Centrifugal pump

The invention relates to a centrifugal pump of the type adapted for use in pumping multiple phase liquids, i.e. liquids having entrained gas and liquids having both entrained gas and solids. The pump includes means for effecting an initial separation of gas from the liquid, as by centrifugal action, and a final separation by means of a unique pump-out mechanism disposed rearwardly of a shroud of an impeller of the pump and including pump-out vanes and a repeller shroud cooperating with the impeller shroud and pump-out vanes to define radially opening flow paths, wherein flow openings extend across the impeller shroud for flow communication with the radially opening flow paths. The mechanism may also include repeller vanes carried by the repeller shroud to extend rearwardly of pump-out vanes.

BACKGROUND OF THE INVENTION 
The invention relates to centrifugal pumps adapted for use in pumping 
liquids having a gas content, and more particularly to centrifugal pumps 
adapted to effect removal of gas from a pumped liquid in order to improve 
pump performance or processing of the pumped liquid. 
It is well known that the presence of a gas, such as air, in a pumped 
liquid may tend to decrease the hydraulic efficiency of a centrifugal 
pump, and that gas separated from the pumped liquid may collect in 
sufficient quantity adjacent the front of the hub or eye of the pump's 
impeller, as to cause the output of the pump to cease. Separation of gas 
from the pumped liquid may be due to centrifugal action imparted to the 
liquid by the pumping vanes of an impeller of the pump or at a point 
upstream of the impeller adjacent the suction inlet of the pump, such as 
for instance where it is necessary to employ a separate fluidizer or 
centrifuge to fluidize or render free flowing a high consistency fibrous 
pulp suspension for pumping purposes. 
It is also known that gas tending to collect in proximity to the hub of the 
impeller of a centrifugal pump may be removed by providing a flow path 
defined by flow openings arranged to extend through a shroud or hub of the 
impeller for purposes of placing front and rear surfaces of the impeller 
in flow communication; a vent chamber opening through a rear wall of a 
pumping chamber of the pump adjacent an impeller drive shaft for receiving 
gas passing through the flow openings and a vent conduit for placing the 
vent chamber in flow communication with a gas receiving or collecting 
reservoir, such as the atmosphere, either directly or via an auxiliary 
vacuum pump, depending on the difference between the suction head, i.e. 
the pressure existing at the suction inlet of the pump, and the pressure 
of the gas receiving reservoir. In that there is rarely complete 
separation of gas from liquid at that point adjacent the front of the 
impeller with which the flow openings communicate, there is a tendency for 
a quantity of liquid to escape with the gas through the flow openings, and 
for this reason it is common practice to provide pump-out vanes on the 
rear surface of the impeller shroud, which are intended to preferentially 
act on the liquid component of the gas-liquid mixture passing through the 
flow openings for purposes of pumping same to the discharge of the pump 
and thus prevent passage of liquid from the pumping chamber into the vent 
conduit. Prior pumps of this general type are disclosed for example in 
U.S. Pat. Nos. 1,101,493; 3,944,406; 4,410,337 and 4,435,193, and Canadian 
Pat. No. 1,158,570. 
It has also been proposed, as in U.S. Pat. No. 3,230,890, to provide an 
auxiliary pump or centrifugal separator for separating gas from liquid 
escaping from the pumping chamber of a centrifugal pump. 
Under certain steady state conditions, a centrifugal pump fitted with 
properly sized pump-out vanes may be operated to effect removal of gas in 
quantities sufficient to permit the pump to operate at an efficiency level 
corresponding to that characteristic of pump operation with liquid having 
essentially no gas content without loss of liquid through the vent 
conduit. 
In actual practice, steady state pump inlet conditions are rarely 
encountered and slight changes in the pressure differential existing 
across the pump from some predetermined value will either adversely affect 
the efficiency of a centrifugal pump or allow for loss of liquid through 
the vent conduit. For example, if the pressure differential across the 
pump should decrease, due to either a decrease of the suction head and/or 
an increase in pressure in the gas receiving reservoir, there would be a 
reduction in the amount of gas removed from the pumped liquid and this 
would result in a reduction in pump efficiency. There would, however, be 
no loss of liquid through the vent conduit. Conversely, if the pressure 
differential across the pump should increase, due either to an increase in 
suction head or a reduction in pressure in the gas receiving reservoir, 
pump efficiency would remain essentially the same, but liquid would escape 
through the vent conduit. 
In that in practice it is difficult or impossible to provide a constant 
suction head and ensure that liquid to be pumped has a uniform gas 
content, it has been proposed for instance in U.S. Pat. Nos. 3,944,406; 
4,410,337 and 4,435,193 and Canadian Pat. No. 1,158,570 to arrange a gas 
flow control valve in the vent conduit leading to a constant speed vacuum 
pump and to continuously adjust the valve in a manner determined by sensed 
changes in various pump operating parameters in an attempt to vary the 
pressure drop across the pump as required to maximize pump efficiency, 
while avoiding loss of pumped liquid through the vent conduit. 
SUMMARY OF THE INVENTION 
The present invention is directed towards an improved centrifugal pump 
particularly adapted for use in pumping multiple phase liquids, i.e. 
liquid having entrained gas and liquids having entrained gas and solids, 
without requiring precise control of the pressure drop across the pump. 
A pump formed in accordance with the present invention is of conventional 
construction from the standpoint that it includes a housing defining a 
pumping chamber having an axially opening suction inlet and a radially 
opening discharge outlet; an impeller mounting for rotation within the 
pumping chamber by a drive shaft, which is aligned with the suction inlet 
and arranged to extend rearwardly of the impeller through an opening 
formed in a rear wall of the pumping chamber; and a gas removal system for 
removing gas tending to collect within the pumping chamber rearwardly of 
the impeller. More specifically, the impeller includes a shroud extending 
radially of the drive shaft and having flow openings extending between 
front and rear surfaces thereof; pumping vanes carried by the front 
surface of the shroud for pumping liquid between the suction inlet and 
discharge outlet; and pump-out vanes carried by the rear surface of the 
shroud for pumping liquid passing rearwardly of the impeller through the 
flow openings for discharge through the discharge outlet. The gas removal 
system includes a vent chamber opening through the rear wall of the 
pumping chamber annularly of the drive shaft, a vacuum pump, a gas vent 
conduit connecting the vent chamber to the vacuum pump, and a gas flow 
control valve arranged in the gas discharge conduit for varying the 
pressure within the vent chamber. Where the pump is intended to pump 
liquids from which gas is difficult to extract by reliance only upon the 
centrifugal action of the impeller, such as for the case of relatively 
high consistency fibrous suspension and relatively homogeneous liquid-gas 
mixtures, a fluidizer or centrifuge may be fitted on the drive shaft 
forwardly of the impeller to aid in separation of gas from the suspension 
or mixture under centrifugal action and the collection thereof in a core 
or "gas bubble" forwardly of the eye of the impeller. 
In accordance with the present invention, an impeller to be employed in a 
centrifugal pump is fitted with a repeller shroud mounted to extend 
radially from the hub of an impeller and cooperates with its shroud and 
pump-out vanes to define radially extending flow paths communicating with 
flow openings extending through the impeller shroud for purposes of 
imparting a well defined radial flow component to gas and liquid passing 
rearwardly of the impeller through the flow openings. The repeller shroud 
may be fitted with rearwardly extending repeller vanes adapted to project 
into an annular chamber formed in a rear wall of a pumping chamber of the 
pump concentrically outwardly of its vent chamber. 
In the practice of the invention, it is required that the repeller shroud 
be of a diameter which at least equals and preferably exceeds the diameter 
of the vent chamber and the radial position of rear ends of the flow 
openings at the point they enter the flow paths, but which is less than 
the diameter of the pump-out vanes and impeller shroud. In turn, the 
diameters of the pump-out vanes and the impeller shroud must equal or 
exceed the diameter of the pumping vanes of the impeller for purposes of 
generating a head no less than that generated by the pumping vanes. 
The utilization of an impeller modified in accordance with the present 
invention allows an otherwise conventional centrifugal pump to be employed 
for example in a typical pulp mill installation for pumping a high 
consistency fibrous suspension from a reservoir subject to variation in 
suspension level at maximum pump efficiency and with no loss of suspension 
through the vent conduit of the pump without requiring continuous 
adjustments of the pressure differential across the pump.

DETAILED DESCRIPTION 
Reference is first made to FIG. 1, wherein a pump formed in accordance with 
the present invention is generally designated as 10 and shown in 
association with a reservoir, such as a standpipe 12, from which a liquid 
is to be pumped. Pump 10 generally includes a housing 14 defining a 
pumping chamber 16 having an axially opening suction inlet lB 
communicating with an outlet opening 20 formed in standpipe 12 and a 
radially opening discharge outlet 22 connected to a discharge conduit 24; 
and an impeller 26 mounted for rotation within the pumping chamber by a 
drive shaft 28, which is axially aligned with the suction inlet opening 
and extends rearwardly of the impeller through an opening 30 in a rear 
wall 32 of the pumping chamber for connection with a suitable motor, not 
shown. 
Impeller 26 is shown in FIGS. 1-3 as including a central hub 34 mounting a 
radially extending impeller shroud 36 having front surface 36a and a rear 
surface 36b arranged to face towards suction inlet 18 and pumping chamber 
rear wall 32, respectively; at least one and preferably a plurality of 
flow openings 38 extending between the front and rear surfaces of the 
shroud; a plurality of pumping vanes 40 carried by the front surface of 
the shroud for pumping liquid from suction inlet 18 to discharge outlet 
22; and a plurality of pump-out vanes 42 for pumping liquid passing 
rearwardly of the impeller through the flow openings radially towards the 
discharge outlet. The diameters D.sub.P02, D.sub.s and D.sub.2 of pump-out 
vanes 42, shroud 36 and pumping vanes 40, respectively, have the 
relationship of D.sub.P02 .gtoreq. D.sub.s .gtoreq. D.sub.2. Diameters 
D.sub.P02 and D.sub.S are preferably greater than D.sub.2, but pump-out 
vanes 42 may otherwise be of a number, axial dimension and shape to ensure 
that the head generated by the pump-out vanes will equal and preferably 
exceed the head generated by the pumping vanes in order to prevent flow of 
pumped liquid towards the rear of impeller 26 about the periphery of 
shroud 36. In that the head generated by pump-out vanes 42 decreases, as 
the axial gap or spacing 44 between the radial surface 32a defined by rear 
wall 32 and the rear edges 42b of pump-out vanes 42 increases, as for 
instance would be the case when the impeller is moved forwardly within the 
pumping chamber to accommodate for wear, it is desirable to initially 
maintain the gap as small as possible, such as for example on the order of 
between 0.015 and 0.050 inch. 
For those instances where pump 10 is employed to pump liquids from which 
entrained gases are not readily removed or separated from the liquid 
solely by the centrifugal action imparted to the liquid by impeller 26, 
such as for the case of high consistency fibrous suspensions and certain 
homogeneous liquid-air mixtures, it is necessary to impart rotation to the 
liquid upstream of the impeller, such as by means of a fluidizer or 
centrifuge 46 conveniently mounted for rotation with the impeller. 
Fluidizer 46 may consist of a plurality of radially extending blades 48 
interconnected at their radially inner edges and adjacent their rear and 
leading ends by a mounting ring or plate 50 and a connecting or 
stabilizing ring 52. Blades 48 may be straight or curved in a direction 
extending axially of shaft 18 depending upon whether it is desired to 
impart only radially directed or both radially and axially directed forces 
to the liquid to be pumped. The axial length of blades 48 may vary 
depending upon the nature of the liquid to be pumped, but when the pumped 
liquid is a high consistency fibrous suspension, it is preferable to size 
the blades to project into the confines of standpipe 12, as shown in FIG. 
1, in order to ensure fluidization of the fibrous suspension prior to 
entry thereof into suction inlet 18. In any case, blades 48 are intended 
to act on the liquid to be pumped in a manner providing for separation of 
gas from the liquid and allow the gas to concentrate or collect in a core 
area of pumping chamber 16 in front of impeller 26 and adjacent to its 
axis of rotation. Gas or liquid rich in gas then is allowed to pass 
through flow openings 38 whereupon it is subjected to centrifugal action 
imparted by pump-out vanes 42 to allow gas to collect adjacent shaft 28 
and any liquid passing through the flow opening to be forced outwardly 
towards discharge opening 22. 
An enlarged vent chamber 60 defined by an annular recess opening through 
pumping chamber rear wall 32 adjacent shaft opening 30 provides a space 
for accumulating gas tending to collect rearwardly of impeller 26 and gas 
may be withdrawn from the vent chamber by a vent conduit 62 for delivery 
to a suitable gas collection reservoir, either directly or indirectly, via 
a gas removal system of the general type designated as 66 in FIG. 6. 
It is desirable to prevent pump 10 from running dry, and accordingly for 
installations subject to substantial, periodic changes in suction head, 
e.g. the height of liquid in standpipe 12 above suction inlet 18, 
discharge conduit 24 would be provided with a liquid flow control valve 70 
operated by a signal(s) from a programmable controller 72 in response to 
the level of liquid within the standpipe, as sensed by a suitable level 
measuring or sensing device 74. Under normal operating conditions, flow 
control valve 70 would be adjusted in a manner tending to maintain the 
height of liquid within standpipe 12 at some predetermined value. 
In the gas removal system 66 shown in FIG. 6, gas vent conduit 62 
communicates with a suitable gas collection reservoir, not shown, such as 
the atmosphere for the case where the gas to be removed is air, and a gas 
flow control valve 80 and a motor powered vacuum pump 82 are connected 
thereinto. Where the withdrawn gas is air, a vacuum regulator, such as may 
be defined by a manually adjustable atmosphere air bleed valve 84, may be 
connected into conduit 62 immediately upstream of vacuum pump 82 to permit 
the vacuum pump to be run continuously when gas control valve 80 is 
closed. 
The construction and mode of operation of pump 10, as thus far described, 
is conventional and known to be alternatively subject to loss in pumping 
efficiency or leakage of pumped fluid through vent conduit 62 incident to 
variation in operating conditions of the pump, such as variations in 
suction head and gas content of the liquid to be pumped, in the absence of 
precise control of the pressure differential existing across the pump, 
such as by precise adjustments of the setting of gas flow control valve 
80, or alternatively, the operating conditions of vacuum pump 82. 
In accordance with the present invention, an otherwise conventional pump 10 
is modified to permit efficient pump operation in the absence of loss of 
pumped liquid through vent conduit 62 over a substantial range of pump 
operating conditions without requiring precise control of the pressure 
differential existing across the pump. 
More specifically, the invention contemplates an improved impeller 
construction, which is shown in FIG. 2 for the case of a partially open 
impeller as including a repeller shroud 90 arranged to extend radially 
from hub 34 rearwardly of impeller shroud 36 and a plurality of repeller 
vanes 92 arranged rearwardly of the repeller shroud and to project 
rearwardly beyond rear edges 42b of pump-out vanes 42 for receipt within 
an additional annular chamber 94 opening through pumping chamber rear wall 
32 concentrically outwardly of vent chamber 60. 
Repeller shroud 90 is required to be sized and arranged, such that it 
cooperates with impeller shroud rear surface 36b and the inner ends 42c of 
pump-out vanes 42 to provide a plurality of well defined radial flow paths 
96, which receive gas and liquid passing through the rear ends of flow 
openings 38 and then serve to propel same radially outwardly of the flow 
openings towards discharge outlet 22. Thus, the diameter D.sub.R of 
repeller shroud 90 is required to be no less than D.sub.BH and preferably 
to exceed the latter sufficiently to ensure that all materials passing 
through flow openings 38 are caused to experience radial acceleration 
prior to reaching the outer rim 90a of the repeller shroud. Diameter 
D.sub.R is also required to equal and preferably exceed the diameter 
D.sub.VC of vent chamber 60, and for the construction shown in FIG. 2 
would preferably correspond essentially to the diameter of additional 
chamber 94. Where pump-out vane rear edges 42b are required to cooperate 
with rear surface 32a for head generation purposes, D.sub.R should not 
exceed a value required to properly define flow paths 96, since otherwise 
the presence of repeller shroud 90 would diminish the head producing 
capability by pump-out vanes 42. The axial spacing between repeller shroud 
90 and rear edges 42b of pump-out vanes 42, and thus rear surface 32a, 
does not appear to be critical, so long as it is sufficient to permit free 
passage of gas over rim 90a and then radially inwardly towards vent 
chamber 60. 
Repeller vanes 92 are shown in FIGS. 2 and 3 as having rear edges 92b 
spaced from the rear wall 94a of chamber 94 by an amount corresponding 
essentially to gap 44, and as being arranged to extend radially of hub 34 
in alignment one with each of pump-out vanes 42. 
Flow openings 38 must be spaced radially of the axis of rotation of 
impeller 26 such that they are located at a diameter D.sub.BH, which does 
not exceed the value of 
##EQU1## 
wherein D.sub.1 and D.sub.2 are the mean inlet and outlet diameters of 
pumping vanes 40. The shape, size, number and placement of flow openings 
38 appear to be matters of choice depending on impeller design and pump 
requirements. However, flow openings 38 must be of sufficient overall area 
to allow for withdrawal of gas tending to collect forwardly of impeller 26 
and individually be of sufficient size to minimize the likelihood of 
blockage by solids, if present in the liquid being pumped. Moreover, flow 
openings 38 would desirably be placed as close as possible to the 
rotational axis of impeller 26 and may pass through hub 34, and thus 
across impeller shroud 36, if allowed by the design and size of the 
impeller. In FIG. 3, flow openings 38 are shown as being placed in 
alignment with alternate flow paths 96 and with such flow paths arranged 
in communication adjacent hub 34 by spacing inner ends 42c of pump-out 
vanes 42 from the hub with a common inside diameter D.sub.P01, which is 
preferably smaller than D.sub.BH. 
The operation of impeller 26 is essentially similar to a like sized/shaped 
standard impeller fitted with flow openings 38 and pump-out vanes 42, 
except that repeller shroud 90 blocks direct axial flow communication 
between the flow openings and vent chamber 60 and causes initial flow of 
all material passing through the flow openings to be directed radially 
outwardly along flow paths 96. By the time the flow of materials reaches 
rim 90a, it is sufficiently well defined to ensure that its heavy 
constituents, i.e. liquid and solids, if any, will tend to continue to 
move radially outwardly under the continuing influence of pump-out vanes 
42 even though exposed to the reduced pressure condition present in vent 
chamber 60. However, the reduced pressure condition present in vent 
chamber 60 is sufficient to deflect the relatively lightweight gas 
constituent of the flow and cause same to pass around rim 90a for flow 
radially inwardly towards the vent chamber. Repeller vanes 92 function as 
a secondary centrifugal separator normally serving to radially expel 
droplets of liquid, which might be entrained in the separated gas, and 
when required serving to generate a head opposing inward flow of liquid 
towards the vent chamber. 
FIG. 4 illustrates the utilization of the present invention in a fully 
enclosed impeller, that is, an impeller having a front shroud 98 connected 
to the leading edges of pumping vanes 40. With this type of impeller, 
forwardly directed adjustment of impeller 26 would normally not be 
required such that gap 44 would remain essentially constant during the 
operational life of pump 10. Thus, with this type of impeller, the need 
for providing separate repeller vanes 92 projecting rearwardly of pump-out 
vane rear edges 42b and into chamber 94 is avoided. The position of 
repeller shroud 90 axially of pump-out vanes is determined in large part 
by the size of gap 44. Thus, for a relatively small gap on the order of 
0.015 inch, it would be preferable to position repeller shroud 90 slightly 
forwardly of pump-out vane rear edges 42b, as shown in FIG. 4, in order to 
provide sufficient clearance between the repeller shroud and rear wall 
surface 32a to allow unobstructed passage of air over rim 90a and radially 
inwardly toward vent chamber 60. On the other hand, for a relatively large 
gap on the order of about 0.050 inch, it may be possible to arrange the 
rear surface of the repeller shroud 90 essentially flush with vane rear 
edges 42b. FIG. 4 also illustrates an alternative pump-out vane 
construction, wherein the inner ends 42c' of all of pump-out vanes 42 
extend into contact with hub 34. With this type of construction, flow 
openings 38 may be of a number sufficient to permit one to communicate 
with each flow path 96. However, where the number of flow openings 38 
allowed for instance due to the chosen number of pumping vanes 40 is not 
sufficient to supply each flow path 96, the flow openings may be arranged 
to connect with only alternate flow flow paths, as depicted in FIG. 3, for 
which case the unconnected flow paths serve only to generate head for 
pumpout purposes. Operation of the impeller depicted in FIG. 4 is similar 
to that shown in FIG. 2, except that repeller vanes need not be employed. 
Where the gap between repeller shroud 90 and rear wall surface 32a is 
maintained relatively small, the rear surface of the impeller is 
particularly effective in expelling droplets of liquid, which might be 
extrained with the separated gas passing inwardly towards vent chamber 60. 
FIG. 5 illustrates the utilization of the invention in a partially open 
impeller of the type permitting axial adjustments thereof without loss of 
head generated by pump-out vanes 42 by the expedient of attaching a 
further shroud 104 to the rear edges of the pump-out vanes and fitting 
such further shroud with a rearwardly projecting annular choke flange 106 
closely received within an annular recess 108 opening through rear wall 
surface 32a to prevent flow of high pressure liquid inwardly past the 
choke flange. Choke flange 106 and recess 32a may be eliminated, if the 
impeller need not be axially adjusted and a relatively small gap or 
spacing can be maintained between the rear surface of such shroud and rear 
wall surface 32a. Further, shroud 104 cooperates with rear surface 36b of 
impeller shroud 36 and pump-out vanes 42 to define additional flow path 
96' disposed in alignment with flow paths 96, and has an inner rim 104a 
cooperating with repeller shroud outer rim 90a to define openings 110 
facilitating escape of gas towards vent chamber 60. If desired, shrouds 90 
and 104 may be of integral construction and openings 110 defined by 
suitably formed apertures. 
FIG. 5 also illustrates an alternate pump-out vane construction, wherein 
the inner ends 42c of staggered ones of, i.e. alternate, pump-out vanes 42 
are spaced from hub 34 and the inner ends 42c' of intermediate pump-out 
vanes connect with hub 34, thus creating a pair of adjacent flow paths 
connected to a common flow opening. Operating characteristics of the 
impeller depicted in FIG. 5 are similar to those depicted in FIGS. 2 and 
4. 
Laboratory tests have been conducted using a standard 4.times.8-14 
centrifugal pump manufactured by Goulds Pumps, Incorporated of Seneca 
Falls, N.Y. to pump water having 15% air content. The standard pump 
employed a semi-open impeller fitted with pump-out vanes and its vent 
chamber was exhausted directly to the atmosphere. It was determined that 
the pump was capable of generating a discharge head and flow rate 
comparable to that obtainable when pumping pure water and without loss of 
water through its vent conduit for a steady state condition where the 
pressure differential across the pump, i.e. the difference between the 
suction head or pressure existing at its suction inlet and the vent 
pressure existing in its vent chamber, was maintained equal to a reference 
value of about fifteen feet of water. It was observed that, when the 
pressure differential was decreased below the reference value, which may 
occur either as a result of a reduction in suction head or the 
introduction of a positive pressure in the vent chamber, a reduced 
quantity of air was vented from the pump, thereby causing the efficiency 
of the pump to fall below that obtainable when pumping pure water. On the 
other hand, when the pressure differential was increased about the 
reference value, which may occur either as a result of an increase in 
suction head or the introduction of a negative pressure in the vent 
chamber, the efficiency of the pump was comparable with that obtainable 
when pumping pure water, but a loss of water through its vent conduit was 
observed. 
Tests were also conducted using a 4.times.8-14 pump fitted with an impeller 
modified in the manner depicted in FIG. 2 to pump water having a 15% air 
content connected into a supply providing a suction head of about fifteen 
feet. The vent conduit was connected to a vacuum pump and tests conducted 
to determine the effect of different pressure differentials across the 
pump. It was observed that the performance of the thus modified pump 
corresponded essentially to that of the standard pump for pressure 
differential conditions equal to and below the reference value of the 
standard pump. However, it was determined that the modified pump was 
capable of performing at an efficiency comparable to that obtainable when 
pumping pure water and without loss of water through its vent conduit for 
pressure differentials in a range exceeding the reference value by upwards 
of fifteen feet of water. 
Standard centrifugal pumps may be readily adapted for use in pumping any 
given liquid having entrained gas under steady state conditions of suction 
head and gas content by simply ensuring that the pressure existing in its 
vent chamber is maintained at a constant value, which is correlated with a 
constant value of the suction head to maintain a pressure differential 
across the pump equal to some predetermined reference value at which the 
pump operates at maximum possible efficiency without loss of liquid 
through its vent conduit. The predetermined reference value would be 
expected to vary depending upon the type of liquid being pumped and the 
size and operating characteristics of the pump itself. However, for pump 
installations not enjoying steady state conditions, such as those 
encountered in pulp mills in connection with the pumping of high 
consistency fibre stock suspensions, it is necessary to provide a control 
for continuously varying the pressure existing in the vent chamber of a 
standard pump in an effort to maintain the pressure conditions across the 
pump at its predetermined reference value, as the available suction head 
raises and falls relative to some average design value. 
As by way of illustration, in a typical pulp mill installation generally 
depicted in FIG. 6, suspension is fed at a variable rate to a suitable 
reservoir, such as standpipe 12 having a height for example on the order 
of about ten feet, as measured about the suction inlet of the pump; the 
discharge flow rat of the pump is controlled, as by adjustments of flow 
control valve 70, with a view towards maintaining the height of the 
suspension at some design value; and gas flow control valve 80 is adjusted 
as required to vary the pressure differential across the pump, in order to 
accommodate for increases and decreases of the height of the suspension 
relative to the design value. It has been observed that for the case where 
a standard pump is employed to pump high consistency fibrous suspensions 
of on the order of about 12%, vacuum pump 84 should be operated to provide 
a negative pressure in vent conduit 62 of about five feet of water for a 
design value or suspension height of five feet in order to provide a 
pressure differential across the pump, which appears to maximize pump 
efficiency without creating a loss of suspension through the vent conduit. 
While diverse types of controllers 72 are presently in use, a typical 
controller would be of the programmable variation, wherein the setting of 
liquid flow control valve 70 is determined by the height of suspension, as 
measured by sensor 74, as a function of time. For example, a set point, 
such as a suspension height of five feet, is established and when upon 
initially filling of standpipe 12 the suspension reaches five feet, valve 
70 would start to open and thereafter might settle for movement within a 
range of 30% to 35% open condition as the suspension height varies between 
four and a half feet and five and a half feet. Valve 70 would typically 
open 100%, if the suspension height was to approach eight feet, and might 
become fully open at lesser suspension height under certain conditions. 
Valve 70 would be fully closed when the level of the suspension dropped to 
an undesired level, during an intended period of pump operation, or when 
the pump was shut down upon completion of an intended draining of the 
reservoir. 
It is proposed to employ a pump modified in accordance with the present 
invention in a pulp mill installation of the type described and to modify 
operating conditions by reducing the negative pressure provided by vacuum 
pump 82 from five feet of water given in the above example to an arbitrary 
selected low value, such as ten feet of water, to allow pump operation 
throughout essentially the whole of possible variations of height of 
suspension within standpipe 12 without adjustments of gas control valve 80 
other than alternatively placing same in fully open or fully closed 
condition incident to an arbitrarily selected setting of liquid control 
valve 70. Specifically, it is proposed to operate vacuum pump 84 at a 
negative pressure sufficient to create a pressure differential across the 
pump when the height of the suspension is at its design value, which 
exceeds the pressure differential required by the modified pump to 
maximize withdrawal of gas therefrom, whereby to permit the pump to 
operate at maximum efficiency throughout the range of obtainable 
suspension levels within standpipe 12 without loss of suspension through 
vent conduit 62. 
To carry operation of the modified pump into effect, a relatively low 
liquid control valve setting, such as 20% open, may be selected on the 
basis that such setting would normally be encountered only during initial 
filling of standpipe 12 and subsequent emptying of such standpipe, as an 
incident to shutdown of operation, such as for maintenance purposes. Thus, 
it is contemplated that controller 72 would cause gas control valve 80 to 
be fully closed at the start-up of pump operation and to become fully open 
when liquid control valve 70 initially is opened to a setting of 20%, 
whereafter the gas control valve would remain fully open until the liquid 
control valve returned to a setting of 20% normally again encountered at 
the time of shutdown. 
It is also contemplated that the modified pump may be employed in extremely 
tall reservoirs, wherein suspension levels typically exceed a range of 
between twenty-five and thirty-five feet at which the suction head is 
sufficient to reduce the amount of air separated from the suspension by 
the fluidizer to a point at which the air does not adversely effect 
operation of a centrifugal pump. When used in this type of installation, 
controller 72 would serve to effect closure of gas control valve 80 when 
the height of the suspension would be sufficient to produce a pressure 
differential across the pump at which suspension would otherwise be lost 
through vent conduit 62. Thus, for this type of installation, the modified 
pump would only serve to effect removal of gas during start-up and 
shutdown of the system. 
The term liquid, as used herein and in the appended claims, is meant to 
include liquids having entrained gas and liquid having both entrained gas 
and solids, such as fibres.