WO2010117312A1

WO2010117312A1 - Hydraulic system for a pressure diffuser

Info

Publication number: WO2010117312A1
Application number: PCT/SE2009/050367
Authority: WO
Inventors: Allan Jonsson
Original assignee: Metso Fiber Karlstad Ab
Priority date: 2009-04-07
Filing date: 2009-04-07
Publication date: 2010-10-14
Also published as: WO2010117312A9

Abstract

The invention is related to a pulp treating apparatus such as a pressure diffuser. The apparatus incorporates a screen having a considerable weight that is moved by a connecting rod 5 in a reciprocating manner between an upper and lower position. The connecting rod 5 is maneuvered by a hydraulic cylinder (61) for vertically moving said screen in first slow stroke concurrent with pulp flow and moving said screen in a second fast stroke countercurrent to pulp flow. According to the invention is a second force actuator means (71) arranged in parallel with the first hydraulic cylinder arrangement (61), and wherein the second force actuator means provides a counterforce balancing the weight of said screen. The second hydraulic cylinders also provides for recovery of energy during the slow stroke. The invention enables a reduction in necessary design pressures of the hydralic system, and reduction in power consumption for operating the hydraulic system.

Description

Hydraulic system for a pressure diffuser

FIELD OF THE INVENTION

The present invention relates to paper pulp treatment operations and particularly to a pressure diffuser for washing pulp wherein a screen basket is moved by a hydraulic system in a slow stroke at pulp speed and reversed in a fast stroke back to initial position. The typical pressure diffuser was designed by Kamyr AB, now known as Metso Fiber Karlstad AB, and was shown already in US 4,944,167.

BACKGROUND OF THE INVENTION

Generally, a pressure diffuser useful in the paper pulp industry comprises an elongated generally vertically upstanding vessel which mounts an elongated annular screen for vertical movement within the vessel. Pulp under pressure flows into one end of the vessel and into the annular space between the screen and the exterior vessel wall and through an outlet adjacent the other end of the vessel. As the pressurized pulp traverses the height of the vessel, displacement liquid is introduced into the annular chamber by a plurality of vertically spaced header assemblies. The displacement liquid flows generally radially inwardly through the pulp, treating the pulp, and through the screen into the interior of the vessel furnished with a liquid outlet. The screen is moved in a slow stroke concurrently with the pulp a limited distance of travel and is then returned quickly in a fast stroke to clean the screen by a combined wiping and back flushing action. The different diameters of the upper and lower ends of the screen create filtrate compression and thus back flushing (e.g. see U.S. Pat. No. 4,396,509), during screen movement thus forcing the liquid through the screen holes to back flush the screen.

While such pressure diffusers have been used successfully, a drawback of this type of hydraulic system for moving the screen basket is the extensive load on the hydraulic system. The pressure diffusers have been delivered in different sizes from 1980 and a total of more than 100 units have been installed world wide. Pressure diffusers having a moderate size, i.e. pressure diffuser type TD70 with a washing capacity of above 1000 ADMT/24h, have a total screen basket weight of about 16 ton, and most recent pressure diffusers for pulp mills of higher production capacity, for example pressure diffuser type TD140 with a washing capacity of above 2000 ADMT/24h, have a total screen basket weight of about 40 ton. On top of these weights of the screen basket, must also the captured wash filtrate volume be managed, which for these sizes TD70/TD140 amounts to about 4,6 ton and 8,1 ton respectively. The typical design pressure for the hydraulic system for a pressure diffuser type TD70 is between 120 and 170 bar, i.e. an average mean pressure of 145 bar. The installed power rating and energy consumption is thus rather high.

SUMMARY OF THE INVENTION According to the present invention, the above-identified drawback is overcome by an additional second force actuator means that is arranged in parallel with the first hydraulic cylinder arrangement, and wherein the second force actuator means provides a counterforce balancing the weight of said screen. By using this design could the hydraulic unit be designed in such a way that the necessary force for moving the screen is reduced to the requirements for accelerating the screen and moving the screen against the friction from the pulp. The second force actuator means could in its simplest form be implemented as counterweights attached to the screen mass, possibly via pulleys and wires, thus balancing the entire mass weight from the screen.

Particularly, the present invention provides, in a preferred embodiment, that the second force actuator means includes at least one truss beam arranged horizontally and connected with the cylinder rod of the first hydraulic cylinder, and having force actuator means attached at each end of the truss beam. By using this truss beam could the additional force be connected to a structure holding the entire mass weight from the screen on both sides of the hydraulic lifting cylinder that is mounted in line with the center axis of the screen body. The truss beam could then be added as an upgrade in existing pressure diffusers. According to a preferred embodiment could the second force actuator means comprise a second hydraulic cylinder arrangement. This means that the entire actuator structure of the screen body is made in a common design type, and enables a compact and efficient lay-out. In yet a preferred embodiment using a common hydraulic design is the second hydraulic cylinder arrangement including a high pressure accumulator connected to receive and supply hydraulic fluid to the second type of hydraulic cylinders via hydraulic lines. In yet another preferred embodiment the high pressure accumulator is connected to receive and supply hydraulic fluid also to the first type of hydraulic cylinders via hydraulic lines. This enable a simplified and integrated system where the high pressure hydraulic fluid is used both for the first and second type of hydraulic cylinders.

In order to have a full integration of a common hydraulic platform is preferably also the same hydraulic pump used for supplying hydraulic fluid to both the first and second type of hydraulic cylinders via hydraulic lines. And in order to be able to install the system as a rebuild kit in installed pressure diffusers, then preferably could the truss beam be connectible to an existing piston rod of a first hydraulic cylinder arrangement by clamping means on said truss beam. Said clamping means could either be implemented as a clamp coupling mountable on any piston rod without any destructive interference of the rod structure, or connected in-between flanges of a flange-coupling between the screen shaft and the hydraulic piston rod. Accordingly, it is a primary object of the present invention to provide novel and improved apparatus for washing pulp in a pressure diffuser in a manner to reduce the necessary installed power rating, and reduce the energy consumption for continuous operation. Yet another objective is to be able to reduce the top pressures needed for the hydraulic unit, which will decrease wear on components as well as decrease the installation costs for a hydraulic unit with lower capacity but still able to perform the intended functions.

These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 a and 1 b is a vertical cross-sectional view through the center line of a pressure diffuser with the screen in the upper and lower positions respectively;

FIG. 2a and 2b is a schematic layout of the conventional hydraulic unit used to control movement of the screen member; and

FIG. 3a and 3b is a schematic layout of the hydraulic unit according to the invention used to control movement of the screen member.

DETAILED DESCRIPTION OF THE DRAWINGS Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawing figures. In the embodiments shown is a pressure diffuser shown with a pulp flow direction moving from top to bottom, i.e. flows downwardly, but the invention could equally well be applied in a pressure diffuser where the pulp flow direction is reversed, i.e. flows upwardly. In the embodiments shown is a pressure diffuser shown with the hydraulic unit arranged at the top, which lifts the screen body upwardly against the gravity, but the invention could equally well be applied in a pressure diffuser where the hydraulic unit is arranged at the bottom, which pushes the screen body upwardly against the gravity.

Referring now to the drawing figures, particularly to FIG. 1 a and 1 b, there is illustrated a pressure diffuser according to conventional design and comprised of a generally vertically upstanding pressure vessel. The vessel is closed except for a pulp inlet 1 adjacent a top portion of the vessel and a pulp outlet 2 adjacent a bottom portion of the vessel. Fixed within the upper and lower portions of the vessel and spaced inwardly of the vessel wall are upper and lower housing members. The upper housing member is generally shaped as a "Chinese hat" with conduits 4 for wash filtrate passing this upper housing member, while the lower housing member is located immediately above the outlet scraper 3. The housing members are spaced axially one from the other and have a movable screen body 8 mounted in- between.

The elongated annular screen body 8 is generally slightly conical in shape and tapers radially inwardly in a downward direction. Thus, as illustrated in FIG. 1 a and 1 b, pulp enters the vessel through the pulp inlet 1 and passes downwardly through an annular interior volume defined between the inwardly facing pressure vessel wall and the outwardly facing surface of the screen 8 for flow out of the vessel through pulp outlet 2.

A plurality of headers 7 each having an inlet are disposed about the exterior of the vessel wall for adding displacement liquid, i.e. wash liquid WL, in a radial directions trough the pulp bed caught between the inside of the pressure vessel wall and the screen member surface. Thus, liquid is introduced by said headers at vertically spaced positions trough the pressure vessel wall by nozzle bores in said wall located substantially circumferentially about the interior volume at each vertical location.

The screen 8 is reciprocal in a generally vertical axial direction, preferably by means of a hydraulic unit 6. The hydraulic unit 6 is coupled to the screen body 8 via a connecting rod 5 that displaces the screen body 8 in a vertically upward or downward direction as desired. It will be appreciated that in normal operation, the screen conventionally moves downwardly in the direction of the flow of pulp and then is rapidly moved upwardly in order to obtain both back flushing of the screen and reversal to a new start position for a washing stroke. In figure 1 a is the screen body shown in the uppermost position and in figure 1 b is the screen body shown in the lowermost position, with the total screen height indicated with reference sign SH and the total stroke length indicated with reference sign SL.

At the bottom of the vessel, there is provided a rotary scraper 3 driven by a motor via a transmission located exterior of the pressure vessel. Consequently, scraper 3 facilitates flow of pulp from the interior volume through outlet 2.

In operation, it will be appreciated that pulp is provided through inlet 1 and into the annular chamber between the pressure vessel wall and screen body 8 and flows outwardly from the vessel through outlet 2. Displacement liquid WL is introduced through the headers 7 and passes generally radially inwardly through the pulp and then through the outer screen surface of the screen body 8 where displaced liquid is separated from the pulp and collected in the interior volume within the screen body 9. Spent displacement liquid FL is removed from the interior volume by an outlet conduit 4. Generally, the screen body 8 is pushed during the wash displacement stroke in the direction of pulp flow, i.e. downwardly, and at a similar speed as the flow of pulp. This is accomplished by the hydraulic unit 6 pushing the connection rod 5 into pressure vessel to displace the screen body 8 downwardly with the pulp.

Referring now to FIG. 2a, is a principal drawing of the conventional hydraulic unit 6 used, activated during the downwardly motion of the connecting rod 5, i.e. during the slow washing stroke. The main elements of the hydraulic system are the pump 65 and associated valves 66,67,68,69 in the hydraulic lines between the main hydraulic cylinder 61 , the high pressure accumulator 62, the low pressure accumulator 63 and the hydraulic oil tank 64. During the down-stroke as shown in figure 2a the higher force on the top-side of the main cylinder piston 60 moves the connecting rod 5 down at slow speed. During this stroke the valve 66 is closed and valves 67 and 68 are open, and in the meantime is the pump loading the high pressure accumulator 62. In order to hold the weight of the screen member is the valve 67 throttled, and the developed pressure drop heats the hydraulic oil, i.e. mechanical energy is converted to heat energy. The valve 67 is the speed control valve, controlled by throttling order, for the downward stroke. Hydraulic fluid is during this stroke replenishing the oil tank 64 from the low pressure accumulator 63. The high pressurized hydraulic fluid is supplied to both the upper and lower chambers of the main hydraulic cylinder 61 , but as the exposed lower surface of the cylinder piston 60 is smaller than the exposed upper surface of the cylinder piston 60, due to the piston rod, is a smaller net resulting force driving the piston 60 downwardly. In FIG.2b the same hydraulic unit 6 shown but activated during the upwardly motion of the connecting rod 5, i.e. during the fast return and back flushing stroke. During the fast upstroke as shown in figure 2b stroke the valve 67 is closed, and the full pressurization in the main cylinder is only made in the lower chamber of the main hydraulic cylinder 61. The valve 68 is the speed control valve, controlled by throttling order, for the upward stroke. The upper chamber of the main hydraulic cylinder 61 is drained to the low-pressure accumulator via the open valve 66. This results in a high relative force on the piston and a fast movement upwardly. And in the meantime is the high pressure accumulator 62 replenishing the high pressure side with pressurized hydraulic fluid as the volumes of hydraulic fluid in motion are quite higher in this stroke. The usage of a high pressure accumulator 62 enables usage of a pump with lower capacity, not necessarily designed for the fast stroke flows, as this smaller pump could charge the high pressure accumulator 62 during the slow stroke where the flow of fluids in the hydraulic circuit is lower.

Referring now to FIG. 3a and FIG. 3b, there is illustrated the same basic hydraulic system as shown in FIG. 2a and 2b, but with the inventive additional force actuator means arranged in parallel with the main hydraulic cylinder 61 . Here is the additional force actuator means installed as a second type of hydraulic cylinders 71 arranged at each end of a horizontal truss beam 70 rigidly connected to the connecting rod 5. The truss beam 70 could be mounted to the connecting rod using any kind of non destructive clamping means, such as a shrink disc coupling or alternatively mounting the truss beam in between a flanged end of the main hydraulic piston rod and a flanged end of the connecting rod 5. The second type of hydraulic cylinders 71 comprises a piston 73 connected to a rigid support via piston rods, and thus divides each cylinder 71 into an upper and lower chamber. As could be seen in figure 3a is the lower chamber connected via hydraulic lines to the low pressure side of the hydraulic system, via (B), and the upper chamber connected via hydraulic lines to the high pressure side of the hydraulic system via (A).

The function of the inventive additional hydraulic cylinders 71 is as follows. In FIG. 3a during the slow downward stroke are the upper chambers in cylinders 71 in a compression mode and delivers the hydraulic fluid to the high pressure accumulator 62, and the downward stroke is not only throttled by valve 67. The advantage is that less energy losses occur as less mechanical energy is converted to heat energy, and instead is the energy stored in the high pressure accumulator 62. During the downward stroke are the lower chambers in cylinders 71 in an expansion mode and are replenished by hydraulic fluid from the low pressure side, via hydraulic lines and connection points (B). In FIG. 3b during the fast upward stroke are the upper chambers in cylinders 71 in an expansion mode and is supplied by pressurized hydraulic fluid from the high pressure accumulator 62. The advantage is that additional force for the upward movement is obtained. If the main hydraulic cylinder 61 is unaltered, this means that a lower pressure is needed in the system if the same lifting force is to be applied, as the total piston area of the hydraulic pistons increases. During the upward stroke are the lower chambers in cylinders 71 in a compression mode and are drained from hydraulic fluid to the low pressure side, via hydraulic lines and connection points (B).

IMPLEMENTATION If the inventive concept is installed in a pressure diffuser of the type TD70 (having a washing capacity of 1000 ADMT pulp /24h), and with the main hydraulic cylinder 61 unaltered, could the necessary design pressure for the system be lowered from 120-170 bar, at an average hydraulic pressure of 145 bar, down to a design pressure of 90-140 bar, at an average hydraulic pressure of 1 15 bar. This is an immediate reduction in design pressure in the order of 20%; (= (145-1 15)/145 ^* 100). This will reduce loading on components such as seals, pistons etc, and will increase operation time between service overhaul. The additional hydraulic cylinders will also enable a saving in power consumption, as the previous losses in heat development (needing coolers for the hydraulic fluid) is replaced by storing the energy in the high pressure accumulator. A total saving of more than 400.000 kWh/year is possible. If the cost per kWh is 0,1 Euro, then annual savings in operation costs could reach 40.000 Euro.

In all the embodiments, the pulp suspension subjected to the displacement wash will be under pressure-that is a pressure significantly greater than one atmosphere. It will be under pressure the entire time it is in the vessel, from the time it enters the inlet 1 until when it leaves the outlet 2.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

PATENT CLAIMS

1 . A pulp treating apparatus comprising: a generally upright, liquid-tight, pressurized vessel defining a first interior volume for containing pulp to be treated under pressure; a pulp inlet (1 ) to the vessel; a pulp outlet (2) from the vessel, the pulp flowing generally vertically between said pulp inlet (1 ) and said pulp outlet (2); a screen (8) defining a surface of revolution upstanding within the vessel and defining, in part, said interior volume containing pulp and having upper and lower ends of different cross-sectional area; extraction means (4) for withdrawing liquid from the pulp, through said screen (8), a first hydraulic cylinder arrangement including at least one first type of hydraulic cylinder (61 ) for vertically moving said screen via a connecting rod (5) in first a slow stroke concurrent with pulp flow and at pulp flow speed and moving said screen in a second fast stroke countercurrent to pulp flow characterized in that a second force actuator means (71 ) is arranged in parallel with the first hydraulic cylinder arrangement, and wherein the second force actuator means provides a counterforce balancing the weight of said screen (8).

2. Apparatus according to claim 1 characterized in that the second force actuator means includes at least one truss beam (70) arranged horizontally and connected with the connecting rod (5) of the first hydraulic cylinder, and having force actuator means attached at each end of the truss beam.

3. Apparatus according to claim 2 characterized in that the second force actuator means comprises second type of hydraulic cylinders (71 ).

4. Apparatus according to claim 3 characterized in that the second hydraulic cylinder arrangement includes a high pressure accumulator (62) connected to receive and supply hydraulic fluid to the second type of hydraulic cylinders (71 ) via hydraulic lines.

5. Apparatus according to claim 4 characterized in that the high pressure accumulator (71 ) is connected to receive and supply hydraulic fluid also to the first type of hydraulic cylinders (61 ) via hydraulic lines.

6. Apparatus according to claim 5 characterized in that the same hydraulic pump (65) is used for supplying hydraulic fluid to both the first and second type of hydraulic cylinders via hydraulic lines. Apparatus according to claim 2 characterized in that the truss beam (70) is connectible to an existing piston rod of a first hydraulic cylinder arrangement by non destructive clamping means on said truss beam.