conditioning chemicals.

It is the object of this invention to provide a filtration system wherein the problems referred to above have been overcome, and there is provided a filtration system wherein operating personnel are adequately informed as to changes in operating parameters sufficient in content and in time to maintain desired levels of operation.

In accordance with this invention we provide a method of monitoring operation of a pressurised filtration system comprising: deriving from said system electrical signals representative of the volume V and the rate of flow Q through or into the filter of the system, concentration Cs of the material being filtered, density Dp of the particles of the material and elapsed time te from commencement of the filtration cycle; combining said signals V, O, Cs and Dp to derive CS, representative of the percentage of cake solids then existing in the filter; and sensing the pressure in the system and when said pressure reaches a preselected value computing the quotient CSa2/t213 to provide a value K indicative of the performance of the system.

The invention will now be described by way of example with reference to one embodiment thereof which is illustrated in the accompanying drawings, wherein: Figure 1 is a schematic illustration of a basic portion of the system of this invention; Figures 2-7 are electrical schematic diagrams, each directed to the determination of a particular parameter and the provision of it as an output of the system; Figure 8 is a graphical illustration of a feature of operation of the system of this invention.

Figure 1 generally illustrates a filtration system including a performance analyser as contemplated by this invention, a display for indicating outputs of the analyser, and an initial portion of the circuitry of the analyser. Source 1 0 represents a source of slurry, that is, a liquid having suspended in it certain solids to be filtered out. A typical example of such a slurry would be sludge resulting from the treatment of sewage. A pipe from source 10 is connected to pumping device 12, which then delivers the slurry through pipe 14 to an appropriate filter 16, wherein the solids are trapped and the remaining liquid exits through pipe 18. We will assume, for the purposes of illustration, that initially, that is, at the commencement of a filtration cycle of operation, filter 16 is empty, that is, that it does not contain any filter cake.

Flow-volume meter 20 is coupled to pipe 14. It is conventional and includes means for providing two output signals, a flow rate signal Q in gallons per minute, and an accumulated flow, that is, volume, signal V in gallons. The concentration of solid matter in the slurry is monitored by a conventional concentration meter 22 coupled to pipe 14. It provides a signal Cs, representative of the percent concentration in terms of weight. The Cs signal is supplied directly to "percent concentration" readout 29 on display 30 and to other circuitry of computer 48, as will be described. The pressure in pipe 14 is sensed by a conventional pressure sensor 24, and it provides a signal output P5 to terminal 96, representative of the pressure in pipe 14 in terms of pounds per square inch (PSI) for use as shown in Figure 2.

The density of the solid matter of the slurry is determined periodically, using conventional laboratory techniques, and this density is introduced, by means of an adjustable signal generator 49, as an indication signal Dp, representative of the density of solid matter in terms of pounds per cubic foot. As an example, Dp may be determined by taking a sample of the filtered cake, as representative of the particles of sludge, determining its dry weight and volume and dividing its weight by its volume.

Display 30 includes several other readouts in addition to percent concentration, and these will be referred to as portions of the system providing signals to them are discussed.

Timer 32 is a conventional timer for providing two forms of outputs, an output t on terminal 33 to "now" readout 31 of TIME section 35 of display 30, and to memory 36, representative of real time, and an output t5 to terminal 106, representative of elapsed time from the time of application of a "set" input from set control 34, the latter being either a mechanical or electrical control connected to timer 32. Output t5 is used as shown in Figure 2.

Memory 36 is a conventional memory. It functions to store 3 time signal ti, representative of the initiation time of a filtration cycle, and a flow rate signal Qi, representing flow rate at the time of such initiation. The signal tj is obtained from the signal output t from timer 32, and the U signal is obtained from the Q signal output of flow meter 20, both being entered into memory 36 by the operation of enter control 38. The initial time signal tj is continuously supplied by memory 36 to "start" readout 40 of time section 35 of display 30, whereby the initiation time of a filtration cycle is constantly displayed. The t signal is also supplied to terminal 124, where it is used as shown in Figure 4.An initial flow rate signal U is provided at terminal 42, from which it is provided to other circuitry, as will be described. A current flow signal Q from flow meter 20 is supplied directly to "now" readout 4G of flow section 47 of display 30. It also appears on terminal 44, where it is used as shown in Figure 2.

Slurry conditioning source 51 is any conventional means for adding chemicals or other substances to the slurry in order to vary the composition or conditioning of the slurry or to make corrections to the process, as discussed more fully under Figure 7. Conditioning source 51 can be connected to the hydraulic line between source 10 and pump 12 (as shown) or at any other convenient location.

Figure 1 further and particularly illustrates circuitry for the determination of "percent of cake" of solid matter, CSa, accumulated in filter 16 during a filtration cycle. The first step in this determination is the computation of density of slurry, designated Dss This is accomplished by D5 computer 48, which includes multipliers 50, 52, and 54, difference or sub-traction unit 56, adder 58, divider 60, and signal generators 49, 53, 55, and 57.It has been found that D5 can be solved by these computational elements arranged as shown to solve the equation: 833Dp Do = ~ Dp(1 00 - Cs) + 62.4C5 where the terms of this equation are outputs of the measuring devices heretofore described or constants or selected values introduced through signal generators. Thus, a value Dp p from signal generator 28 would be provided as a signal input to multipliers 50 and 52 and to terminal 73.

First, the signal Dp is multiplied in multiplier 52 by a signal value representing the density of the liquid phase of the slurry expressed in pounds per gallon times (8.33 > r 100 in the case of water at 700 F), this signal value being supplied by signal generator 53. Where he fluid is other than water, an appropriate substitution would be made.

Next, difference unit 56 subtracts a signal Cs, representative of percent concentration of the slurry as expressed by weight, from a signal representative of 100%, thc 100 or 100% signal being supplied by signal generator 55. The difference signal is then multiplied by signal D in multiplier 50, and the resultant product is added by adder 58 to a 62.4C, signal, this being obtained as an output of multiplier 54.The constant 62.4 represents the density of the liquid phase of the slurry expressed in pounds per cubic foot (62.4 pounds per cubic foot for water at 700 F), as provided by signal generator 57, this constant also appearing on terminal 59 for use as noted later. if the fluid is other than water, the density constant would be that of the other fluid.

The sum signal output of adder 58, D (100-C5) + 62.4C5 is then civided into the product output signal 833dip of multiplier 52 by divider 60 to provide a signal D5, representative of the density of the slurry expressed in pounds per gallon of slurry.

Next, using the signal value D5, a signal value w, representing weight of dry solids per unit of volume of slurry, is determined by computer 62.

First, the signal D5 is multiplied by a signal Cs in multiplier 64, and second, the resulting product is divided in divider 66 by a signal representative of 100 provided by signal generator 101.

The final computation in this series is accomplished by CS, computer 67, which employs multiplier 68, memory 70, divider 72, subtraction unit 74, multiplier 76, adder 78, divider 80, multiplier 82, and signal generators 75 and 83. Initially, signal wis multiplied in multiplier 68 by a signal V, representing accumulated flow up to this point in the filtration cycle, to provide an output I, representative of the total weight of solids pumped so far into filter 16. As the value I is employed at succeeding steps in the computation of CSa, memory 70 includes means for periodically sampling, at a selected rate, the I output of multiplier 68 and storing it for a sufficient period to complete the computations shown. In the first calculation, the total weight of solids I is divided by a signal from terminal 73, representing particle density Dp, obtained as described previously, resulting in a signal representative of the volume of solids in filter 1 6. This volume of solids signal is then subtracted from a reference signal V, from signal generator 75, representative of the volume of filter 1 6. The subtraction is accomplished by subtraction unit 74, resulting in an output representative of the volume of liquid in filter 16.

The Vf output from signal generator 75 also appears on terminal 71, where it is used shown in Figure 5. Next, the volume of liquid signal is multiplied in multiplier 76 by an output from terminal 59, representative of the density of the liquid phase of the slurry expressed in pounds per cubic foot (62.4 for water at 700 F, or other density constant where the liquid is other than water), resulting in an output of multiplier 76, representative of the weight of liquid in filter 1 6.

next, the weight of liquid signal from multiplier 76 is added in adder 78 to the weight of solids signal Ifrom memory 70, and the sum represents the total weight of cake in filter 16. This total weight signal is next divided into the weight of solids signal / in divider 80 to provide an output, which is then multiplied by an output from signal generator 83, representative of 100%, to provide the signal CSa, representative of actual percentage of dry cake solids in the filter. Signal CS, is transmitted to and displayed by "percentage of cake" readout 84 on display 30. The CS, signal also appears on terminal 102, where it is used as shown in Figure 2.The quantity "percentage of cake" is significant in order to make the operator aware of the status of the filtration cycle and the conditions of the forming cake in the filter at all times, allowing intelligent decisions to be made, for example, should it become necessary for mechanical reasons to terminate the cycle prematurely.

Figure 2 illustrates signal circuitry which combines with the circuitry shown in Figure 1 to provide a signal which is believed to be representative of a new measurement parameter.

It is designated K and is uniquely indicative of the rate of buildup of solids that will occur in filters of filtration systems under terminal pressure conditions. It is, therefore, a single value representing a given set of conditions including filter area, filter chamber thickness, filter volume, viscosity of the slurry, permeability of the slurry solids to flow, and terminal pressure across the filter.

Figure 8 illustrates graphically the relation between the variables which determine K, the percent of dry cake solids (CS), and the elapsed filtration time (t5), with the solid line a being representative of the relationship of these quantities as defined by the illustration equation for the relation, and the dashed line B being representative of them as operating parameters of a typical filtration system. The reason for the difference is that the equation is based upon filtration pressure being constant and as existing at the end of a filtration cycle. In practice, in order to generate such a pressure at the beginning of a filtration cycle, an inordinately high flow rate would be required. Since such rates can frequently not be attained in the early phases of a cycle, solids buildup in the filter will lag behind those indicated by curve a during these phases.As the rate of buildup defined by curve a begins to drop, the actual pumping capability will begin to catch up and the curves will converge as shown.

Beyond the point of convergence, the two curves will be synonymous and the filter will operate at a terminal pressure differential.

The circuitry of Figure 2 determines a signal value K by first solving the equation: cs2 a K-t2513 As shown, a signal CS5 from terminal 102 of Figure 1 is squared in squaring device 100 and is provided as a numerator to divider 98. A signal t5 from terminal 106 of Figure 1 is raised to the 2/3 power by computational element 104 and is fed as a denominator to divider 98. In order to insure that the criteria referred to above with respect to pressure is met, divider 98 is enabled only when the pressure signal as provided from pressure sensor 24 (see Figure 1) indicates a near terminal pressure. A pressure criteria signal is generated as follows.Signal generator 93 provides a pressure signal Pt, representative of an anticipated operating pressure at the termination of a filtration cycle, and it is supplied as an input to multiplier 92, where it is multiplied by a signal from signal generator 95, representative of the number 0.9.

Thus, there is provided as an output of multiplier 92 a signal representative of 0.9Pt, or 90% of the anticipated terminal pressure. This quantity is then compared in comparator 94 with a current pressure signal P5 from terminal 96, and there is provided an output from comparator 94 when current pressure exceeds the 90% figure, and thus there would be present essentially terminal operating pressure in accordance with the dictates of the equation illustrated above. While the output of comparator 94 may be used directly as a gating or enabling signal to divider 98 to effect an output K when such pressure is achieved, a safeguard is provided to protect against triggering an output K response to an artificially created pressure value.

Thus, for example, a value might be accidentally closed downstream of pressure sensor 24 and create an abnormally high pressure unrelated to filter operation. The safeguard involves sampling flow rate through pipe 14 and inserts the requirement that there be at least one-half the rate of flow that existed at the commencement of the filtration cycle before the pressure output of comparator 94 is effective. This is achieved by providing a signal Qi from terminal 42 of Figure 1, representative of the flow at the beginning of the filtration cycle, to multiplier 86, where it is multiplied by a signal from signal generator 97, representative of the number 0.5. The product of multiplier 86,0.5Q,, is then fed as one input to comparator 88, together with a second input from terminal 44 of Figure 1, representative of current flow Q. Comparator 88 then provides an output only when current flow is at least equal to 0.5 flow at the commencement of the filtration cycle. In accordance with the considerations discussed, the outputs of both comparators 88 and 94 are fed to AND gate 90 which, accordingly, provides a gating or enabling signal to divider 98 only when there is both near terminal pressure in the system, and there is adequate flow through the system to provide assurance that there are no obstructions in the line.Under these circumstances, divider 98 is gated, and the signal value, representative of K, is provided as an output on terminal 111 and is displayed by "is" readout 110 of the K section 108 of display 30. Additionally, the K section of the display includes a "want" readout 112 and a "was" readout 114. "Want" readout 112 may simply be a programmable display wherein a selected value is either electrically or mechanically inserted. "Was" readout 11 4 is activated, as will be described with respect to Figure 6.

Referring to Figure 3, signal K is employed in the determination of the predicted duration of a filtration cycle. This prediction may be made for selected values of percentage of cake solids (CSt) desired or permissible at the end of a filtration cycle. A selected such signal is obtained as an output of signal generator 11 9 and fed to squaring unit 118 and as CSt2 is supplied as the numerator of divider 11 6, wherein it is divided by signal K from terminal 111 of Figure 2. The quotient output of divider 11 6 is raised to the 3/2 power by computational element 120. The result is a signal representative of the length of a filtration cycle in minutes required to achieve a selected percentage of cake solids.This time signal is fed to terminal 122, and, as shown in Figure 4, the signal is added to adder 126 to a signal t from terminal 124 of Figure 1, which is representative of the time of commencement of the current filtration cycle. The sum of these times, labelled tt, is thus representative of the real time projected end of filtration cycle, and as such is fed to and displayed on "end" readout 128 of TIME section 35 of display 30.

An additional parameter of value to an operator of a filtration system is the flow rate which will exist at the end of the filtration cycle. This can be determined by subtracting the weight of dry solids contained in the filter cake one minute before the termination of the cycle DS2 from the weight of dry solids in the cake at the time of termination DS, and dividing the result by the weight of dry solids per unit of volume w. The circuitry for accomplishing this is shown in Figure 5.

As shown, a constant I, as provided by signal generator 189, is subtracted from the signal representing the projected length of filtration cycle tc, provided from terminal 122 of Figure 3 by difference unit 185, resulting in signal tc - 1, representing the projected length of the filtration cycle minus one minute. Time tc - 1 is raised to the 2/3 power by computational element 186 and is multiplied by K, as provided by terminal 111 of Figure 2(1) by multiplier 187. The square root of this product is taken by computational unit 188, resulting in signal CS2, representative of the percentage of dry solids in the filter cake at time tc -1.

It has been found that the dry solids DS contained in a filter cake at any point in time can be determined by the equation 62.4 (CS) (Vf (Dp) DS Dp(100~CS) + 62.4 CS where 62.4 represents the density of the liquid phase of the slurry in pounds per cubic foot.

Dry solids DS2 at time tc - 1 is, therefore, determined as follows: Cake percentage concentration CS2 at time tc - 1 is multiplied by a signal value representing the density of the liquid phase of the slurry (62.4 in the case of water at 700 F) as provided by terminal 59 Figure 1 in multiplier 150. The product is then multiplied by a signal representing the volume of the filter Vf (as provided by terminal 71 of Figure 1) by multiplier 148. This product is then multiplied by a signal representing the density of the slurry particles Dp (as provided from terminal 73 of Figure 1) by multiplier 146, resulting in a signal n, representative of 62.4 (CS2) (Vf) (Dp).

Cake percentage concentration CS2 is also multiplied by a signal representative of the density of the liquid phase of the slurry (as provided by terminal 59 of Figure 1) by multiplier 197, resulting in a signal e representative of 624CS Cake concentration CS2 is substrated from a constant 100, provided by signal generator 101, by difference unit 1 81. The difference is multiplied by a signal representing particle density Dp (as provided by terminal 73 of Figure 1) by multiplier 182. This product is added to signal e by adder 183, resulting in a signal d, representative of Dp (100 - CS2) + 62.4 CS2.

This signal d is divided into signal n, described previously by divider 142, resulting in a signal DS2, representative of the dry solids weight of the filter cake at time to~1.

Dry solids DS weight of the cake at the termination of the cycle, and thus after a corresponding elapsed time tc, is determined as described previously for the determination of DS2, using multipliers 132, 134, 191, 192, 193, adder 1 94, difference unit 195 and divider 196, except that desired cake percentage concentration at the end of the cycle CSt, supplied by a selected output of signal generator 119, is used in place of percentage concentration one minute from cycle end CS2. Thus, divider 196 provides an output DS, representative of dry solids weight of the cake at cycle end.

Dry solids weight one minute before cycle end DS2 is subtracted from dry solids weight at cycle end DS by difference unit 140, resulting in a signal representative of the weight of dry solids added during the last minute of flow. This is divided by signal value w, representing weight of dry solids per unit of slurry volume (provided by terminal 138 of Figure 1) by divider 190, resulting in a signal representative of predicted flow rate Qt at the end of the filtration cycle. This signal is fed to and displayed by "end" readout 152 of FLOW section 47 of display 30. "Now" flow readout 46 of display 30 is activated, as previously described with respect to Figure 1, to allow the operator to compare current flow rate with that which should exist at the cycle end.

Figure 6 illustrates circuitry for displaying the value of K from a previous cycle which is stored in memory 156. Values of K are made available to memory 156 from terminal 111 of Figure 2, and a discrete signal K is entered into memory 156 responsive to a selected elapsed time signal tet supplied memory 156 from terminal 106 of Figure 1. Typically, the selected time signal for gating would be one which would be near or at the end of a filtration cycle. Thus, during a given filtration cycle, there would be stored in memory 156 a value of K from the last portion of the previous filtration cycle. Then, at the end of the current cycle, and upon the receipt of a selected elapsed time signal, the K input of memory 156 would again be gated, and a new value of K would be inserted in memory 156 in place of the old value.

In each instance, it would be displayed during the following cycle of filtration. It is designated K1 and is fed to terminal 158 and is provided to and displayed by "was" readout 114, the K section 108 of display 30. As stated above, the K section of the display also includes "is" display readout 110 and "want" readout 112. By the presence of the three readouts, "want", "is", and "was", an operator is quite fully advised as to the relation between current operating conditions, previous operating conditions, and desired operating conditions.

In order to more fully provide an awareness on the part of an operator of operating trends which may assist him in the determination of equipment adjustments, departures from normal operating conditions are indicated by CONDITIONING section 172 of display 30. Accordingly, the circuitry of Figure 7 measures departures of K from a selected range and the direction of departure. The range illustrated (it may differ) is 110% from a Kl value when is obtained from terminal 1 58 of Figure 7, representative of the value of K prevailing at the end of the previous filtration cycle, or some other selected value of K1.

The + 10% reference signal is obtained by multiplier 160, which multiplies K1 by a signal representative of the 1.1 from signal generator 164. The product is then compared in comparator 168 with a current value K from terminal 111 of Figure 2. A --10% reference is obtained by multiplier 162, which multiplies signal K1 by a 0.9 value signal from signal generator 166. The product of 0.9 K1 is fed to one input of comparator 174. A second input to comparator 174 is provided by gate 180, which gates through to comparator 174 a current value of K when gate 1 80 is enabled by an output from comparator 168. Comparators 168 and 174 are identical, and each is connected to operate as follows.

Assuming that the signal applied on the left side terminal, in this case being a signal which is a function of K1, is less than a signal applied at the top terminal, in this case a value of K, there will be an output on the right side terminal. In the event that the Kt signal is greater than the K signal, then there will be an output from the lower terminal.

Accordingly, assuming that the value K applied to comparator 1 68 is greater than 1.1 K1, indicating that the value K represents an increase from a normal range, there is provided an output from comparator 168 to DECREASE readout 176 of the CONDITIONING section 172 of display 30.

Assume next that a signal K is less than 1.1K1, there is an output on the lower terminal of comparator 168 to gate 180, which then gates through signal K to comparator 174. Next, assume that a .9K, signal input to comparator 174 is less than K. This would, of course, mean that, in fact, the value of K is within the range of from .9K, to 1.1 K1, which would be deemed a normal operating range. In accordance with this condition and the logic described, there will be an output from the right side terminal of comparator 1 74, and this is provided (as shown) to the "OK" readout 178 of CONDITIONING section 172 of display 30 to indicate normal operation.If, however, the input to the .9K1 input to comparator 174 is of a greater value than K, as provided through gate 180, this below-normal operating range signal will be evidenced by an output signal on the lower side of comparator 1 74, and this signal will be applied to INCREASE readout 170 of CONDITIONING section 172 of display 30 to, accordingly, signal a lower-than-normal operating condition.

The system which has been described clearly provides for much improved monitoring of filtration processes. As a matter of act, for the first time, operating personnel are actually provided significant data as to the status of a filter on a moment-by-moment basis during the filtration cycle. Further, and beyond this, they are provided forecasts as to the time of termination of a cycle under actual or selected conditions of termination.

From all this, effective control of the filtration process is now realisable.

Claims (20)

1. A pressurised filtration system comprising: a source of fluid material to be filtered; a filter having an inlet and an outlet; transfer means connected to said source of material for transferring said material under pressure from said source to said filter; flow monitoring means responsive to flow through said filter for providing an electrical signal Q, representative of the rate of said flow, and an electrical signal V, representative of the accumulated flow from the time of commencement of a filtration cycle; concentration monitoring means coupled in circuit between said source of material and said filter for providing an electrical signal C5, representative of the concentration of said material; density registration means for effecting an electrical signal D representative of the density of particles of said material; filter volume registering means for effecting a signal Vf, representative of the volume of said filter; first signal processing means responsive to said signals V, Cs, Dp, and V, for providing an output electrical signal CSa, representative of the percentage of cake solids existing in said filter at a discrete point in time during the filtration cycle; pressure sensing means coupled in circuit with fluid flow to the inlet of said filter for providing a signal Pat representative of the pressure of said flow; timing means for generating a signal tet representative of the elapsed time since the commencement of a filtration cycle; second signal processing means comprising; first signal means responsive to said signal t5 for providing an output t2t3, representative of the twothirds power of said time, second signal means responsive to the signal CSa for providing a signal representative of a signal CS52, and third signal means responsive to the said outputs of said first and second signal means and an enable signal for providing a signal representative of the output of said first signal means divided into the output of said second signal means, this last-named signal being a signal K, representative of the fundamental dynamics of the functioning of the filtration cycle of the system; and third signal processing means comprising: fourth signal means for providing an output signal xP,, representative of a selected fraction of selected pressure at termination of a filter cycle, fourth comparator means responsive to the output of said fourth signal means and a current pressure signal P5 for providing e compared output when said pressure signal P5 is at least equal to or greater than the signal xPt, and fifth signal means responsive to the presence of said compared output for providing said third signal means said enable signal: whereby a signal K from said third signal means is provided as an output.

2. A system as set forth in Claim 1 wherein: said third signal processing means comprises memory and signal means responsive to the rate of flow signal Q for providing as a continuing output a signal xQ", representative of a selected fraction of the flow rate at the approximate beginning of a filtration cycle: and said system further comprises: second comparator means responsive to said signal xQj and the signal Q for providing a second compared output when the signal Q is at least equal to the signal xQi, fifth signal means for providing an output signal xpt, r#epresentative of a selected fraction of selected pressure at the termination of a filtration cycle, said forth signal means comprises coincidence means responsive to the presence of both said compared outputs from said comparator means for providing to said third signal means said enable signal.

3. A system as set forth in Claim 2, further comprising: sixth signal means for providing a signal CSt2, representative of the square of the desired percentage of cake solids at the end of a filtration cycle: and seventh signal means responsive to the signal K, and said signal CSt2 from said sixth signal means for providing as an output a time tc, representative of the projected duration of a filtration cycle.

4. A system as set forth in Claim 3 comprising eighth signal means responsive to the output of said seventh signal means and a signal representative of the commencement of said cycle for indicating the termination time tt of said cycle.

5. A system as set forth in Claim 4 further comprising ninth signal means responsive to: the duration time signal tc: the signal K; the signal Dp, the signal representing the density of the liquid phase of the slurry; the signal Vf; the signal CSt; the signal w; for providing an output Qt, representative of the flow rate at the termination of a filtration cycle

6. A system as set forth in Claim 1 further comprising memory means responsive to said third signal means fGr periodically storing the output signal K, and for providing as an output during a given filtration cycle a stored signal K from a previous filtration cycle.

7. A system as set forth in Claim 6 further comprising comparison means responsive to said signal K and said signs K i for Droviding a difference signal output when a present value signal K differs from signal K, by a selected amount.

8. A system as set forth in Claim 7 wherein said comparison means includes means for providing a first discrete difference signal when the said difference is of a positive sign. and a second discrete difference signal when said difference is of a negative sign.

9. A system as set forth in Claim 8 wherein said comparison means includes means for providing a third discrete signal when the s:gna! difference is less than said selected amount

10. A system as set forth h in Claim 1 comprising display means responsive to the output of said third signal means for displaying a said signal K, representative of the fundamental dynamics of the functioning of the filtration cycle of said system.

11. A system as set forth in Claim 4 including means responsive to said eighth signal means for displaying a termination time tt of said cycle.

12. A system as set forth in Claim 5 comprising display means for displaying a signal Qt, representative of the flow rate at the termination of a said cycle.

13. A system as set forth in Claim 9 comprising display means for displaying a first signal indication when said difference signal from said comparison means is of a positive sign; means for displaying a second signal indication when said difference signal is of a negative sign: and means for displaying a third signal indication when said difference signal is less than said selected amount.

14. A system as set forth in Claim 13 wherein said display means includes: means responsive to the output of said third signal means for displaying a said signal K, representative of the fundamental dynamics of the function of the filtration cycle of said system: means responsive to said eighth signal means for displaying a termination time tt of said cycle; and means for displaying a signal Qt, representative of the flow rate at the termination of a said cycle.

15. A method of monitoring operation of a pressurised filtration system comprising: deriving from said system electrical signals representative of the volume V and the rate of flow Q through or into the filter of the system, concentration Cs of the material being filtered, density Dp of the particles of the material and elapsed time t5 from commencement of the filtration cycle; combining said signals V, Q, Cs and Dp to derive a signal CS, representative of the percentage of cake solids then existing in the filter; and sensing the pressure in the system and when said pressure reaches a preselected value computing the quotient CS52/t213 to provide a value K indicative of the performance of the system.

16. A method as claimed in Claim 15 wherein the value K is displayed to provide a visual indication of the performance of the system.

17. A method as claimed in Claim 15 or 16 including computing the formula (CSt5K)3/2.

where CSt is a preselected value of percentage of cake solids, in order to derive a value to representative of the projected duration of the filtration cycle necessary to achieve the value CSt.

18. A method as claimed in Claim 17 wherein the value to is used to provide a visual indication of a projected termination time for the filtration cycle.

19. A pressurised filtration system substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

20. A method of monitoring operation of a pressurised filtration system substantially as hereinbefore described with reference to the accompanying drawings.