in which the subindex Y recites to a variable measured upstream of a pump and subindex Y to a variable measured downstream of a pump. In the above equation p is die pressure, z is the height of die point of the pressure measurement, g is gravitational acceleration, p is the density of die flow medium and w is die flow velocity of the flow medium at die point of the pressure measurement. The differential between pressure head (H) and enthalpy (h) is such diat enthalpy is a sum of pressure head (the first, "p" term in eq. 1), so called geodesic head (the second, "z" term in eq. 1) and velocity head (die diird, "w" term in eq, 1 ) all divided by gravitational acceleration (g)(see also eq. 3).

Next, the efficiency of die pump is calculated from equation

η =Δi p-|₍₂₎

And, die correlation between the change in enthalpy Δh and total differential head H is given by equation H= Δi_

(3)

However, the tiieory concerning the operation and use of centrifugal pumps goes still further. It is known that when the rotational speed of a pump is changed also die numerical values of the variables in the characteristic curves of die pump change. In otiier words, one could plot different pump curves by means of running the pump test with different rotational speeds. However, tiiere is an easier way to find out die effect of the changes in rotational speed in d e pump curves. It is a generally accepted fact that, especially when it is a question of Newtonian liquids, and when die change in rotational speed is not large die so called law of kinematic similarity holds true with substantial accuracy, in other words

Qa~O_b'~ n

ΔΛ_a=Δ⁽ ^ > (4)

n.

P =Pκ^* ( -^³

In die above equations subindex "a" refers to a situation where die rotational speed is n, and subindex "b" to a situation where the rotational speed is n..

The above group of equations (4) makes it possible to determine die volume rate of flow, die specific enthalpy and die power input in a situation where die pump is rotating at anotiier speed tiian where die characteristic curves were determined.

It is also known to determine, or measure, die volume rate of flow when die pressure differential p₂ - p. and/or die power input P is known. In certain cases, i.e. where the terms containing Δz (height differential) and Δw (flow velocity differential) in equation ( 1 ) are identically equal to 0 die total differential head H is given by using equation (3) whereby substituting δh from equation (1) dierein gives

(5)

provided mat the density p of a flow medium is known.

In accordance witii Fig. 2 and using die published characteristic curves of a pump where die head H of a pump and die power input P is given as a function of volume rate of flow Q one may have an estimate for the volume rate of flow Q. If for instance die differential between die squares of die flow velocities

("\v" term in equation (1)) is not 0 one may reach me solution by using an iterative approach. The values marked witii superscript "^*" are measured values for the head in accordance with equation (5). In Fig. 2 there has been illustrated how die erroneous values eitiier in the measurement or in the characteristic curve may lead to two different values as an estimate for die volume rate of flow, Q^' = Q_H^" or Q^'

= Qp'. In this kind of a case, if no other way to solve the problem exists an average value

may be used. It is only up to this point that conventional pump characteristic curves have been used so far. For instance, it has been taken for granted tiiat the density of the medium to be pumped is known. However, when it is a question of an medium consistency pulp changes in botii die consistency and gas content have an effect on the density. However, this problem has been overcome by the invention because of a large amount of empirical data tiiat has been gathered from hundreds of test runs that can accurately and reliably determine tiiese values from other values that are easily and reliably measured. For each pump type and size die manufacturer can easily plot graphs that have been developed from doing hundreds of tests which show the correlation between die pressure differential and volume rate of flow, and die power input and volume rate of flow, for a number of different consistencies (see Figs. 4a and 4b) and for a number of different gas contents (see Figs. 6a and 6b). In die utilization of conventional pumps for pumping medium consistency pulp, such as fluidizing centrifugal pumps sold under die trademark MC^® by Ahlstrom Pumps, Inc. of Easley, South Carolina, U.S.A. and Ahlsϋrom Pumps Corporation of Karhula, Finland, it is possible to readily measure die power input to d e pump (power consumption) and also die pressure differential over (pressure head of) die pump. The power input may be determined in several different ways. A simple way is to monitor the power the drive unit takes from the electrical network and since die efficiency of die drive unit is known calculate the power taken by the pump itself. Another way is to mount a torque sensor on die shaft and monitor botii the torque and die rotational speed of die shaft. In otiier words, die power input may be determined in a manner known per se. Utilizing this information collected in a manner described above in, for instance, a computer, and using standard formulas based upon this empirical data, it is possible to easily automatically calculate one or both of the volume rate of flow and the consistency as well as die density.

According to one aspect of die present invention a metiiod of determining physical variables of a slurry or liquid in industrial applications using a radial flow or axial flow pump, comprises d e steps of: (1) performing test runs with said pump for receiving information on die operation of die pump as a function of at least one of die density of die slurry or liquid, die solids consistency of the slurry or liquid and die gas content of die slurry or liquid, said information including at least two of the power input P, die volume rate of flow Q of die slurry or liquid, die rotational speed n of die pump, the pressure head H of die pump, and die efficiency η of die pump,

(2) processing said received information witii die data fed into the test system i.e. witii at least one of die density of the slurry or liquid, die consistency of die slurry or liquid and die gas content of die slurry or liquid; (3) providing a mill scale installation using a similar pump as the pump in step 1) with said processed information from step 2);

(a) supplying power to die pump to pump die slurry or liquid tiirough the pump in a padiway in which there is a pressure differential between first and second points in the padiway; (b) measuring die power supplied to die pump;

(c) measuring die pressure differential between die first and second points in die padiway;

(d) inputting said power and said pressure differential into a computer; and (e) using said computer, automatically calculating one or several physical variables using at least one of the measured values from steps (b) and (c).

The physical value tiiat was calculated in sub-step (e) may be at least one of the volume rate fo flow, die consistency, die density, and die gas content of the slurry or liquid. Preferably the slurry is medium consistency (i.e. between about 6-18%) pulp. Sub-step (c) may be practiced by measuring die pressure difference on opposite sides of the pump, die first and second points being in die padiway on opposite sides of die pump. Sub-step (e) may be practiced to calculate botii die volume rate of flow and die consistency of die pulp at an actual mill scale application. In step (1), preferably at least two of the power input P, d e pressure head H, and die efficiency η of die pump are received as a function of die volume rate of flow. When d e liquid or slurry is a slurry, in step (7) the information may be processed so tiiat the consistency is given as a function of both pressure head and volume rate of flow c= c(H,Q), and power input and volume rate of flow c = c(P,Q); die processed information whan illustrated in graphical form may be given as a set uf curves, a curve for each consistency value at desired intervals in both a QH graph and a QP graph. In step (2) die information may be also or alternatively processed so tiiat the gas content is given as a function of both pressure head and volume rate of flow k= k(H,Q), and power input and volume rate of flow k = k(P,Q); and die processed information when illustrated in graphical form may be given as a set af curves, a curve for each gas content value at desired intervals in both a QH graph and QP graph.

In step (2) die information fed into die test system of step (1) as a constant value i.e. at least one of the density of die pumpable slurry or liquid, die consistency of the pumpable slurry or liquid and the gas content of die pumpable slurry or liquid and as a function of which die test runs are performed and die information received as results of the test runs i.e. die measured values for at least some of the power input of die pump, die volume rate of flow of die slurry or liquid, die efficiency of die pump, die rotational speed of the pump, and die pressure head are processed in such a manner that die processed information is readily usable in mill scale applications. The processing may mean, e.g. forming of sets of curves e.g. characteristic curves for each pump type and size showing the needed correlations between different variables. The physical variables may tiien be calculated by a certain type of software. The processing may also mean forming of a mathematical model for die pumping whereby anotiier type of software may be used for calculating the physical variables.

Sub-step (d) may be further practiced by inputting at least two of entiialpy, power input, and efficiency of die pump into the computer. Alternatively or in addition sub step (a) may be further practiced by inputting into die computer each of the constant consistency curves c = c(Q,H) received from die test runs of step (1) as a function of volume rate of flow and pressure head. Sub- step (d) may be alternatively or further practiced by inputting each of die constant consistency curves c = c(Q,h) received from die test runs of step (1) as a function at volume rate of flow and entiialpy into die computer. Sub-step(d) may be alternatively or additionally further practiced by creating a matiiematical function for each consistency curve. Sub-step (d) may be alternatively or additionally be practiced by inputting each of the constant gas content curves k = k(Q,H) received from die test runs of step (1) as a function of volume rate of flow and pressure head into die computer. Sub-step (d) may be alternatively or additionally further practiced by inputting each of the constant gas content curves k - k(Q,h) received from die test runs of step (1) as a function of volume rate of flow and entiialpy into the computer. Sub-step (d) may be further practiced by creating a mathematical formula for each gas content curve.

The calculations for sub-step (e) may be used to control operation of the pump, to reconfigure it or surrounding equipment, to change (automatically or manually) one or more physical parameters of the liquid or slurry, or to perform a number of other functions.

According to one aspect of die present invention an apparatus for determining at least one physical variable of a slurry or liquid comprises a pump housing with an inlet channel forming part of a so called inlet piping, an outlet channel forming part of a so called outiet piping and an impeller arranged witiiin said housing and attached to a shaft rotatably connected to drive means. Said pump inlet piping and said pump oudet piping are provided witii means for determining a pressure differential between said pipings. Said drive means are provided witii means for determining the power supplied to die pump to effect pumping of the slurry or liquid tiiereby. A computer is connected to the pressure differential and power determining means to use information supplied tiiereby to calculate said at least one physical variable of a slurry or liquid flow.

Said pressure differential determining means may be connected to two points in the flow pathway of which points one is disposed in one of the inlet channel and in die outiet channel. The pressure difference determining means may alternatively be connected to two points, of which one is disposed in die inlet channel and die other in die outiet channel. The pressure difference determining means may be preferably connected to two pressure sensors of which at least one is disposed in one of the inlet channel and the outlet channel. The pump is preferably a non- positive displacement pump, such as a centrifugal pump (e.g. a fluidizing centrifugal pump, used for pumping medium consistency pulp).

According to another aspect of the present invention an apparatus for determining physical variables of a slurry or liquid comprises a non-positive displacement pump housing with an inlet channel forming part of a so called inlet piping, an outlet channel forming part of a so called outiet piping and an impeller arranged witiiin said housing and attached to a shaft. Said apparatus comprises further means for determining a pressure differential between said pump inlet channel and said pump outiet channel. A drive may be an electric motor and include means for determining the power used for pumping the pulp. A computer may be connected to the pressure difference determining means and to die power determining means to use information supplied thereby to calculate tiie physical variables.

The invention also relates to a method of operating a non-positive displacement pump pumping a liquid or slurry, comprising the steps of using the non-positive displacement pump as a sensor for determining die density, solids consistency, volume rate of flow, and/ or gas content of the liquid or slurry being pumped.

It is the primary object of the present invention to provide for optimum operation or utilization of a pump, and an apparatus which also allows optimization of the pump operation. This and other objects of the invention will become clear from an inspection of the detailed description of die invention and from me appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic graphical representation illustrating the correlation between the pressure head, power input, the volume rate of flow and d e efficiency for a centrifugal pump;

Figure 2 is a schematic graphical representation for the same pump as in Figure 1 showing the way how volume rate of flow is determined when die power input and die pressure head is known;

Figure 3 is a schematic representation of a preferred novel way of defining die volume rate of flow of a centrifugal pump using the information received from trial runs of a cenϋrifugal pump;

Figures 4a and 4b are schematic graphical representations for the same pump as in Figure 1 showing the correlation between the pulp consistency, the power input, the pressure head and d e volume rate of flow; and Figures 5a and 5b are schematic graphical representations showing a novel way of determining pulp consistency when die pressure head and die power input are known,

Figures 6a and 6b are schematic graphical representations for the same pump as in Figure 1 showing the correlation between the gas content of the pulp, die pressure head, die power input and the volume rate of flow; and

Figure 7 is a schematic representation of exemplary apparatus according to die invention used in die practice of the exemplary metiiod according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS As it has been explained above in connection witii the prior art the density of d e flow medium has to be eitiier known or has to be measured in order to be able to use die prior art methods. However, when die density of die slurry or liquid to be pumped may change and when it has not been measured one could proceed as follows. There are two unknown variables, volume rate of flow (Q) and density (p). Therefore both p₂ - P) and power input P have to be measured. After having measured die botii variables the problem can be solved and both density and volume rate of flow be determined as soon as tiiere are two independent equations. The equations which could be used are equation (5), the simplified equation for the head of a pump and equation (2) defining die efficiency of die pump. Applying first definition (3) to equation (5) and then substituting tiie results to equation (2) gives

P _ _?₂ - ι η (6)

In accordance with Fig. 1 die efficiency η is a function of volume rate of flow Q, ~ = n(Q)- Equation (6) is diagrammatically illusu-ated in Fig. 3. The ratio Q/η(Q) may be determined in connection witii, for instance, die test run of a pump. The measured values P", p₂" and p,^* give in accordance witii Fig. 3 a value Q' for volume rate of flow. An estimate for the density is received by means of applying die curve h for die head (H) of die pump (corresponding to specific entiialpy) of Fig. 1. When the volume rate of flow is Q'

thereby solving the equation gives

In other words, measuring botii die pressure differential and the power input and using the above described metiiod both die volume rate of flow and die density can be solved for die slurry or liquid. The result is that the user of a pump has at his/her disposal a combined sensor for both volume rate of flow and density.

The conventional pressure sensors (e.g. diaphragm, pisto, piezoelecuic, or other conventional type) for determining die pressure differential are preferably positioned in die pump housing. Though it may seem an obvious choice to position die pressure sensors in the pipe line upstream (die inlet conduit) and downstream (the outiet conduit) of the pump tiiat should not be done. With such an approach one would lose die accuracy of the measurements as the measured values would hardly correspond to tiiose received in pump manufacturer's own test runs whereby die pump manufacturer^* s data which could otiierwise be used, in accordance witii our invention, loses it accuracy. Merely the change in positioning of the sensors would lead to remarkable errors in the final results. The inlet and outlet conduits in mill applications are hardly ever identical to those of die pump manufacturer's test stations. Naturally, if the sensors are, for some reason, to be positioned outside die pump itself it is highly desirable to design die inlet and outiet conduit as exactly as possible in accordance witii the pump manufacturer's instructions. Another fact speaking in favour of the positioning of the sensors in the pump housing is die possibility to simplify die overall structure of die pipe line in die nearhood of die pump. For instance, it would be possible to fasten a valve directly to d e pump outiet witiiout a short piece of pipe with the pressure sensor arranged between die pump outlet and the valve.

There are other variables having an effect on pumping process other tiian the density of die flow slurry or liquid. The most important characterising variable in MC^® pumping is the consistency of the pulp. Consistency, or solids consistency, being understood as the relative amount of solids (particularly fibers) in the fiber-water mixture (slurry). For the sake of simplicity, the easiest way to study die situattion is by leaving other variables (like density and gas content) out of die evaluation. Figs. 4a and 4b illustrate the correlation of the consistency (c) to the characteristic curves of an MC^® pump. The curves have been plotted from tests where, in each test, the consistency was maintained constant and, for instance, die volume rate of flow was changed and botii die power input and die pressure head was measured. In this way, the desired accuracy of die measuments defined the number of test series driven. For instance, if the desired consistency interval is 0.5 % and die consistency range from 6 to 16 % tiiere are 21 test series to run to have 21 consistency curves in both QH- and QP-graphs. Between each series of tests the consistency of die pulp was changed by die desired interval (e.g. 0.5%). The subindexes Y, Y anti '3^' in Figs. 4a and 4b show die direction of die increase in consistency - the higher die subindex die higher is die consistency. For example, for a relatively low value of medium consistency, such as about 7%, die graph c, indicates die relationship, while for a relatively higher consistency, e.g. about 14%, the graph c₄ indicates the relationship, with a large number of graphs Cι - c„ providing empirical data for a given pump, type and size input into a computer. In other words, each of the curves in Figs. 4a H = H(Q,c) and 4b P = P(Q,c) have been plotted at constant consistency. These curves may be fed into a computer's memory using conventional techniques and it is a simple and common practice for a computer programmer to develop a program tiiat solves the desired values.

For determining die consistency die user of a pump is able to make the. following two measurements, again, p₂^* - pi" (corresponding to pressure head H) and P^*. The graphic solution of the problem i.e. for determining die consistency with die help of the volume rate of flow Q' is illusu"ated in Figs. 5a and 5b.

Positioning the pressure differential i.e. the pressure head H" in Fig. 5a gives a first horizontal line, parallel to x-axis, and positioning the power input P^* in Fig. 5b gives a second horizontal line. The consistency in question is found when a ruler is placed in a vertical position on Figs. 5a and 5b and moving it across die consistency curves until the H^* line and P^* line intersect the ruler on consistency curves c_H = c_P giving the desired consistency c\ The solution can also be calculated numerically as follows. First functions H and P are created, for instance by feeding d e values of the curves like the one shown in Figs. 5a and 5b into a computer program which is programmed to find die equations of the type shown in (9) giving the inputted values

H-H {Q, c)

(9)

P^~P (Q, c)

For the sake of simplicity, the most simple feasible functioning model could be of linear form as follows

H=a₀+a₁-Q+a₂-c

(10)

And, substituting H = H^* and P = P" gives

b₂- (H* -a₀) -a₂^■ ( P* -b_Q )

<?'- a_xb₂ a₂b

(I D a_χ- ( P_* -b₀) -b_χ- (H* -a₀)^ai *2 ~^a2 '^Λl

However, since the shape of the curves in Figs. 4a and 4b is typically non-linear it should be understood tiiat die linear model is too simple. However, die above shows die basic principle of determining die consistency c.

in any case die above described metiiod of determining volume rate of flow and consistency is based on die pump tests made by the manufacturer. In other words, the present invention relates to taking full advantage of die possibilities die pump manufacturer has since die manufacturer tests their pumps with regard to all die needed variables. Now, die present invention makes it possible for the user of the pump to define a number of physical variables just by using the pump itself as a sensor for those variables, for example for volume rate of flow and/or consistency. The above metiiod can be used in all kinds of centrifugal pumps i.e. both pumps for MC^® pulp, low consistency pulp and water. Also die metiiod may be applied for determining die gas content, too. In such a case some additional measurements relating to the gas discharge or to die size of die gas bubble at the eye of the pump impeller are needed.

Figs. 6a and 6b illustrate the correlation of the gas content k to the pressure head H and die power input. The curves have been plotted from tests where, in each test, the gas content value was maintained constant and, for instance, die volume rate of flow was changed and botii the power input and d e pressure head was measured. In this way, the desired accuracy of the measurements defined the number of test series driven. For instance, if the desired gas content interval is 0.5 % and die gas content range from 5 to 20 % tiiere are 31 test series to run to have 31 gas content curves in both QH- and QP-graphs. Between each series of tests die gas content of the pulp was changed by die desired interval, naturally. The subindexes Y,Y and Y in Figs. 6a and 6b show die direction of the increase in gas content the higher the subindex die higher is the gas content. In other words, each of die curves k,, k₂ and k₃ in Figs. 6a H = H(Q,k) and 6b P -

P(Q,k) have been plotted at constant gas content. The gas content can be defined in a manner similar to the consistency determination.

Figure 7 shows a flow pathway 10, such as, for example only, a standard pulp conduit having an inlet conduit 12 and an outiet conduit 14, between which a pump 16 is provided, preferably a fluidizing MC^® pump. The pump 16 has a housing with an inlet channel 18 and an outiet channel 20. The inlet channel 18 of the pump being attached to said inlet conduit 12 forming together so called inlet piping. In a corresponding manner the outlet channel 20 of the pump is attached to die outiet conduit 14 forming together a so called outiet piping. The impeller of the pump 16 is disposed within said housing, attached to a shaft 22 and driven by a conventional drive, such as an electric motor 24.

The power supplied to the motor 24 to effect rotation of the impeller may be measured utilizing a conventional power measuring device 26. Naturally it is also possible to manually read the power input, process it further, basically manually, and then feed die information manually further e.g. into a conventional computer ot any suitable type. In the padiway 10 diere is a first point 28 in said inlet piping 12,18 and a second point 30 m said outlet piping 14,20 i.e. on opposite sides ot the pump 16 with a conventional pressure differential measuring device 32 disposed m line 34 between die points 28, 30.

There are several options tor determining the pressure differential. I.e. diere may be two pressure sensors one positioned to point 28 and the other in point 30 whereafter the differential is determined by a processor unit (for instance 32 could be such a unit) or d e computer 36 or die differential could even be calculated manually (e.g. using a hand held calculator, slide rule, or pen and paper). There may also be a unit 32 determining die pressure differential directly. In such a case die unit 32 could be connected with two pipes to points 28 and 30 and sense die pressure differential. The data from die measuring devices 26, 32 is fed, eitiier automatically or manually, to a conventional computer 36. Because of the empirical data known from the above discussed figures 1 through 6b which has been input into the computer 36, tor each different pump type and size, and because die pump type and size is also input into the computer 36, it is possible to automatically calculate one or both of die volume rate of flow and d e solids consistency, or die gas content, or die density, because die variables can be solved in such a manner that there are two equations and two variables, for instance, volume rate of flow (Q) and consistency c (m percent) to be solved.

The means for determining pressure differential may be any suitable conventional device, typically of fluidic, mechanical, electro- mechanical, piezoelectric, or substantially solely electrical, construction, which is capable of measuring pressure differential. The means 26 for determining die power supplied to the pump 16 to effect pumping of the liquid or slurry may also be of any suitable conventional construction.

The most practical places for the pressure sensing means are believed to be die inlet channel 18 and die outiet channel 20, primarily due to die facts discussed earlier in this description.

It should be, again, kept in mind tiiat the above specification is to be understood as an example only, at least witii regard to die discussion concerning centrifugal, or even closer MC^® pumps, since the same principle of operation may be used witii all pumps except the so called positive displacement pumps. A way to describe the pumps included in this category is to talk about non-positive displacement pumps which would tiien include at least so called radial flow pumps i.e. centrifugal pumps and helico-centrifugal pumps; and axial flow pumps. It should also be understood that the above specification as well as the appended claims describe die invention in a simplified manner referring more to a way how the invention would be used manually, or graphically. Therefore, it is clear tiiat all ways of utilising the invention by using a computer and special software are witiiin die scope of the invention.

Claims

1. A method of determining physical variables of a pumpable slurry or liquid in industrial applications using a radial flow or axial flow pump, characterized by the steps of:

(1) performing test runs with said pump for receiving information on the operation of the pump as a function of at least one of the density of die pumpable slurry or liquid, die consistency of the pumpable slurry or liquid and die gas content of the pumpable slurry or liquid, said information including at least two of die power input P, the volume rate of flow Q of tiie pumpable slurry or liquid, the rotational speed n of die pump, the pressure head H of die pump, and die efficiency η of die pump,

(2) processing said received information with the data fed into the test system i.e. with at least one of the density of die pumpable slurry or liquid, die consistency of die pumpable slurry or liquid and die gas content of the pumpable slurry or liquid;

(3) providing a mill scale installation using a simtiar pump as in step (1) with said processed information from step (2);

(a) supplying power to the pump to pump the pumpable slurry or liquid through the pump in a pathway in which there is a pressure differential between first and second points in the pathway;

(b) measuring the power supplied to the pump in sub-step (a);

(c) measuring die pressure differential between the first and second points in the pathway; (d) inputting said power and said pressure differential into a computer; and

(e) using said computer, automatically calculating one or several physical variables using at least one of the measured values from steps (b) and (c).

2. A metiiod as recited in claim 1. characterized in that said physical variables calculated in sub-step (e) are at least one of the volume rate of flow, the consistency, the density and the gas content of the slurry or liquid.

3. A metiiod as recited in claim 1 characterized in that the pumpable slurry or liquid is cellulose pulp, preferably so called MC^® pulp, i.e. a slurry having a solids content or a consistency between about 6 - 18% .

4. A metiiod as recited in claim 1 , 2 or 3 characterized in that, in step (1), said at least two of the power input P, the pressure head H and die efficiency η of die pump are received as a function of the volume rate of flow.

5. A method as recited in claim 1, 2, 3 or 4 characterized in tiiat, in step (2) the information is processed so tiiat said consistency is given as a function

• of both pressure head and volume rate of flow c = c(H,Q), and power input and volume rate of flow c = c(P,Q).

6. A metiiod as recited in claim 5 characterized in that said processed information, when illustrated in graphical form, is given as a set of curves, a curve for each consistency value at desired intervals botii in the QH-graph and in die QP- graph.

7. A method as recited in claim 1 , 2, 3 or 4 characterized in that, in step (2) die information is processed so that said gas content is given as a function of both pressure head and volume rate of flow k = k(H,Q), and power input and volume rate of flow k = k(P,Q).

8. A method as recited in claim 7 characterized in that said processed information, when illustrated in graphical form, is given as a set of curves a curve for each gas content value at desired intervals botii in the QH-graph and in die QP- graph.

9. A method as recited in claim 3 characterized in that sub-step (e) is practised to calculate both the volume rate of flow and die consistency of the pulp in actual mill scale application.

10. A method as recited in claim 3 characterized in that sub-step (c) is practised by measuring the pressure differential on opposite sides of die pump, the first and second points being in the pathway on opposite sides of die pump.

11. A metiiod as recited in claim 1 characterized in tiiat sub-step (d) is further practiced by inputting at least two of enthalpy, power input, and efficiency of said pump into the computer.

12. A method as recited in claim 1 characterized in tiiat sub-step (d) is further practiced by inputting each of the constant consistency curves c = c(Q,H) received from manufacturer's test runs as a function of volume rate of flow and pressure head into the computer.

13. A method as recited in claim 1 characterized in tiiat sub-step (d) is further practiced by inputting each of the constant consistency curves c - c(Q,h) received from manufacturer's test runs as a function of volume rate of flow and entiialpy into the computer.

14. A metiiod as recited in claim 12 or 13 characterized in that sub-step (d) is further practiced by creating a mathematical function for each consistency curve.

15. A metiiod as recited in claim 1 characterized in that sub-step (d) is further practiced by inputting each of the constant gas content curves k = k(Q,H) received from the test runs of step (1) as a function of volume rate of flow and pressure head into die computer.

16. A method as recited in claim 1 characterized in that sub-step (d) is furher practiced by inputting each of the constant gas content curves k = k(Q,h) received from the test runs of step (1) as a function of volume rate of flow and entiialpy into the computer.

17. A method as recited in claim 15 or 16 characterized in that sub-step

(d) is further ptactices by creating a mathematical function for each gas content curve.

18. An apparatus for determining physical variables of a slurry or liquid flow comprising: a pump (16) housing with an inlet channel (18) forming part of an inlet piping (12, 18), an outlet channel (20) forming part of an outiet piping (20, 14) and an impeller disposed within said housing and attached on a shaft (22) rotatably driven by drive means (24), characterized in that said pump inlet piping (12, 18) and said pump outlet piping (20, 14) is provided with means (32) for determining a pressure differential between said pipings, said drive means (24) being provided with means (26) for determining die power supplied to the pump (16) to effect pumping of tiie pumpable slurry or liquid thereby; and a computer (36) connected to die pressure differential (32) and power (26) determining means to use information supplied thereby to calculate at least one physical variable of a liquid or slurry flow.

19. An apparatus as recited in claim 18 characterized in tiiat said physical variable is at least one of the volume rate of flow of the pumpable slurry or liquid, the density of die pumpable slurry or liquid, the gas content of said pumpable slurry or liquid and die consistency of die slurry or liquid flowing through the pathway.

20. Apparatus as recited in claim 18 characterized in that said pressure differential determining means (32, 36) is connected to two pressure sensors of which one is disposed in one of said inlet channel (18) and said outlet channel (20).

21. Apparatus as recited in claim 18 characterized in that said pressure differential determining means (32) is connected by means of two pipes to points of which one is disposed in one of said inlet channel (18) and said outlet channel (20).

22. Apparatus as recited in claim 18 characterized in that said pressure differential determining means (32) is connected to two pressure sensors which are disposed in said inlet channel (18) and in said outiet channel (20).

23. Apparatus as recited in claim 18 characterized in tiiat said pressure differential determining means (32) is connected by means of two pipes to points of which one is disposed in said inlet channel (18) and die other in said outlet channel (20).

24. Apparatus as recited in claim 18 characterized in that said pump (16) is a non-positive displacement pump.

25. Apparatus as recited in claim 18 characterized in that said pump (16) is a centrifugal pump.

26. Apparatus as recited in claim 24 characterized in tiiat said centrifugal pump (16) is a medium consistency pump for pumping medium consistency pulp.

27. An apparatus for determining physical variables of a slurry or liquid said apparatus having a non-positive displacement pump (16) housing witii an inlet channel (18) forming part of a so called inlet piping (12,18), an outiet channel (20) forming part of a so called outiet piping (20, 14) and an impeller arranged within said housing and attached to a shaft (22), characterized in thet said apparatus comprises means (32) for determining a pressure differential between said pump inlet channel (18) and said pump outiet channel (20).

28. Apparatus as recited in claim 27 characterized in that said pressure differential determining means (32, 36) is connected to two pressure sensors of which one is disposed in said inlet channel (18) and the other in said outiet channel (20).

29. Apparatus as recited in claim 27 characterized in that said pressure differential determining means (32) is connected by means of two pipes to points of which one is disposed in said inlet channel (18) and die other in said outiet channel (20).

30. Apparatus as recited in claim 27 characterized in that said pump (16) is a centrifugal pump.

31. Apparatus as recited in claim 30 characterized in that said centrifugal pump (16) is a medium consistency pump for pumping medium consistency pulp.

32. An apparatus as recited in claim 27 characterized in drive means (24) connected to said shaft (22), said drive means (24) being provided witii means (26) for determining the power supplied to the pump (16) to effect pumping of the pumpable slurry or liquid thereby.

33. An apparatus as recited in claim 27 characterized in a computer (36) connected to the pressure differential determining means (32) and power determining means (26) to use information supplied thereby to calculate said physical variables.

34. An apparatus as recited in claim 27 characterized in a computer used for calculating said physical variables from die information received from the pressure differential determining means (32) and power determining means (26).

35. The use of a non-positive displacement pump as a sensor for determining die density of the pumpable slurry or liquid to be pumped.

36. The use of a non-positive displacement pump as a sensor for determining die consistency of the pumpable slurry or liquid to be pumped.

37. The use of a non-positive displacement pump as a sensor for determining the volume rate of flow of the pumpable slurry or liquid to be pumped.

38. The use of a non-positive displacement pump as a sensor for determining the gas content of the pumpable slurry or liquid to be pumped.