WO2000058802A1

WO2000058802A1 - Fluid flow control method and apparatus for filtration system

Info

Publication number: WO2000058802A1
Application number: PCT/IB2000/000272
Authority: WO
Inventors: Paul Fritz Fuls; André Keith JOUBERT; Diederik Arnoldis Kapp
Original assignee: Technology Finance Corporation (Proprietary) Limited
Priority date: 1999-03-25
Filing date: 2000-03-14
Publication date: 2000-10-05
Also published as: JP2002540519A; AU2934700A; EP1088260A1

Abstract

A method and controller (10) are provided for controlling a fluid flow rate along a line (21). Fluid is fed along the line to a control valve (20) having a controllably variable aperture. Pressure is sensed by pressure sensing means (14) in the line upstream of the valve. Instantaneous flow rate of the fluid is determined based on a knowledge of the sensed pressure, of the valve aperture setting, of the flow characteristics of the fluid and of the fluid pressure downstream of the valve. The aperture setting is varied to match the flow rate with a required flow rate. This is done by control means (16) responsive to the sensed fluid pressure and to the aperture setting.

Description

FLUID FLOW CONTROL METHOD AND APPARATUS FOR FILTRAΗON SYSTEM

THIS INVENTION relates to controlling the flow rate of a fluid. In particular it relates to a method of, and a controller and valve assembly for, controlling the flow rate of a fluid, eg in a filtration system.

A filtration system may be controlled so as to meet a certain range of recovery ratios with a given system input. A recovery ratio is defined as the ratio of a quantity of feed which is filtered out by a filter, known as the filtrate, to the total feed to the filter. The quantity of feed which is not filtered out by the filter is known as the retentate. The recovery ratio is usually expressed as a percentage, eg 80% recovery means that there is 80% filtrate and 20% retentate after filtration. Thus an 80% recovery ratio will take place when the retentate flow rate is 20%, and the filtrate flow rate is 80%, of the feed rate into the system. During use, the filter becomes clogged and it is necessary to control the recovery ratio by controlling the exit rate of the retentate from the filter.

Prior filtration systems of which the applicant is aware have made use of expensive flow meters in order to control the recovery ratio. It is an object of the invention to offer a solution to this problem which avoids the necessity of a flow meter.

In accordance with the invention a method of controlling the flow rate of a fluid along a flow line includes the steps of: feeding the fluid along the fluid flow line to a control valve having a controllably variable aperture; sensing the pressure in the fluid upstream of the control valve; determining the instantaneous flow rate of the fluid along the fluid flow line based on a knowledge of the sensed pressure, of the aperture setting of the valve, of the flow characteristics of the fluid and of the pressure in the fluid downstream of the valve; and controllably varying the aperture setting of the control valve to match the flow rate of the fluid along the fluid flow line with a required flow rate.

In the various embodiments of the invention described hereunder, various combinations of features are described or defined as simultaneously forming part of the invention. It is explicitly noted, however, that it is not essential that the features of these combinations be used simultaneously, and the invention contemplates that any one or more of such features can be omitted or altered, while one or more of the remainder are retained. In particular, any feature may be omitted, without enlarging the invention or extending its scope, provided that the features defined hereinabove are present.

The method may act to control the recovery ratio of a filtration system for filtering a fluid, the system having a filter to which a fluid feed is supplied, filtrate passing through the filter and retentate bypassing the filter, the method including the step of supplying the fluid feed to the filter at a known feed rate, the retentate being fed from the filter along the fluid flow line to the control valve and the sensing of the pressure in the fluid being at a position intermediate the filter and the valve, the controllable varying of the aperture acting to match the flow rate along the fluid flow line with a required retentate flow rate which gives a required recovery ratio. Usually, the pressure in the fluid downstream of the control valve is atmospheric pressure, so that it is sufficient merely to sense gauge pressure, ie the pressure above atmospheric pressure, upstream of the control valve. However, when the fluid pressure at the vale outlet varies materially from atmospheric pressure, it may be necessary to sense fluid pressure both upstream and downstream of the control valve, to establish the pressure drop across the valve, the determining of the instantaneous flow rate being based on a knowledge of this pressure drop instead of merely the fluid pressure upstream of the valve.

When the pressure is sensed at a position intermediate the filter and the valve to which the fluid is fed, the pressure will be sensed upstream of the valve.

When the density and/or viscosity of the fluid is materially affected by temperature, the method may include the further step of sensing the temperature of the fluid and controllably varying the aperture of the control valve in accordance with both the pressure and the temperature of the fluid. Accordingly, the method may include sensing of the temperature of the fluid, the determining of the instantaneous flow rate being based on a knowledge also of the sensed temperature and of its effect on the flow characteristics of the fluid.

The instantaneous flow rate may be estimated by means of a predetermined algorithm derived from experimental data providing the fluid flow rate as a function of pressure and density for that fluid, and derived from the setting of the aperture of the control valve. Instead the instantaneous flow rate may be estimated from a look-up table for a particular valve aperture setting. In other words, the instantaneous flow rate may be determined by calculation using an algorithm derived from experimental data and giving the instantaneous flow rate as a function of the pressure drop across the valve, of the temperature of the fluid and of the aperture setting of the valve; or, instead, the instantaneous flow rate may be determined from a look-up table derived from experimental data and giving the instantaneous flow rate as a function of the pressure drop across the valve, of the temperature of the fluid and of the aperture setting of the valve.

The method may be iteratively executed until the required flow rate is obtained . More particularly, the controllable varying of the aperture may be iteratively executed until the required flow rate is matched by the flow of fluid along the flow line.

Further according to the invention there is provided a controller for controlling the flow rate of a fluid along a fluid flow line, the controller including: a control valve having an aperture setting which is controllably variable; pressure sensing means located in the fluid flow line upstream of the control valve for sensing the pressure of fluid in the flow line; and control means responsive simultaneously to a fluid pressure sensed by the pressure sensing means and responsive to the aperture setting of the control valve, the control means being operable to determine the instantaneous flow rate of the fluid through the valve based on a knowledge of the flow characteristics of the fluid, and the control means being operable to control the aperture setting of the valve to match the flow rate with a required fluid flow rate through the valve. In other words, the controller may include: a control valve having an aperture which is controllably variable; pressure sensing means located in the flow line upstream of the control valve; and control means responsive to the pressure sensing means and responsive to the aperture setting of the valve at that time, and operable to determine the instantaneous flow rate of the fluid through the valve from known characteristics of the fluid, and operable to vary the aperture setting of the valve, thereby to control the fluid flow rate through the valve.

The controller may further include temperature sensing means operable to sense the fluid temperature of the known fluid, thereby to determine the density and/or viscosity of the fluid. Thus, the controller may include temperature sensing means for sensing the temperature of the fluid in the flow line, the control means being responsive also to the fluid temperature sensed by the temperature sensing means.

The control means may be an analog control means. Instead the control means may be a digital control means. Typically the control means is a PID control means. The control means may include a data interface operable to receive input data from, and to output data to, peripheral equipment such as a hand-held programmer, a personal computer, a mainframe computer, a data logger, or the like. The data interface may be in the form of an RS232 interface, of the like. In particular, the control means may be a PID control means which includes a data interface operable to receive input data to which it is responsive and to output data to the control valve. The control means may be operable to control a plurality of control valves and, accordingly, to control the flow rates of fluid along a corresponding number of fluid flow lines. More particularly, the controller may include a plurality of said control valves, each control valve having its own pressure sensing means individually associated therewith, the control means being operable to control the aperture settings of all the valves simultaneously.

Preferably, the control means is flow/pressure control means which may be configured to receive analog inputs, strain gauge inputs, digital inputs, or the like. Typically, the control means provides analog and digital outputs to peripheral electronic devices. The or each control valve may be a needle valve, in which case the control means may provide automatic datum settings for its needle. In addition to control of the needle valve, the control means may control other filtration system parameters.

Each control valve may be connected to a stepper motor responsive to a control signal from the control means, the valve being connected to aperture setting means for setting the aperture of the valve, the setting means being mechanically coupled to an output shaft of the stepper motor to permit setting of the aperture by the motor; and the stepper motor may be mounted on the valve, the controller including a stepper drive unit mounted on the stepper motor for driving the stepper motor, and the drive unit having a serial interface connected to the control means.

The invention extends also to a valve assembly for use as part of a controller, the valve assembly including: a control valve having a housing provided with an inlet, an outlet and a controllably variable valve aperture; a stepper motor connected to the valve and being responsive to a control signal from control means; and aperture setting means for setting the valve aperture, the setting means being mechanically coupled to an output shaft of the stepper motor to permit selective setting of the aperture by the motor.

In this way the aperture setting means is able to regulate fluid flow through the aperture from the inlet to the outlet in response to a said control signal.

The valve assembly may include sensing means for sensing temperature and/or pressure, remote from the aperture, upstream or downstream of the aperture.

As indicated above, the stepper motor may be mounted on the valve and the valve assembly and controller may include a stepper drive unit for driving the stepper motor; and the stepper drive unit may be mounted on the stepper motor in a modular fashion. Typically, the stepper drive unit has an RS232 serial interface or an RS422 serial interface, or the like, which is connected to the control means. The control means may also be in modular form, thereby to permit selective attachment together of various modules to form the controller.

The control valve may have a fluid passage defined in the housing and extending from the inlet into a chamber arranged at an end of the passage remote from the inlet, its aperture setting means being in the form of a spindle rotatably mounted in the chamber and defining an axially extending passage or bore in communication with the outlet, the spindle having at least one radially extending passage or bore in communication with the axially extending bore, the radially extending bore having a mouth co-operating with the remote end of the passage and defining, with the remote end of the passage, the valve aperture, which is controllably variable by rotation of the spindle to cause variation of the alignment of the mouth with the remote end of the passage.

The housing may have transverse ports therein, leading into the fluid passage defined in the housing and extending from the inlet into the chamber, the transverse ports being for receiving temperature- and pressure sensors.

However, instead, the control valve of the assembly may be a needle valve, the aperture setting means of the needle valve being located within the housing and being in the form of a tapered needle received in an annular seat which needle restricts the aperture, the needle being coupled to an input shaft by a screw drive whereby rotation of the shaft is translated into displacement of the needle in the direction of taper through the aperture of the seat, thereby to regulate the flow of fluid through the aperture.

The invention refers also to a novel control valve being a spindle, or being a needle, as described herein.

Embodiments of the invention are now described, by way of example, with reference to the accompanying diagrammatic drawings.

In the drawings,

Figure 1 shows a schematic block diagram of a controller in accordance with the invention; Figure 2 shows a longitudinal section of a valve used in the controller of Figure 1 ;

Figure 3 graphically shows the relationship between the flow rate along a flow line, controlled by the controller of Figure 1 , and actuator positions of the valve for different pressures in the flow line;

Figure 4 shows a schematic block diagram of a further embodiment of a controller in accordance with the invention;

Figure 5 shows a longitudinal section of a valve assembly including a stepper motor and a needle valve used in the controller of Figure 4; Figure 6 shows a view similar to Figure 5 on a reduced scale of a stepper drive (and power supply) module coupled to the valve assembly of Figure 5;

Figure 7 shows a view similar to Figure 6 of a multiple input/output valve/plant control means module coupled to the stepper drive module shown in Figure 6; and

Figure 8 shows a view similar to Figure 6 of an optimal control module connected to the valve plant control means module depicted in Figure 7.

In Figure 1 reference numeral 10 generally indicates a controller. The controller 10 has temperature sensing means 1 2, pressure sensing means 1 4, control means 1 6 responsive to the temperature sensing means 12 and the pressure sensing means 14, a valve actuator 18 and a variable valve generally indicated by reference numeral 20 responsive to the valve actuator 1 8.

The temperature sensing means 1 2 senses the temperature of a known fluid, eg the retentate emitted from a filtration system, flowing along a flow line 21 from an inlet 21 .1 to an outlet 21 .2. A signal representative of the temperature sensed is fed into the control means 1 6 via a line 24. The sensed temperature is processed by the control means 1 6 to provide an estimation of the fluid density of the known fluid at the sensed temperature. When the fluid density does not vary substantially over the temperature range of operation it may be taken as constant thereby eliminating the need for a temperature sensing means 1 2.

The pressure sensing means 14 senses the pressure of the fluid and a signal representative of the pressure is fed via a line 26 into the control means 1 6.

The control means 1 6 is a microprocessor-based controller deriving its input power via a connector 22. In another embodiment of the invention the control means 1 6 is an analog controller. Typically, the control means 1 6 is a PID controller. The control means 1 6 uses the known fluid characteristics in conjunction with the temperature and the pressure data and the known position of the valve opening or size of the valve aperture to estimate the flow rate along the flow line. Thus the fluid flow rate is determined without the use of a flow rate sensor, as is described in more detail below.

The valve actuator 1 8 is connected to the control means 1 6 via a line 30 and a connector 28 and mechanically to a valve 30 via a line 31 . The control means 1 6 transmits a control signal, appropriate for the estimated size of the valve opening which is required to ensure the desired flow rate, to the valve actuator 1 8. The valve actuator 1 8 mechanically opens or closes the valve 20 in response to the signal received from the control means 1 6. The actuator 1 8 is a stepper motor and thus the position of the valve opening is known at all times. More particularly, an algorithm, which is derived from experimental data of the known fluid passing along the flow line and programmed into the controller 10, provides an estimation of the flow rate along the flow line 21 as a function of pressure, temperature and actuator setting. In another embodiment of the invention the estimation of the flow rate is determined by means of a look-up table.

The control means 1 6 senses the pressure and the temperature of the known fluid along the flow line 21 and, using the known fluid characteristics, as well as the current valve opening, estimates the flow rate along the flow line 21 by means of the algorithm pre-programmed into the controller 10. The estimated flow rate is then compared with a required flow rate which is also pre-programmed into the control means 1 6. A signal proportional to the difference between the required flow rate and the estimated flow rate is transmitted via line 30 to the valve actuator 1 8 thereby to increase or decrease the flow rate depending on whether the required flow rate respectively exceeds or is lower than the estimated flow rate. This procedure is iteratively repeated by the control means 1 6 to ensure that the flow rate along the flow line 21 is commensurate with the required flow rate. Thus the control means 1 6 predicts the size of the opening of the valve 20 in order to achieve a particular preselected or required flow rate.

Figure 3 graphically shows the relationship between the actuator position (shown on the horizontal axis) and the flow rate (shown on the vertical axis) along the flow line 21 for different fluid pressures. The plot is determined from experimental data when the known fluid is passed along the flow line 21 and the flow rate characteristics are measured. An algorithm is then determined to fit the experimental data. The valve is selected to operate along the linear portions of the pressure curves 70 for a particular fluid.

The algorithm has the following form:

P R ex L^Z x p

where R = Actuator setting;

P = Pressure;

Q = Flow rate; and

P = Fluid density.

The valve actuator 18 is conveniently in the form of a conventional stepper motor. The commands transmitted to the stepper motor may thus be recorded and thus the position of the stepper motor is known at all times and, consequently, the size of the valve opening 20 is also known at all times. Thus the valve actuator 18, in response to the control signal, controllably determines the size of the opening of the valve 20.

The control means 16 can interface with peripheral equipment (not shown) such as a hand-held programmer, a personal computer, a mainframe computer, a data logger or the like. The peripheral equipment is interfaced with the control means 16 by means of a connector 32. Thus the connector 32 may have an analog output line, a digital output line, an analog input line, a digital input line and a RS232 line.

Referring now to Figure 2, the valve 20 is shown in greater detail. The valve 20 has a housing 42 having an inlet 44 and an outlet 46. A passage 48 extends from the inlet 44 and terminates at its remote end 50 in a chamber 52. The chamber 52 is circular in cross-section and has a spindle 54 rotatably mounted therein. The spindle 54 has an axially extending passage or bore 56 in flow communication with the outlet 46 and a transverse radially extending passage or bore 58 interconnecting the bore 56 with the passage 48. The transversely extending passage or bore 58 has a mouth which co-operates with the end 50 of the passage 48 so that by rotating the spindle 54 via the stepper motor (not shown) which is drivably connected to a shaft 60, the mouth of the transverse bore 58 can be aligned or misaligned with the end of the passage 50 to a variable degree.

The valve 20 illustrated in Figure 2 also makes provision for the location in the housing 42 of the temperature sensor 1 2 and the pressure sensor 14 described above and which are inserted respectively in ports 62 and 64 of the housing 42.

The spindle 54 is sealingly rotatably mounted in the chamber 52 by means of suitable O-ring seals 66. The shaft 60 has bearings 68 and engages the spindle 54 by means of an engagement formation provided on the end of the shaft 60 which engages with a corresponding slot in the end of the spindle.

In use, rotation of the shaft 60 by means of the stepper motor causes the mouth of the transverse bore 58 to be aligned or misaligned to a varying degree with the end 50 of the passage 48 thereby controlling the flow rate through the valve 20. If desired several transverse bores 58 may be provided in the spindle 54, each of the bores 58 being of different size. One of the bores 58 is then selected for a particular application thereby facilitating handling of a wider range of fluid flow rates and fluids. Instead, the transverse bores 58 may have identical sizes so that if the bore becomes worn, the spindle 54 can be rotated to use another bore.

Referring to Figures 4 to 8 of the drawings, the same reference numerals have been used in Figures 4 to 8 to indicate the same or similar parts as those of Figures 1 and 2, unless otherwise specified.

The controller of Figure 4 is, however, designated 100 and includes a valve assembly which has a needle valve 20 and a stepper motor 101 , a stabilizer tube unit 1 02, a stepper drive (and power supply) module 1 04, a multiple input/output valve/plant control means in the form of a control module 1 6 and an optimal control module 108.

The stabilizer tube unit 1 02 has a housing 1 20 which defines a flow path 1 22 having an inlet 1 26 and an outlet 1 24. The inlet 1 26 is connected to a source of fluid such as the retentate from a filter and the outlet 1 24 is connected in flow communication with an inlet 1 36 of the needle valve 20. Transverse ports 1 28, 1 30 are provided in the housing 1 20 and respectively receive a pressure sensor 14 and a temperature sensor 1 2, as shown in Figure 4. The stabilizer tube unit 102 stabilises the flow of fluid into the needle valve 20, by reducing turbulence and pressure fluctuations therein.

The needle valve 20 has a housing 1 34 defining a passage between the inlet 1 36 and a valve outlet 1 38, as shown in more detail in Figure 5. The needle valve 20 has aperture setting means in the form of an interchangeable tapered needle 1 40 and a valve seat in the form of an orifice disc 1 42 having a central aperture 143 through which a tapered portion 144 of the needle 140 extends. The orifice disc 142, in conjunction with the needle 140, throttles fluid flow from the inlet 1 36, through the interior 1 37 of the housing 1 34, to the outlet 1 38.

The needle 140 is removably attached to a spindle 146 which is drivingly coupled to the stepper motor 1 01 . A precision screw drive 1 48 is provided to translate rotational displacement of an output shaft 1 50 of the stepper motor 101 into axial displacement of the spindle 146 which, in turn, displaces the needle 140 axially through the aperture 1 43 in the orifice disc 142 thereby controlling fluid flow from the inlet 136, through the aperture 143 in the orifice disc 142, to the outlet 1 38. The rotational position of the shaft 1 50 of the stepper motor 1 01 is apparent at all times and, accordingly, the axial position of the needle 140 is also apparent. Thus, the needle valve 20 functions as an absolute positioning control valve.

The spindle 146 is sealingly and rotatably mounted in the housing 1 34 by means of a suitable seal pack 1 52. An O-ring seal 1 54 is provided to facilitate the mounting of the stepper motor 101 on the housing 1 34 of the needle valve 20 in a modular fashion. The housing 1 34 of he needle valve 20 is typically made of stainless steel and the seal pack 1 54 has a plastics housing, for example, a PTFE scraper and triple seal. The output shaft 1 50 of the motor 1 01 is rotationally keyed to the screw of the screw drive 148, which screw is axially movable relative to the shaft 1 50. The needle 140 and orifice disc 142, which is also interchangeable, are selected to suit a chosen rate of fluid flow from the inlet 1 36 to the outlet 1 38. Accordingly, both the needle 1 40 and orifice disc 1423 are selectively interchangeable to allow operation of the needle valve 20 in an optimal flow control position.

If desired, the valve shown in Figures 2 and 4 to 8 can be replaced by a vane-, a butterfly-, a ball-, a diaphragm-, a pinch-, a cylindrical-, a Saunders valve, or the like, provided that the valve opening is determinable at any given time. The relationship between flow rate and valve opening size of the valve for a particular pressure drop across the valve, need not necessarily be linear, provided that the relationship therebetween is known and is recorded, eg in a look-up table, or is capable of being generated by one or more algorithms (see Figure 3).

When used in a filtration system, the valve 20 is connected to the retentate outlet of a filter (not shown) via the stabilizer tube unit 102

(Figure 4) or directly to the filter outlet (Figure 2); and the valve outlet 1 38 (Figure 4) or the outlet 46 of the valve 20 (Figure 2) serves as a waste outlet or feeds into a tank, in each case at atmospheric pressure. The controlled 1 0, 100 controls the flow rate of the retentate and thereby the recovery ratio of the filter.

The stepper motor 1 01 is mounted in modular fashion on the needle valve 20 and connected, via stepper lead plug 1 32, to the stepper drive module 1 04 (Figure 6) . The stepper drive module 104 has a connector 106 connected to a RS232 interface, thereby to permit position control of the stepper motor 1 01 . The stepper drive module 104 is also mechanically coupled to the needle valve 20 thereby controlling the opening and closing of the needle valve 20. As is clearly shown in Figure 6, the stepper drive module 1 04 is mounted in a modular fashion on the stepper motor 1 01 to form a compact unit.

The control module 1 6 and optimal control module 1 08 are both in electrical communication with the stepper drive module 104. The pressure and temperature sensing means 1 4, 1 2 respectively, located in the ports 1 28 and 1 30 of the stabiliser tube unit 102 (see Figure 4) provide temperature and pressure data inputs to the control module 1 6.

The control module 1 6 processes these data and, in conjunction with the preprogrammed information on the fluid, calculates the instantaneous flow rate along the retentate flow line. The valve 20 then has its aperture setting varied in response to this calculation.

The control module 1 6 is connectable, via connectors 1 10, to up to eight pressure sensing means 1 4 and to up to eight temperature sensing means 1 2, or other analog inputs (as shown by broken lines 1 1 2 in Figure 4) . The control module 1 6 has further connectors 1 14 which provide up to eight analog outputs and is also configured to interface sixteen digital inputs as well as sixteen digital outputs (as shown by broken lines 1 1 6 in Figure 4) . The digital outputs/inputs 1 1 6 can be used to drive several stepper drive units 104 which, in turn, are mechanically coupled to an equal number of stepper motors 101 and needle valves 20. Accordingly, the controller 1 00 can control the flow rates along several independent flow lines.

The optimal control module 1 08 is programmable via a stepper drive serial interface to provide optimal plant control. In order to achieve this, the optimal control module 1 08 takes capital, running, life and product cost factors into account in order to provide an improved control signal thereby contributing to an improved yield of the filtration system.

As is the case with the stepper drive module 104 and control module 1 6, the optimal control module 108 is modular in form and mounted on the control module 1 6 (see Figure 8) .

In certain circumstances, software is installed to enable automatic valve characterisation thereby automatically determining the flow rate/valve setting relationship. Typically, the user provides measured flow rates in response to screen requests thereby building up the characterisation graphs/tables. Varying the fluid temperature in response to screen requests, enables the derivation of fluid temperature/viscosity parameters.

The valve assembly can be used anywhere a flow control valve is required to control the flow of a fluid.

The invention illustrated provides a control system that allows determination of the flow rate from the know fluid characteristics, ie from the fluid temperature and pressure as well as the setting of the control valve, thereby avoiding the necessity of a flow meter, which is usually employed to measure the retentate flow rate. Furthermore, it should be noted that the present invention can be used, not only to control flow rate by measuring temperature and pressure and setting the valve aperture accordingly, but can similarly, in principle, be used to control fluid pressure, by setting the valve aperture, in response to pressure measurements, at a setting which provides a desired pressure upstream of the valve, the valve being calibrated accordingly. 20

1 . A method of controlling the flow rate of a fluid along a flow line, the method including the steps of: feeding the fluid along the fluid flow line to a control valve having a controllably variable aperture; sensing the pressure in the fluid upstream of the control valve; determining the instantaneous flow rate of the fluid along the fluid flow line based on a knowledge of the sensed pressure, of the aperture setting of the valve, of the flow characteristics of the fluid and of the pressure in the fluid downstream of the valve; and controllably varying the aperture setting of the control valve to match the flow rate of the fluid along the fluid flow line with a required flow rate.

2. A method as claimed in claim 1 , which acts to control the recovery ratio of a filtration system for filtering a fluid, the system having a filter to which a fluid feed is supplied, filtrate passing through the filter and retentate bypassing the filter, the method including the step of supplying the fluid feed to the filter at a known feed rate, the retentate being fed from the filter along the fluid flow line to the control valve and the sensing of the pressure in the fluid being at a position intermediate the filter and the valve, the controllable varying of the aperture acting to match the flow rate along the fluid flow line with a required retentate flow rate which gives a required recovery ratio.

Claims

1 9

It should be noted, in particular, that pressure measurements are made upstream of the valve, and are gauge pressure measurements (and not absolute pressure measurements) the pressure drop to atmosphere from said upstream site being measured, as such pressures tend to be more stable, with fewer fluctuations, than pressure measurements downstream of the valve. Thus, when the valve is downstream of a filter, this pressure is affected only by the pump feeding the filter, whereas measuring pressure downstream of the valve can be influenced by what is being done downstream of the valve, and can be less reliable.

Finally, it should be noted that inherent advantages of the use of a stepper motor include its absolute positioning of valve aperture setting and its resistance to over-running. Thus, the valve has good resistance to fluid forces applied to its closure element (the needle or spindle) and has a reduced tendency, if any, to slam shut. Stepper motors are also known for their simplicity, ruggedness, low cost, low maintenance requirements and open-loop operation.

3. A method as claimed in claim 1 or claim 2, which includes sensing of the temperature of the fluid, the determining of the instantaneous flow rate being based on a knowledge also of the sensed temperature and of its effect on the flow characteristics of the fluid.

4. A method as claimed in any one of claims 1 - 3 inclusive, in which the instantaneous flow rate is determined by calculation using an algorithm derived from experimental data and giving the instantaneous flow rate as a function of the pressure drop across the valve, of the temperature of the fluid and of the aperture setting of the valve.

5. A method as claimed in any one of claims 1 - 3 inclusive, in which the instantaneous flow rate is determined from a look-up table derived from experimental data and giving the instantaneous flow rate as a function of the pressure drop across the valve, of the temperature of the fluid and of the aperture setting of the valve.

6. A method as claimed in claim 4 or claim 5, in which the controllable varying of the aperture is iteratively executed until the required flow rate is matched by the flow of fluid along the flow line.

7. A controller for controlling the flow rate of a fluid along a fluid flow line, the controller including: a control valve having an aperture setting which is controllably variable; pressure sensing means located in the fluid flow line upstream of the control valve for sensing the pressure of fluid in the flow line; and control means responsive simultaneously to a fluid pressure sensed by the pressure sensing means and responsive to the aperture setting of the control valve, the control means being operable to determine the instantaneous flow rate of the fluid through the valve based on a knowledge of the flow characteristics of the fluid, and the control means being operable to control the aperture setting of the valve to match the flow rate with a required fluid flow rate through the valve.

8. A controller as claimed in claim 7, which includes temperature sensing means for sensing the temperature of the fluid in the flow line, the control means being responsive also to the fluid temperature sensed by the temperature sensing means.

9. A controlled as claimed in claim 7 or claim 8, in which the control means is a PID control means which includes a data interface operable to receive input data to which it is responsive and to output data to the control valve.

10. A controller as claimed in claim 9, in which the controller includes a plurality of said control valves, each control valve having its own pressure sensing means individually associated therewith, the control means being operable to control the aperture settings of all the valves simultaneously.

1 1 . A controller as claimed in any one of claims 7 - 1 0 inclusive, in which each control valve is a needle valve.

1 2. A controller as claimed in any one of claims 7 - 1 1 inclusive, in which each control valve is connected to a stepper motor responsive to a control signal from the control means, the valve being connected to aperture setting means for setting the aperture of the valve, the setting means being mechanically coupled to an output shaft of the stepper motor to permit setting of the aperture by the motor.

1 3. A controller as claimed in claim 1 2, in which the stepper motor is mounted on the valve, the controller including a stepper drive unit mounted on the stepper motor for driving the stepper motor, the drive unit having a serial interface connected to the control means.

14. A valve assembly for use as part of a controller as claimed in any one of claims 7 - 1 3 inclusive, the valve assembly including: a control valve having a housing provided with an inlet, an outlet and a controllably variable valve aperture; a stepper motor connected to the valve and being responsive to a control signal from control means; and aperture setting means for setting the valve aperture, the setting means being mechanically coupled to an output shaft of the stepper motor to permit selective setting of the aperture by the motor,

1 5. A method as claimed in claim 1 , substantially as described and as illustrated herein.

1 6. A controller as claimed in claim 7, substantially as described and as illustrated herein.

1 7. A valve assembly as claimed in claim 14, substantially as described and as illustrated herein.