SE535003C2

SE535003C2 - Device for degassing control

Info

Publication number: SE535003C2
Application number: SE1050462A
Authority: SE
Inventors: Peter Danielsson; Magnus Monsell
Original assignee: Metso Paper Inc
Priority date: 2010-05-10
Filing date: 2010-05-10
Publication date: 2012-03-13
Also published as: BR112012028840A2; CN102869833A; SE1050462A1; WO2011142711A1; CN102869833B; EP2569482A1

Abstract

ABSTRACT Method and arrangement (1) for pumping pulp, Which arrangementcomprises a housing (2) in Which a pulp pump (4) for pumping pulp isarranged, Wherein a vacuum pump (5) for degassing the pulp inside thehousing (2) is connected via a conduit (6) to a gas outlet (7) of the housing(2). A control unit (13) is arranged to control the pump speed of the vacuumpump (5) and in that the control unit (13) is connected to a sensor (8) that isarranged in connection to the gas outlet (7) for detecting the presence of pulpin the conduit (7), Wherein the control unit (13) is arranged to lower thepump speed of the vacuum pump (5) if pulp is detected in the conduit (6). (Fig. 1)

Description

lO ARRANGEMENT FOR DEGASSING CONTROL TECHNICAL FIELD The invention relates to an arrangement in a pulp pumping process, and inparticular the invention relates to an arrangement for degassing control in apulp pumping process where pulp is pumped and simultaneously degassed.

BACKGROUND In pulp pumping arrangements, gas (e.g. air) accompanying the pulp causesproblems. Typically, the gas tends to cumulate to bubbles that areaccumulated upstream of the pump. Therefore, a vacuum pump is oftenconnected via a conduit to the pulp pumping area, in order to create a subpressure to evacuate the unwanted gas.

A problem with such arrangements is that pulp may follow the gas into thevacuum pump conduit and cause problems. Firstly, the pulp may completelyor at least heavily impede the evacuation of gas such that the gas bubblesstill may be accumulated in the pulp that is being pumped. Further, thepulp may get sucked into the vacuum pump and thereby compromise itsfunction.

In WO 02 / 075047 this problem is solved by arranging a means for supplyingﬂushing water into the conduit, such that unwanted pulp is ﬂushed awayfrom the conduit. However, this solution is not fully satisfactory as it resultsin an unwanted dilution of the pulp, or, if the ﬂow of water is too small, anunsatisfactory removal of pulp from the conduit. Hence, a compromise ariseswhere the operator has to choose between an unwanted dilution, and anunsatisfactory removal of pulp from the conduit.

WO 03/ 014572 discloses a centrifugal pump for pumping a pulp suspensionin which the amount of liquid added to the pulp may be reduced in thatliquid is only added to the system upstream of the vacuum pump, whereasin conventional arrangements liquid has also been added downstream of thevacuum pump in order to guarantee its well function.

Both of these arrangements do however imply a dilution of the pulp, andfurther, they allow very small possibilities to control the evacuation of gasfrom the pulp pumping chamber. lO SUMMARY An object of the invention is to provide an improved arrangement andmethod for pumping pulp, which eliminate or reduce the problems of theprior art cited above, and which allows better control of the evacuation of gasfrom the pulp pumping chamber.

According to a first aspect the invention relates to an arrangement forpumping pulp, which arrangement comprises a housing in which a pulppump for pumping pulp is arranged, wherein a vacuum pump for degassingthe pulp inside the housing is connected via a conduit to a gas outlet in thehousing. A control unit is arranged to control the pump speed of the vacuumpump and the control unit is connected to a sensor that is arranged in theconduit for detecting the presence of pulp in the conduit, wherein the controlunit is arranged to lower the pump speed of the vacuum pump if pulp isdetected in the conduit.

According to a second aspect the invention relates to a method of pumpingpulp by means of a pulp pump located inside a housing, which methodcomprises: degassing the pulp that is being pumped inside the housing bymeans of a vacuum pump, which is connected via a conduit to said housing.The method further comprises: sensing if pulp is present in the conduit,indicating to a control unit if pulp is present in the conduit, and controllingthe vacuum pump in response to the indication received by the control unit.

With the invention according to the first and second aspect the problems ofthe prior art are eliminated or at least reduced, and further, a better controlof the evacuation of gas from the pulp pumping chamber may be achieved.

In an advantageous embodiment of the method according to the inventionthe control unit is arranged to lower the pump speed, i.e. the rotation speedof the vacuum pump if pulp is detected in the conduit.

In another advantageous embodiment the sensing of pulp comprises the stepof measuring the temperature in the conduit, wherein a sensed temperatureexceeding a first threshold value indicates the presence of pulp in theconduit, which triggers the lowering of the pump speed of the vacuum pump.

Another advantageous embodiment of the method involves a step of closing avalve in the conduit if the sensed temperature exceeds a second thresholdvalue or if the sensed temperature stays above the first threshold value for apredetermined period of time.

Further embodiments of the invention are presented in the detaileddescription and the dependent claims.

The invention, and further objects and advantages of it, is best understoodfrom the following detailed description with reference to the appended drawing,wherein: Fig. 1 is a schematic illustration of an embodiment of the arrangement inaccordance with the invention.

DETAILED DESCRIPTION Fig. 1 schematically shows an embodiment of a pump arrangement 1 inaccordance with the invention. The arrangement comprises a housing 2 inwhich a pulp pump 4 for pumping pulp is arranged. The pulp pump 4 hasan inlet 10 into which pulp is fed, and an outlet 11 from which pulp is fedunder pressure. The pulp pump 4 is driven by a motor 12 via a shaft 3.

In this application the housing 2 is defined as the housing of the whole pulppump 4, including a pump housing 2a in which the pulp is affected by thepump action of the pump wheel (not shown), and a shaft housing 2bcovering the part of the shaft 3 adjacent the pump housing 2a.Conventionally, the overall housing may consist of several interconnectedhousings, but in this application the term housing 2 is meant to include allthese different housings, e.g. the pump housing 2a and the shaft housing2b, even though the housing 2 may consist of separate but interconnectedparts.

As described above, problems may arise due to gas, e.g. air, surrounding thepulp getting mixed into the pulp. Therefore, a vacuum pump 5 for degassingthe pulp inside the housing 2 is connected to the housing 2 via a gas outlet 7of the housing 2 and a conduit 6. A problem that may occur in connection tothe use of such vacuum pumps is that pulp may follow the gas into theconduit 6 and impede the evacuation of gas. Therefore the gas outlet 7 hasto be located at a location of the housing 2 where pulp will normally notenter. In the shown embodiment the pump is a centrifugal pump thatpresses the pulp in the radial direction by centrifugal forces towards thecircumferential walls of the pump housing 2a and out through the outlet 11.Hence, in order to minimize the pulp in the conduit 6, the gas outlet 7should be located in an axial extension of the housing 2 (not its radialextension). Therefore, the gas outlet 7 may be located at the inlet 10 or, as isconventional, in the shaft housing 2b which covers the connection of theshaft 3 to the pump housing 2a. Of course this implies that there is aconnection for the gas to pass from the pump housing 2a to the shafthousing 2b. Gas may then be sucked by the vacuum pump 5 from inside the pump housing 2a, via the shaft housing 2b and into the gas outlet 7.However, if an efficient degassing is to be achieved, it is not possible tototally prevent pulp from following the sucked out gas, no matter where thegas outlet 7 is located. Hence, there will always be a risk that pulp enters theconduit 6.

One way of solving the problem of pulp in the conduit 6 would be to arrangea spring loaded valve in order to let in air into the conduit when the pressurein the conduit reaches below a certain value. Such a control system wouldhowever only allow for two control levels, one in which the valve is closedand a second in which the valve is open. Further, it would not be powerefficient, as the vacuum pump 5 would be fully active regardless of theneeded suction effect.

In the arrangement according to the invention this is instead solved bymeans of a control unit 13 and an interconnected sensor 8. The connectionis in the figure indicated by a dotted line. The sensor 8 is arranged inconnection to the gas outlet 7, preferably inside the conduit 6, for detectingthe presence of pulp in the conduit 6. Further, the control unit 13 isarranged to lower the pump speed of the vacuum pump 5 if pulp is detectedin the conduit 6. The connection between the control unit and the vacuumpump 5 is in the figure indicated by a dotted line. In this manner, thesuction force of the vacuum pump 5 may be lowered by simply reducing theeffect by which the vacuum pump 5 is working. Hence, electrical energy maybe saved with respect to conventional solutions. Also, the control unit 13may be set to keep down the pump speed of the vacuum pump 5 for acertain period of time, such as e.g. approximately 5, 10, 15 or 20 seconds, inorder to allow sufficiently time for the pulp that has entered the conduit toslip down by gravity and out of the conduit.

A further problem is how to detect that pulp enters the conduit 6. In aspecific advantageous embodiment of the invention the sensor 8 for detectingpulp in the conduit is a temperature sensor, e.g. a thermometer, for sensingthe temperature in the conduit 6. Normally, the temperature of the pulpsuspension is between 45 and 95°C and most often between 70 and 95°C.The temperature of the gas inside the housing 2 is more or less the same asthat of the pulp. However, the conduit 6 is cooled by the surrounding airsuch that the temperature of the gas inside the conduit 6 is lower than thetemperature of the pulp in the housing 2. The temperature of the gasdecreases continuously in the conduit 6 from the gas outlet 7 of the housing2 to the vacuum pump 5. If pulp enters the conduit 6 the temperature in theconduit will rise, which will be detected by the temperature sensor.

In order to detect the presence of pulp in the conduit in reliable way thetemperature sensor should be placed relatively close to the gas outlet 7, i.e.2-20 cm from the gas outlet 7 and preferably 5-10 cm from the gas outlet 7.If the detector is placed too close to the outlet 7 it may be inﬂuenced bynormal deviations in the pulp ﬂow and if it is placed too far from the outlet itmay detect the presence of pulp at a too late stage. However, the mostadvantageous location of the sensor 8 is dependent of the width of theconduit 6 and other site specific parameters. Therefore the mostadvantageous location of the temperature sensor is site specific and maytherefore advantageously be determined by the skilled person by means ofempirical tests.

When a raise of temperature above a predetermined threshold value isdetected by the sensor 8 in the conduit 6, this indicates the presence of pulpin the conduit 6, which in turn triggers the control unit 13 to lower thepump speed of the vacuum pump 5. The threshold value for thistemperature increase may typically be set to 5 to 30°C. Also, since thetemperature of the pulp often remains the same, the threshold value mayalso be set to correspond to a specific temperature that is e.g. 5 to 30°Chigher than the normal operation temperature in the conduit. Hence, thesensor is either set to send an indication to the the control unit 13 whenevera certain increase of temperature in the conduit 6 has been detected, orwhenever the sensed temperature in the conduit 6 has reached above acertain predetermined temperature. In other words, the threshold value iseither a specific temperature, or a specific temperature raise with respect tothe normal operation temperature.

In correspondence to the distance of the sensor from the outlet, thethreshold value should not be set to small, or to close to the normaloperation temperature, as it then may be inﬂuenced by normal deviations inthe pulp ﬂow, but on the other hand it should not be set too high as it maythen risk to detect the presence of pulp in the conduit at a too late stage.

Also, the control unit 13 may be set to respond to a sudden increase oftemperature instead of an increase of a certain magnitude. In fact, due tocombined effect of the control unit 13 and the sensor, the possibility ofcontrolling the vacuum pump 5 may be achieved in a great number of waysthat has not been possible until now.

As an alternative to a temperature sensor the sensor 8 may instead be anoptical sensor or any other kind of suitable sensor.

In an advantageous embodiment of the invention, the control unit 13 isconnected to a frequency converter, which is adapted to control the pump lO speed of the vacuum pump 5. An advantage of using a frequency converterin this manner is that it allows continuous and stepless control of thevacuum pump in a most energy efficient Way, since no energy is Wasted.

As a complement to the vacuum pump, a control valve 9 may be arranged inthe conduit for closing the passage of the same. The control valve 9 may e.g.be closed at a start up procedure of the pulp pump, in order to avoid thatpulp enters the conduit 6 due to eXcessive pressure inside the pulp housing2. In an advantageous embodiment of the invention, the control valve 9 mayhowever also be controlled by the control unit 13, e.g. the control valve 9may be automatically closed in the event of detection of the presence of pulpin the conduit 6 over a certain period of time, i.e. When the loWering of thepump speed of the vacuum pump 5 is not enough to obtain the desired effectof returning the pulp from the conduit 6 into the housing 2. As analternative, the control valve 9 may be arranged in the conduit 6 for closingits passage in the event of detection of a raise of temperature above apredetermined second threshold value, Which is higher than the firstthreshold value. This Would have generally the same effect as closing thevalve after a certain period of time of high temperature, i.e. to obtain thedesired effect of safely returning the pulp from the conduit 6 into thehousing 2.

Above, a specific embodiment of the invention has been described, in order toexplain the invention. HoWever, the invention is not limited to thisembodiment. Instead it the scope of the invention is defined by the appendedclaims.

Claims

1. An arrangement (1) for pumping pulp, Which arrangement comprises ahousing (2) in Which a pulp pump (4) for pumping pulp is arranged, Whereina vacuum pump (5) for degassing the pulp inside the housing (2) isconnected via a conduit (6) to a gas outlet (7) of the housing (2),characterised in that a control unit (13) is arranged to control the pumpspeed of the vacuum pump (5) and in that the control unit (13) is connectedto a sensor (8) that is arranged in connection to the gas outlet (7) fordetecting the presence of pulp in the conduit (6), Wherein the control unit(13) is arranged to lower the pump speed of the vacuum pump (5) if pulp isdetected in the conduit (6).

2. The arrangement (1) according to claim 1, Wherein the sensor (8) is atemperature sensor for sensing the temperature in the conduit (6) andWherein a detected raise of temperature eXceeding a predetermined thresholdvalue indicates the presence of pulp in the conduit (6) that triggers thecontrol unit (13) to lower the pump speed of the vacuum pump (5).

3. The arrangement according to claim 2, Wherein a control valve (9) isarranged in the conduit for closing the same in the event of detection of araise of temperature above a predetermined second threshold value, Which ishigher than the first threshold value.

4. The arrangement (1) according to any of the preceding claims, Whereinthe control unit (13) is connected to a frequency converter adapted to controlthe pump speed of the vacuum pump (5).

5. The arrangement (1) according to any of the preceding claims, Wherein acontrol valve (9) is arranged in the conduit for closing the same in the eventof detection of the presence of pulp in the conduit (6) over a certain period oftime.

6. A method of pumping pulp by means of pulp pump (4) located inside ahousing (2), Which method comprises: - degassing the pulp that is being pumped inside the housing (2) by meansof a vacuum pump (5), Which is connected via a conduit (6) to said housing(2), characterised in that the method further comprises: - sensing if pulp is present in the conduit (6), - indicating to a control unit (13) if pulp is present in the conduit (6), - controlling the vacuum pump (5) in response to the indication receivedby the control unit (13). lO

7. The method according to claim 6, Wherein the control unit (13) isarranged to lower the pump speed of the vacuum pump (5) if pulp is detectedin the conduit (6).

8. The method according to claim 7, Wherein the sensing of pulp comprisesthe step of measuring the temperature in the conduit, and Wherein a sensedtemperature raise that exceeds a first threshold value indicates the presenceof pulp in the conduit (6), Which triggers the lowering of the pump speed ofthe vacuum pump (5).

9. The method according to claim 8, further comprising a step of closing avalve in the conduit if the sensed temperature exceeds a second thresholdvalue or if the sensed temperature stays above the first threshold value for apredetermined period of time.