Digester control

Abstract

Claims

Dec. 6, 1949 J. M. M ALEAR DIGESTER CONTROL Filed April 25, 1947 Zmventor: (Ittorneg. Patented Dec. 6, 1949 DIGESTER CONTROL James M. McAlear, NeedhamQMasS assignor to Mason-Neilan Regulator Company, Boston, Mass, a voluntary association of Massachusetts Application April 25, 1947, Serial No. 743,868 15 Claims. 1 This invention relates to the control of pulp digesters more particularly in respect to the relief of gases during the cooking process, and the device embodying this invention is especially suited for use in connection with digestcrs wherein the sulphate or soda process is utilized. As is Well known to those skilled in the art in the manufacture of paper pulp, a digester is filled with wood chips of a character suited to the product and is then charged with cooking liquor until th major portion of the chips are covered, the liquor being in the iorm of water to which carbonates, sulphides and hydroxides have been added to provide a caustic solution best adapted for the purpose. Heat is then applied to the mass, either directly by the introduction of steam at the bottom of the digester with or without forced circulation, or indirectly by forcibly circulating the liquor from the digester to a heat exchanger and thence back to the digester, it being desirable with either method to raise the temperature rapidly to a predetermined maximum at which the cook temperature is preferably maintained until the pulp has attained the consistency desired. It will be understood by those skilled in the art that whether a direct or indirect method of heating is employed, the efiicient operation of a digester depends upon penetration of the cooking liquor into all of the chips and that this in turn depends on proper circulation throughout the entire mass. And it has been found that during the heating up process of a sulphate digester, quantities of air, together with volatile substances such as terpentine and the like which vaporize as the cooking process proceeds, seriously impede circulation, and that these gases, although in gradually diminishing amounts, continue to accumulate for the balance of the cook. Owing to this accumulation of gases during the cook cycle, the actual temperature of the mass fails to adhere to the vapor pressure-temperature curve of the boiling solution, since the noncondensibles and volatile fluids must be replaced by vapors from the boiling solution if the digester temperature is to conform with that of the vapor pressure-temperature curve. Thus the heat input is retarded whether the heat is provided directly by the introduction of steam or indirectly by forced circulation. With a view to improving circulation, it has been customary to vent the gases from the digester from time to time in an attempt to maintain the digester temperature in correspondence with the vapor pressure-temperature of the liquor, but there has been a tendency to carry over a portion of the liquor solution together with particles of pulp entrained therewith as well as a substantial portion of the liquor vapors, thereby adding to the load of the apparatus which is used to recover cooking liquor and by-products as terpentine. It will be understood that in the cooking of wood pulp by the sulphate process, a strainer is required at the inner end of the relief pipe to prevent the carry-over of considerable quantities of pulp, and while the carry-over tendency may be materially decreased by improved venting means, it is not only impracticable to operate without a strainer, but means must be provided for cleaning the strainer when it becomes clogged in order that the gases may be relieved as they accumulate during the cook period. And while automatic means have been provided for blowing out the strainer, so far as known to me such means are largely independent of the gas-off control mechanism and function on a motor driven cyclical basis requiring additional equipment which must be designed for the purpose and adds to the cost of the control mechanism. It is an object of this invention to provide gasoffand strainer cleanout control mechanism wherein pressure and temperature responsive elements of a differential controller function to operate a valve in the digester relief line to bleed ofi gases as required to maintain the digester temperature in correspondence with the vapor pressure-temperature of the cooking liquor regardless of the pressure obtaining in the digester, and wherein the same pressure and temperature responsive elements also function to govern valve actuating mechanism, whereby intermittent blasts of steam are applied to the digester strainer for cleaning the same so long as it remains clogged. I have found from extensive tests with successive cooks, wherein various kinds of wood chips were used, that by means of the device embodying this invention, the carry-over of liquor together with particles of pulp associated therewith is negligible. Thus the quantity or fluid to be passed through the liquor recovery system is reduced to a minimum and the load on the terpentine recovery equipment is correspondingly cut down, thereby materially decreasin the capacity requirements of that apparatus and accordingly lowering the cost of producing paper pulp. Furthermore, by eliminating protracted clogging of the relief line strainer and ridding the digester substantially continuously of air and other volatile vapors only, a digester 3 temperature best suited to the process is constantly maintained, gas pockets are prevented, and a maximum circulation of cooking liquor is provided, whereby a high grade pulp may be produced at a minimum cost. The invention will be more fully understood from the following description when taken in connection with the accompanying drawings, in which: Fig. 1 is a view partly in diagram showing a controller with associated valves and valve actu ating mechanism embodying my invention as applied to a pulp digester of the direct heated type. Fig. 2 is a front elevation of the operating mechanism of the controller illustrated in Fig. 1; and Fig. 3 is a detail in section of the valve actuating mechanism shown in Fig. 1. I Having reference to the drawings and particularly to Fig. 1, there is shown a digester l having the usual detachable cover II at the top to provide for charging the register with wood chips, and having a pipe [2 at the bottom equipped with a hand valve [3 through which cooking liquor may be introduced and the digester filled to a level, for example, as indicated by a dotted line I4. The pipe I2 and Valve I3 are also adapted to be used to discharge the 'finished pulp at the end of the cook. As herein shown the digester I9 is arranged to be heated directly by the introduction of steam and for this purpose a perforated ring type inani- 'fold [5 may be employed which is connected by a plurality of pipes It with a steam inlet line ll. The introduction of steam through the line H, from a source not shown, is governed by a "control valve l8 which, as illustrated, is operated pneumatically and is governed by a controller I9 to which it is connected by means of a pipe 20. The controller l9 responds to changes in pressure in the digester I!) through a pipe 2| and serves to vary the digester pressure in accordance with a selected schedule as for example, a schedule wherein the digester pressure is raised from atmosphere to 100 p. s. i. during a period of substantially three quarters of an hour, is maintained at that pressure for approximately three hours, and is then reduced at the end of the cook. For this purpose the time cycle pressure controller I9 may be of any usual construction, and it will be understood that while I have illustrated a time cycle pressure controller for governing steam input, the introduction of heat may be governed by a rate of flow controller or other means if desired. As the heating up process proceeds, the air which is trapped in the digester after the charging operation is completed, together with other non-aqueous vapors which are released during the process, must be vented if the digester temperature is to conform to the vapor pressuretemperature of the sulphate solution in accordance with the selected pressure schedule afforded by the controller [9. For this purpose the digester is provided with a relief line 25 which is controlled by a relief valve 26 and is arranged to discharge into apparatus, not shown, in which products such as cooking liquor and turpentine are recovered. It has been found that considerable surging occurs in the digester during the boiling process and that some of the pulp tends to enter the relief line 25. To prevent this, a strainer 21, which may be in the form of aperforated plate, is provided at the inner end of the relief line. It has also been found that the tendency to surge is materially accentuated by the intermittent release of relatively large quantities of gas and this not only tends to carry over slugs of liquid, but also tends to cause some of the pulp to b come impinged against the strainer screen thereby causing th'e'screen 'tobecdr'ne clogged. It therefore becomes necessary to provide means for gradually bleeding off the air and non-aqueous vapors as they are released, thereby materially reducing the surging tendency and, by reason of the restricted flow through the relief line, lessening the tendency to clog up the strainer. Furthermore, by this method of operation, circulation is improved and the average digester temperature may be made more nearly to adhere to the vapor pressure-temperature of the cooking liquor. And in accordance with my invention I provide automatic means which function as a gas-off control to vent the gases at the rate at which they are released and which also function with a simple form of valve actuating mechanism as an automatic device for cleaning out the strainer Whenever it becomes clogged. The gas-01f control consists of a pressuretemperature relation controller 30 which ma be of any usual construction, the controller herein illustrated being with slight modification similar to that disclosed in the United States patent to N. Belaef, No. 2,284,795, dated June 2, i942, and being adapted to actuate the relief valve 26 as required to maintain a set relation between the digester pressure and temperature. The relief valve 26 is preferably of substantially less capacity than the steam valve l8, being sized to take care of the m aximum relief gas requirements and when fully open, to have no appreciable affect on the digester pressure. It includes an operating pressure chamber 3| which is partially defined by a diaphragm 32, the pressure in the chamber being opposed by a spring 33 in the usual manner. As herein illustrated, the parts are so arranged that the valve is normally closed, i. e. the valve is opened by an increase in operating pressure. I Means for connecting the pressure chamber '31 of the valve with the output 'pressu're on the controller 30 is in the form of a line 34 which is governed by a valve I90, to be more fully described. The controller 3%] is responsive to changes in digester pressure througha pipe 36 'whichis preferably connected to the digester at a point near the connection with the relief line '25 as indicated at 31. It is also responsive 'to changes in temperature at a oint downstream of the strainer 2'! by means of a temperature sensitive bulb 38 "which is connected to the controller by flexible tubing 39, 'the bulb beingprefe'r'ably mounted in a side connection 40 with which an elbow 4|, forming a part ofthe relief line, is provided. Having reference to Fig. '2'of the drawings, the pressure-temperature relation controller 33, which is herein illustrated as of the indicating type, will'serve to describe the operating principle embodied in 'my invention'but'the'controller may be of the two-pan recorder type if desired. The controller mechanism is mounted on a suitable back plate and is provided with indicating pointers 46 and 41 and a setting indicator 48 which are adapted to cooperate with a scale 49. At 50 is an assembly support including a circular portion 5| and an extension 52, the circular portion being rotatably mounted on the back plate 45 by means of a fixed bearing ring 53 and having amazes a gear segment 54 meshing with a pinion 55 which in turn is mounted on a shaft 56. The shaft 56 may be provided with a squared end suitable for a key, not shown, by which the assembly support arm may be rotated clockwise or in a counterdirection. The setting indicator 48 is attached to the circular portion 5| and serves to indicate the ratio setting between the digester pressure, and the actual temperature, in terms of pressure, of the vapors being released through the relief line 25 in a manner to be described. The extension 52 may carry a nozzle 60 mounted on a support member 6| made fast to the extension. Cooperating with the nozzle 66 is a flapper 62 in the form of a flat spring secured at one end to a flapper arm 63, as by rivets 64. The flapper arm 63 is pivotally mounted at one end on the extension 52 and at the free end is herein shown as integral with a cam follower arm 66 which rests on a cam 61, to be more fully referred to. The nozzle 66 may extend into an opening 68 in the flapper arm for cooperation with the flapper 62 which is adapted to yield and prevent injury to the parts in the event that the normal operating range of movement is exceeded. While I have herein shown one type of nozzle-flapper arrangement, it will be understood that any usual construction may be employed. A convenient operating fluid, for example air, may be supplied under pressure to the nozzle 60 through the line 34 from a pipe 69 having a restricted orifice 10 supplied from a source not shown. The nozzle-flapper arrangement constitutes a simple form of pressure system for operating the relief valve 26 either directly, or indirectly through a pilot of any standard construction. The ratio movement by which a selected ratio between the temperature, in terms of pressure, of fluids as they leave the digester and the fluid pressure in the digester may be established and maintained, is comprised essentially of four members which are so arranged in respect to their relative positions and movements as substantially to constitute a parallelogram under all conditions of setting and control. These members comprise an arm 1| rigidly secured to a pivot shaft 12 to which the pointer 41 is also rigidly secured; an arm 13 rigidly connected to the pointer 46 which is in turn pivotally mounted on the shaft 12, and a lever 14 and link 15, the lever being pivoted at one end to the free end of the arm 1| and at its other end being arranged to carry the cam 61, and the link 15 being pivotally connected to the lever 14 and arm 13 as shown. The cam 61 may be adjustably secured to the lever 14 by means of a frictional mounting such as a screw 16 or the like. The pointers 46 and 41 may be actuated by helical coils 86 and 8|, respectively, secured to the plate 45 by supports 82, the former coil being responsive to the digester pressure by means of the pipe 36, and the latter coil being sensitive to the relief gas temperature, which it is desired to maintain at a definite ratio value relatively to the digester pressure, by means of the tube 39 and bulb 38 which together with the coil may be filled with a suitable expansible fluid. The free end of the coil 80 is adapted to rotate an arm 82 which is pivoted on a shaft 83 at the center of the coil and is arranged to move the arm in a counterclockwise direction on an increase in digester pressure. The pointer 46 may be operatively connected to the arm 82 by a link 84, by which means the pointer is positioned on the calibrated scale 49 in accordance with the pressure in the digester as maintained by the steam input controller l9. The pointer 41 may be in operative connection with the helical coil 8| by means of an arm and link 86 arranged in a similar manner to the connecting parts just described. The coil 8| positions the pointer 41 over the calibrated scal 49 and is adapted to indicate the relief gas temperature in terms of pressure. The cam 61 and ratio movement, including the pointer arms 1| and 13, link 15 and lever 14, cooperate to position the flapper 62 in relation to the nozzle 66 in accordance with the relative positions of the pointers 46 and 41. As shown in the drawings the cam surface is laid out substantially on the arc of a circle and the cam is positioned in such a way relatively to the lever 14 and the follower 66 that a change in value of the pressure is followed by the same change in value of the temperature in the relief line 25. This is accomplished by a change in output pressure in pipe 34 which results in a change in relief valv opening in a manner to be described. The cam action may be varied by swinging the cam about its mounting 16 or by changing the shape of the cam. The adjustment and operation of the gas-off control is as follows: The time cycle pressure controller I8 is set to maintain the desired time-pressure cook schedule. The gas-off controller 36 is set to maintain the desired ratio between the temperature of the gases at the bulb 38 and the temperature of saturated vapors of the liquor solution at the pressure in the digester. The last mentioned setting is accomplished by turning the pinion 55 to establish the initial relation between the nozzle 66 and the flapper 62. The rotation of the assembly support 56 counter-clockwise to move the setting indicator 48 from zero on the scale 49 increases the ratio, and as the pointer is turned clockwise and approaches zero, the differential is decreased. A simple method of establishing the ratio setting for a given liquor solution is to set the indicator 48 at a position at which the liquor solution vapors just start to bleed through the relief valve. When the cook is started steam is introduced into the bottom of the digester through the inlet valve l8 as governed by the time cycle pressure controller IS in accordance with a prescribed time-pressure schedule. In accordance with this schedule, the digester pressure is raised to a predetermined maximum, is maintainedv at that maximum over a selected period and is then reduced at the end of the cook. As the pressure is raised, air and other non-aqueous vapors collect at the top of the digester and enter the relief line 25. It may be assumed, for example, that the digester pressure is 10 p. s. i. and that the temperature of saturated liquor vapor at 10 p. s. i. is 240 degrees F. As air and other nonaqueous vapors enter the relief line 25, the temperature at the bulb 38 will fall below 240 degrees F., the helical coil 8| will wind up turning pointer 41 and arm 1! clockwise, thereby raising cam 61 and flapper 62. The nozzle pressure will thus increase and open the relief valve 26 proportionally to increase the flow of gases from the digester. As the air and volatile vapors escape and are replaced by vapors of the liquor solution, the temperature at the bulb 38 rises, coil 85 unwinds turning pointer 41 counter-clockwise to lower cam 61 and flapper 62 and reduce the opening ofthe relief valve until the ratio established by the setting pointer 48 is restored. Now let it be assumed that the pressure in the digesteris raisedzaboveulop. s.i;. by thecontrollens. i9; As this takesaplaceathe pressure responsive i. coil 80 unwinds turning pointerrdfi counterclockaw wise.- and with itarnr J3,;-lever,;'|4, andscam;61:1. thereby tending ,to .raise the .flappentoincrease the operating pressure andopen thereliefwalvenl However, if the temperatureyat.thewbulbincreases at thesame ratio, the temperature;.responsivel coil 8| will also unwind and. turnthespointer ll: counter-clockwise so-that. the armr'll lowers theeio cam Eland tends to reduce .the nozzle pressurev and close the relief valve." Undentheconditions; just referred to, the partsare smarrangedthat, the counter-clockwise movement of. the came tending to raise the flapper 62=and= open=therelief valve is offsetby-the downward movements of the cam tending to lower-the-flapper andclosee; the relief valve. Thussso long-.asvthe set'ratio is maintained. the flapper-nozzle .relation-uremains undisturbedandthe opening-of the relief valve is not varied. From the above it will-be. noted that irrespectiveof theapressure-inihe digester the opening assumed bythe reliefvalve will remain unchangedsotlongasthe desired. pressure-temperature .ratio .is maintained. And 2. in this connection it will be understoodthatthe. correct functioning of the, gas-off control mech; anism just described is dependent on maintain-l ing a reasonably unrestricted flow. of relief gases through the straineril, and thedevice embodying my invention is so arranged and constructed that it also functions to operate a simple form of valve actuating mechanism whereby the strainer is automatically cleaned whenever .the flow is sufficiently restricted 'atthat point. to .in- terfere withthe proper operation of thegas-ofi 0 control. Having reference to Fig.1 of thedrawingsll preferably provide a Y connection'fiflat the'di gester outlet beyond the strained 27; 'I'he'connection 9G includes a through "pipe "9! forming part of the relief line 25 and-a side"*connection"' 92 which maybe at a"'degree angle'thereto. The side connection may be supplied with steam through-a pipe EB-at a pressure; for-example; of;- 150 p. s. i. from a source not shown and is ar-' ranged to direct steamagainst the strainer 21 to free it frompulpwhichhas become'lodged onthe inner surfacethereof; Apneumatically operatedvalve 95 serves to governtheflow of steam r threegh the pipe 83. As hereirrshown thevalve includes a diaphragm 95' -partially defining' a chamber 96 to whichoperatin-g 'fluid'undera'se lected pressure is supplied-through a pipe91 from a source not shown. Fluid'pressure in'the chamber 'is opposed by a spring 98, the valve being arranged to-be normally closed; lie. the valve is opened on an increase of operatingjpressure. When the steamvalve is opened "to admit a blast of steam against the strainer 'Z'L'it is pref which are the t e-W p eed-tare nsw ieh 1.5 a 331 41 t h vh exce ds the upper mounted :in=the-.output' pressure line 34 from the l controller '33 to the. relief valve 26 'and-intheoperatingpressure line- 9T to the steamrvalve 94, respectively. The three-way valves maybe of any. usual construction and .as herein shown the valve lflfi, .comprises:a body having pipe connecr tion-s E62 and. I631'and a bleed connectionnlfik The:valve includes a solenoid lilfigwhichzis in, operative connection with a valve member. 106 which cooperates with two oppositelyrdisposedn oi'ifices lil'l, whereby the bleed connection |04. is.. closed and operating fluid is permitted to pass through the valve when the solenoid is deenergized, and when the solenoid is ,energizedn fluid- -fiow. through the, output; pressure line, 34 is shutoff and the pressure chamber. 3| of the. I relief valve isvented to atmosphere. The three-p way valve lei which is operated bya solenoid m8 is of a similar construction; However, the operating pressure line 9'1 is connected with the;- valve body as indicatedat I09 and I It, the bleed being providedat Ill, so that when the solenoid i88 is de-energized, the supply ofoperating fluid 1 is cut off and the pressure chamber 96 of the steam valve is vented to atmosphere, and when the solenoid is energized, the bleed is closed and i the steam valve is'opened wide. Means for governing the operation of the solenoid valves is inthe form of a pressure switch H2 which is in turn controlled by the pressure and temperaturecoils and BI and the operating mechanism associated therewith in a manner to be described. The switch includes a frame H3- in which a bellows I I4 .is mounted and is provided at its free end-with a T-shaped switch actuating member H5. Surrounding thememher -I I5 is a helical spring H6 which is retained between a head H? on the member H5 and the lower end of the frame H3 thereby tending to ,,collapse the bellows i M. The member I I5, which i is slideably mounted in the frame, carries an adjustable switch engagingmember H8 at its lowerend A switch member H9 in the form of a leaf-spring resiliently engages the adjustment member vi 153, beingrigidlysecured at one end to r a .connection 126.. The. switch member H9 is adaptedto engagea contact IZI which togetherwith connectionaml form part of a circuit whereby the solenoids and l08-may be energized so from a source of electric current not shown. By varying the adjustment- HB the switch member H9 may be initially positioned in respect to the contact I2| and thus any desired pressure setting may be provided within the operating range of the device. The switch ll2,is so arranged that it is under the control of the same elements which function to maintain a selected pressure-temperature ratio in the digester. For this purpose the pressure switch bellows ll4 is connected with the controller output pressure line 34-by means of a pipe I22. Thus the bellows responds tothe same fluid pressure '.by which the relief valve 26 is operated. I It, will beunderstood that the pressureswitch 5;;memberi If! is normally open and that unless the pressure in the bellows H4 exceeds the set value, solenoids I05 and Therefore output pressure will normally be admitted to thepressure chamber 3| of the relief 7oo-valve 26 and the operating pressure in line 91 i will normally be shut 01f fromthe pressure chamber..96 .of-the-steam valve '94. Under these con ditions,.the relief'valve will function and the steam valve willbeclosed provided the pressure I08 will be de-energized limit of the overall operating pressure range of the relief valve. If it be assumed that the operating pressure range of the relief valve and steam valve is from 2 p. s. i. when the valves are closed to 14 p. s. i. when the valves are fully open, the switch I I2 may be set to energize the solenoids I05 and I08 at a pressure, for example, of 16 p. s. i. Thus unless the output pressure of the controller exceeds the set value, the controller will function to operate the relief valve to maintain the desired pressure-temperature ratio in the digester. Under the conditions just described the operation will be as follows: The time cycle controller [9 will vary the steam input to the digester to maintain a selected time-pressure schedule throughout the cook period. During this period the relief valve 26 will relieve the gases as they accumulate to maintain the desired pressure-temperature ratio in the digester so long as the relief valve is permitted to govern the flow of gases without interference by the clogging of the strainer 21. Should the strainer become clogged sufficiently to interfere with the flow of gases and render the relief valve ineffective to maintain th required pressuretem'oerat re ratio, the temperature at bulb 38 will fall thereby causing the controller 30 to increase the out ut pressure proportionally to the increase in the pressure-temperature ratio. When the pressure-temperature differential expands to a value at which an output pressure of 14 p. s. i. is provided, the relief valve will be fully o ened. If the temperature at the bulb still continu s to de rease. the controller will continue to increase the operating pressure to which the bel ows I 4 is also responsive. When the pressure reaches 16 p. s. 1.. the pressure switch H9 will enga e the contact IN to close the outnut pressure line 34 and vent the pressure chamber 3| 0* the relief valve to atmosphere. t me the three-way valve Hll will close its bleed connection and will admit fluid operatin pressure to the pressure chamber 96 of the steam valve. Thus the steam valve will o en wide to admit steam to the strainer 21 and the relief valve ill close to prevent the escape of steam through the relief line to the recovery apparatus. The blast of steam into the relief line will not onlv tend to clean the strainer screen but at the same time wi l raise the temperature at the bulb 38. and when t e temperature at the bulb arrives at a value within the selected pressure-temperature range at which the controller reduces the output pressure below 16 p. s. i.. the switch H9 will o en, thereby closing the steam valve and actuating the valve member of the relief valve to fully open position. Under these conditions if the strainer has been freed by the blast of steam. th b lb 38 will continue to function together with its associated mechanism to vary the opening of the relief valve as required to maintain the selected temperature-pressure ratio. However, should the blast of steam fail to free the strainer, the temperature at the bulb will drop again and the output pressure will build up until the pressure switch is again actuated. The relief valve will again be closed and the steam valve will open to repeat the cycle just described. It will be noted that the mechanism herein described provides means for controlling the relief of gases from a digester at varying pressures throughout the cook period in accordance with a prescribed schedule, which means also function to provide intermittent blasts of steam against the strainer until it is free of obstructions when- At the same ever the clogging of the strainer interferes with the effective operation of the gas-off control. While I have shown and described in detail a preferred form of apparatus embodying this invention, I wish it to be understood that modifications may be employed without departing from the spirit and scope of my invention. And I wish it to be further understood that While I have referred both in the description and in the claims to steam as a suitable medium for cleaning the strainer and for providing an intermittent cleaning action, the term steam is intended to refer to any medium having characteristics suited to the purpose. Having herein described my invention, what I claim and desire to secure by Letters Patent. is: 1. In a gas-01f control for a digester having heat input means which vary the pressure in the digester, a strainer in the relief line. and a steam line connected to said relief line for directing steam against thestrainer from a source of steam supply, the combination with a first valve in said relief line, a second valve in said steam line, a first element responsive to fluid pressure in said digester, a second element responsive to fluid temperature in said relief line, and means associated with said elements for actuating said first valve to maintain the aforesaid pressure and temperature within a selected ratio value, of a third element governed by said first and second elements to close the first valve and open the second valve whenever a relative drop in temperature causes said ratio to deviate from a predetermined value. 2. In a gas-off control for a digester having heat input means which vary the pressure in the digester, a relief line for venting gases from the digester, a strainer in the relief line, and a steam line connected to said relief line for directing steam against the strainer from a source of steam supply, the combination with a first valve in said relief line, a second valve in said steam line, a first element responsive to fluid pressure in said digester, a second elementresponsive to fluid temperature in said relief line, and means associated with said elements for actuating said first valve to maintain the aforesaid pressure and temperature within a selected ratio value, of a third element governed by said first and second elements to close the first valve and open the second valve when a relative drop in temperature causes said ratio to deviate from a predetermined value, and to reverse the operation of said valves when said ratio returns to said predetermined value. 3. In a gas-off control for a digester having heat input means which Vary the pressure in the 'digester, a relief line for venting gases from the digester, a strainer in the relief line, and a steam line connected to said relief line for directing steam against the strainer from a source of steam supply, the combination with a first valve in said relief line downstream of said strainer, a second valve in said steam line, a first element responsive to fluid pressure in said digester, a second element responsive to fluid temperature in said relief line at a point between said strainer and said first valve, and means associated with said elements for actuating said first valve to maintain the aforesaid pressure and temperature within a selected ratio value, of a third element governed by said first and second elements to close said first valve and open said second valve when a relative dropin temperature causes said ratio to deviate from a predetermined value, and to reverse the operation of said valves when said ratio returns to said predetermined value. 1 1 1. In a gas off control fora digester havin heat input means whichvary the' pressure in the digester, a relief line for ventinggases'from the digester, a strainer in thejre'lief line, and a steam line connected to said relief linei for directing steam against the strainerfrom a source of steam supply, the combination with a pneumatically operated first valve in said relief linehaving" apredetermined working range of fluid operating pressure change, a second -valve saidsteamline, a first element responsiveto fluid pressure in said dige'ster, a second element responsive to fluid tem- *perature in said relief line, a source of operating fluid pressure, and'means' associated with said elements for varying fluid pressure from said source to said first valve to maintain, the digester presifatio value, of a t hird" fl'uidoperating pressure asva men-ts, and meansas'sociated a ment for varying the operating, pr essure to said first valve and (for actuatingfisaid second valve to sure and relief-line temperature within a selected eirient responsive .tor said 'loyisaid eleh said third ele- 7 close said first-valve and" open saidj second valve i when the fluid operiatingj pressurevaries from'said -predetermined working range" by a sel'ected value, and to reverse the-operation for s'aid'v alves when said operating pressure returns to said selected value. 5. In a- 'gas-ofi' control for a Y digester having heat input means which varythe pressure in a the di'gester; a relief line for venting gases-from thedigester, a strainer iirthereliefline, and a stea mline connected to' said 'elief line for dire'cting steam against thestrainer from a source valve to maintain a ratio between said ldigester pressure and relief line temperature within a "selected value, ai' three wayvalve in said fluid connection having a jnoifmally openithrough passage and a normally closed bleed, and' mechanism for actuating said'steam 'valve;of an element sensitive to changesin said output" pressure and inoperative connection with 'saldthree-way valve and with said steam valve "actuating niechanism, said element being operative to close thethrough passage and open the' bleed of said three-way valve, to permit jsaid relief'valveto'iclose, and simultaneously to"operate said"valve actuating mechanism to open said steam v'alve when said output pressure reaches a selecte'd'value, andto reverse the operation of said valves when said output pressure returnsto said selected value. 6. In a digester having :heat input means, a relief line provided with a strainer fol-venting gases from the digester, a pipe throughwhich steam may be passed'to clean said strainer, in combination, a relief valve in said relief line downstream of said strainer, a steam valve in said steam line, electrical'means for operating said valves, a ratio controller sensitive to changes in digester pressure, and sensitive to changes in relief line temperature at a point intermediate said strainer and relief valve, said controller being in operative connection with said relief valve, and Varying thevalueof avalve actuating medium from asource of supply proportionally 12 to changes in theratio value of said pressure and temperature to'maint'ain. said .ratio within. a selectedvalue, aswitch governing the operation iofsaid electricalmeans, and an element sensitive 5 to changes in thevalue of said valve actuating jlmedium operative to actuatesaid switch when said mediumis at aselected value. 7.1 Inj ajgas-off control'for a digester .having f'heat input means' iorvarying the digester pressure, arelief line, a strainer in said, relief line, a Qpipe through which-steam maybe passed to clean 1 saidfstra inenijin, ,combination, a relief valve in the relief line; ajsteam, valve .in the. steam line, an element responsive to digester pressure,- an 5: e lement responsive; to relief line temperature, '; said elements bein Operative tovary thelopening :}of said,relief v'alvelto maintain the ratio hetween said pressure and temperature within. a "selected value,and means for independentlyopi'opelfatipg said; steam valve,-,said means .bein j governed by said elements and operative to close jj said; relief valveandlopensaid steam valve when 1 a; relative dropjiri temperature causes said pressuregtemperature, ratio to exceed a predetermined valuaand to reverse ,theoperation of said valves when, said -ratio returns .to .said. predetermined I v lu 8; Inapparatus foriregulating the ventin flof 3083 through, a relief line proVided; withwa [strainer and extending-from aJpressure-vessel {evolving a heated-normally -gaseous fluid. and having a conduitithrough .which steam may pass O {clean the strainer,, first. control .means sensij1 ti velq the fluid prssurelin the vesselupstream *ofthejstrainer and. to. the. fluid temperature, in ;the,re1ief line downstream of the strainermfor varyingthe venting of .gases through the, relief line to imaintain the saidpressure and the tem- 40 perature corresponding thereto w atua. selected di fierential value, and second control meansincludingan element broughttintoaction .by-said f'first control .means, ,vvhenisaidsidiiferential-exce eds a A predetermined value, i occasioned.v by a i .3 relativeidrop ,initemperature for admitting steam ;to said conduit tQ-cIeanLthe. strainer. : 9 In'apparatus for regulatinglthewentin :of 1 gasesfthrough; a relief, line. i provided witha jfstrainer .and extending fIOm- KJDI'ESSUIB vessel to evolving a heated normallyl gaseous fiuidanol having ajcon duitithrough. which steamimay pass to clean the strainer, first. control: means sensitive to thefiuid pressure inthe vessel upstream of Qthe [strainer-i and i to the Qfluid temperature: in varying -the venting of. gases -.through--the -relief fline to maintain the. said ipressure and the temi peraturefcorrespondiiag.theretoat a'seleoted 'differential value, and .secondcontrol-means includw' i ing an element brought .intoaction by said first controlimeans when said differential exceeds a predetermined value, occasioned by a relative drop in temperature, forradmitting steam to'said -.conduit to clean the strainer, said element being :35 again'actuatedby said first control meanson the return of said differential :to said predetermined value for shutting off steamto said conduit. "10. In apparatus for regulating the ventingflof gases through a relief L line ,.p1'ovided-- with a strainer and extendingfrom a pressure vessel evolving a heated normally gaseous fluid and having aconduitthroughwhich steam maytpass to clean the strainer, first controlrmeans sensitive lto .thefiuid pressure. inltheyessel upstream of the ,75 "strainer and to thefiuid temperature, in ,the relief line at a point downstream of said strainer for varying the venting of gases through the relief line to maintain the said pressure and the temperature corresponding thereto at a selected differential value, and second control means including an element brought into action by said first control means when said differential exceeds a predetermined value, occasioned by a relative drop in temperature, for shutting off the venting of gases through the relief line and admitting steam to said conduit to clean the strainer. 11. In apparatus for regulating the venting of gases through a relief line provided with a strainer and extending from a pressure vessel evolving a heated normally gaseous fluid and having a conduit through which steam may pass to clean the strainer, first control means sensitive to the fluid pressure in the vessel upstream of the strainer and to the fluid temperature in the relief line at a point downstream of said strainer for varying the venting of gases through the relief line to maintain the said pressure and the temperature corresponding thereto at a selected differential value, and second control means including an element brought into action by said first control means when said differential exceeds a predetermined value, occasioned by a relative drop in temperature, for shuttng off the venting of gases through the relief line and admitting steam to said conduit to clean the strainer, said element being again actuated by said first control means on the relative rise of said temperature to said selected differential value for venting gases through the relief line as governed by said first control means and for shutting off steam to said conduit. 12. In apparatus for regulatin the venting of gases through a relief line provided with a strainer and extending from a pressure vessel evolving a heated normally gaseous fluid and having a conduit through which steam may pass to clean the strainer when it becomes clogged, differential temperature-pressure control means having a source of valve actuating medium, said means being sensitive to the pressure in the vessel upstream of the strainer and to the temperature in the relief line downstream of the strainer and varying the value of the valve actuating medium proportionally to the difference between said pressure and the relief line temperature corresponding thereto, a valve actuated by said controlled medium for governing the venting of said relief line to maintain said temperature-pressure difference at a selected value, a steam valve in said conduit, and an element actuated by said controlled medium and in operative connection with said relief valve and said steam valve to close said relief valve and open said steam valve when a relative drop in temperature causes the temperature-pressure difference to exceed a predetermined value. 13. In apparatus for regulating the venting of gases through a relief line provided with a strainer and extending from a pressure vessel evolving a heated normally gaseous fluid and having a conduit through which steam may pass to clean the strainer when it becomes clogged, differential temperature-pressure control means having a source of valve actuating medium, said means being sensitive to the pressure in the vessel upstream of the strainer and to the temperature in the relief line downstream of the strainer and varying the value of the valve actuating medium proportionally to the difference between said pressure and the relief line temperature corresponding thereto, a valve actuated by said controlled medium for governing the venting of said relief line to maintain said temperature-pressure difference at a selected value, a steam valve in said conduit and an element actuated by said controlled medium and in operative connection with said relief valve and said steam valve to close said relief valve and open said steam valve when a relative drop in temperature causes said controlled medium to vary beyond a predetermined value and to open said relief valve and close said steam valve when said controlled medium returns to said predetermined value. 14. In apparatus for regulating the venting of gases through a relief line provided with a strainer and extending from a pressure vessel evolving a heated normally gaseous fluid and having a conduit through which steam may pass to clean the strainer when it becomes clogged, differential pressure control means having a source of valve actuating medium, a valve in the relief line downstream of said strainer, said control means being sensitive to the temperature in the relief line at a point between said strainer and said relief valve and to the pressure in said vessel and varying the value of the valve actuating medium proportionally to the difference between said pressure and the relief line temperature corresponding thereto, and said valve being normally actuated by said controlled medium for governing the venting of said relief line to maintain said temperature-pressure difference at a selected value, a steam valve in said conduit, and an element actuated by said controlled medium and in operative connection with said relief valve and said steam valve to close said relief valve and open said steam valve when a relative drop in temperature causes the temperature-pressure difference to exceed a predetermined value, and to open said relief valve to vent gases from the relief line as governed by said control means and simultaneously close said steam valve when said difference returns to the predetermined value, 15. In apparatus for regulating the venting of gases through a relief line provided with a strainer and extending from a pressure vessel evolving a heated normally gaseous fluid and having a conduit through which steam may pass to clean the strainer when it becomes clogged, differential pressure control means having a source of valve actuating medium, a relief valve in the relief line downstream of the strainer, said control means being sensitive to the temperature in the relief line at a point between said strainer and said relief valve and to the pressure in said vessel and varying the value of the valve actuating medium proportionally to the difference between said pressure and the relief line temperature corresponding thereto, and said valve bein normally actuated by said controlled medium for governing the ventin of said relief line to maintain said temperature-pressure difference at a selected value, a steam valve in said conduit, and an element sensitive to the value of said controlled medium and in operative connection with said relief valve and steam valve to close said relief valve and open said steam valve when the controlled medium exceeds a predetermined value, and to open said relief valve to vent gases from the relief line as governed by said control means and simultaneously close said steam valve when said controlled medium is returned to said predetermined value. JAMES M. MCALEAR. (References on following page) The following :references are of 'record" 1n;the REFERENCES .CITED file of this patent: UNITED STATES :PA'IENTS Number Name fiDate 'De' Mers May 23', 1933 Number

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    US-2887427-AMay 19, 1959Worthington CorpMethod and apparatus for controlling pulp digesters
    US-2896709-AJuly 28, 1959Foxboro CoDigester relief system
    US-2909239-AOctober 20, 1959Worthington CorpDigester control systems
    US-2980207-AApril 18, 1961Simon Ltd HenryAir filters
    US-2993827-AJuly 25, 1961Worthington CorpMethod and apparatus for controlling wood pulp digesters
    US-2996422-AAugust 15, 1961Ed Jones CorpDigestion of cellulosic material
    US-3350012-AOctober 31, 1967Foxboro CoBoil control system
    US-3788472-AJanuary 29, 1974Carter R CoWaste water process tank control facility
    US-4923068-AMay 08, 1990Cdp Product Development CorporationAutomatic filter system