Arrangement and method for degassing a pump

Abstract

An arrangement and a method for controlling a gas flow separated from a suspension of medium consistency pulp. The pulp is treated in a pulp treatment apparatus including a first pump and a second pump, wherein the second pump is a degassing centrifugal pump provided with a degassing system including a degassing conduit in which a degassing valve is arranged for regulating a pressure difference between an inlet of the second pump and the degassing conduit. The degassing system further includes a pressurized degassing vessel working under overpressure and having an inlet and an outlet, wherein the outlet of the vessel is connected to a pressure control valve for maintaining a desired overpressure in the vessel.

Claims

1. An arrangement configured to control a gas flow separated from a suspension of medium consistency pulp in a pulp treatment apparatus, the arrangement comprising: a first pump and a second pump, wherein the first pump pumps the suspension as the suspension flow to a first reactor and the second pump pumps the suspension as the suspension flows from the first reactor to a second reactor, wherein the second pump is a degassing centrifugal pump with a degassing system, and the degassing system comprises: a degassing conduit configured to receive gases from the second pump; a degassing valve connected to the degassing conduit and the degassing valve is configured to regulate a pressure difference in a flow of the gases between an inlet of the second pump and the degassing conduit, and a pressurized degassing vessel downstream of the degassing valve in the flow of the gases through the degassing conduit, wherein the pressurized degassing vessel is pressurized to above atmospheric pressure in an overpressure condition, and the pressurized degassing vessel includes an inlet and an outlet, wherein the outlet of the degassing vessel is connected to a pressure control valve configured to maintain the overpressure condition of the degassing vessel.

2. The arrangement according to claim 1, wherein the pressurized degassing vessel includes a horizontal pipe, wherein an inlet of the horizontal pipe is connected to the degassing conduit, and an outlet of the horizontal pipe is connected to an outlet conduit connected to the degassing valve.

3. The arrangement according to claim 2, wherein the degassing system further comprises: a water separator connected to the degassing conduit and downstream of the degassing valve, wherein the water separator includes an unpressurized chamber, a vent conduit in an upper part of the unpressurized chamber, and a drainage conduit connected to an outlet of the unpressurized chamber and configured to adjust a water level in the unpressurized chamber.

4. The arrangement according to claim 1, wherein the pressurized degassing vessel includes a pressurized water separator chamber having a liquid level and a gas space above the liquid level, and a vent conduit, in which the degassing valve is arranged to maintain the overpressure condition in the pressurized water separator chamber, and the pressurized water separator chamber further comprises a drainage conduit and a valve configured to regulate the liquid level in the pressurized water separator chamber.

5. The arrangement according to claim 2, wherein a diameter of the horizontal pipe is at least 50 mm.

6. The arrangement according to claim 1, wherein a volume of the pressurized degassing vessel is in a range of 0.01 m.sup.3 to 1.0 m.sup.3.

7. The arrangement according to claim 1, wherein the pressurized degassing vessel is configured to operate at a pressure in a range of 0.1 bar(g) to 4.0 bar(g).

8. The arrangement according to claim 1, wherein pressure in the pressurized degassing vessel includes a vent to release pressurized gas to atmosphere and the overpressure condition in the pressurized degassing vessel is regulated by a vent valve coupled to the vent.

9. The arrangement according to claim 8, further comprising a controller configured to actuate the vent value to maintain the overpressure condition in the pressurized degassing vessel and the controller is configured to adjust pressure in the pressurized degassing vessel based on a pressure of a suction side of the second pump.

10. The arrangement according to claim 9, wherein the controller is configured to adjust the pressure in the pressurized degassing vessel by increasing the pressure in the pressurized degassing vessel in response to an increase in the pressure that the suction side of the second pump.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is described in more detail by means of an embodiment according to the invention and with reference to the accompanying schematic drawings, in which:

(2) FIG. 1 is a schematic illustration of an oxygen delignification process in connection which the new method and arrangement may be applied; and

(3) FIGS. 2a and b is a schematic illustration of known degassing arrangements of a booster MC pump, and

(4) FIG. 3 shows an arrangement of an embodiment of the present invention, and

(5) FIG. 4 shows an arrangement of another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(6) FIG. 3 shows an embodiment of the new method. Pulp having a consistency of 6-16% in line 30 is typically discharged from a pulp treatment reactor, as shown in FIG. 1, in which the pulp is discharged from the oxygen delignification reactor 5. The pulp in line 30 is typically fed by a degassing MC pump 31 through line 32 to a second reactor (reactor 8 in FIG. 1). The pump 31 works as a booster pump for raising the pressure of the pulp.

(7) In the MC pump 31 air or gas is separated from the pulp, and then discharged from the pump to a degassing conduit 33 by creating a certain pressure difference between the inlet side and the degassing chamber of the pump. Typically the degassing system of the booster MC pump 31 consists of pressure measurements in the inlet 30 of the pump and in the degassing conduit 33 that is connected to the degassing chamber of the pump. The pulp is removed from the pump through a conduit 32. The degassing conduit 33 (gas discharge conduit) comprises a degassing valve 34. The difference in pressure between the inlet of the pump and the degassing (gas discharge) is measured by a pressure gauge. The difference is compared with a set value and the degassing valve is controlled by a controller, such as a pressure differential indicator controller (PDIC), to maintain differential pressure at a desired level.

(8) The degassing conduit 33 is connected to a pressurized degassing vessel 35, which is also a water separation chamber. The water separator chamber in which water is removed from the separated gas is also equipped with a drainage conduit 37 for regulating a liquid level in the water separator vessel 35. The water separator chamber 35 has a vent conduit 36, which is connected to a ventilation system or atmosphere. According to the present invention the water separation vessel is working under overpressure. The vent conduit is provided with a valve 38 which is adjusted to maintain a desired overpressure in the water separation chamber. The pressure is adjusted to be high enough to control and damp unforeseen changes over the degassing valve 34. The pressure set point is adjusted so that the pressure in the water separation chamber is in a range of 0.1-2.0 bar (g), preferably 0.3-1 bar (g). The water separation chamber is located at a lower elevation than the valves 34 and 38.

(9) The water separator chamber in which water is removed from the separated gas is also equipped with a drainage conduit 37 for regulating a liquid level in the water separator vessel 35. There has to be a liquid level in the water separator chamber so that the chamber can be pressurized and the gas flow can be directed to the top of the chamber and further to the vent conduit. The gas volume is controlled with the liquid level control (LIC) circuit in the drainage conduit 37. The gas volume is also be adjusted within certain limits.

(10) The pulp fed to the booster pump may contain fluctuating amounts of air or other gases, which are separated in the pump and tend to accumulate to the pump, if the gases are not discharged from the pump in suitable amounts. The pressurization of the water separation vessel decreases the volume flow of the off-gas from the degassing valve 34, which will reduce the capability of the off-gas flow to take along pulp fibers. The opening of the degassing valve is typically in the same range as in a corresponding atmospheric system, although the gas volume flow rate is smaller but the pressure difference is also lower. This means that the off-gas velocity in the degassing conduit 33 of the pump and in the degassing valve 34 is smaller.

(11) FIG. 4 shows another embodiment of the new method. Pulp having a consistency of 6-16% in line 40 is typically discharged from a pulp treatment reactor, as shown in FIG. 1, in which the pulp is discharged from the oxygen delignification reactor 5. The pulp in line 40 is fed by a degassing MC pump 41 through line 42 to a second reactor (reactor 7 in FIG. 1). The pump 41 works as a booster pump for raising the pressure of the pulp.

(12) In the MC pump 41 air or gas is separated from the pulp, and then discharged from the pump to a degassing conduit 43 by creating a certain pressure difference between the inlet side and the degassing chamber of the pump 41. Typically the degassing system of the booster MC pump 41 consists of pressure measurements in the inlet 40 of the pump and in the degassing conduit 43 that is connected to the degassing chamber of the pump 41. The pulp is removed from the pump through a conduit 42. The degassing conduit 43 (gas discharge conduit) comprises a degassing valve 44. The difference in pressure between the inlet of the pump and the degassing (gas discharge) is measured by a pressure gauge. The difference is compared with a set value and the degassing valve is controlled by a controller, such as a pressure differential indicator controller (PDIC) 45, to maintain differential pressure at a desired level.

(13) The degassing conduit 43 is connected to a pressurized degassing vessel 46. The vessel is a pipe 46 which is located horizontally and has a sufficient diameter and length. The pressure is controlled by a pressure control valve 47 in the outlet conduit 48 of the vessel 46. The overpressure in the vessel and volume of the vessel allows damping quick pressure changes in the degassing valve 44. The pressure control valve 47 is adjusted to maintain a desired overpressure in the vessel 46. The pressure is adjusted to be high enough to control and damp unforeseen changes over the degassing valve 44. The pressure set point is adjusted so that the pressure in the water separation chamber is in a range of 0.1-2.0 bar (g), preferably 0.3-1 bar (g).

(14) The pressurized degassing vessel 46 is further connected through the conduit 48 to a separate unpressurized water separation chamber 49 having a liquid level 50 and a gas space above the liquid level and a drainage conduit 51 for regulating a liquid level in the chamber. The water separator chamber further comprises a vent conduit 52 for providing an outlet for separated gas.

(15) In FIG. 4, at least a portion of the degassing conduit 43 and at least a portion of the outlet conduit 48 travel vertically. Preferably, the degassing valve 44 of the degassing conduit is located at a higher elevation than the pressure control valve 47 of the outlet line. The degassing vessel 46 is located at a lower elevation than the valves 44 and 47.

(16) The new arrangement allows a stable operation of a booster MC pump in a pulp treatment system.

(17) While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms comprise or comprising do not exclude other elements or steps, the terms a or one do not exclude a plural number, and the term or means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise.