CA2058325A1 - Positive displacement pumps - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Improvements in screw pumps, and especially positive displacement pumps, are disclosed. The clearance between the screw flight and the bore is increased over an increased period. The screw is balanced at preferred locations on the flight. Wear hard material such as tungsten carbide is emplaced on a portion of the outer surface of the screw flight. A wear measurement port extends from the outside surface of the pump to the inner surface of the bore.
Pressure regulating ports and controls provide an active pressure feedback control system to control the feeding of pressure from the outlet end of the screw back toward the inlet end of the screw. Pump wear, and pressure pulsations in the operation of the pump are thus reduced. The wear measurement port enables a method of monitoring wear on the pump screw during operation without disassembling the pump, namely by manipulating a wear measurement device through the port. The wear measurement device can be a proximity sensor which can be connected to a monitor. The wear measurement port can be sealed with the proximity sensor in place in the port, such that wear of the screw can be monitored while the pump is operating.

Description

POSITIVE DISPI.ACEMENT PUMPS

BAC~GROUND OF THE INVENTION

This invention relates to pumps and pumping methods.
It especially relates to pumps which are operated at high pump speeds in pumping low viscosity liquids, especially in large volumes.
As used herein, high speed operation of a pump means operation at speeds of at least 500 revolutions per minute (rpm), preferably at least 900 rpm, and most preferably at least 1200 rpm. It is anticipated that pumps disclosed herein can be operated at sustained speeds hetween about 1500 rpm and about 1900 rpm.
It is known to use positive displacement pumps, having counter-rotating twin screws, for pumping higher viscosity products, such as pastes, creams, oils and the like.
It is known to use pumps which are not generally considered to be positive displacement pumps for pumping low viscosity fluids, such as water at high speed and in large volumes.
In the papermaking art, it is known to use fan pumps for pumping two and three phase media which are part liquid and part gas, for example foamed liquid containing 50 to ~0 percent air by volume, which optlonally include some solid material. Foamed fiber furnishes containing solid cellulo-sic fibers for use in papermaking processes are well known as disclosed, for example, in United States Patent 4,443,297 to Cheshire et al, herein incorporated by reference. Pumping of such multi-phase foam media has presented a plurality of problems as the pump speed, the volume of flow, and outlet pressure have been increased.
Using conventional fan pumps, the increase in flow volume has not corresponded well with increase in speed of the pump because of the compressibility of the foamed media.
Accordingly, in order to achieve efficient pumping at higher volumes and higher pump speeds, applicants have found it expedlent to use a positive displacement pump for pumping such foamed media. Such pumps have conventionally been used for pumping higher viscosity products, generally 2~$,~

product5 which provide some fluid lubricity between the stationary pump casing and the moving impeller (e.g. screw).
An advantase of such positive displacement pumps is that they generally create isolated batches of the media being pumped, isolating the batches essentially at, or near, the inlet pressure, whereby batches of foamed media are susceptible to being pumped from the inlet to the outlet in essentially predictable volume, wherein the volume pumped, as measured at the pump inlet, is essentially linearly related to the speed of operation of the pump.
Applicants have found that, when pumping the above mentioned foamed media at e.g. 900 rpm, the pumping opera-tion creates pressure pulses which are transmitted thro~gh the outlet of the pump. Essentially, the fluid in a isolated pumping cell is at a pressure below the outlet pxessure of the pump. Upon reaching the outlet, the pressure in the fluid at the outlet suddenly rushes into the open cell and compresses the fluid in the cell. The sudden rushing OI the fluid into the newly opened cell causes a ra?id, temporary, pressure change at the pump outlet. This pressure change is transmitted out of the pump, throush the pump outlet, as a pressure pulse in the fluid in the enclosed pressurized system downstream of the pump .
While such pressure pulses are of little consequence in operations which comprise only transfer of the media, where the output of the pump is intimately connected with the formation of the web in a papermaking process, such pressure pulses directly a~~fect the uniformity of flow of furnish onto the paper-ma~ing fabric, and accordingly, the unifor-mity, in the machine direction, or the paper so made.
Applicants have also found that some conventionally-produced screw type positive displacement pumps experience excessive rates of wear when operated for sustalned periods at their rated sDeed of 900 rpm for pumping the above recited tkree phase media, containing about 1% to about 4%
by weight cellulose fiber. For example, a typical such pump, having a designed clearance or 0.020 inch (0.5 mm.) ~ ~ ~ r~

between the rotati~g pumping screw and the stationary bore, had a measured clearance of 0.040 inches (1.0 mm.) after sustained operation ~or only three hours.
It is an object of this invention to provide improved screw pumps which can withstand high speed operation over extended periods of time with substantially reduced wear between the stationary and the rotating members.
It is another object to provide means to measure the wear of the pump parts without disassembling the moving member from the stationary member.
It is a further object to provide means to monitor the wear of the pump parts over time without disassembling the moving member from the stationary member.
It is another object to provide a method o monitoring the wear of the pum? parts without disassembling the moving member from the sta~ionary member.
It is still another object to provide pumps which can pump low viscosity --luids at hiyh speed operation with lower amplitude pressure ?ulses.
- It is yet another object to provide pumps which can pump low vlscosity fluids a~ high speed operation with lower rates of change of ~ressure.
It is still another object to provide a method of pumping low viscosity fluids at high speed operation with lower amplitude pressure pulses.
It is a further object to provide a method of pumping low viscosity fluids at high speed operation with lower rates of changes of pressure.
It is yet another object to provide a method of balancing a screw for a screw pump without increasing the leakage between the higher pressure outlet end of the screw and the lower pressure inlet end of the screw, at the loci of removing material for achievement of balance, or other-wise reducing the pumping capacity of the screw over a typical 360 degree rotation of the screw; and without significantly wea~ening the screw.

SUM~IARY OF THE DISCLOSURE

Certain of the objectives are achieved in a family of positive displacement pumps, comprising an outer casing, and first and second pumping screws. The outer casing comprises an inlet, an outlet, and internal passage means adapted to convey material between the inlet and the outlet. The pumping screws have first portions contained in the passage means. The passage means has an inner surface adjacent the pumping screws, portions of the inner surface defining a space therebetween, the space comprising a bore. Each pumping screw has a minor circumference defined by a central shaft, and a major circumference defined by at least one screw flight. A second portion of each of the pumping screws is contained in the bore over a length of the bore wherein ~he bore and the flisht are coextensive. The screw flight extends radially and longitudinally about the shaft, and outwardly from the shaft, to an outer surface of the flight, at least a portion of the outer surface being defined in the major circumference. Front and rear surfaces of the flight extend between the shaft and the outer surface, and define a thickness of the flight there-between. Each flight has a height, as measured between the shaft and the outer surface.
The screw flights are secured to the respective shafts and have inlet and outlet ends. Each screw fligh~ terminates at the outlet end thereof.
Certain of the objectives are obtained in a family of positive displacement pumps, also comprising an outer casing and first and second pumping screws, the casing comprising an inlet, an outlet, and the internal passage means. The pumping screws have the same minor and major circumferences, and the same general arrangements of the fli~hts on the shafts. The passage means comprises the bore, having an inner surface, and adapted to receive portions of the pumping screws. The flights are similarly received in the boxe with similar clearances. First and second bearings mount each of the pumping screws to the casing at spaced locations thereo.. The flights are disposed between the bearings. The screws comprise one or more balancing holes 2 ~

in corresponding flights thereo~, tHe balancing holes being disposed on surIaces of the flights, and being adapted to reduce an~ tendency of the screws toward imbalance, the balancing holes being disposed on, or below, surfaces such that they will not materially reduce the pumping capacity of the pump.
The balancing holes may be plugged with material having a density less than the average density of the respective ones of the flights. The holes are preferably capped and ground such that the outer surface of the flight is continuous and smooth over the holes.
Certain of the objectives are achieved in a more generically ~efined screw pump comprising an outer casing and a screw. The outer casing comprises an inlet, an outlet, and inte~nal passage means adapted to convey material between the inlet and the outlet. The pumping screw has a minor circumference defined by a central shaft, and a major circumference defined by at least one screw flight. The screw flight extends radially and longitudi-nally about the sh2-t, and outwardly from the shaft, to an outer surface of the flight, at leas~ a portion of the outer surface being defined in the major circumference. The passage means comprises a bore adapted to receive at least a portion of the pum2ing screw, the bore comprising an inner surface. A irst portion of the flight is received in the bore over a length of the bore wherein the bore and the flight are coextensive. The clearance between the outer surface of the flight and the inner surface of the bore is sufficiently small as to promote efficient pumping of material through the bore by the screw. A wear measurement port extends through the outer casing, from an outer end thereof on the outside surface of the pump to an inner end thereo~ at the inner surface of the bore propinquant the major circumference defined by the outer surface of the screw, upon rotation of the screw. The wear measurement port is positioned such that a portion of the outer surface o, the flight can be disposed adjacent the inner end of the ~3~ ~3 port, whereby a measurement device can be manipulated in or through the port, and the wear of the scr~w can thus be measured.
The screw and the outer casing preferably include indicators which indicate that angular position of the screw at which the outer surface is disposed in measurement position at the wear measurement port.
In preferred embodiments within this family, a proxim-ity sensor is dis?osed in the wear measurement port, and means for transmitting a signal generated ~y the proximity sensor extends from ~he proximity sensor and the wear measurement port, the wear measurement port being closed and sealed sufficient for normal operation of the pump.
Certain of the objects or the invention are achieved in a screw pump, pre-^erably a positive displacement pum?
wherein the flights comprise elongate hardened wear strips as components of the outex surfaces of the flights, each wear strip having a width corresponding in direction to the corresponding width of the corresponding one of the outer surfaces, less than 50% of the width of a given length portion OI the outer surface being comprised of the wear strip.
In some embodiments, a portion of the wear strip is prererably overlain by the softer material comprising the main body OL the corresponding one of the screws, the overlying softer material being adapted to hold the wear strip against movement out of ~he flight in a direction perpendicular to the longitudinal axis of the screw both before and after initial wear o~ the wear strip at a surface thereof corresponding to the outer surface of the screw.
In some embodiments, the widths of the wear strips vary along the lengths of the flights such that the width of a given wear strip at a given locus is related to the poten-tial amount of wearing contact at the l~cus between the respective one of the outer surfaces and the bore. In those embodiments where the width of the strip varies according to location on the flight, the wider widLhs or the wear strips can e~compass greater than 50~ of the width of the outer surface at a given locus, up to 100~, especially ~J ~

adjacent the outlet end of the flight at loci where the width of the flight is tapering toward the zero thickness at the outlet end of the flight.
Certain objects of the invention are achieved in a method of monitoring wear in a screw pump. The method comprises the ste?s of selecting an appropriate screw pump having a port therein suitable for receiving a wear measure-ment device, manipulating a measuring device through the port, and measuring the wear of the respective one of the screws, using the measuring device as manipulated through the port. In the p~mp so used, the port is positioned such that a portion of the outer surface of the respective flight can be disposed adjacent the inner end of the port, along a lonsitudinal axis extending along the length of the port, whereby a measuring device can be manipulated through the port and the wear o_ the screw can thus be measured.
In some embodiments, the wear measurement port has a plug in it, and the method comprises, between the steps of selecting the pump and manipulating the measurlng tool therethrough, the steps of operating the pump, stopping the opera.ion of the ?um?, and removing the plug.
In a preferred embodiment, a preferred wear measure-ment device is a proximity sensor, and the method comprises emplacing the proximity sensor in the port, sealing the port closed, operating the pump by turning the screws with the proximity sensor so emplaced, and sensing dynamic changes of proximity of the flight to the sensor.
Still others of the objects are achieved in a method of balancing screws used in the pumps. The method of balanc-ing the above described screw comprises the steps of determining the locus and amount of imbalance of the screw;
and removing material from the flight, in correcting the imbalance, along that portion of the flight which is between the bore and the bearings.
Certain of the objects are achieved in methods of controlling the amplitude and/or the rate of change or pressure pulses in a low viscosity pumping operation, wherein the viscosity is no greater than 5 centipoise, and wherein a positive displacement screw pump, as desc_ibed 2~$~ ~J'~

above, is used, an~ is operated at high speed, whereby the pum? has an operating inlet pressure and an operating outlet pressure. The purn?ing cells between facing portions of the front and rear surfaces of the flight have cross sections defined between the major and minor circumferences.
Indi~idual batches of the material ~eing pumped are substan-tially isolated, in the pumping cells, from both the outlet pressure and the inlet pressure during traverse of the screw by the batches between the inlet and the outlet. The individual batches of material are sequentially opened to the outlet pxessure during operation of the pump and the associated pumping of the material.
The method of controlling the pressure pulses in the last described o?eration comprises opening the cells to the outlet ?ressure by creating an opening between a respective cell ar.d the outlet, wherein the cross-section of the opening is at least as great as the cross-section of the cell, over a period of time greater than the time during which, at the speed of operation of the pump, the flight traverses an arc, as measured transverse to the sha-t, wherein the cross-section oî the s?ace defined between the arc at the major circumference and the shaft is ecual to the cross-section of the corresponding pumping cell.
Pre_erably, the cells are opened over a period of time at least two times, more preferably at least six times, in some embodirnent preferably at least ten times, and up to at least _ifteen times, as great as the time during which the flight txaverse the arc.

$ ~

BRIEF D~SGRIPTION OF THE DRAWINGS

FIGURE 1 shows a front view of a pump of the invention with portions cut away.
FIGURE 2 shows a transverse cross-section of a portion of the pump, and is taken at 2-2 of FIGURE 1.
FIGURE 3 is a fragmentary cross-section showing an alternative combination o~ bore and screw, to achieve reduced pressure pulsation effect.
FIGURE 4 shows a fragmentary cross-section including a dynamic control system adapted to reducing pressure pulsa-tion by providing control apparatus for controlling the amount of pressure lea~age, from the screw outlet area back toward the screw inlet ends.
FIGUR~ S shows a cross-section of a balancing hole and is taken at 5-5 of FIGURE 1.
FIGURES 6 and 7 show fragmentary cross-sections of the screw and illustrate the wear strips on the screw.
FIGURr 8 is a view Oc the right side of the pump of FIGURE 1~ showing wear strip variation with respect to position on the screw.

~ 3 DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Turning now to FIGURES 1 to 4, a pump 10 has an outer casing 12, including an inlet 14, an outlet 16, and internal passages generally designated 18, traversing the casing 12 between inlet 14 and outlet 16. Internal passages 18 include a bore 20 adapted to receive therein a pair OL-pumping screws 22 and 24. The cross-section of bore 20, and thus the space defined therein, is defined at any given point along its length by an inner surface 26. The bore 20 further comprises an outer surface 27, and a wall 29 between inner and outer surfaces 26 and 27.
Pumping screws 22 and 24 comprise minor circumferences defined by shafts 28 and 30 respectively. Each shaft has a longitudinal axis 33 extending along the length thereof.
Screws 22, 24, further comprise major circumferences 31 defined by the outer surfaces 32, see Figure 6, of helical screw f1ights 34, 36, 38, and 40, at the maximum circumferences of those flights. In the embodiments illustrated, each shaft has an attached pair of flights disposed toward opposing ends thereof.
Sturfing boxes 42 and 43 are attached to opposing ends of outer casing 12 and contain stuffing seals 44 and 45.
The stuffing ~oxes 42 and 43 support screws 22 and 24 on bearings 46, 48, 50, and 52, the seals 44 and 45 being disposed between the bearings and the pumping chamber which is generally defined by internal passages 18. The bearings and seals can, of course, be reversed in position for use in pumping material which is adapted to lubricate the bearings.
A gear box 54 is mounted to the right end of the right stuffing box 43 and houses gears 56 and 58 which are preferably integral parts of the respective screws 22, 24 as illustrated. Gears 56, 58 are arranged with the teeth or gear 56 meshed with those of gear 58 such that the rotation of shaft 28, by a motor or other prime mover(not shown), causes counter-rotation of the screws 22, 24 whereby one o screws 22, 24 rotates clockwise and the other screw rotates counter-clockwise.

~ 3 Each of the screw flights extends radially and lon-gitudinally about the respective shaft, and extends out-wardly from the shaft to the outer surface 32 of th~
respective shaft. Each flight comprises a front surface 6C
disposed toward outlet 16 and a rear surface 62 disposec away from outlet 16, front and rear surfaces 60 and 6 defining the thickness of the respective fllght therebetweer at any point along the length of the respective flight. At any point along a given flight, the flight has a height "H"
as measured between the respective shaft 28 or 30 and the respective outer surface 32.
Pumping call spaces 64 and 66 are defined betweer.
facing portions of the front and rear surfaces of respective flights 34 and 38, between the shaft 28 and the outer surfaces 32 of the flights 34 and 38. Pumping cell spaces 68 and 70 are defined between facing front and rear surfaces of respective flights 36 and 40, between the shaft 30 and the outer surfaces 32 of flights 36 and 40.
As seen by the combined teachings of FIGURES l, 2 and 4 the flights 34, 36, 38, and 40 of the screws 22, 24 are meshed between shafts 28, 30, by the design and positioning of the screws, such that the flights 34, 38 on screw 22 reach into the pumping cell spaces 68, 70 on screw 24, essentially closing off the cell spaces 68, 70 between the shafts Z8 and 30. Similarly flights 36, 40 on screw 24 reach into the pumping cell spaces 64, 66 on screw 22, essentially closing off the cell spaces 64, 66 between shafts 28 and 30. The thicknesses of the fliyhts and the spacing of the corresponding front and rear surfaces (the width "W" of the cell spaces) on the adjacent flights cooperate such that the clearances between the flights at the meshed screws are small enough that the flight OI one screw serves as an effective closure of the cell spaces of the opposing screw at the loci where the screws are meshed between their shafts.
The clearances between the outer surfaces 32 of the respective fLights and the bore 20 are small enough, and comprise enoush length of bore 20, to promote efficient pumping of a low viscosity material, such as water, a foamec ~ ~ ,;d ~

liquid containing, (e.g. 50~ to 80~ air by volume,) and the same media when including papermaking fibers dispersed therein. Typical clearances are between about 0~02 and about 0.04 inch ~about 0.5 to about 1.0 mm.). The typical seal clearance is normally effective over at least one turn of each flight, thereby effectively isolating the inlet end of the flight from the outlet pxessure at the outlet and of the flight.
The closure of the pumpi.ng cell spaces 64, 66, 68, and 70 by the meshing of the screws between shafts 28 and 30 creates at least one, and preferably a plurality of isolated and distinct pumping cells 72 at each flight on each screw.
Each pumping cell 72 is defined within a pumping cell space (e.g. 64) between facing front and rear surface portions 60, 62 between the respective shaft (e.g. 28) and the ad~acent inner surface 26 o~ the bore 20, about generally one turn of the pumping cell space (e.g. 64) between consecutive extensions of the opposing flight (e.~. 36) into the pumping cell at the locus of meshing of the screws. Accordinsly, each pumping cell extends approximately one turn (a bit less) about the shaft between adjacent turns of the cor-responding flight. The cell has a beginning and an end where the screws are meshed. Accordingly, the pumping cell is isolated from the inlet, from the outlet, and from the other cells. Upon turning of the screw (e.g. 22) in the pumping direction, the pumping cells are advanced along the-screw toward the outlet 16 at the center of screw 22.
The portions 7 4 of the screws which are in internal passages 18 comprise the portions between the seals 44 and 45. The portions 74 include first lengths over which the screws and the bore are coextensive (i.nside bore 20) and second lengths between bore 20 and the respective seals 44, 45 .
Each screw flight 34, 36, 38, 40 begins at an inlet end 76 disposed toward the respective stu~fing seal 44 or 45 and ends at an outlet end 78 disposed toward outlet 16. Inlet end 76 of the flights may, and preferably does, extend at least part or the distance between bore 20 and seals 44, 45.
Outlet end 73 is preferably configured as in FIGURES 1 and 2 ~ 3 ~

2, to reduce the pressure pulses in the pumped fluid. The clearance between the outer surface 32 of the flight and the inner surface 26 of the bore may be increased to the outlet end 78 or the fli~ht over a portion of the coexistent length of the bore and the screw along the longitudinal axis of the shaft of the respective screw.
The e~bodiment illustrated in FIGURES 1 and 2 shows an increase in the clearance between the outer surface of the flight, at outlet end 78, and the inner surface 26 of the boxe, over a full turn of 360 degrees of the flight on the sha't (FIGU~E 2~.
The progressive increase in the clearance between the outer surface 32 of the flight and the inner surface 26 of the bore represents a progressive (gradual) opening of the pumping cell to the operating pressure at the outlet 16 of the pump, whereby the pulsating effect of the opening of the cell is spread over a longer period of ti~e than if the height of the flight came ~o an end, from its full height at the major circumference, over a shorter distance during high speed operation of the p~mp, which can develop about 50 psi pressure dif e ence between the pump outlet and the pump inlet even at the lower speed of 909 rpm when pumping paper making furnishes based on water or foamed llquid as herein disclosed.
The provision of the progressive increase in clearance ls illustrated in FIGURES 1 and 2 as a progressive decrease in height "~" or the screw flights at the outlet ends 78 of the flights, the size of the bore 20 being kept constant.
FIGURE 3 illustrates another method of achieving the increase in clearance, namely an increase in the cross-section of bore 20 adjacent the outlet end of the flight, while the height n~ of the flight is maintained constant to outlet end 78, in accordance with the respective major circumference 31, or is tapered in height ovPr a relatively shorter distance.
Both changes may, of course, be made, whereby the height of the flight is progressively reduced and the cross-section of the bore is changed (increase or decrease) to acco~modate the desired rate of change in the clearance.

At the higher speeds of operation contem~lated for the pumps of this lnvention, any imbalance in the screws will be evidenced by vi~ration and wear in the screws and bores or the pump. Accordingly the screws are prererably balanced.
The balancing is accomplished by defining the angle of imbalance (the radial angle about the shaft whereat the screw is heavy) and removing an appropriate amount of material from the flights, to reduce that imbalance, at the appropriate radial angle, at locations, and in such ways, that the ins'antaneous efficiency of the ongoing pumping operation will not be reduced.
Preferr-d locations for removing materi~l are shown in FIGURE 1. Hidden holes 82 are located at distances approxi-mately 25~ (and respectively 75%) of the distance between the bearings 46, 52, whereby they reduce the tendency of the screw to set up a standing one-cycle sine wave vibration.
Holes 82 are generally made by removing the calculated amount of material with a drill. As illustrated in FIGUR~
;, the holes 82 are preferably capped with a cap 84 which is welded in place over the hole, after which the cap 8~ is machined or ground flush so that it becomes an integral part OL the outer surface 32 of the flight. Accordingly, the balancing holes 82 do not penetrate outer surface 32, and thus have no ef ~c~ on the ability of the outer surface 32 to create isol2ted pumping cells with appropriate control of ~-luid leakage between outer surface 32 and inner surface 26 o~ bore 20 The balancing holes can be plugged with material of lower density than that of the flight in which they are made.
As seen in FIGURE 2, wear measurement ports 86 extend through outer casing 12, from inner ends 88 thereof at inner surface 26 to outer ends 90 thereof at the outside surface of the pump. Ports 86 have longitudinal axes 87, and are closed by re~ovable plugs 92 which maintain the seal of the pump during normal pump operation. The simplest plug 92 (left side in ~IGURE 2) is typically threaded, and is removed when the pump is out of service, whereupon a measuring tool is manipulated through the port 86 and against the outer sur.ace 3~ or the respective flight. For such use, it is necessary ~hat the screw be positioned such that the outer surface 32 is aligned wi~A the hole 86, ,, D3 between hole 86 and the shaft of the screw. Preferably the shaft 28 and the outside of the gear box, or other appro-priate stationary surface, are marked at matching locations thereof, as at 94 on shaft 28, such that alignment of mark 94 on shaft 28 with the matching mark on the outside surface or the gear box brings the outer surfaces 32 of the flights into alignment with the wear measurement ports 86 which are so used.
The port 86 indicated on the right side of FIGURE 2 is closed by a plug 96 which lncludes a dynamic proximity sensor 98, connected by signal carrier 100 (e.g. wire cable) to a monitoring device 102. Plug 96 is in place in port 86 while the pump is in operation. Upon each rotation of the respective screw, the outer surface 32 passes proximity sensor 98 whereupon a discreet signal is sent to monitor 102, which provides for monitoring of the wear of the pump while the pump is in operation.- An acceptable proximity t-ansducer system, including proximity sensor 98 and monitor 102 is the 7200 Proximitor, available from Kaman Instrumen-tation Cor?oration, Colorado Springs, Colorado.
With such constant input of information from the proximity sensor, it is now possible to record the proximity as sensed over a ?eriod of time, and thereby to detect changes in the proximity, which indicate wear or imminent failure of the pump before the even~ occurs, whereupon the complex papermaking process (or other process) can be shut down in an orderly manner, and indeed such maintenance, repair, or rebuild can be efficiently planned.
Referring now to FIGURF 4, there is shown therein a fluid pressure feedback control system comprising fluid pressure receiving ports 104, pipes 106, valves 108, manifold 110, pipe 112, and fluid sending port 116. Fluid pressure receiving ports 104 are similar to wear measure-ment ports 86 in that they extend through wall ~7 of bore 20 to the inner suxface 26 of the bore. Ports 104 are typi-cally spaced such that any pluxality of fluid pressure receiving ports 104 open into different pumping cells.
Ports 104 are connected by pipes 106, through valves 108 and maniîold 110 to pipe 112 which is in fluid communication with the high pressure portion 114 of bore 20, through rluid pressure sending port 116. 3y manipulating valves 108, fluid at ~he outlet pressure of the pump can be controllably and adjustably red back into the pumping cells, as indicated at pressure gauges 118, whereby the pressure dirferential between the high pressure portion 114 and the next ooening one of the cells can be reduced by building the pressure in the cell before it is fully opened to the outlet pressure at the outlet end of the flight.
Similarly, the higher pressure fluid, and thus the fluid pxessure, can also be fed by way of manifold 110, and the respective valve 108, to any or each of the pressure receiving ports 104. The pressure in the respective pumping cells (e.g. 64) can thus be controlled, and increased progressively in a given cell as the fluid traverses the length of the flight, such that the pressure change ex-?erienced by the fluid in a cell when the cell opens at outlet end 78 is controlled, and the pressure change upon opening or the cell to the outlet pressure can be minimized when desired. And as the pressure change at flight outlet end 78 is reduced, the pressure pulses associated with that pressure change are also reduced, whereby the vibration and resonance caused, in the downstream piping, by such pressure pulses, is reduced. Thus, the pressure feedback control system ll9 illustrated in FIGURE 4 can be efective to reduce the fluid stress, es?ecially the vibration and resonance stress, on the pump and on the piping and other apparatus which is subjected to the output pressure of the pump downstream from the pump.
One pressure feedbac~ system ll9 is shown in FIGURE 4, for controlling the pressure along flight 34, it being unders~ood that a similar feedbac~ control system is used for each flight in the pump (e.g. 36, 38, and 40).
The resonance, vibration, etc., in the operating system or which the pump is a part, as set up by high speed operation of ~he several components thereof, can be somewhat accommodated by manipulating the valves 108 to change the intensi~y and duration of pressure pulses produced by pU-~?
10 in order to provide a dynamic control c~pabili.y in the 2.~

dynamic vibration aIId resonance environment. Further, as vibration and resonance change in the operating system (e.g.
in response to chan5es in pump speed), the control valves can be manlpulate~ to provide the changed optimum amounts of fluid and pressure feed-back.
While the pumps of this invention are designed to operate wit~. a clezrance of e.g. 0.02 inches (0.5 mm.) between the screws 22, 24 and the bore 20, wherein the screws 22, 24 are supported by bearings 46, 48, 50, 52 near opposing ends thereof, and whereby the screws should never touch the inner sur.ace 26 of bore 20, in actual practlce, such touchins does occur, in part due to vibration and resonance. In addition, the pumps which applicants con-template using in paper making processes are large, e.g.
major an~ minor circumferences of 50 inches (127 cm.) and 25 inches (63.5 cm.) respectively, and unsupported distances between bearinss 46, 48 and bearings 50, 52, of over 10 feet (3.05m). A variety of forces apply bending moments on screws 22, 24. For example, the mass of the material in screws 22, 24 encourages a certain amount of sag, in response to ~ravity, in the middles of the screws, midway between the su?portins bearings, centering around the locus of outlet 16. This sas contributes to eccentric rotation or the screw. As the screws rotate, they set up resonance vibrations that contribute to eccentricity of the rotation.
Also, as taught in ~nited States Patent 2,463,460, the screws tend to shift toward the discharge opening, in a direction transverse to the lengths of the shafts 28, 30, when the pump is operated.
With such forces being applied to the screws 22, 24, the screws do experience some contact between the outer surfaces 32 of the flights and the inner surrace 26 of bore 20. The momentum of the impacting contact is, of course, related to the mass of the respective screw and the velocity of the respective movement. Accordingly, outer surface 32 is effectively hardened to reduce the wear of the screw.
The hardenings preferred for screws 22, 24 are illustrated in FIGURES 6-8.

3 2 ~

In FIGURE 6, a groove 120 is cut along the middle of outer surface 32 of the flight, and is filled with a wear hard material 122 suc:~ as tungsten carbide or stellite. The wear hard material 122 is mechanically locked in place, against movement in a direction transverse to the longi-tudinal axis of the shaft 22, by the overlying material at wedge 128. Hard material 122 can be applied, as conven-tional, with a plasma arc. However, inserting a pre-formed strand of material 122 into a groove 120 is preferred, followed by locking the hard mate~ial in place by modest deformation of the surrounding material to secure the lock at wedge 128.
Since subs.antially less than half of the thickness, overall, of the outer sur~ace 32 of the flight is occupied by the hard material 122, the cost and difficultly of finish grinding outer sur~ace 32, to form a unitary and uniform surface as shown, is commensurately reduced as compared to finish grinding the surface after applying hardening material over the entire width of outer surface 32. The mechanical locking at wedge 128 enables the use of a less e~?ensive pre-formed strip of hard material rather than the conventional more ex?ensive hot melted spray application of hard material 2s a' 43 in United States Patent 3,841,805.
Alternative loci and orientations of attachment are shown in FIGURE 7, wherein the hard material 122 is shown at the leading and trailing edges 126 and 12~ respectively of the outer surface 32, and can be at either or both edges, leading edge preferred. At leading edge 126, the wear hard material 122 is mechanically locked in place, against movement in directions both transverse to the longitudinal axls of the shaft 22 and parallel to the longitudinal axis OI the shaft, by the overlying material at wedge 128, similar to the mechanical locking illustrated in FIGURE 6.
In FIGURE 6, approximately half of the hard material 122 will be worn away by the time the mechanical locks OI the corresponding wedges 128 are worn away. Similarly, a significant amount of material 122 will be worn away at leading edge 126 before the mechanical locks of the respec-tive wedges 128 are worn away there.

?~

r IGURE 8 illustrates varying the width of the hard material 122 along the length of the flight, according to the proba~le rate of wear at any given locus on the screw.
Since the greateSt moment arm in the screws is adjacent the outlet, the probabilities for wear are greatest adjacent outlet 16, and become rather progressively less when one moves in the directions toward the supporting bearings.
Accordingly, at the outlet ends 78 of the flights 38, 40 the entire outer surraces 32 thereat are surfaced with hard material 122. As one progresses along flights 38, 40 toward their inlet ends, the fractions of the surfaces 32 which are occupied by the wear hard material 122 become progressively smaller as shown in FIGURE 8.
As described above, the pumps of this invention are improved by incor?oration of a variety of modifications, each of which can be used alone, or in any combination with the others. Preferably, all the improvements are incor-porated into a given pump whereby the cumulative benefits thereof are obtained. Such pumps are improved by introduc-ing some of the fluid at the outlet pressure to an isolated pumping cell ?rior to completely opening the cell to the outlet ?ressure, as illustrated in FIGURES 1-4. They are improved by balancing the screws as illustrated in FIGUR~S 1 and 5, by careful selection of the locus of the balancing holes as illustrated FIGURE l, and by capping, and grinding smooth the caps on the balancing holes as seen in FIGURrS l and 5. They are improved by providing sensors and monitors to sense and monitor screw wear. They are improved by providing pressure feedback control system 119 which allows dynamic control o- the pumping pressure in the isolated pumping cells 72.
Typical materials contemplated to be pumped by pumps of this invention are low viscosity fluids such as water and compositions containing a substantial amount o~ water.
Other materials could, of course, be pumped. A preferred material is a foamed fiber furnish for paper making as dis-closed in United States Patent 4,443,297 Cheshire et al.
Suitable papermaking foamed fiber furnishes contain from about ;0 to about 80 percent air by volume. The overall 2 ~ 2 ~

li¢uid medium contains about 200 to about 300 ppm surfac tant, such as ~he sur~actant sold under the trade name A-OK
by Arco Chemicals, Inc. Papermaking fiber is dispersed in the foam at ~ concentration up to about 2~ to about 3~ by weight.
In the pumping operation, a prime mover such as an electric motor~ is coupled with shaft 28 of screw 22 at the right end of the pump (FIGURE 1). As the motor rotates shaft 28, the entire screw 22, including flights 34 and 38, rotates, and is supported for rotation in bearings 46,52, as well as end bearing 130~ As screw ~2 rotates, engaged gears 56, 58 cause simultaneous rotation of shaft 24 in the opposite direction. Each of the flights 34, 36, 38, 40, are pitched such that the rotation of the respective screw causes material, which is entrapped in pumping cells defined by the flights of the respective screws, to be advanced, in the res?ective pumping cells, toward the outlet ends 78 of the respective flights, and accordingly, toward the outlet 16 of the pump 10.
Material to be pumped (e.g. three phase foamed fiber rurnish) enters the pump at inlet 14 and follows dive_gent paths to opposing inlet ends 76 of the flights on opposing ends of bore 20. As the screws turn, the material is drawn into bore 20 by the turning screws. As the material enters bore 20, individual units of the material are entrapped in pumping cells 72, where they are isolated from the inlet, from the outlet, and from each other. The isolation is, of course, violated in small part by the ordinary lea~age tolerated by the small clearances between the several respective parts of the pump. Continued turning of the screws causes advancement of the pumping cells, and accord-ingly the liquid entrapped therein, toward the outlet ends 78 of the llight, and ul~imately out of the pump at outlet 16.
While the pumps of this invention have been described with respect to pumping water and aqueous foamed liquid dispersions at contemplated conditions, ~hey can clearly be used with other pumpable materials and at other pumping conditions.

Claims (19)

1. A twin screw positive displacement pump comprising:
(a) an outer casing, said outer casing comprising an inlet, an outlet, and internal passage means adapted to convey material between said inlet and said outlet (b) first and second pumping screws, each said pumping screw having (i) a minor circum-ference defined by a shaft, and (ii) a major circumference defined by at least one screw flight, said screw flight extending radially about said shaft and longitudinally along said shaft, and outwardly from said shaft, to an outer surface of said flight, a portion of said outer surface being defined in said major circumference, front and rear surfaces of said flight extending between said shaft and said outer surface and defining a thickness of said flight therebetween, said screw flights being secured to said shafts;
said passage means comprising a bore adapted to receive portions of said first and second pumping screws, said bore comprising an inner surface; first portions of said flights being received in said bore along lengths of the respective ones of said screws wherein said bore and said flights are coextensive clearances between said outer surfaces of said flights and portions of said inner surface of said bore being sufficiently small to promote efficient pumping of material through said bore by said screws; and (c) first and second bearings mounting each said pumping screw to said casing at spaced locations thereof, said flights being disposed between said bearings, said screws comprising one or more balancing holes in corresponding flights thereof, said balancing holes being disposed on surfaces of said flights and being adapted to reduce any tendency of said screws toward imbalance.

2. A positive displacement pump as in Claim 1, said balancing holes being disposed at locations on said flights adjacent the inlet to said bore.

3. A positive displacement pump as in Claim 1, the entrances to said one or more balancing holes being located on said outer surfaces of said flights, and being spaced from said front and rear surfaces sufficient distances that said holes accommodate no significant reduction in pumping capacity of material to be pumped.

4. A positive displacement pump as in Claim 1 wherein said one or more balancing holes are plugged with material having a density less than the average density of the respective ones of said flights.

5. A positive displacement pump as in Claim 1 wherein said one or more holes are capped such that said surfaces are continuous over said holes.

6. A twin screw/type positive displacement pump, comprising:
(a) an outer casing, said outer casing comprising an inlet, an outlet, and internal passage means adapted to convey material between said inlet and said outlet;

(b) a pumping screw, said pumping screw having (i) a minor circumference defined by a shaft, and (ii) a major circumference defined by at least one screw flight, said screw flight extending radially about said shaft and longitudinally along said shaft, and out-wardly from said shaft, to an outer surface of said flight, a portion of said outer surface being defined in said major circum-ference; said passage means comprising a bore, adapted to receive a portion of said pumping screw, said bore comprising an inner surface, an outer surface, and a wall therebetween; a first portion of said flight being received in said bore over a length of the respective one of said screws wherein said bore and said flight are coexistent;
clearance between said outer surface of said flight and said inner surface of said bore being sufficiently small as to promote efficient pumping of material through said bore by said screw;

(c) at least one wear measurement port extending through said wall of said bore, form an outer end of said wear measurement port on said outer surface of said bore to an inner end of said wear measurement port at said inner surface of said bore propinquant said major circumference defined by said outer surface of said pumping screw upon rotation of said screw, said wear measurment port being positioned such that a portion of said outer surface of said flight can be disposed adjacent said inner end of said port, whereby a measuring device can be mani-pulated through said port, and the wear of said screw can thus be measured; and (d) a proximity sensor in said wear measurement port;

said wear measurement port being closed and sealed sufficient for normal operation of said pump.

7. A screw pump as in Claim 6, including means extending from said proximity sensor for transmitting a signal generated by said proximity sensor, and monitoring device means for monitoring signals, transmitted from said proximity sensor through said signal transmitting means, during routine operation of said pump.

8. A twin screw positive displacement pump, compris-ing:
(a) an outer casing, said outer casing comprising an inlet, an outlet, and internal passage means adapted to convey material between said inlet and said outlet:

(b) first and second pumping screws, each said pumping screw having (i) a minor circum-ference defined by a shaft, and (ii) a major circumference defined by at least one screw flight, said screw flight having a length extending radially about said shaft and longitudinally along said shaft, and out-wardly from said shaft, to an outer surface of said flight, a portion of said outer surface being defined in said major circum-ference; front and rear surfaces of said flight extending between said shaft and said outer surface and defining a thickness of said flight therebetween; said screw flights being secured to said shafts, said passage means comprising a bore adapted to receive portions of said first and second pumping screws, said bore comprising an inner surface, first portions of said flights being received in said bore over lengths of said screws wherein said bore and said flights are coextensive; and clearances between said outer surfaces of said flights and said inner surface of said bore, said clearances being sufficiently small to promote efficient pumping of material through said bore by said screws; said flights comprising effectively elongate hardened wear strips as components of said outer surfaces, each said wear strip having a width less than half the thickness of said outer surfaces.

9. A positive displacement pump as in Claim 8 wherein portions of said wear strips are overlain by softer material comprising the main body of the corresponding ones of said screws, said overlying softer material being adapted to hold said wear strips against movement out of said flights in directions perpendicular to the longitudinal axis of the corresponding ones of said screws both before and after initial wear of said wear strips at surfaces thereof corresponding to said surfaces of said screws.

10. A positive displacement pump as in Claim 9, said screws comprising balancing holes in corresponding flights thereof, said balancing holes being disposed on surfaces of said flights.

11. A positive displacement pump as defined in Claim 8 wherein said widths of said wear strips vary along said lengths of said flights such that the width of a given wear strip at a given locus is related to the potential amount of wearing contact at that locus between the respective one of said outer surfaces and said bore.

12. A positive displacement pump as defined in Claim 11 wherein portions of said wear strips are overlain by softer material comprising the main body of the corresponding ones of said screws, said overlying softer material being adapted to hold said wear strips against movement out of said flights in directions perpendicular to the longitudinal axis of the corresponding ones of said screws both before and after initial wear of said wear strips at surfaces thereof corresponding to said surfaces of said screws.

13. A screw type positive displacement pump compris-ing:
(a) an outer casing, said outer casing comprising an inlet, and outlet, and internal passage means adapted to convey material between said inlet and said outlet;

(b) a pumping screw, said pumping screw having (i) a minor circumference defined by a shaft, and (ii) a major circumference defined by at least one screw flight, said screw flight having an inlet end and an outlet end, and extending radially about said shaft and longitudinally along said shaft, and out-wardly from said shaft, to an outer surface of said flight, a portion of said outer surface being defined in said major circum-ference; said passage means comprising a bore, adapted to receive a portion of said pumping screw, said bore comprising an inner surface, an outer surface, and a wall therebetween; a first portion of said flight being received in said bore over a length of said screw wherein said bore and said flight are coexistent: clearance between said outer surface of said flight and said inner surface of said bore being sufficiently small to promote an operative pumping seal for pumping material through said bore by said screw; and (c) a fluid pressure feedback control system, said control system comprising;
(i) a fluid sending port, said sending port being disposed on the higher pressure side of said outlet end of said flight and extending through said wall of said bore from a first portion of said outer surface of said bore to said inner surface thereof, (ii) a fluid receiving port, said receiving port extending from a second portion of said outer surface of said bore through said wall of said bore to said inner surface of said bore and being disposed at said inner surface of said bore between said inlet end of said flight and said outlet end of said flight, and (iii) a conduit, connecting said fluid sending port to said fluid receiv-ing port.

14. A screw pump as in Claim 13 and including valve means adapted to control flow of fluid through said conduit between said sending port and said receiving port.

15. A screw pump as in Claim 13, including a plurality of said receiving ports, each said receiving port having an inner end thereof at said inner surface of said bore, said receiving ports being spaced such that portions of said flight adjacent said inner surface of said bore are disposed between said inner ends of said receiving ports; conduits connecting aid receiving ports to aid sending port; and value means adapted to distribute flow of fluid from said sending port among said receiving ports, said valve means being adapted to control the pressure at each of said receiving port whereby pressure in the fluid being pumped an be increased progressively during traverse along the length of said flight such that the pressure change ex-perienced by the fluid upon expression of the fluid from said flight at the outlet end thereof is substantially less than the pressure differential between said pump outlet and said inlet end of said screw of the fluid being pumped.

16. A method of monitoring wear in a screw pump, said method comprising the steps of:
(a) selecting a screw pump having (i) an outer casing, said outer casing comprising an inlet, an outlet, and internal passage means adapted to convey material between said inlet and said outlet, (ii) first and second pumping screws, each said pumping screw having a minor circumference defined by a shaft, and a major circumference defined by at least one screw flight, said screw flight extending radially about said shaft and longitudinally along said shaft, and outwardly from said shaft, to an outer surface of said flight, a portion of said outer surface being defined in said major circumference; said passage means comprising a bore adapted to receive portions of said first and second pumping screws, along coexistent lengths of said bore and said screws, said bore comprising an inner surface, an outer surface, and a wall there-between; first portions of said flights being received in said bore; clearances between said outer surfaces of said flights and said inner surfaces of said bore being sufficiently small to promote efficient pumping of material through said bore by said screws; and (iii) a wear measurement port extending through said wall of said bore, from an outer end of said wear measurement port on said outer surface of said bore to an inner end of said wear measurement port at said inner surface of said bore propiquant one said major circumference defined by one said outer surface of one said pumping screw upon rotation of said screw, said wear measurement port being positioned such that a portion of said outer surface of said one flight can be disposed adjacent said inner end of said port, along a longitudinal axis extend-ing along the length of said port, whereby a measuring device can be manipulated through said port and the wear of one said screw can thus be measured;

(b) manipulating a measuring device through said one port; and (c) measuring the wear of the respective one of said screws, using said measuring device as manipulated through said port.

17. A method as in Claim 16 said wear measurement port having a plus therein, said method comprising, between steps (a) and (b), the steps of (i) operating said pump;
(ii) stopping said operating of said pump;
(iii) removing said plug from said port; and (iv) turning said one screw such that a portion of said outer surface is positioned adjacent said inner end of said port.

18. A method as in Claim 16, said measurement device comprising a proximity sensor, said method comprising emplacing said proximity sensor in said port, sealing said port closed, operating said pump, by turning said screws, with said proximity sensor so emplaced, and sensing dynamic changes of the proximity of said flight to said proximity sensor.

19. A method of balancing a screw for a screw pump, said pump comprising (i) an outer casing, said outer casing comprising an inlet 9 an outlet, and internal passage means adapted to convey material between said inlet and said outlet, (ii) a pumping screw, said screw having a minor circumference defined by a shaft, and a major circumference defined by at least one screw flight, said screw flight extending radially about said shaft and longitudinally along said shaft, and outwardly from said shaft, to an outer surface of said flight, a portion of said outer surface being defined in said major circumference, front and rear surfaces of said flight extending between said shaft and said outer surface and defining a thickness of said flight therebetween, said passage means comprising a bore adapted to receive a first portion of said screw, said bore compris-ing an inner surface; a first length of said flight being received in said bare along a length of said bore wherein said bore and said flight are coexistent, and (iii) first and second bearings mounting said screw to said casing at spaced locations thereof, said flight being disposed between said bearings; said flight comprising a second portion thereof between said bore and one said bearing, said method of balancing said screw comprising the steps of:
(a) determining the angular locus of imbalance of said screw about the circumference of said screw, and the amount of imbalance; and (b) adjusting the amount of material on said flight, in correcting said imbalance, at an appropriate angular locus, along said second portion of said flight.