Centrifugal pitot pump with means for improving net positive suction head

A centrifugal pitot pump has a rotor driven in rotation within a casing. A pitot tube pickup in a rotor chamber within the rotor intercepts rotating fluid and withdraws the fluid through a duct. The duct mounts the pitot tube and passes through the rotor casing. Fluid to be pumped passes through the annulus between the rotor and the duct and up through generally radial passages in the rotor into the rotor chamber. A leakage path from the rotor chamber to the inlet annulus between a hub of the pitot tube pickup and the rotor permits fluid to pass from the chamber into the annulus. A ring interrupts line-of-sight communication between the chamber and the annulus along the leak path and dissipates considerable of the leaking fluid velocity head to thereby improve the net positive suction head of the pump. The entrances of the radial passages in the rotor are large in the axial direction to reduce the sharpness of the turn from the annulus into these passages, to thereby reduce the net positive suction head required to operate the pump without cavitation.

BACKGROUND OF THE INVENTION 
The present invention relates in general to centrifugal pumps of the pitot 
tube type, and, more in particular, to an improvement in such pumps that 
reduces the net positive suction head required to prevent cavitation. 
Centrifugal pumps of the pitot type are well known. In general, these pumps 
include a drive that drives a rotor in rotation within a casing. A pitot 
pickup in a chamber of the rotor and stationary relative to the rotor 
intercepts fluid within the chamber and draws that fluid from the chamber. 
The exiting fluid has a head larger than its inlet fluid head because of 
energy imparted to the fluid by the rotor. Typically, fluid enters the 
rotor chamber along a path that includes an annulus surrounding the pitot 
tube mount and inside the rotor. From this annulus the fluid passes 
through a plurality of generally radial passages in the rotor to exit near 
the outer radial limit of the rotor chamber. The pitot inlet in the 
chamber may be comparatively close to the outer radial limits of the 
chamber or comparatively close to the axis of rotation of the rotor, 
depending on the application. 
Typically, the pitot tube mount is in the form of a duct or tube extending 
along the axis of rotation of the rotor and through a wall of the rotor, 
usually a rotor cover. The duct is attached to the casing, or some other 
stationary support. 
Pitot pumps are noted for their ability to impact large increase in head in 
the fluid being pumped. Adaptations of these pumps into separators and 
cleaners are possible because of the opportunity to stratify fluids within 
the rotor chamber and sort materials according to their density. 
Stratification, of course, comes from the large centrifugal force field 
present within the rotor chamber. An example of this application is a 
separator for separating solids from a liquid. In petroleum applications 
it is not uncommon to use production fluid from a petroleum well to power 
downhole machinery. This fluid must be free of solids. A pitot separator 
separates solids from the power fluid by centrifugal action and removes 
the solids either through a pitot pickup or nozzles in the wall of the 
rotor. A second, clean pitot tube pickup draws solid-free material from 
the chamber. Thus, in this application, it is possible to have more than 
one pitot pickup within the chamber. Separators, too, can use multiple 
head pitot pickups, as well as weir-like take-offs from the chamber in the 
walls of the rotor. 
Known pitot pumps include those described in the following U.S. Pat. Nos.: 
3,384,024; 3,776,658; 3,795,459; 3,817,659; 3,838,939; 3,926,534; 
3,960,319; 3,977,810; and 3,994,618. 
In these pitot-type pumps and separators, the duct mounting the pitot tube 
extends through a wall of the rotor. The duct is stationary while the wall 
rotates. Fluid inside of the rotor has a considerably higher head than 
incoming fluid in the annulus on the outside of the duct. Fluid leaking 
from the rotor chamber into the annulus has a deleterious effect on the 
net positive suction head of the pump. The net positive suction head 
(NPSH) is that pressure over and above the vapor pressure of the fluid 
being pumped within the inlet of the pump required to prevent cavitation 
in the pump inlet. Cavitation is localized vaporization of fluid. 
Cavitation adversely affects pump performance by reducing flow rate and 
discharge head. Cavitation also physically degrades the pump, often quite 
quickly. In previous designs, the interface between the rotor and the duct 
provided a labyrinth path for fluid through a plurality of axially spaced, 
circular grooves on the outside of the duct. Nonetheless, line-of-sight 
communication between the rotor chamber and the duct above the lands of 
the grooves and within the bore of the rotor receiving the duct permitted 
fluid from within the rotor to enter the duct resulting in a high velocity 
head, even jet-like, with the harmful impact on net positive suction head. 
SUMMARY OF THE INVENTION 
The present invention provides in a pitot type pump means for improving the 
net positive suction head by blocking a direct leak path of fluid from a 
rotor chamber of the pump into a fluid inlet passage to the chamber but 
outside that chamber and along an interface between the pitot tube and the 
rotor. 
One form of the present invention provides a pitot pump with a rotor having 
a chamber in which a pitot tube pickup is disposed. A duct passing through 
an end wall of the chamber and the rotor supports the pitot tube. An 
interface between the duct and wall provides a leak path between the rotor 
and an inlet passage into the rotor chamber. A barrier in this leak path 
prevents line-of-sight communication between the inlet and the rotor 
chamber. 
It has been found that despite the presence of a leak path between the 
rotor chamber and the inlet, interrupting line-of-sight communication 
between the two results in an improvement in the net positive suction head 
for the pitot pump. It is thought that the problem has been with the 
line-of-sight communication, permitting the fluid from the rotor chamber 
to jet into the entrance with adverse consequences on net positive suction 
head. By interrupting line-of-sight communication, the jet dissipates and 
the suction head improves. 
To further enhance net positive suction head, radial passages of the rotor 
communicating with the entrance passage have large openings in the 
direction of fluid flow in the entrance, almost invariably axial. 
In a detailed form, the present invention contemplates a centrifugal pump 
of the pitot tube type that employs a rotor havng a rotor chamber within 
it. The rotor is adapted to be driven in rotation by some prime mover, 
such as an electric motor. A casing houses the rotor and provides a 
shroud. The pitot tube pickup within the chamber mounts on a duct that 
extends coaxially with the rotor through an end wall of the rotor and 
anchors to a stationary part of the pump. The pitot tube and duct form a 
pitot tube assembly. The duct has an axial passage in communication with 
the entrance of the pitot tube for the discharge of fluid from the pump. 
An annulus around the duct provides the entrance into the rotor chamber, 
and it is this annulus that receives discharge through the leak path 
between the rotor chamber and the inlet. A circular lip received in a 
groove prevents line-of-sight communication through the leak path from the 
rotor chamber to the inlet. Generally, radial passages extend from the 
annulus to outlets proximate the outer radial limit of the chamber. These 
radial passages at their entrances are wider in cross section than in 
their medial portions in order to further improve net positive suction 
head. 
These and other features, aspects and advantages of the present invention 
will become more apparent from the following description, appended claims 
and drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference in general to FIG. 1, pitot pump 10 is shown in side 
elevation. The general organization of the pump includes a rotor assembly 
12 disposed within a casing 14 for rotation. A drive shaft 16 drives the 
rotor in rotation. The drive shaft is adapted to be coupled to a prime 
mover, such as a motor. A pitot tube assembly 18 is stationary relative to 
the rotor, the assembly extending from outside the rotor into a rotor 
chamber 20 within the rotor. Outside the rotor, the pitot tube anchors to 
a stationary part of the pump by attachment to manifold 22. 
Rotor 12 includes a rotor cover 24. A plurality of generally radial 
passages 26 in the cover open at their outer ends into chamber 20 and open 
at their inner ends into an annulus 28. Annulus 28 feeds fluid to passages 
26. Pitot tube assembly 18 includes a tube or duct 30 that defines the 
inner wall of annulus 28. A pitot tube arm 32 extends from an interior end 
of duct 30 (with respect to the rotor) radially within chamber 20. A scoop 
34 caps the arm and provides the entrance for fluid to enter the pitot 
tube assembly. Duct 30 has a passage 36 extending through it to a 
discharge chamber 38. 
Manifold 22 receives inlet fluid from a source and passes that fluid into 
annulus 28. Fluid passes through annulus 28 and into radial passages 26 
for discharge into rotor chamber 20. Rotor 12, rotated by the prime mover, 
increases the head of the fluid as it passes up passages 26. Fluid within 
chamber 20 will have a higher head than fluid within annulus 28. This 
fluid will eventually be taken off by scoop 34, passed through arm 32 and 
through duct 30, and into discharge chamber 38. 
With reference to FIG. 2, a leak path exists along the interface between 
rotor 12 and pitot tube assembly 18 communicating rotor chamber 20 and 
inlet annulus 28. This leak path is indicated in general by reference 
character 40. Fluid passes from chamber 20 into annulus 28 through path 
40. An annular, circular ring 42 in the wall of cover 24 prevents 
therefore line-of-sight communication between chamber 20 and annulus 28. 
An annular, circular channel 44 radially inside of ring 42 but on pitot 
tube assembly 18 receives the ring. The axial width of channel 44 exceeds 
the axial width of ring 42 to provide tolerance clearance between the 
radial walls of the channel and the ring. Fluid will then flow from 
chamber 20 into annulus 28 about ring 42 but will be intercepted by the 
ring. It has been found that with the provision of the ring, this fluid 
flow is considerably less energetic and velocity head is dissipated. The 
ring interrupts the jet-like flow that would otherwise exist. With the 
interruption, an adverse effect of net positive suction head is 
attenuated. 
A further improvement in the net positive suction head results from 
increasing the axial reach of the mouth of passages 26. A widened mouth 46 
for these passages parallel to an axis 50 of the pump reduces corner 
losses of fluid passing from entrance 28 into passages 26 by reducing the 
sharpness of the corner. As will be developed subsequently, this increase 
in the axial span of the mouth for passages 26 is accompanied by a 
decrease in the rotary or circumferential span of the mouth. See FIG. 3. 
In greater detail and with reference to both FIGS. 1 and 2, pitot tube 
assembly 18 is an integral assembly of arm 32 and duct 30. This assembly 
must pass through rotor cover 24. Thus the diameter of the duct cannot 
exceed the inside diameter of ring 42. Pitot tube assembly 18 in the 
vicinity of leak path 40 includes a hub or an elbow 52 at the base of arm 
32. The elbow ends in a radial shoulder 54 that steps the diameter of the 
pitot tube assembly down to about the same as the inside diameter of ring 
42. An annular, axially extending land 56 extends from shoulder 54 to 
channel 44. A shoulder 58 of channel 44 extends from the base of the 
channel to land 56. On the opposite end of channel 44, a flange 60 extends 
radially from duct 30 to a diameter about the same as the inside diameter 
of ring 42. A shoulder 62 of channel 44 and a flange 60 extends radially 
of axis 50. Flange 60 thus forms a land for channel 44. A tapered shoulder 
64 extends from the major diameter portion of flange 60 to a reduced 
diameter section of duct 30 that exists away from leak path 40. Thus, 
flange 60 also acts as a dam in reducing the effect of the fluid from 
chamber 20 entering annulus 28. 
With reference again to FIG. 1, duct 30 extends along axis 50 away from the 
zone of leak path 40 towards chamber 38. A flange 66 extends radially away 
from the adjacent portion of duct 30 to provide an anchor for the duct in 
manifold 22. This anchor is effected through a plurality of fasteners 68 
that secure the duct to an interiorly extending radial flange 70 of the 
manifold. An O-ring 72 between the manifold and the duct seals the 
outgoing fluid in the duct from escaping into incoming fluid in annulus 
28. A stub passage section 74 in the manifold extends between a chamber 76 
of the manifold and passage 36 of the duct. Chamber 76 opens into an exit 
passage 78. 
Annulus 28 within cover 24 opens into a plurality of passages 26, as can be 
seen in FIG. 3. Considered generally radially of axis 50, mouths 46 for 
passages 26 are narrow at their entrances and widen in the direction of 
rotor rotation as the radial distance from axis 50 increases to a fully 
developed passage section 80. With this increase in width of passages 26 
in the direction of rotor rotation, passages 26 narrow in the direction 
along axis 50 in an amount corresponding to the widening in rotational 
direction so as to present substantially the same cross-sectional area to 
the flow of fluid through the passage. Passages 26 turn axial at their 
ends to exit through exit ports 82 into chamber 20. Passages 26 are 
slightly off radii from axis 50 to compensate for rotation of the passages 
with respect to annulus 28. The direction of rotation is indicated by the 
arrow in FIG. 3. 
With reference to FIG. 1, pitot tube arm 32 extends from elbow 52 radially 
within chamber 20. The outside of the arm progressively narrows with 
increases in radius in a known fashion. Initially, the arm reaches a 
minimum thickness facing the rotary motion of fluid within chamber 20 so 
as to reduce drag. Scoop 34 that caps the arm intercepts the fluid and 
directs it down through the arm and into passage 36 for discharge. 
With continued reference to FIG. 1, rotor 12 is formed of a deeply dished 
drum 84 that forms the radial boundaries of chamber 20 and one end 
boundary. Cover 24 attaches to drum 84 as through a plurality of fasteners 
86 between the two and provides the other end boundary. Drum 84 has a hub 
88 that secures the rotor to a mounting flange 90 at the end of drive 
shaft 16. Attachment is through fasteners 92. 
Drive shaft 16 extends from a prime mover to mounting flange 90 and steps 
up before meeting the flange at a shoulder 93 of an axially extending 
section 94. 
Casing 14 has a cylindrical section 100 that spans the axial extent of 
rotor 12. An end plate 102 attaches to cylindrical section 100 through 
fasteners 104. A second end plate 106 attaches to cylindrical section 100 
through fasteners 108. End plate 106 has a large diameter hole 110 that 
receives a bearing retainer plate 112. This plate extends radially of 
section 94 of drive shaft 16. The retainer plate nests within a hub 118 of 
a lubricant reservoir and journal assembly for the drive shaft. This 
reservoir assembly has been largely omitted because it is of standard 
configuration. A bearing 120 for the interior end of drive shaft 16 is 
received on the drive shaft. An oil slinger 122 directs oil at the 
bearing. A second oil slinger 124 on the opposite side of bearing 120 does 
the same thing. A bearing retainer 126 receives bearing 120. A bearing 
mount spring ring 128 in turn receives the bearing retainer. A sleeve 130 
receives this entire assembly. Sleeve 130 is received within hub 118. Hub 
118 has a radially extending flange 140 that secures to plate 106 through 
fasteners 142. A breather passage 144 in end plate 106 communicates the 
volume outside of rotor 12 and within casing 14 to atmosphere. A lubricant 
bleed passage 146 through plate 112 and hub 118 drains lubricant to 
outside the pump. 
At the other end of the pump, a seal 150 is disposed radially within a seal 
adapter 152 that is in turn received in a bore manifold 22. An O-ring 
between seal adapter 152 and this bore prevents leakage along the 
interface between the two. A second O-ring between the seal adapter and 
the seal prevents leakage along the interface between these two items. A 
seal clamp ring 156 on the inside of adapter 152 bears on one axial end of 
the seal and the adapter bears on the other axial end of the seal. These 
three elements are stationary. A ring 158 within a retainer 160 seals 
against seal 150. A spacer 162 positions ring 160 relative to seal 150. 
O-rings between the interfaces of this spacer and the rotor cover and the 
ring seal these interfaces. 
Manifold 22 secures to end plate 102 as through fasteners 166. Chamber 76 
of manifold 22 is capped by a plug 168 which is secured to the manifold as 
by fasteners 170. An O-ring may be provided between the plug and the walls 
of chamber 76 to effect a seal. 
FIG. 4 illustrates the improvement in the net positive suction head 
attendant with the present invention. The ordinate shows net positive 
suction head required to operate the pump without cavitation. The abscissa 
shows the flow rate of the pump. The upper, dashed line shows the net 
positive suction head of the pump without the lip and channel of the 
present invention. The lower, solid line shows the net positive suction 
head requirements with the lip and channel of the present invention. It is 
clear from the plot that the improvement in net positive suction head 
obtains for a large range of flow rates. 
The operation of the present invention has been described earlier in 
connection with specific structural functions but an overall description 
will be presented here. 
Rotor 12 is caused to rotate within casing 14 and this causes fluid to 
enter chamber 20 of the rotor through the following path. The fluid enters 
manifold 22 and from there flows into annulus 28. There, the fluid flows 
axially of the pump outside duct 30. Then the fluid enters generally 
radial passages 26. Within these passages, the fluid picks up head. The 
fluid discharges out outlets 82 and into chamber 20. There, the head can 
be further increased. The net positive suction head required to maintain 
satisfactory fluid flow through the pump varies considerably with the 
fluid losses into the chamber. With large losses due to fluid head losses 
in the inlet path, the net positive suction head requirement for the pump 
increases. 
Assuming an adequate net positive suction head to operate the pump, fluid 
is drawn off from chamber 20 through scoop 34 and flows down through arm 
32, out passage 36, into chamber 76, and out exit passage 78. During 
operation, a substantial pressure differential exists between chamber 20 
and annulus 28. 
Production tolerance requirements require that a fairly substantial leak 
path exist between chamber 20 and inlet annulus 28 along path 40. With the 
substantial driving pressure differential between chamber 20 and annulus 
28, fluid flows along a leak path 40 at a high velocity. If this leaking 
fluid is allowed to enter annulus 28 with its velocity unabated, the net 
positive suction head requirements for the pump increase substantially. 
The presence of the lip and channel arrangement of lip 42 and channel 44 
substantially attenuates the adverse effect of leakage on the net positive 
suction head. The lip and channel reduce the jet-like flow of the escaping 
head, and an effective restriction in the mouth of passages 26 and annulus 
28 is removed. It is noted that a head loss exists with or without the lip 
from the loss of the velocity head of the fluid flowing from the chamber 
to the annular passage. This loss with the invention, however, takes place 
away from the mouth and annulus. 
The present invention has been described with reference to a preferred 
embodiment. The spirit and scope of the appended claims should not, 
however, necessarily be limited to the foregoing description.