US2909239A

US2909239A - Digester control systems

Info

Publication number: US2909239A
Application number: US556958A
Authority: US
Inventors: Andrew E Bennett
Original assignee: Worthington Corp
Current assignee: Worthington Corp
Priority date: 1956-01-03
Filing date: 1956-01-03
Publication date: 1959-10-20
Anticipated expiration: 1976-10-20

Description

Oct. 20, 1959 A. E. BENNETT v DIGESTER CONTROL SYSTEMS 3 Sheets-Sheet 1 Filed Jan. 5, 1956 (1ttomeg Oct. 20, 1959' A. E. BENNETT 9,

I DIGESTER CONTROL SYSTEMS Filed Jan. 3, 1956 5 Sheets-Sheet 2 Amok/5w E. BEAM Err VB L14. M

(Ittorneg Oct. 20,1955

A. E. BENNETT 2,909,239

DIGESTER CONTROL SYSTEMS Filed Jan. 3. 1956 3 Sheets-Sheet 3 Inventor ANDREW E; BENNETT BB c wM U it d tate m Q 2,909,239 DIGESTER CONTROL SYSTEMS Andrew E. Bennett, We'ym'outh, Mass., ass'ignor, by mesne assignments, to Worthington Corporation, New York, N.Y., a corporation of Delaware Application January 3, 1956, Serial No. 556,958 4 Claims. (Cl. 18361) This invention relates to the control of wood pulp digesters and particularly concerns means, for use in connection with the relief of gases, for clearing a relief line screen in the digester, said screen being commonly employed to prevent solid and semi-solid material from entering the relief conduit.

In the manufacture of wood pulp, it is customary to fill a digester with wood chips, of a character suited to the product, to which is added a charge of cooking liquid of a selected volume, and thereafter to set up a reaction by the admission of steam or other heating agent. It is common practice to raise the cook temperature, as rapidly as possible without impairing the product, to a selected maximum value in accordance with a predetermined vapor-pressure-temperature curve, at which value the cook temperature is preferably maintained until the pulp has attained the' required fiber separation.

In the course of this cooking process, gases in the form of air and volatile material, such as turpentine or the like, are given off, and it is usual to vent these from the top of the digester as required to maintain predetermined temperature and pressure values therein. It will be understood that the eflicient operation of a digester depends upon penetration of the cooking liquor into all of the chips, and that this in turn depends upon proper circulationthroughout the entire mass. The characteristics of the cook mass are such that circulation tends to be extremely sluggish, particularly during the early stages of the cook, and due to the chemical constituents of the liquor solution, and to the reaction taking place by the application of heat, there is a tendency toward foaming which under certain conditions results in carrying liquor over into the relief line and lodging masses of solid or semi-solid material against the screen, thereby clogging the screen and seriously interfering with the operation of the cooking process. It therefore becomes necessary to clear the screen as soon as it becomes clogged, and for this purpose it has been customary to blow steam back through the relief line into the digester to free the screen of clogging matter. 7

I am aware that various means have been utilized for clearing the screen, such as by employing a hand operated steam valve, or automatic devices for manipulating the valve. Among such automatic devices it has been proposed to measure the pressure differential across the screen, and when the differential pressure has reached a predetermined value, a purging cycle is introduced by opening a steam valve and closing a relief valve under the control of instrumentalities which include electrical connections and a time-relay, whereby steam or other purging fluid is admitted to the relief line to purge the screen for a selected period of time. If the screen is not cleared, the cycle is repeated.

The difiiculty with screen purge or blow-back systems as utilized heretofore, so far as known to me, is that the blow-back medium enters the dome of the digester in excessive amounts, thereby raising the dome temperature and pressure which results in disrupted circulation. Furthermore, when the purge cycle is terminated and digester relief is again established, the excessive steam must be relieved which increases the load on the vapor condensing system. This is due to the fact that there can be no predetermined time for a purge cycle to function as it Patented Oct. 20, 1959 2 should, since a proper purge cycle varies with the existing digester pressure and has no relation to a definite uniform time period.

I have discovered that a surge of pressure on the relief line side of the digester screen resulting in a hammerlike shock, the force of which exceeds the digester pres sure, not only is very effective to clear the screen, but does not interfere with the digester pressure, temperature, or circulation, provided the force of the shock is controlled and immediately terminated. As is hereinafter more fully pointed out, this is accomplished by means, embodying this invention, which respond immediately under some cook conditions to a selected pressure drop across the screen, and under other cook conditions to varying selected pressure drops across the screen to close: the relief valve and open the purge valve to introducea blast of steam into the relief line, thus raising the relief? line pressure sufliciently to provide the necessary shock and thereafter functions to abruptly close the purge valve. and open the relief valve to complete the purge'cycle be-- fore the digester pressure, temperature, and circulation; have been affected. If the screen has not been sufficiently cleared by the blow-back cycle, the purge is immediately" repeated until the screen has been adequately cleaned.

Experience has indicated that a series of repetitive short:

duration pressure pulses is more effective in clearing the; screen'than a prolonged purge.

In actual practice, it has been found that since circula tion of the cook mass is particularly sluggish during: the initial stages of the cook, there is a greater tendency at this part of the cook cycle for surging and foaming to occur which tend to carry over into the relief line solid and semi-solid matter, thereby causing the screen to become clogged. On the other hand, I have found that as the digester pressure approaches and arrives at the maximum pressure utilized during the holding period of the cook, while the circulation is less sluggish than at the early stages, there is a tendency for the pressure to fluctuate somewhat due to various causes depending on the type of digester employed and the constituents which make up the cook mass. Thus the cooking conditions change during the cooking process and when required, in order to meet this situation and provide effective means for clearing the screen under these changing conditions, I govern the operation of the purge valve and relief valve by means which respond to a pressure drop across the a screen. The means include a differential pressure respouse mechanism combined with a ratio adjustment for said mechanism and with mechanism which provides a so-called differential gap-action, whereby the pressure drop across the screen required to initiate the purge cycle is gradually increased during the pressure rise period, and whereby the purge cycle is terminated at a selected pressure in excess of the digester pressure at the initial stage of the cook, which excess pressure is gradual- 1y reduced to some selected value at the maximum pressure maintained in the digester during the holding period. Thus at a digester pressure of 5 p.s.i., for example, and a gap-action range of 20 p.s.i., the purge cycle may be initiated when the relief line pressure drops to 0 p.s.i. and the cycle may be terminated When the relief line pressure reaches 20 p.s.i. By this means the cycle is initiated at the available relief line pressure drop and is terminated when the relief pressure is suiiiciently in excess of the digester pressure to clear the screen at a time in the cook period when there is the greatest tendency for the screen to become clogged. On the other hand, when the digesterpressure reaches its maximum value, for example IOO p.s.i., the control mechanism may be arranged so that the purge cycle is not initiated until the relief pressure 7 drops to p.s.i. and is terminated when the relief pres-- sure reaches p.s.i. Thus the pressure drop required to initiate the purge cycle is such that fluctuations which may occur in the digester pressure at this period of the cook do not cause so-called false blow-backs. Furthermore at this stage of the cook, better circulation is obtained so that there is less tendency for the screen to become clogged, and again the consistency of the pulp is such that less excess relief pressure is required to clear the screen.

While it is preferable to control the purge cycle in the manner just described, the control mechanism embodying my invention may be arranged, when conditions permit, to initiate the purge cycle at a selected pressure drop across the screen and to terminate the cycle at a selected excess of relief line pressure above the digester pressure, which drop in pressure and excess of pressure remain constant throughout the digester pressure rise period.

It is an object of this invention to provide means for clearing the screen of a pulp digester, whereby when the screen becomes sufiiciently clogged to interfere with the relief of volatile fluids, the relief valve is closed and purging fluid under pressure is admitted to the relief line to provide to the screen a hammer-like shock without introducing purging fluid into the digester in quantities sufficient to interfere with the cooking process.

It is an object of this invention to provide means for clearing the screen of a pulp digester, whereby when the screen becomes sufliciently clogged to interfere with the relief of volatile fluids, the relief valve is closed and purging fluid under pressure is admittted to the relief line to provide a sudden increase in relief line pressure, which increase exceeds the pressure obtaining in the digester by a selected amount, the admission of purging fluid being then abruptly terminated, whereby the screen is subjected to a hammer-like shock for the purpose of clearing the screen and the abrupt termination of purging fluid prevents sufficient purge fluid from entering the digester to interfere with the cooking process.

It is an object of this invention to clear the screen of a pulp digester by means which respond to varying pressure drops across the screen during the pressure rise period of the cook to close the relief valve and admit purge fluid under pressure to the relief line, thereby providing relief line pressures which exceed the digester pressure by amounts which vary during the said pressure rise period.

It is still another object of this invention to provide purge cycle control mechanism which functions to govern a purge valve and relief valve to initiate a purge cycle for the purpose of clearing the screen and, if necessary, to repeat said cycle until the screen has been cleared, at which time the relief valve may again be manipulated by any suitable means for governing the relief of volatile fluid throughout the cooking process.

These and other objects of this invention will be more fully understood from the following description when taken in connection with the accompanying drawings, wherein:

Fig. 1 is a view, partly in diagram, of a pulp digester of the direct steam heated type, showing automatic blowback mechanism, embodying my invention, for clearing the digester screen, together with means for governing the steam input to the digester and with means for varying the relief of volatile fluids therefrom;

Fig. 2 is an enlarged fragmentary view showing the gas relief screen mounted in the neck of the digester;

'Fig. 3 is a front elevation, enlarged and partly in diagram, of the controller shown in Fig. 1 for governing the blow-back mechanism;

Figs. 4 and 5 are details of a portion of the mechanism shown in Fig. 3, illustrating various operating positions; and

Figs. 6 and 7 are graphs showing the effect of'eertain adjustments which may be made with the control mechanism shown in Fig. 3.

Having reference to the drawings and particularly to Fig. 1, there is shown a digester, generally indicated at 10, which includes a body portion 11, a cone-shaped bottom '12, a dome 1 3, and a neck 14, mounted on the dome, having the usual detachable cover 15 to provide for charging the digester with wood chips. The bottom 12 has connected therewith a pipe 16 equipped with a hand valve 17 through which heated cooking liquor may be introduced and the digester filled to a level as indicated, for example, by dotted line 18. The pipe 16 and valve 17 are also utilized to discharge the finished pulp at the end of the cook.

The digester 10 is provided with a relief line 20 which is connected with the neck 14, the neck being herein shown as including a removable screen 21. As more clearly shown in Fig. 2, the screen 21 may be of cylindrical shape and at the lower end may have an annular flange 22 secured thereto which is adapted to rest on an internal flange 23, suitably attached to the neck 14. The upper end of the screen 21 has an annular flange 24, also secured thereto, which engages the inner surface of a gasket 25 disposed between the cover 15 and the neck 14 to which the cover may be secured, as by stud bolts 26.

The relief line 20 includes a relief valve 27 which may be actuated by a diaphragm 28 partially defining a pressure chamber 29, said chamber being in connection with a three-way pressure operated valve 30, by means of a pipe 31. The relief valve 27 is herein shown as of the reverse acting type, and preferably includes a double seated valve member 32 which is connected with the diaphragm 28 by means of a stem 33, pressure in the chamber 29 being opposed by the usual compression spring 34. The three-way valve 30 has a valve member 35 which cooperates with oppositely disposed valve seats 36 and 37 and may be actuated by a diaphragm 38, partially defining a pressure chamber 39, to which the output pressure from a controller 40 is connected by means of a pipe 41. The diaphragm 38 of the three-way valve is connected with the valve member 35 by a stem 42 and is backed by the usual compression spring 43. The threeway valve 30 has three connections, namely, a connection 44 for operative connection with the relief valve 27, a connection 45 for receiving a regulated supply of operating fluid pressure, and a connection 46 to atmosphere. The connection 45 for receiving operating fluid pressure communicates through a pipe 47 with a pressure reduction valve 48 in turn connected with a source of operating fluid pressure, not shown, by means of a pipe 49. The valve 48 may be provided with the usual valve member 50 which cooperates with a seat 51 and functions to vary pressure through the valve as governed by a diaphragm 52. The diaphragm 52 responds to changes in pressure in an outlet chamber 53 of the valve and functions to maintain a selected pressure in the pipe 47 as determined by a compression spring 54, disposed at the upper side of the diaphragm, the tension of which may be suitably varied by a handwheel 55 to provide varying pressures, as indicated by a suitable pressure gauge 56, for actuating the relief valve 27. The three-way valve 30 is so arranged that the

pipes

47 and 31 are normally connected, and the connection 46 with atmosphere is closed. Thus unless output pressure from the controller 40 is applied to the three-way valve diaphragm 38, the relief valve 27 may be manually governed by the reduction valve 48.

While I have herein shown a manually operated pressure reduction valve 48 for governing the relief valve to vary the relief of gases from the digester as required for the effective operation of the cooking process, it will be understood that instead of the manually operated pressure control valve, any suitable form of automatic gas relief control mechanism may be utilized for the purpose.

The relief line 20 has connected therewith a blow-back line 59 which communicates with a supply under pressure, not shown, of a suitable purge fluid, such as steam, for

aeoaasa the purpose of clearing the screen 21 when it becomes clogged with solid or semi-solid material. The flow of purge fluid through the blow-back line 59 is governedby a valve 60 which may be generally similar in construction to the relief line valve 27. The purge valve 60 is also preferably of the reverse acting type and includes a diaphragm 61 which partially defines a pressure chamber 62 connected with the output pressure line 41 from the controller 40 by means of a pipe 63. Pressure in the chamber 62 is opposed by a compression spring 64 which surrounds a stem 65 connecting the diaphragm 61 with a suitable valve member, not shown. As is the case with the relief valve 27, the blow-back valve 60 is normally closed by the spring 64, is opened on an increase of pressure in the diaphragm chamber 62, and functions together with the relief valve, as governed by the controller 40, to initiate and terminate purge fluid cycles for the purpose of clearing the screen 21 in a manner to be described.

The digester is also provided with a steam admission line 70 which connects a source of steam supply under pressure, not shown, with a perforated ring type manifold 71, or an equivalent steam distribution device disposed in the upper portion of the digester bottom '12, by means of a plurality of pipes 72. Steam admission to the digester through the pipe 70 may be varied by a valve 73 as governed by a time cycle pressure controller 75 which responds to pressure changes, preferably at the bottom 12 of the digester, through a pipe 76, and functions to raise the cook pressure to a selected maximum value in accordance with a predetermined pressure rise curve, and to maintain the pressure at said selected maximum value over a predetermined period of time. For this purpose, the controller 75 may be of any well-known construction, and as herein illustrated is of the pneumatic type. It receives a supply of operating fluid under regulated pressure through a pipe 77 from a source, not shown, and responds to changes in digester bottom pressure to vary output pressure to the diaphragm 78 of the steam valve 73 to vary the digester pressure in accordance with the predetermined cook cycle. Pressure from the controller 75 actuates the valve 73, and for this purpose is connected by means of a pipe 80 with a pressure chamber 79, partially defined by the diaphragm 78. The diaphragm 7-8 is backed by the usual compression spring 81 and, by means of a stem 82, actuates a valve member, not shown, to vary steam flow to the digester as required to conform to the cook schedule. And while I have herein shown a preferred form of steam admission control mechanism for governing the digester pressure throughout the cook cycle, it will be understood that any suitable'means may be utilized for this purpose.

The controller 40, which governs the blow-back cycle, responds to varying pressure drops across the screen 21 by means of a pipe 83, connected with the dome 13 of the digester upstream of the screen, and by a pipe 84 which connects with the relief line 20 downstream of the said screen. The controller combines a differential pressure response mechanism with differential gap-action mechanism, whereby under some cook conditions, the blow-back valve 60 and relief valve 27 may be actuated to initiate a purge fluid cycle when a selected pressure drop occurs across the screen 21, and may be again actuated to terminate the cycle when the relief line pressure exceeds the digester pressure by a selected value. However when the cook conditions require that the controller functions to initiate the purge cycle when selected pressure drops occur across the screen 21, which pressure drops increase proportionally with the increase in digester pressure, and said conditions also require that the cycle be terminated when the relief line pressure exceeds the digester pressure by selected amounts WhlCh decrease proportionally with the digester pressure, I provide a ratio adjustment means for the pressure diiferential responsive mechanism, which means function together with the said mechanism and with the differential gap-action 8 device to accomplish this purpose. will now be described.

The controller 40 may be similar to that shown in the United States patent to Robins, No. 2,585,347, with pressure differential response mechanism having a ratio adjustment incorporated therein. When so arranged, the controller referred to will accomplish the objects of this invention when applied to a pulp digester to govern purge cycle mechanism. In order 'to simplify this description however, the controller shown in Fig. 3 is a partially diagrammatic illustration of mechanism suited to the purpose. The controller may be mounted in a case, indicated by broken lines 85, having the usual back plate 86 on which the mechanism is supported. This coinprises dilferential pressure response mechanism including the Bourdon coils and 91, a differential pressure setting device, generally indicated at 92, a ratio adjustment for said mechanism, generally indicated at 88 and a differential gap-action device, generally indicated at 93. The coil 90 is responsive to digester pressure by means of the pipe 83, and the coil 91 is responsive to relief line pressure by means of the pipe 84, each coil having a fixed end 94 to which the said pipes connect. The coil 90 has a shaft 95, disposed centrally thereof, with which the free end of the coil is in operative connection, it being understood that the shaft is rotated by the coil in proportional relation to changes in digester pressure. Similarly the coil 91 is provided with a central shaft 96 which is rotated by the coil in proportional relation to changes in relief line pressure. The coil 90 may have a pressure range from 0 p.s.i. to 150 p.s.i., a range which is ample for digester cooks, and the shaft of the said coil may have secured thereto an indicator 97 which is adapted to cooperate with a scale 98 having a similar pressure range. The pressure relief line coil 91 may also have a range of O p.s.i. to 150 p.s.i., and an indicator 99 may be secured to the shaft 96 to cooperate with a suitable scale 100.

The controller 40 operates to indicate the pressure setting of the controller in a manner to be described.

The controller 40 functions to maintain the relief line pressure on a selected basis in reference to the digester pressure and in the event that the relief line pressure drops below the digester pressure by a selected amount, the coil 91 actuates the gap-action mechanism 93 to initiate a screen purge cycle over a selected range of relief line pressure change, and thereafter functions to abruptly terminate said cycle. To this end, the coils 90 and 91 and the index 102 are mechanically interconnected by a differential linkage. The index 102 is positioned on the scale 101 by means of the digester pressure responsive coil 90 to indicate the setting of the controller. For this purpose, the index 102 is mounted on a fixed pivot 103 at its lower end and has a pivotal connection 104 with a link 105 midway between the ends of the said link. The link 105 has at one end a pivotal connection 106 with the upper end of a link 107, and at the other end is pivoted at 108 with a floating link 109. The link 107 has, between its ends, a pivotal connection 110 with a sector plate 111 and at its lower end has an adjustable pivotal connection 112 with one end of a link 113 the opposite end of which is pivotally connected at 114 with the lower end of the indicator 97. The lower end of the link 109 is pivoted at 117 to one end of a link 118 the other end of which has a pivotal connection 119 with the lower end of the indicator 99. The floating or differential link 109 is pivoted at 115, midway between its ends, to one end of a link 116, herein shown broken for clarity, by which the gapaction mechanism is actuated in a manner to be dmcribed.

The gapaction mechanism 93 includes a nozzle 125, a batlle 126 and a bellows 127. The nozzle receives operating fluid, such as air under regulated pressure, from a source, not shown, through pipes 128 and 129,

In order to simplify the description, a' third scale 101 is shown with which an index 102 cothe former having a restriction 130 of a capacity less than that of the nozzle so that the nozzle pressure is determined by the relative position of the nozzle and bafile. The bellows 127 has a fixed end 131 and a free end 132 to which a post 133 is rigidly secured. The interior of the bellows 127 connects with nozzle pressure in the pipe 129 by means of a pipe 134, and the pipe 129 in turn connects with the pipe 41 by which the nozzle pressure is communicated to the diaphragm chamber 62 of the steam valve 60 and to the diaphragm chamber 39 of the three-way valve 30.

Means for actuating the baflle 126 in respect to a nozzle 125 include an arc-shaped member 135 having at its lower end a pivotal connection 136 with the bellows post 133 and at its upper end a pivotal connection 137 with the link 116. The bafile 126 is pivotally mounted at 138, between its ends, on a fixed post 139, one end of the battle being adapted to cooperate with the nozzle 125 and being normally held in engagement with same by a compression spring 14% disposed at the opposite side of the pivot 138 and confined between the baffle and a fixed member 141. Means for positioning the baffle 126 in respect to the nozzle 125 is in the form of a baffle actuating member 142 rigidly secured to a rotatable arm 143 pivotally connected at its upper end 144 to a fixed bracket 145 and at its lower end having a pivotal connection 146 with one end of a link 147. The other end of the link 147 is connected by an adjustable pivot 148 with the arc-shaped member 135. The radial center of the arc-shaped member 135 is at the pivot 146 and the adjustable pivot 148 cooperates with a slot 149 in the arc-shaped member and is provided with locking means 150 for clamping the pivot at any selected position lengthwise of the slot.

The gap-action operation can best be visualized from Figs. 4 and 5. As is well known, gap-action control is obtained by the use of positive feedback. Let it be assumed, as shown in Fig. 4, that the pivotal connection 115 of the link 116 has been moved in the direction of the arrow from some position B to position A, at which position the bafile actuating arm 142 permits the spring 140 to cause the bafile 126 to start to throttle the nozzle 125. The increase in nozzle pressure will start to expand the bellows 127 which will lift the arm 142 away from the baffle, thereby increasing the nozzle pressure to the supply pressure. The parts will then assume the dotted line position shown. No further action takes place until the pivotal connection 115 moves in the direction of the arrow from position A to some position B, at which position the baffle actuating arm 142 will again engage the bafile and start to decrease the nozzle pressure. This position of the parts is shown in full lines in Fig. 5. The decrease in nozzle pressure starts to collapse the bellows 127, thereby further uncovering the nozzle and immediately reducing the nozzle pressure to p.s.i. The parts position just referred to is shown in Fig. 5 in dotted lines. It is evident that the motion AB of link 116, necessary to change to the nozzle pressure between 0 p.s.i. and supply pressure, may be varied by adjusting the position of the pivot 148 along the slot 149 of the arc-shaped member 135, the gap range being increased by moving the pivot 148 towards the pivotal connection 136 of the arcshaped member with the post 133, and being decreased when the pivot 148 is moved in the opposite direction. By the proper arrangement of parts, the gap distance AB may be centered about point M which is at the center of the gap band, and the point M is always at the center of the band at any band width to which the mechanism may be adjusted.

The differential pressure link 109, shown in Fig. 3, is positioned around the pivot 117 by the coil 90 in response to changes in digester pressure, and the link is positioned around the pivot 193 by the coil 91 in response to changes in relief line pressure. Since the index 192 is positioned on the scale 101 by the indicator 97 of the coil on a selected adjustable basis, to be described, each position of the index 102 will require an identical position of the relief line pressure indicator 97 to position pivot 115 at the mid point M of the differential gap. Thus the index 102 sets the center M of the differential gap-action.

As previously referred to, the index 102 is positioned by the digester pressure indicator 97 on some selected adjustable basis. For this purpose, as herein shown, the sector 111 is utilized to introduce a differential setting between the indicator 97 and the index 102. The sector 111 has a fixed pivotal connection 120 at its lower end and at its upper end is provided with a toothed arc-shaped section 121 which cooperates with a manually operable pinion 122. Thus when the sector 111 is in the mid position shown, the position of the index 102 on the scale 101 corresponds with the position of the digester pressure indicator 97 on the scale 98. However if the sector 111 is rotated clockwise about its pivot, the index 102 is moved up the scale 101 in respect to the indicator 97, and when the sector is turned counterclockwise, the index 102 is moved down the scale in respect to the indicator 97. And since the index 102 is connected with the gap-action link 116 by means of links 105 and 109, the relief pressure coil 91 operates around the index setting to provide relief line pressures to initiate and terminate purge cycles when the screen becomes clogged.

As shown in Fig. 3, the sector 111 is at its mid position of the range of adjustment, and the pivots 112 and 106 are equidistant from the pivot on which the link 107 is mounted to provide

equal leverage arms

151 and 152. Therefore the differential pressure setting between the digester indicator 97 and the index 102 is O p.s.i., and since the

leverage arms

151 and 152 of the link 107 are equal in length, the position of the index 102 on the scale 101 coincides with the position of the indicator 97 on the scale 98 at all pressures obtaining in the digester throughout the cook period. Therefore as shown in Fig. 3, since the differential pressure setting is 0 p.s.i., the drop in relief line pressure across the screen 21 required to initiate a purge cycle, and the amount of relief line pressure in excess of digester pressure required to terminate said cycle, is governed by the width of the differential gap-action band.

Diiferential pressure settings other than 0 p.s.i. are obtained by the manually operated pinion 122, by which the sector 111 is rotated as described above, which positions pivot 110 thus introducing a constant differential between indicator 97 and index 102 over the entire scale range. The index 102 can be made to lead the indicator 97 or lag behind the said indicator by moving the pivot 110 to the right, or to the left, respectively. The introduction of 5 p.s.i. leading pressure differential is illustrated by the graph shown in Fig. 6. The graph shows the relationship of the index 102 to the digester pressure indicator 97 throughout the digester pressure rise. The digester pressure is plotted on the abscissa, indicated by the full line X, and the differential pressure between the indicator 97 and index 102 is plotted on the ordinate of the graph. Plus values indicated that the index leads the indicator and minus values indicate that the index lags behind the indicator. The graph illustrates a 5 p.s.i. lead of the index in relation to the indicator, the position of the index being shown by the dash line Y. If it be assumed'that the controller differential gap setting is 20 p.s.i., a gap band corresponding to this width is illustrated by the shaded area and shows that the index line Y is at a mid position in the band. The upper edge U of the band is at plus 15 p.s.i. and the lower edge L is at minus 5 p.s.i. Therefore if the relief line pressure drops to 5 p.s.i. below the digester pressure at any point in the scale range, the gap-action device 93 will initiate a purge cycle, and when the relief pressure increases to 15 p.s.i. above the digester pressure, the gap-action device will terminate the said cycle. With this control adjust ment the purge cycle is initiated at the same 5 p.s.i. pressure drop across the screen throughout the digester pres sure rise, and also throughout said rise the purge cycle is terminated at the same excess of relief pressure over the pressure obtaining in the digester.

In addition to the differential pressure adjustment herein above described, I provide the ratio adjustment 88. The adjustable pivot 112 cooperates with a slot 160 in the lower end of the link 107 and, by means of a suitable clamp 161, may be adjusted to any selected position therein. Thus when the pivot 112 is adjusted to provide an equal length to leverage arms 151 and '152 a one-one ratio is obtained so that the movement of the index 102 is equal to that of the indicator 97. The ratio may be increased or decreased by moving the pivot 112 up or down, respectively, in the slot 160.

The graph illustrated in Fig. 7 shows the controller adjusted to the same 5 p.s.i. pressure setting as shown in Fig. 6, but with a ratio setting wherein the motion of the index 102 is 0.9 of the motion of the indicator 97. It will be noted from the graph that the index line Y slopes rom a 5 p.s.i. lead at p.s.i. digester pressure to a p.s.i. lag at 100 psi. digester pressure. The differential gap band as shown by the shaded area is again assumed to have a width of 20 p.s.i. If the controller 40 is adjusted as shown in Fig. 7 when the digester pressure is at 5 p.s.i., if the relief line pressure drops to 0 p.s.i., the coil 91 will turn the indicator 99 counterclockwise and, by means of the differential link 109 will operate the gapaction device 93 to provide a sudden increase in output pressure to supply pressure in pipe 41. Thus the threeway valve 30 will disconnect the diaphragm chamber 29 of the relief valve 27 from the pressure reduction valve 48 and connect the said diaphragm chamber with atmosphere, thereby permitting the relief valve to close. At the same time the purge valve 60 will be open Wide. The relief line pressure will rapidly rise to 20 p.s.i. to deliver the required shock to the screen 21 and will thereupon be abruptly terminated before substantially any steam can enter the digester. As the digester pressure is increased during the cook cycle, while the gap band remains at the same Width, the pressure drop across the screen required to initiate the cycle gradually increases and the excess of relief line pressure above the digester pressure gradually diminishes. Thus when the digester pressure reaches 100 p.s.i., the purge cycle is initiated at a pressure drop across the screen of 15 p.s.i. or, namely, at a relief line pressure of 85 p.s.i., whereas the cycle is terminated at an excess relief pressure of 5 p.s.i., or at 105 p.s.i.

It will be understood from the above that the control apparatus embodying this invention combines means responding to digester pressure and to relief pressure with an adjustable diflferential pressure control setting means having ratio adjustment means associated therewith when required by the cooking conditions, and differential gapaction means governed by the aforesaid means for actuating some suitable form of valve means for initiating and terminating a purge cycle for the purpose of clearing the digester screen when required without interfering with the efficient operation of the cook process.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a control for a digester having heat input means to vary the pressure in the digester, a relief line for venting gases from the digester, a screen in the relief line, and conduit means connected to said digester downstream of said screen and to a source of purging fluid for directing said purging fluid against the screen, the combination with valve means in said relief line, means operatively connecting a source of operating fluid to said valve means to regulate the venting of gases from the digester, and valve means in said conduit means normally closed, of a control mechanism coacting with the respective valve means to initiate and terminate a purge cycle for clearing the screen comprising, a first means responsive to fluid pressure in said digester, a second means responsive to fluid pressure in the relief line, linkage means operatively inter-connecting said first and second pressure responsive means to maintain a selected pressure differential between said relief line and said digester pressures respectively, a gap acting control valve means connected to said source of operating fluid and each of said valve means, and mechanical means connecting said gap acting control valve means to said linkage means so constructed and arranged that when the relief pressure drops below the preselected differential pressure setting the relief valve means will be closed and the purge conduit valve means opened and the reverse operation of these valves will occur when the relief line pressure returns to the predetermined ditferential pressure setting.

2. in a control for a digester as claimed in claim 1 including, a ratio mechanism associated with the linkage means for varying the selected differential pressure setting in proportional relation to changes in digester pressure.

' 3. In a control for a digester having heat input means to vary the pressure in the digester, a relief line for venting gases from the digester, a screen in the relief line, and conduit means connected to said digester downstream of said screen and to a source of purging fluid for directing said purging fluid against the screen, the combination with valve means in said relief line, means operatively connecting a source of operating fluid to said valve means to regulate the venting of gases from the digester, and valve means in said conduit means normally closed, of a control mechanism coacting with the respective valve means to initiate and terminate a purge cycle for clearing the screen comprising, a first means responsive to fluid pressure in said digester, a second means responsive to fluid pressure in the relief line, linkage means operatively inter-connecting said first and second pressure responsive means to maintain a selected pressure differential between said relief line and said digester pressures respectively, a gap acting mechanism including, control valve means having a selected range of operating pressure change connected to said source of operating fluid, a positive feedback means responsive to changes in output fluid from said valve, means operatively connecting said gap acting control valve to said relief valve means and said purge conduit valve means, and mechanical means operatively connecting said linkage means and said feedback means with said control valve means to open the purge valve means and close the relief valve means at one limit of said operating pressure range when the pressure in the relief line drops below the predetermined differential pressure setting and to close the purge valve means and open the relief valve means when the pressure in the relief line exceeds the predetermined differential pressure setting.

4. In a control as claimed in claim 3 wherein said mechanical means includes, an arc-shaped member pivotally connected at one end to said feedback means and movable therewith, a connecting means between said arc-shaped member at the end remote from said pivotal end and said linkage means, and an actuating arm adjustably connected to the medial portion of said arcshaped member and to said control valve for actuating said control valve in accordance with signals from said linkage means.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Pulp and Paper Manufacture, Stephenson, c, McGraw- Hill, 1950, pages 453-483, particularly pp. 456, 464, 466, 468-471, 479-481.