AU9133998A - Integrated process control valve - Google Patents

Description

-'A

1

AUSTRALIA

Patents Act 1990 VALTEK INCORPORATED S .5 a.

ORIGINAL

COMPLETE

SPECIFICATION

STANDARD

PATENT

Invern tion Title: In tegrated process control valve The following statement is a full description of this invention including the best method of performing it known to us:- This invention relates to a process control valve, automatic or manual.

having integrated therewith a plurality of sensors for sensing various physical parameters of the fluid flowing through the valve, and a utilisation device for controlling operation of the valve and/or displaying representations of the parameters.

Control valves, used in a variety of environments not only to control the flow of fluids (liquids and gases), but also to regulate pressure and temperature of fluids in fluid processing systems, are a special type of valve having a power positioning actuator which is responsive to externally supplied signals for operating (moving) a throttling or closure Inechanism located in the valve body. Typical of such valves are rotary valves (ball.

plug, eccentric disk. etc.) and linear valves (globe, gate, etc.). An exemplary globe-type control valve includes a valve body having an internal passage formed therein with an inlet opening, for receiving fluid, an outlet opening.

15 for discharging fluid, and a central opening located in the valve body between the inlet opening and outlet opening and forming a valve seat. A valve stem. with a valve plug located on one end thereof, is disposed to extend into the valve body and is movable to selectively move the plug olo "and off from the valve seat to thereby close the central opening and slop the 20 flow of fluid, or unclose the central opening and allow the flow of fluid.

Srespectively. The other end of the valve stem, opposite that on which the plug is located, is coupled to an actuator typically mounted on top of the valve body. The actuator includes a cylinder, and a movable piston disposed in the cylinder and coupled to the other end of the valve stem (or could included a diaphragm for operating the stem). A pressurised source of air is supplied to a positioner often times located at side of the actuator, and the positioner, in response to control signals, directs pressurised air to the cylinder both above and below the piston, to thereby cause the piston to move to selected positions in the cylinder and thus the plug to move to desired positions in the valve body.

By controlling the position of the plug in the valve body, upstream 2 pressure, downstream pressure and temperature of the fluid flowing through the valve (as well as external variables such as pressure or volume of fluids in tanks connected into the system, pH of fluid in the system, etc.) can be 'i 35 controlled. Typical prior art methods for determining theses upstream and downstream pressures and the temperature of the fluid have involved placing 3 sensors in pipes joined both upstream and downstream to the valve. Flow rates through the valve have also been measured in prior art systems by inserting a flow meter into pipes leading to or coming from the valve. (It is has also been proposed that flow rates be determined in microprocessor S controlled valves by calculating the flow rates from flow characteristics of the valves and flow characteristics of the fluids. This sensed information could than be transmitted to a remotely situated control processor for processing which could, in turn, transmit signals back to the positioner to cause it to supply pressurised air to the cylinder of the actuator in an attempt o1 to move the plug to positions in the valve body which would yield desired fluid flow parameters. Of course, with the sensors being located outside of the valve. the precision of the process being carried out would not be colnpletely accurate.

Summary of the Invention 15 In a first aspect the present invention provides a process control valve for controlling the flow of gas. the valve comprising: a valve body having an inlet for receiving gas. an outlet for discharging gas, a gas flow passage connecting the inlet and the outlet, and a controllable throttling element which is movable to selectively vary the cross-sectional 20 area of flow of at least a portion of the passage; actuator means coupled to the valve body and responsive to control signals for selectively moving the throttling element: a first pressure sensor disposed at the inlet of the valve body in the gas flow passage. at a place such that a repeatable, continuous pressure 25 measurement can be made, for producing a first signal representing the pressure P, of the gas at the inlet; a second pressure sensor disposed at the outlet of the valve body in the gas flow passage. at a place such that a repeatable, continuous pressure measurenent can be made. for producing a second signal representing the pressure PZ of the gas at the outlet; utilisation means for receiving said first and second signals and for developing an output dependent upon the received signals; said utilisation means including: means for determining the gas pressure drop across the valve body from the first and second signals; means for storing a predetermined gas pressure drop value; 4 means for comparing the determined gas pressure drop with the stored gas pressure drop value and for producing a difference signal whose magnitude represents a difference between the compared values: and means for producing control signals for application to the actuator means to cause it to move the throttling element to thereby vary the gas pressure drop across the valve body to more closely match the stored gas pressure drop value and reduce the magnitude of the difference signal; a temperature sensor for producing a third signal representing the temperature T, of the gas in the gas flow passage; a throttling element position sensor for producing a fourth signal representing the flow capacity C, of the valve body: wherein said utilisation means further comprises processing means or determining from the first, second, third and fourth signals the flow rate of the gas in the passage.

In a further preferred embodiment the process control valve further comprises at least one further sensor disposed in the fluid flow passage for S" producing at least one further signal representing at least one physical parameter of the valve body and/or fluid present in the valve body. and S, the utilisation means being adapted for receiving said further siginils 20 and for developing an output dependent upon the received further signals.

wherein said utilisation means is disposed on the valve and comprises s orod program control means for receiving said further signals and for producing I control signals for application to the actuator means to cause it to move the throttling element in a manner dependent upon the values of the further 25 signals.

2 In a preferred embodiment. the gas has a molecular weight of M and n J ratio of specific heats k. and wherein said processing means comprises a stored program control means for determining the flow rai Q in accordance with Q= gC,PT 2 where g is a scaling constant determined for the valve body: C, is the flow capacity of the valve body; SY is an expansion factor equal to 1-AP/P,3FXr. where Fk k1.40. Xr is the terminal pressure drop for the valve body, and Z is a compressibility factor of the gas in question determined for a particular T, and P.

The processing means can include: means for storing a predetermined flow rate value; means for comparing the determined flow rate with the stored flow rate value and for producing a difference signal whose magnitude represents the difference between the compared values; and means for producing control signals for application to the actuator means to cause it to move the throttling element to thereby vary the flow rate to more closely match the stored flow rate value and reduce the magnitude of the difference signal.

In vet a further aspect, the present invention provides a method of conttrolling the flow of a gas in a flow controlvalve having a valve body with an inlet, outlet and a gas flow passage therebetween for carrying gas,.a valve throttling means disposed in the pAssageway and being selectively movable to various positions to vary the floi volume of gas tihrugh tile valve body.

an actuator responsive to comnnand ignals for niovingtlhe throttling means in accordance with the conunand sigilals, a first pressure .sensor disposed in Ihe gas flow passage at a place such that a repeatable. conItinuous pressure measurement can be made. for a first pressurem easuring gas pressure P, at the inlet, a second pressure sensor dispcsed in the gas flow passage, at a place such that a repeatable, continuous pressure measurement can be mare., 25 for measuring gas pressure P, at the outlel, and a temperature sensor for measuring temperature T of the gas in the passage a position sensing means for measuring the percentage p at which the throttling means is open.

ranging from 0 to 100 percent, and microprocessor means for receiving and storing the measurements Pi. Po and T and p and for producing command signals to control the operation of the valve, said method comprising: measuring Po and T for a selected value of p and storing said measurements and the value of p for which the measurements were made: calculating the gas flow rate Q, through the valve for the selected value of p using the stored measurements, and storing the calculated Q; storing a predetermined flow rate value and 6 comparing Q: with Q and supplying command signals to the actuator to further open the throttling means by a predetermined percentage if Q: is less than and to further close throttling means by a predetermined percentage if Q, is greater than Q., In a preferred embodiment of the invention, the utilisation device includes a video display screen for displaying representations of the first and second signals. Such representations include graphic "signatures" showing various measured values of the first and second signals for various positions of the throttling element. These "measured" signatures may be displayed on the display screen along side of predetermined and previously stored "correct" signatures for enabling the user to determine if the valve is operating properly. The method according to the second aspect can, accordingly, further comprise the step of: displaying on a video display screen representations of the stored measurements and Q and Q.

Se: Also, signatures may be developed and displayed showing various measured parameters of the valve and valve command signals. For example, if pneumatic control signals re used for causing the actuator to move the S throttling element, signatures may be developed and displayed of the values 20 of the pneumatic control signals for various positions of the throttling element.

SI In the method according to the second aspect, step can comprise the following steps: S- calculating the vapour pressure of the gas P, exp[A-B/(T+C)] for S 25 the selected value p and storing the results of the calculation, where A B and C are Antoings coefficients for the gas, and storing the results of the S| calculation.

comparing T with T, where T, is the critical absolute Stemperature for the particular gas flowing through the valve, and moving to step if T is less than T, otherwise moving to step comparing P with Pi and terminating if P, is less than P,, otherwise moving to step I determining and Po,) representing the fluid pressure at the inlet and outlet of the valve body respectively calibrated to ISA standards.

and storing the results of the determination; 7 calculating P,=Piji/PC and T, TIT, where P, is the critical pressure, psia. For the gas, and then determining the compressibility factor Z of the gas from predetermined and stored tables using P, and and storing the results of the determination; determining the flow coefficient C, and gas pressure recovery coefficient X, for the valve from a predetermined and stored table, for the selected value of p; calculating X= (Pi(i7,-Poial)/Piti and storing the results of the calculation: calculating Y=1-X/(3FX,) where Fk=k/1.40 and k is the ratio of specific heats for the gas in question, and storing the results of the calculation: and S(ni) calculating q=7320C,Pii,, Y iX/ (MTZ) where M is the molecular weight of the gas. and storing the results of the calculation.

5 BriefDescription of the Drawings The above and other objects, features and advantages of the inniovatiol will become apparent from a consideration of the following detailed description presented in connection with the accompanying drawings in which: 20 FIG. 1 shows a side, elevational, partially cross-sectional view of ta process control valve made in accordance with the principles of Ihe present invention: FIG. 2 is a schematic diagram of the process control valve of FIG. 1.

illustrating various parameters of the valve and the fluid flowing through the J 25 valve which may be sensed for later use or display: FIGS. 3 and 4 are graphical representations of the values of various parameters of the valve of FIG. 1 as the valve is opened and closed respectively: FIG. 5 is a flow diagram suitable for calculating flow rate of a liquid through the valve of FIG. 1: and FIG. 6 is a flow diagram suitable for calculating the flow rate of a gas flowing through the valve of FIG.1.

Detailed Description Referring to FIG. 1. there is shown one illustrative embodiment of an integrated process control valve made in accordance with the present invention. The valve includes a valve body 4 having an inlet 8. an outlet 12.

8 "mv2 and a fluid flow passage 16 connecting the inlet and outlet. Formed centrally in the valve body 4 is a central opening 20 circunscribed by a valve seat 24 for receiving a valve plug 28. The valve plug 28 is coupled to the lower end of a valve stem 32 which extends upwardly and out of the valve body 4 to a location within a cylinder 36 which is part of an actuator 40 disposed on the top of the valve body.

The actuator 40 also includes a yoke 44 on which the cylinder 36 is disposed, and which, in turn, is fitted onto a bonnet 48 disposed in an opening 52 in the top of the valve body 4. A bonnet flange 56, and bolts hold the bonnet 48 in place in the opening 52. The stem 32 extends upwardly through the bonnet 48 and is held slidably in place by an upper Sguide 62 and a lower guide 63, which are surrounded by the bonnet. The Svalve stem 32 extends also through the yoke 44 and into the cylinder 36 Swhere a piston 68 is disposed on the upper end of the stem.

15 The cylinder 36 includes openings 72 and 76 for receiving air under S pressure into the interior of the cylinder, both above and below the piston 68.

to thereby control the positioning of the piston 68 in the cylinder and thus the position of the plug 28. That is, when air supplied to opening 72 is under greater pressure than that supplied to opening 76, the piston 68 is forced 20 downwardly, and vice versa.

Mounted on the side of the yoke 44 is a valve positioner 80 which r: eceives air under pressure from an air supply 84 and, in response to an electrical control signal supplied via line 88 from a controller 120. develops a Scontrol pressure by which air under pressure is selectively directed to either 25 the opening 72 or the opening 76 in the cylinder 36.

The positioner 80 includes an arm 92 coupled to a stem clamp 94 which. in turn. is coupled to the stem 32 so that each change of position of the plug 28 and thus of the stem 32 may be detected by the positioner. The position of the stem 32, as detected by the arm 92. is mechanically converted into a force (represented, for example, by distortion of a spring) and then compared to a force representing an input signal from the controller 120 supplied over lead 88. to ascertain the difference in values of the forces. The positioner 80 then causes the piston 68 to move until the signal values match. indicating that the piston, and thus plug 28, has been moved to the correct position. The position of the stem 32. and thus plug 28, is detected Iby the controller 120 via a position sensor arm 96 pivotally mounted to the 9 controller and coupled to the stem clamp 94. An angular position sensor disposed in the controller 120 detects the angular position of the arm 96 and thus the position of the stem 32. This information is made available to the controller 120.

The above description is generally of a conventional process control valve available, for example, from Valtek of Springville, Utah. There are. of course. other types of process control valves on which the present invention could be used. but the valve described in FIG. 1 serves as a suitable representation of well known parts and operation of a process control valve on which the present invention may be utilised.

Disposed in the valve body 4 are a number of sensors, including a pressure sensor 104 disposed in the side wall of the fluid flow passage 16.

near the inlet 8. and at a location where the axial side walls of the passage are generally linear or uncurved. The purpose of this is to locate the sensor 15 104 at a place in the valve body 4 a repeatable, continuous pressure measurement can be made, unaffected by flow problems. This is best accomplished at an axially linear portion of the passage 16. A second pressure sensor 108 is located at the outlet 12 of the valve body 4, again on the side wall. and at a location where the axis of the fluid flow generates 20 substantially repeatable. continuous pressures. The sensors 104 and 108 are located on the side of the passage 16, rather than at the bottom or at the top.

to avoid contamination and interference by debris and scales flowing along the bottom or gas bubbles flowing along the top. The pressure sensors 104 S" and 108 could be any of a variety of fluid pressure sensors such as a 25 conventional diaphragm strain gauge.

The pressure sensors 104 and 108 produce signals which are cariied by lines 112 and 116 respectively to a controller 120 which is mounted on the side of the yoke 44. Advantageously, the pressure s -als would comprise voltage levels for supply to the controller 120 for initial storage and ultimate processing. The controller 120 could advantageously bU.a microprocessor such as the Intel 8032 microcontroller or the Motorola 68 332 i-icroprocessor.

Also disposed in the fluid flow passage 16 of the valve body 4 is a temperature sensor 124 for sensing the temperature of the fluid flowing in the passage. The sensor 124 might illustratively be a thermocouple temperature sensor suitable for measuring temperatures over a wide range.

The temperature sensor 124 produces a signal which is supplied by line 128 also to the controller 120.

Disposed on the downstream side of the valve seat 24 in the fluid flow passage 16 is a cavitation sensor 132. This sensor develops a signal representing the cavitation or turbulence taking place in the narrowest part of the passage 16 (vena contracta) where such cavitation is of concern. The signal so developed, again, is supplied via line 136 to the controller 120. The cavitation sensor 132 is a conventional sensing device such as accelerometer or an acoustic transducer.

Two air pressure sensors are provided in the cylinder 36 of the actuator including a sensor 140 for sensing the air pressure above the piston 68 and a sensor 144 for sensing the pressure below the piston, both in the cylinder 36. Signals developed by sensors 140 and 144, representing the detected air pressures, are supplied via lines 142 and 144 respectively to the controller 120.

Other parameters which may be sensed and utilised by the controller 120 are the pressure of the air from the air supply 84. (characterising signal supplied over line 148 to the controller) the position of the piston 68 and thus stem 32 as determined by the arm 92 (characterising signal supplied 20 over line 152). and a control pressure produced by the positioner 80 in response to the electrical signals supplied over line 88 (characterising signal supplied by the positioner 80 over line 156).

A number of the parameters sensed for the valve shown in FIG. 1 are specific to the particular valve structure thereshown. It should be 25 understood that a variety of other parameters could likewise be sensed or detected by built-in sensing devices if other valve structures were employed.

Signals representing these parameters could also be supplied to the controller 120 as desired by the user.

Referring now to FIG. 2, there is shown a schematic diagram of the integrated process control valve of FIG. 1, with the valve body shown at 4 in diagrammatic form, the valve stem being shown at 32, the cylinder of the actuator being shown at 36. the piston being shown at 68, the positioner at and the positioner arm at 92. Other parts of FIG. 2 corresponding to the FIG. 1 apparatus are the controller 120 (but not shown mounted on the side of the yoke as in FIG. a position sensor line 152 for carrying signals to the controller representing the position of the piston 168, a control pressure line 156 for carrying signals representing the control pressure developed by the positioner 80. a control signal line 88 for carrying the electrical control signals from the controller to the positioner, an air supply pressure line 148 for carrying signals representing the pressure of air supplied by the supply 84, valve inlet and outlet pressure lines 112 and 116 respectively for carrying signals representing the fluid pressure at the inlet of the valve body 4 and at the outlet of the valve body, a temperature sensing line 128 for carrying signals representing the temperature of the fluid in the valve body 4. and a cavitation sensing line 136 for carrying signals representing the cavitation occurring in the vena contracta of the valve body 4. all as previously discussed. Finally, the air pressure in the cylinder 36 above the piston 68 is S, determined by a pressure sensor 140 shown located in an input air pressure line 204 (rather than in the sidewall of the cylinder 36 as in FIG. and the air pressure in the cylinder 36 below the piston 68 is determined by a 15 pressure sensor 144 shown located in an input pressure line 208.

The controller 120 is programmed to read, store and process the signals supplied by the various sensors, as desired by the user, and then may signal the positioner 80 to change the position of the piston 68 and thus the position l of the plug 28 (FIG. 1) to bring the valve in conformance with certain 20 predetermined characteristics. For example, predetermined inlet pressures

P

1 and outlet pressures or a predetermined temperature T could be keyed into the controller 120 on an input device 212 and stored. The controller 120 could then be programmed to successively monitor the inlet and outlet pressures and temperature of the fluid in the valve bodi 4, and then make 25 adjustments in the position of the piston 68 to bring selected measured parameters into conformance with the corresponding predetermined stored parameters.

The controller 120 may also be programmed to develop a variety of data. graphs and "signatures" which may be displayed on a video display screen 216 for viewing by the user. Further, a signature of the valve can be developed prior to use and this signature stored in the controller 120 for comparison against signatures developed at various times while the valve is in use. For example, a signature consisting of correct pressures, temperature and flow rates for various positions of the plug 28 (and thus various flow capacities can be determined and stored in the controller for subsequent 12 comparison with corresponding measured signatures to determine if the valve is operating and process proceeding correctly.

The internal parameters of the valve such as the air pressure in the cylinder have the piston the air pressure in the cylinder below the piston the control of pressure developed by the positioner Pe, the air supply pressure P, and the position of the piston and valve stem, could also be measured, stored and displayed on a display screen 216 (FIG. 2).

Predetermined acceptable ranges for these parameters could also be stored in the controller 120 and then periodically compared with corresponding measured values for the parameters (for example on the display screen 216) to enable the user to determine if the measured parameters are within acceptable ranges.

FIGS. 3 and 4 show representative graphs of various parameter SI measurements of the process control valve of FIG. 1, as the valve is gradually opened (FIG. 3) and as the valve is gradually closed (FIG. Such graphs Smay be displayed on the display screen 216 (FIG. 2) to show the user how the valve is operating. The various lines on the graphs of FIGS. 3 and 4 are Slabelled to correspond to the parameter identification given above, Still other uses could be made of the measured parameters, both of the fluid flowing through the valve and of the internal operation of the valve S such as taking and storing the measurements at one point in time and then at .a later point in time taking and storing new measurements and comparing these with the previously stored measurements to determine if a significant S and possibly defect-indicating change has taken place over the period of time 25 from the taking of the first measurements to the taking of the second measurements. Of course, if a problem is indicated in any of the diagnostic procedures described, corrective action could be immediately implemented.

The integrated process control valve described and shown in FIGS. 1 and 2 could also be used to provide a "signature" of the process and operation taking place in other components of the system in which the valve is installed. For example, pumps, other valves, holding tanks, etc. are typically designed to operate within certain specifications as to pressure, temperature, etc. and pressure and temperature measurements could be made by the valve of the present invention and compared to previously provided specification values for the other components to which the valve is connected to thereby determine if the measured parameters compare with the specification values.

Ii 13 SFIG. 5 shows a flow chart of a program which may be implemented on the controller 120 for calculating the flow rate of liquid in the valve body 4.

The first step, after the start, is to measure the position of the valve plug (piston). and the valve inlet pressure, valve outlet pressure, and temperature of the fluid flowing in the valve for given positions of the valve plug (box 304). These measurements are supplied to the valve controller which stores the measurements for subsequent processing. Next, the pressure drop A P, across the valve body for various valve plug positions is calculated in accordance with the formula of box 308. This pressure drop is corrected for ISA standards since subsequent calculations in arriving at the flow rate are all keyed to these ISA standards. The correction formula is shown in box 312, where a and b are empirically determined constants for various positions of the valve plug for the particular valve in question typically done to comlpensate for change in pressure readings.

15 Next. the vapour pressure P, of the liquid is calculated in accordance with the formula shown in box 316, where A, B and C are Antoines coefficients for the liquid in question. The liquid pressure ratio factor F, is then calculated in accordance with the formula given in box 320, where P. is I the critical pressure for the particular liquid flowing through the valve. The flow coefficient C, and liquid pressure recovery coefficient F, for the valve are deternined from predetermined, stored tables for the particular valve and Sfor various positions of the valve plug.

Next. the valve is checked to determine if it is choked (cavitating) and :T this is done by first calculating the pressure drop across the valve for the 25 choked condition A P(choked) in accordance with the formula shown in box 328. The specific gravity Gf of the liquid is next calculated pursuant to the formula in box 332, where GFB, T. and Ta are known constants for the particular liquid in question. In box 3 3 6, a determination is made as to whether or not the pressure drop calculated in box 312 is greater than the pressure drop for a choked valve, calculated in box 328, and if it is. the Sprocess moves to box 340, otherwise the process moves to box 344. In both Scases, the flow rate q is calculated using formulas shown in the boxes to give the desired current measure of flow rate.

SThe calculated flow rate may either be used to apprise the user by way 35 of a visual display as to the present flow rate of fluid in the valve, or to Sprovide an indication to the controller as to which direction the valve plug "m- Net th valv is cheke .to deemn -if iti -hkd(aiaig must be moved to change the flow rate and thus bring it closer to a predetermined desired flow rate value. The controller would develop the Sappropriate signal for supply to the valve positioner to cause the valve positioner to change the position of the valve plug, after which measurements and calculations to obtain the flow rate would again be made.

SThis process would be repeated until the desired flow rate were achieved.

SFIG. 6 shows a flow diagram for calculating flow rate of a gas in the valve of FIGS. 1 and 2. As with liquids, the first step after starting is to measure the position of the valve plug, and the valve inlet pressure, valve outlet pressure and valve fluid temperature as indicated in box 404. The "vapour pressure of the gas P, is then calculated (box 408) using a formula already discussed. A determination is then made as to whether the measured temperature T is greater than the critical absolute temperature T. (for the particular gas flowing through the valve] as indicated in box 412. If it is, the 15 process moves to box 416; otherwise the process moves to box 420. At box S420. a determination is made as to whether the valve inlet pressure P, is greater than or equal to the vapour pressure P, and if it is, the process is stopped since the fluid flowing in the valve may be a liquid rather than a gas.

S' 2, Assuming that the fluid in the valve is a gas, and that the process 20 moved either from box 412 or box 420 to box 416, the valve inlet pressure I calibrated to ISA standards, and the valve outlet pressure, also calibrated to ISA standards, are determined by table lookup which would have been earlier determined by experimental testing and stored in the controller.

S The next step in the process is to calculate Pr and T, (box 420) which, 25 along with stored tables, are used to determine the compressibility factor Z of the gas (box 424), where P, is the critical pressure (psia) for the gas. In box 428, the flow coefficient C, and gas pressure recovery coefficient X, for various valve positions are determined from predetermined, previously stored tables.

The ratio of the pressure drop across the valve to the valve inlet pressure is then calculated in box 432, followed by a determination as to whether X, is greater than X, (box 436). If it is (meaning that a choked condition exists), X is set equal to Xt, (box 440), and the process moves to box 444. whereas if it is not, the process moves directly to 444. The expansion factor Y is calculated in accordance with the formula given in box 444, where Fk equals K/1.40 and K is the ratio of specific heats for the gas in question. Finally, the flow rate q is calculated in box 448 as shown by the formula.

Following calculation of the flow rate for gas, the controller may either display the calculated rate, use it to adjust the valve plug position to thereby change the flow rate, or both as previously described for the calculation of liquid flow rates.

It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention.

Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the e present invention and the appended claims are intended to cover such .e modifications and arrangements.

o. ee *0*OS Be ft ,0 o> oe

Claims (4)

1. A process control valve for controlling the flow of a gas, said valve comnprising: a valve body having an inlet for receiving gas, an outlet for discharging a gas flo-w passage connecting the inlet and the outlet, and a controllable throttling element which is movable to selectively vary the cross-sectional area of flow of at least a portion of the passage; actuator means coupled to the valve body and responsive to control signals for selectively moving the throttling element; a first pressure sensor disposed at the inlet of the valve body in the gas flow passage, at a place such that a repeatable, continuous pressure measurement can be made, for producing a first signal representing the pressure P, oF the gas at the inlet; ~a second pressure sensor disposed at the outlet of the valve body in the gas flow passage, at a place such that a repeatable, continuous pressure measurement can be made, for producing a second signal representing the pressure P, of the gas at the outlet; utilisation means for receiving said first and second signals and for developing an output dependent upon the received signals; said utilisation means including: means for determining the gas pressure drop across the valve body from the first and second signals; means for storing a predetermined gas pressure drop value; means for comparing the determined gas pressure drop with the stored gas pressure drop value and for producing a difference signal whose magnitude represents a difference between the compared values; and means for producing control signals for application to the actuator means to cause it to move the throttling element to thereby vary the gas pressure drop across the valve body to more closely match the stored gas pressure drop value and reduce the magnitude of the difference signal; Fatemperature sensor for producing a third signal representing the temperature T, of the gas in the gas flow passage; a throttling element position sensor for producing a fourth signal representing the flow capacity C, of the valve body; wherein said utilisation means further comprises processing means for determnijing from the first, second, third and fourth signals the flow rate of the gas iii the passage.

2. The process control valve as in claim 1 wherein said gas has a molecular weight of NI and a ratio of specific heats k, and wherein said processing mocans comprises a stored program control mieans for determining the flow r'ate Q in accordance with Q=gC"P 1 Y FPPMT 1 where g is a scaling constant determined for the valve body;, is the flow capacity of the valve body; Y is an expansion factor equal to 1-AP/P,3FXr,, where Fk k/l1'.40, X-r is the terminal pressure drop for the valve body, and Z is a compress ibility factor of the gas in question determined for a particular T, and P,.

3. The process control valve as in claims 1 or 2 wherein said processing means include: means for storing a predetermined flow rate value; mieans for comparing the determined flow rate with the stored flow rate value and[ for producing a difference signal whose magnitude represents the difference between the compared values; and its, means for producing control signals for application to the actuator means to cause it to move the throttling element to thereby Vary the flow rate to more closely match the stored flow rate value and reduce the magnitude of the difference signal.

4. A method of controlling the flow of a gas in a flow control valve having a valve body with aii inlet, outlet and a gas flow passage therebetween for carrying gas, a valve throttling means disposed in the passageway amid being selectively movable to various positions to vary the flow volume of gas through the valve body, an actuator responsive to command signals for moving the throttling means in accordance with the command signals, a first pressure sensor disposed in the gas flow passage at a place such that a repeatable, continuous pressure measurement can be made, for a first pressure measuring gas pressure P 1 at the inlet, a second pressure sensor -~lu~-rummr*-Il ryp)l~ disposed in the.gas flow passage, at a place such that a repeatable, continuous pressure measurement can be made, for measuring gas pressure P. at the outlet, and a temperature sensor for measuring temperature T of the gas in thepassage, a position sensing means for measuring the percentage p at which the throttling means is open, ranging from 0 to 100 percent, and microprocessor means for receiving and storing the measurements PI, and T and.p and for producing command signals to control the operation of the valve, said method comprising: measuring P, and T for a selected value of p and storing said measurements and the value of p for which the measurements were made; calculating the gas flow rate Q, through the valve for the selected value of p using the stored measurements, and storing the calculated Q; storing a predetermined flow rate value Qs; and comparing Q with Q, and supplying command signals to the actuator 15 to further open the throttling means by a predetermined percentage if Q: is less than and to further close throttling means by a predetermined percentage if Q, is greater than Q. The method as in claim 4 further comprising: displaying on a video display screen representations of the stored measuremejts and Q and Qs. S6 The method as in claims 4 or 5 wherein step comprises: calculating the vapour pressure of the gay i exp[A-Bi/T+C)] for the selected value p and storing the results of the O:iS calculation. where A, B and C are Antoines coefficients for the gas, and lstoring the results of the calculation; S(g) comparing T with T where T. is the critical absolute temperature for the particular gas flowing through the valve, and moving to step if T is l;:es than otherwise moving to step S (hi) comparing P, with P, and terminating if P, is less than P 1 otherwise moving to step Aletermining P, and.Po(,a) representing the fluid pressure at the inlet and outlet of the valve body respectively calibrated to ISA standards, and storing the results of the determination; calculating P,=P(InPc and Tr T/T, where P, is the critical pressure, psia, for the gas, and then determining the compressibility factor Z of the gas 19 from predetermined and stored tables using P, and and storing the results of the determination; determining the flow coefficient C, and gas pressure recovery coefficient X, for the valve from a predetermined and stored table, for the selected value of p: calculating X= [Pi(isa -Po(i,/Pii and storing the results of the calculation: calculating Y=1-X/(3FkX,) where Fk=k/1.40 and k is the ratio of specific heats for the gas in question, and storing the results of the calculation: and calculating q=7320C,Pisa, Y/FX (MTZ) where M is the molecular weight of the gas, and storing the results of the calculation. DATED this fourth day of November 1998 t VALTEK INCORPORATED Patent Attorneys for the Applicant: F.B. RICE CO. a i.