SE545288C2 - Method for converting a plug screw feeder - Google Patents

Abstract

The invention relates to a method for converting a first plug screw feeder (11), which is suitable for feeding a first material, to a second plug screw feeder (21), which is suitable for feeding a second material, said first plug screw feeder (11) comprising: a housing (12) comprising an interior screw zone (13), a plug pipe (14) having an inner diameter (D), and a first plug screw (15) comprising a first screw flight (17), which extends a distance (Li) in the screw zone (13); wherein the method comprises the step of replacing the first plug screw (15) with a second plug screw (25) comprising a second screw flight (27), which extends in the screw zone (13) and also extends a distance (L4) into the plug pipe (14), where D ≤ L4 ≤ 2D.

Description

METHOD FOR CONVERTING A PLUG SCREW FEEDER TECHNICAL FIELD The present invention relates to a method for converting a plug screw feeder, which is designed for the transport one type of material, such as wood chips, to a plug screw feeder suitable for the transport another type of material, e.g., a more granular material with lower network strength and higher compressibility.

BACKGROUND Plug screw feeders are widely used. In the pulping industry, plug screw feeders are used for transporting wood chips or other raw materials, such as sawdust, bagasse, or straw, from one part of the pulping process to another. When used in a pulping process, a plug screw feeder is typically not only used for conveying and feeding chips but is also used for dewatering of the wood chips and/or to provide a pressure-tight plug to a pressurized zone, such as a digester. When a plug screw feeder is used to provide a pressure-tight plug to a pressurized zone, the quality and reliability of such a pressure-tight plug are always a concem, since the risk of so-called blow-back, i.e. when pressurized fluids or gases suddenly go backwards (i.e. opposite the material feeding direction) through the plug screw feeder, is something that always needs to be taken into account and minimized as far as possible. In some applications, the pressure inside a zone at the inlet side of the plug screw feeder can be higher than the pressure at the outlet side of the plug screw feeder, but the purpose of the plug screw feeder is the same, i.e. to convey material from a first zone to a second zone and, at the same time, to create a pressure-tight plug between the first zone and the second zone, where the pressure prevailing in the first zone differs from the pressure prevailing in the second zone.

Now, the pulping industry is typically a truly large-scale process industry including very big vessels, such as digester and bins, for the containing raw materials or processed materials being in different phases of the pulping process. To optimize, e.g., the efficiency and economy of the processes in a pulping mill, each device is carefully designed and dimensioned to fit into the overall layout of the mill. The process layout of a particular mill becomes therefore in practice also designed to handle rather specific raw materials having more or less specific characteristics. Thus, the sheer scale of this industry and the optimized process lines lead to less versatility, which may cause process problems if, for example, the raw material for some reason is changed. One such potential problem area is the plug screw feeder.

An object of the present invention is therefore to provide a method for converting an existing plug screw feeder designed for handling materials having a first set of material characteristics to a plug screw feeder designed for handling materials having a second set of material characteristics, which second set of material characteristics is different from the first set of material characteristics.

SUMMARY OF THE INVENTION The above-mentioned objects are achieved with a method for converting a first type of plug screw feeder to a second type of plug screw feeder, and with a plug screw feeder of the second type. Preferred embodiments are set forth in the dependent claims.

According to a first aspect, the invention relates to a method for converting a first plug screw feeder, which is suitable for feeding a first material, to a second plug screw feeder, which is suitable for feeding a second material, said first plug screw feeder comprising a housing, which comprises an interior screw zone, a plug pipe, which has an inner diameter D, and a first plug screw, which comprises a first screw flight, which extends a distance L1 in the screw zone, wherein the method comprises the step of replacing the first plug screw with a second plug screw, which comprises a second screw flight, which extends in the screw zone and also extends a distance L4 into the plug pipe, where D 5 L4 S 2D.

As used herein, the term "a first material" is a material characterized by a first set of characteristics and the term "a second material" is a material characterized by a second set of characteristics, where the first set of characteristics differs from the second set of characteristics. The sets of characteristics can comprise granularity, compressibility and network strength, such that, for example, the first set of characteristics comprises low granularity, low compressibility and/or high network strength, whereas the second set of Characteristics comprises high granularity, high compressibility and/or low network strength, where the terrns "high" and "low" in this context are relative terrns. By these def1nitions it should be clear that "the first material" and "the second material" can be the same type of material, e.g. wood chips of a certain variety, but which have different qualities (characteristics). It is, however, more likely that the first material is wood chips and the second material is sawdust or residues from agriculture such as straw, bagasse, or empty fruit bunches.

Further, the feature that the first plug screw feeder comprises "a first plug screw, which comprises a first screw flight, which extends a distance L1 in the screw zone" means that the screw flight does not extend into the plug pipe, or extends only a small distance into the plug pipe, or in all event that the screw flight extends less than the distance D into to the plug pipe, as will be explained further below.

To appreciate the usefulness of the present method according to the invention one may envisage a situation in which a plug screw feeder of a first type, i.e. a plug screw feeder which is suitable for feeding a first type of material, i.e. a material characterized by a first set of characteristics, has been installed between two (large) vessels having different pressures prevailing therein, and the dimensions and other features of the plug screw feeder have been designed and optimized for the feeding of this first type of material from the first vessel to the second vessel and, at the same time, have been designed and optimized for the creation of a pressure-tight plug into the inlet of the second vessel. Then, for some reason, the material is changed into a second type of material, i.e. a material characterized by a second set of characteristics, the second set being different from the first set, and it is discovered that the plug screw feeder no longer is capable of forrning a pressure-tight plug. Instead of installing a completely new plug screw feeder suitable for feeding the second type of material and, possibly, moving the first vessel or the second vessel to thereby change the distance between the first vessel and the second vessel, the inventors have realized that it is instead possible to let the screw flight of the plug screw extend further into the plug pipe than in a conventional design. Apparently, this measure effectively reduces the plug-forrning length of the plug pipe (since the screw flight rotates in a first or rear portion of the plug pipe and thereby prohibits the forrning of a pressure- tight plug in this first or rear portion), and this measure is therefore in contradiction with current thinking because a (relatively) long plug-forrning length of a plug pipe has been considered necessary for the forrning of a reliable pressure-tight plug.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be further explained hereinafter by means of non-limiting examples and with reference to the appended drawing, wherein: Fig. 1 is schematic illustration of a plug screw feeder according to the prior art, and Fig. 2 is a schematic illustration of a plug screw feeder according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 illustrates very schematically a longitudinal cross-section of a plug screw feeder 11 of a conventional design. For clarity of illustration and easier understanding of the present invention, many details, which are included in an operational plug screw feeder, have been excluded, such as motor, gear box, bearings, inlet and outlet openings, etc. The plug screw feeder 11 comprises a housing 12, which comprises an interior screw zone 13, a plug pipe 14, which has an inner diameter D and a length Lz, and a plug screw 15, which, in tum, comprises a screw shaft 16 and a screw flight 17, which, in the interior screw zone 13, is provided on a first or rear portion of the screw shaft 16. The screw flight 17 has, in the interior screw zone 13 of the housing 12, a longitudinal length L1. The length Li is thereby equal to or slightly shorter than a longitudinal length of the housing 12. In Fig. 1, the latter case has been illustrated by a gap provided between a first or rear end of the screw flight 17 and a rear end wall 18 of the housing 12, i.e. in the embodiment depicted in Fig. 1, the screw flight 17 is not provided all the way to a first or rear end of the screw shaft 16. In other plug screw feeders, a screw flight can, however, extend all the way to a first or rear end of a screw shaft, such that there is no gap provided between a first or rear end of the screw flight and a rear end wall of a housing.

Further, in most conventionally designed plug screw feeder, a screw flight ends at the level of a first or rear end of a plug pipe, or the screw flight may end a small distance before the level of the first or rear end of the plug pipe. It is, however, also possible that a screw flight extends a small distance into a plug pipe. This latter situation has been illustrated in Fig. 1, where the screw flight 17 extends a distance Lg into the plug pipe 14. This distance Lg is typically very short, i.e. Lg 2 0, but it may be slightly longer, such that, e.g., Lg S 0.5D, and, in all event, Lg is shorter than the diameter of the plug pipe 14, i.e. Lg < D.

Further, the screw shaft 16 is in this embodiment j oumalled in bearings at the rear end wall 18 of the housing 12 and in bearings at a second or front end of the plug pipe 14, such that the screw shaft 16 extends through the plug pipe 14. It is, however, possible to joumal a screw shaft inside the housing of a plug screw feeder in a special arrangement, such that the screw shaft does not extend into a plug pipe.

Now, according to the present invention, the plug screw feeder 11, which is suitable for feeding a first type of material, can be converted to a plug screw feeder 21, which is suitable for feeding a second type of material, the second type of material being different from the first type of material. The invention therefore provides: a method for converting a first plug screw feeder 11, which is suitable for feeding a first material, to a second plug screw feeder 21, which is suitable for feeding a second material, said first plug screw feeder 11 comprising a housing 12, which comprises an interior screw zone 13, a plug pipe 14, which has an inner diameter D, and a first plug screw 15, which comprises a first screw flight 17, which extends a distance L1 in the screw zone 13, wherein the method comprises the step of replacing the first plug screw 15 with a second plug screw 25, which comprises a second screw flight 27, which extends in the screw zone 13 and also extends a distance L4 into the plug pipe 14, where D S L4 S 2D.

In one embodiment of the invention L4 > 0.5 Lz, where L; is the length of the plug pipe The resulting plug screw feeder 21 is schematically illustrated in Fig. 2, where it in particular can be noted that all parts except the plug screw 25 and its screw shaft 26 and screw flight 27 are the same as in Fig. 1, and have therefore been given the same reference numerals. Thus, by replacing the plug screw 15, which comprises a screw flight 17 that does not extend into the plug pipe or only extends a small distance Lg into the plug pipe 14, where Lg less than about 0.5D and is at least less than D, with the plug screw 25, which comprises a screw shaft 26 and a screw flight 27, which extends a considerable distance L4 into the plug pipe 14, where D S L4 S 2D, it has been possible to feed another type of material, e. g. a more granular material, a material having higher compressibility, and/or a material having lower network strength. Instead of replacing the whole plug screw 15, including the screw flight 17 and the screw shaft 16, it is possible to keep the screw shaft 16 and only provide, e. g. by welding, more screw flights to the second or front portion of the screw shaft 16. Further, here it has been assumed that the first or rear end portion of the plug screw 26 is remained unchanged, but it possible to also extend the screw flight 27 towards the first or rear end, i.e. towards the end wall 18, the distance L1 in Fig. 2 would then differ from the distance L1 in Fig. 1. In one embodiment L4 > 0.5 Lz, where L; is the length of the plug pipe Typical dimensions for a plug screw feeder in which the present method can be applied would be that L1 is in the order of 1500 mm, L; is in the order of 750 mm, and D is in the order of 250 mm.

Although the present invention has been described with reference to specific embodiments, also shown in the appended drawings, it will be apparent to those skilled in the art that many variations and modif1cations can be done within the scope of the invention as described in the specification and defined with reference to the claims below.

Claims (3)

1. A method for converting a f1rst plug screw feeder (11), which is suitable for feeding a f1rst material, ufšsítfilï; is tïÉï-tzsrac,tetizafti bv a first of tïlïzsractetistics. to a second plug screw feeder (21), which is suitable for feeding a second n1aterial, »aflxich is chfarattttfrizaftl hv saftïoattl att" cfnaraattefristíafs. strid secort being difïferfsziti tfmrtt said first". set :sf afhaattaat:tetiàstics: said first plug screw feeder (11) con1prising: a housing (12) con1prising an interior screw zone (13), a plug pipe (14) an inner diameter (D) anti wsfltich. in a .ffcetíiinßfï direction. positioned the Etousing tlgïë and comsected to so thaat and a first plug screw (15) con1prising a first screw flight (17), which extends a distance (L1) in the screw zone (13) ztnd :vfníatlt cïoes tï-tot extentis into tfne :slag pärm ätt/få) ofextentíls titan the distatace D itxtan the :sluf-t :sitta t' 4); said n1ethod being characterized by the step of replacing the first plug screw (15) with a second plug screw (25) con1prising a second screw flight (27), which extends in the screw zone (13) and also extends a distance (L4) into the plug pipe (14), where D 5 L4 S 2D.

2. The n1ethod according to clain1 1, wherein the f1rst screw flight (17) extends a distance (L3) into the plug pipe (14), where Lg < D.

3. The n1ethod according to clain1 1 or clain1 2, wherein the plug pipe (14) has a length (Lz) and L4 > 0.5L