A method and apparatus for degassing fiber suspension containing gas with a medium consistency pump

Abstract

The invention relates to a method for degassing fiber suspension containing gas with a medium consistency pump (11), wherein the medium consistency pump (11) is a centrifugal pump driven by an electric motor (12) and having an impeller rotating in a pump housing (13), an inlet (14) connectable to a fibrous suspension supply line, an outlet (15) connectable to a fibrous suspension discharge line, and at least one degassing channel (16) for discharging gas from the medium consistency pump (11). In the method according to the invention the medium consistency pump (11) is controlled by a controller (17) controlling the degassing differential pressure level based on the operation point of the medium consistency pump's (11) theoretical position on its pump curve. The invention relates also to an apparatus (10) for degassing fiber suspension containing gas.

Description

[0001] A METHOD AND APPARATUS FOR DEGASSING FIBER SUSPENSION

[0002] CONTAINING GAS WITH A MEDIUM CONSISTENCY PUMP

[0003] Field of the invention

[0004] The present invention relates to a method and apparatus for degassing fiber suspension containing gas with a medium consistency pump.

[0005] Background of the invention

[0006] When pumping medium consistency pulp by using medium consistency pump (hereinafter also “MC pump’’) degassing is needed to ensure good enough pumping performance. Although the degassing has very important role in such a pumping process, it’s drawback is that it can also cause so called “fiber loss” meaning that in certain circumstances some pulp is escaping from the pump through its degassing piping. Typically, degassing arrangement in a medium consistency pump consists of pump internal degassing channels, degassing piping, a vacuum pump, a control valve, and an on / off valve, as well as a bleed air valve for controlling the degassing pressure level. The vacuum pumps may be external or internal vacuum pumps. In many known MC pumps the internal vacuum pump is realized by installing the vacuum pump to the common shaft rotating the other pump impellers.

[0007] Furthermore, to improve degassing many known MC pumps are typically equipped with a fluidizer which is accomplished e.g., by mounting a fluidizing impeller to the same shaft as the pump main impeller prior to the main impeller. The fluidizing impeller causes, when rotating the pump, gas to separate from the suspension, and to collect adjacent the shaft from where it is exhausted by a vacuum pump which in the known MC pumps may be an internal or an external vacuum pump.

[0008] Degassing in pumps applied in pumping pulp are traditionally controlled by setting a constant pressure value on the pump’s degassing channels according to pulp’s properties (e.g., consistency). This can be achieved, for instance, by means of controlling the pumping action of the degassing pump or by means of a control valve (i.e., a bleed-air valve) provided at the degassing piping or channel. Since the properties of the pulp can vary such a constant pressure level is always a compromise between good pumping performance and minimizing the fiber loss. Thus, since those times, there has been a clear need to develop the pumps and their control methods further to perform pumping process as effectively as possible without above mentioned drawbacks. Therefore, some improved pumping arrangements with advanced control methods for fulfilling such needs have been developed during the last decades. Earliest of them still apply the traditional control principle with constant pressure and some of them such which differ somehow from it.

[0009] A known solution that applies the above-mentioned traditional control method for degassing fibrous suspension is disclosed in patent publication US 4,410,337. In that case the apparatus (i.e., the pump) has a first pump chamber with a rotor for pumping the fibrous suspension, and a second pump chamber with a rotor forming an internal vacuum pump as well as internal degassing channels from the vacuum pump to the first pump chamber and a degassing outlet to discharge the gas. In that solution pressure difference between fiber suspension at an inlet of the first pump chamber and in the outlet of the degassing outlet is controlled by a control valve and a pressure controller to maintain the pressure difference within a predetermined limit values to keep the said pressure difference as constant as possible.

[0010] Patent publication US 4,435,193 discloses corresponding solution to the patent publication US 4,410,337 except it is further controlled the head and / or the discharge flow rate of the pump by controlling the differential pressure between the liquid or suspension inlet pressure and the gas discharge pressure. The differential pressure is then controlled by controlling the discharge rate of the gas, and by determining the pressure head and varying the set point for a dP-controller in response to the pressure head to achieve as low energy consumption as possible with a constant discharge rate. Patent publication US 4,673,330 discloses a method for controlling a centrifugal pump for pumping a liquid or suspension (such as MC pulp) which contains gas. In this document the difference in electrical conductivity between the liquid or suspension and the gas is utilized in determining the size of the gas bubble formed at the center of the of the centrifugal pump. This is done through measuring electrical conductivity of the liquid or suspension in different radial positions inside the pump’s chamber by one or more electrodes spacing radially from the rotation axis of the impeller. The voltage passing through the electrodes is then used to control the pump to regulate the size of the gas bubble so that it does not impede the pump’s operation.

[0011] In international patent application publication WO 2011 / 142711 it is disclosed a method and an arrangement for pumping pulp which have a pulp pump and an external vacuum pump connected to the pulp pump by a degassing conduit for degassing the pulp to be pumped. In this method and arrangement, the arrangement includes a control unit which controls the pump speed of the external vacuum pump according to a signal obtained from a sensor that is placed at the connection of the gas outlet of the pulp pump. The sensor detects whether any pulp has entered in the gas discharge conduit through the detected gas outlet and reduces the pump speed of the vacuum pump if it observes that any pulp has entered in the gas discharge conduit. Thus, the control philosophy here is to minimize the fiber loss by adjusting the performance of the vacuum pump. In the other hand, pumping performance reached by the pulp pump may not be optimal since the control arrangement does not consider how much gas remains within the pulp to be pumped.

[0012] Furthermore, in US patent application publication US 2014 / 0023485 it is disclosed a centrifugal pump for pumping pulp that can be e.g., medium consistency pulp. It is a type of a centrifugal pump having an impeller chamber (at the front side) and an internal degassing chamber (at the back side) within the same pump housing, and which has a build in separation unit formed between the pump impeller chamber and a degassing chamber. The separation unit which purpose is to reduce the fiber loss includes a disc rotating together with the pump shaft, and which has a closed surface without openings as well as one or more ribs on the side of disc being on the side of the degassing chamber. The rotating disc causes that during the operation of the pump the pulp that is pressed in the direction of the degassing chamber is conveyed back into the impeller chamber. Gas (i.e., air) that is also conveyed into the degassing chamber can flow to the opposite direction to the pumping action of the separator unit and hence escape from the pump through a gas discharge opening at the degassing chamber. This arrangement reduces fiber loss remarkably and makes pump more stable with or without an internal or external degassing pump.

[0013] Summary of the invention

[0014] The aim of the present invention is to provide a new method and an apparatus for controlling a medium consistency pump by means of which performance and efficiency of the medium consistency pump can be further increased and lower fiber loss can be reached. Furthermore, the aim of the present invention is to bring forth a method and an apparatus for controlling a medium consistency pump which can reach the above-mentioned objects with a simpler control arrangement than the known solutions described above.

[0015] The aim of the invention is reached because the degassing of the medium consistency pump is controlled by controlling the degassing differential pressure level based on the operation point of the pumps theoretical position on the pump curve. This can be achieved e.g., such that the operation point of the pump (i.e., the pressure as function of the flow rate) is estimated by using rotational speed and electrical power values obtained from a variable speed drive (VSD) system of the pump’s electric motor, or by current and / or power measurements of the pump’s electric motor as well as calculated consistency of the fibrous suspension obtained from pressure measurement. The degassing pressure setting is then calculated from a predefined function accordingly. An advantage of the present invention is that adjusting the degassing differential pressure in the above-mentioned manner, optimal performance at any given time is reached and the fiber losses that can occur at certain operational points are remarkably reduced. Furthermore, since the control is based on values of the parameters that is obtained from the existing control system of the pump’s electric motor it can be realized in its simplest form without any additional measurement sensors. This makes it possible to apply simpler and cheaper control arrangements than those in the above- mentioned known degassing solutions.

[0016] Brief description of the drawings

[0017] In the following, the invention will be described in more detail with reference to the appended drawing in which:

[0018] Fig. 1 shows an apparatus according to the present invention seen obliquely from the direction of the first side of the electric motor and the pump, and

[0019] Fig. 2 shows the apparatus shown in the figure 1 seen obliquely from the direction of the second side of the electric motor and the pump.

[0020] Detailed description of some advantageous embodiments

[0021] Figures 1 and 2 shows an embodiment of an apparatus 10 according to the invention. The apparatus shown in the figures 1 and 2 is intended to be used in pumping of the fibrous suspensions being processed in a pulp mills (i.e., pulp being preferably medium-consistency pulp). However, it may be alternatively used for pumping also other fibrous suspensions such as e.g., fibrous suspensions based in waste or circulated fiber materials such as waste wood based and / or textile based fibrous suspensions.

[0022] The apparatus 10 shown in the figures 1 and 2 comprises a medium consistency pump 11 that is a centrifugal pump driven by an electric motor 12 and which has a pump housing 13, an impeller rotating in the pump housing 13, an inlet 14 connectable to a fibrous suspension supply line, outlet 15 connectable to a fibrous suspension discharge line, and at least one degassing channel 16 for discharging gas from the MC pump 11. The apparatus 10 according to the figure 1 comprises also a controller 17 that controls the MC pump 11 . The controller 17 is configured to control the MC pump 11 such that it controls the degassing pressure level based on the operation point of the MC pump’s theoretical position on its pump curve. In such control degassing set point in the degassing channel 16 may be determined e.g., by a three-phase estimation which is based on MC pump’s 11 power and rotation speed as well as consistency of the fibrous suspension. In the first phase the flow rate of the MC pump 11 is estimated based on speed of rotation, power, and consistency of the fibrous suspension values by applying a pump model and size specific model. Consistency value applied herein can be a value either from a consistency meter, a consistency estimate based on pressure measurement, or from calibrated consistency control in the same process step. In the second phase operation point of the MC pump 11 is estimated through the estimated flow rate by means of the pump model and the size specific model. Then in the third phase a degassing pressure set point is determined from the operation point estimate, suction side pressure (i.e., standpipe pressure) and the consistency of the fibrous suspension by applying a universal correlation between these parameters. In case the consistency value being used in above-described degassing pressure set point determination at the controller 17 is an estimated value it can be calibrated based on determination of the actual consistency of the fibrous suspension by measurement or by testing (i.e., e.g., analyzing the fibrous suspension in a laboratory).

[0023] The MC pump 11 is in this embodiment a type of centrifugal pump which has impeller chamber 18 at first in the front side of the pump housing 13 and after that a degassing chamber 19 at the rear side (i.e., at the side of the electric motor 12). The degassing chamber 19 includes a gas outlet 20 to which the degassing channel 16 is connected.

[0024] The electric motor 12 is connected to the MC pump 11 through a power transmission device 23. In this embodiment the power transmission device 23 is an enclosed drive shaft assembled in between the electric motor’s 12 drive shaft and the MC pump’s 11 drive shaft so that mechanical power produced by the electric motor 12 is transmitted to the MC pump 11. However, in the other embodiments the power transmission from the electric motor to the MC pump may be arranged also any other possible ways so that the MC pump’s shaft can be rotated by the electric motor in appropriate manner.

[0025] In some embodiments being kind of shown in the figures 1 and 2 there may be in the MC pump 11 , in between the impeller chamber 18 and the degassing chamber 19, a separation unit which purpose is to prevent fibrous material from entering to the degassing chamber 19 and to the degassing channel 16 from there. In such embodiments the separation unit may be e.g., corresponding separation unit as it is described in the document US 2014 / 0023485.

[0026] In the embodiment shown in the figures 1 and 2 there may be also an external degassing pump (not shown in the figures). Typically, it is connected the degassing piping 21 being connected to the degassing channel 16 as it is shown in the figures 1 and 2.

[0027] Furthermore, the apparatus 10 shown in the figures 1 and 2 may optionally comprise at least one of the following: A measurement sensor to determine consistency of the fibrous suspension, a pressure transmitter, a production rate meter, and / or a flow rate meter.

[0028] When controlling the MC pump 11 differential pressure of the MC pump 11 is adjusted based on the power load of the electric motor 12, the flowrate, and the MC pump’s speed of rotation with the consistency of the fibrous suspension. The power load of the electric motor 12 and the MC pump’s 11 speed of rotation can be obtained from a variable speed drive (VSD) 22, or they can be measured with a separate measurement means. To ensure correct determination of the differential pressure the apparatus 10 may include an inlet pressure measurement. It can be accomplished by a suitable pressure measurement means (e.g., a pressure transmitter) arranged at the region of the inlet 14 of the MC pump 11 . In an embodiment of the apparatus shown in the figures 1 and 2 the flow of the electrical energy to the electric motor 12 is controlled by the variable speed drive (VSD) 22. In such embodiment the method operation point of the MC pump 11 is determined by the controller 17 based on rotational speed and electrical power values of the VSD 22 and consistency (calculated or estimated).

[0029] In some other embodiments current and / or electric power to be fed to the MC pump’s electric motor is measured and the degassing pressure setting is then calculated from the measured current and / or electric power, the estimated or measured consistency of the fibrous suspension and the flow rate by applying a predefined function. The predefined function can be formed e.g., by applying suitable polynomial or other kind equations that are determined by substituting to these relationships the respective actual current and / or power load values that have been obtained from e.g., the VSD of the electric motor and by comparing the obtained theoretical degassing pressure values to the respective measured degassing pressure values.

[0030] As can be seen from the above descriptions consistency has effect on the degassing operation. Thus, it is important to know the actual consistency of the fibrous suspension to be pumped so that the theoretical position of the operational point can be determined accurately enough. For that purpose, in some embodiments, the consistency of the fibrous suspension may be measured. Alternatively, the consistency of the fibrous suspension may be determined based on data obtained from a pressure transmitter. Furthermore, in some other embodiments the consistency of the fibrous suspension being used in the control of the MC pump may be alternatively or in addition determined by calculating it from the production rate and / or the flow rate of the fibrous suspension.

Claims

Claims1. A method for degassing fiber suspension containing gas with a medium consistency pump (11 ), wherein the medium consistency pump (11 ) is a centrifugal pump driven by an electric motor (12) and having-an impeller rotating in a pump housing (13),-an inlet (14) connectable to a fibrous suspension supply line,-an outlet (15) connectable to a fibrous suspension discharge line and-at least one degassing channel (16) for discharging the gas from the medium consistency pump (11 ), and wherein in the method the medium consistency pump (11 ) is controlled by a controller (17) controlling the degassing differential pressure level based on the operation point of the medium consistency pump’s (11 ) theoretical position on its pump curve.

2. The method according to claim 1 , wherein degassing the differential pressure of the medium consistency pump (11 ) is adjusted based on power load of the electric motor (12), flowrate, and the medium consistency pump’s (11 ) speed of rotation with the consistency of the fibrous suspension.

3. The method according to claim 1 or 2 wherein a degassing pressure set point is determined through three phase estimation process and wherein in the first phase flow rate of the medium consistency pump (11 ) is estimated on the basis of the medium consistency pump’s (11 ) rotation speed, the medium consistency pump’s (11 ) power and the consistency of the fibrous suspension; in the second phase an operation point the medium consistency pump (11 ) is estimated on the basis of the estimated flow rate of the medium consistency pump (11 ); and in the third phase the degassing pressure set point is determined from an universal correlation between the estimated operation point of the medium consistency pump (11 ), a standpipe pressure and the consistency of the fibrous suspension.

4. The method according to any of the preceding claims, wherein the consistency of the fibrous suspension is measured.

5. The method according to any of preceding claims, wherein the consistency of the fibrous suspension is determined based on data obtained from a pressure transmitter (23).

6. The method according to any of preceding claims, wherein the consistency of the fibrous suspension is determined by calculating the consistency from the production rate and / or the flow rate of the fibrous suspension.

7. The method according to any of preceding claims, wherein the method comprises calibrating the consistency value used in the degassing pressure set point determination at the controller (17) based on determination of the actual consistency of the fibrous suspension by measurement.

8. The method according to any of the preceding claims, wherein flow of the electrical energy to the MC pump’s (11 ) electric motor (12) is controlled by variable speed drive (22), and wherein the method comprises a method step of determining operation point of the MC pump (11 ) based on rotational speed and electrical power values of the variable speed drive (22).

9. The method according to any of the preceding claims wherein flow of the electrical energy to the MC pump’s (11 ) electric motor (12) is controlled by measuring current and / or electric power to be fed to the electric motor (12) and by calculating the degassing pressure setting from measured current and / or electric power and flow rate values by applying a predefined function.

10. An apparatus (10) for degassing fiber suspension containing gas with a medium consistency pump (11 ), wherein the apparatus (10) comprises a medium consistency pump (11 ) being a centrifugal pump driven by an electric motor (12) and having-an impeller rotating in a pump housing (13),-an inlet (14) connectable to a fibrous suspension supply line,-an outlet (15) connectable to a fibrous suspension discharge line and-at least one degassing channel (16) for discharging gas from the pump, and wherein the apparatus comprises further a controller (17) configured to control the medium consistency pump (11 ) by controlling the degassing pressure level based on the operation point of the medium consistency pump’s (11 ) theoretical position on its pump curve.

11. An apparatus (10) according to claim 10, wherein the apparatus (10) comprises at least one of the following: a measurement sensor to determine consistency of the fibrous suspension; a pressure transmitter, a production rate meter, or a flow rate meter.