Note: Descriptions are shown in the official language in which they were submitted.
s~
This invention relates to an improved me~hod and
apparatus for flushing the plunger of a posi~ive displacement
pump.
Although the invention is not thus limited, our
flushing method and apparatus are particularly useful as ~pplied
to slurry pumps which handle water suspensions of abrasive
particles. A conventional positive displacement slurry pump
includes a plurality of cylinders, xespec~ive plungers in each
cyli~der, and packing rings surr~unding ~he plungers where they
enter~the cylinders. In operating such pumps, it is conventional
to flush the plungers with ~lean water or other flush liquid
applied immediately in front of the packing rings to wash away
abrasive particles and prevent such particles from being drawn
into the packing during suction ~trokes of the plungers.
Several types of flush systems have been used
heretofore, among which are:
~ a) Low pressure flush, in which a small quantity of
clean flush liquid flows around the plunger through a narrow
annular passage in the cylinder wall only during suction strokes
of the plunger. A check valve in the flush liquid supply line
prevents back flow during discharge strokes.
(b) Synchronized flush, in which a small flush pump is
synchronized to discharge flush liquid into the annular ~rea
ahead of the packing during suction strokes of the slurry pump.
tc) High pressure flush, in which clean flush liquid is
introduced to an annular passage around the plunger at a pressure
slightly higher than the pump discharge pr~ssure. This system
enables a large portion of the flush liquid to flow around the
plunger during suction strokes when the pressure differential
is greatest and the flush liquid is needed to exclude abrasive
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particles from the packing. A small flow is maintained during
discharge strokes, thus eliminating need :fDr check valves or
synchronizing means and assuring ~lean packing at all times.
An object of ~he present invent:ion is to p~ovide an
improved flushing methc~ and apparatus of th~ high pressure type
in which we effectively wash particles fro~ a pump pl~r.aer and
exclude them from the packing and at the same time obtain a
ready indication of ~he condition of the packing.
According to the present invention, there is provided,
in a pumping operation in which a suspension of particles is
handled in a positive displacement pump, the plunger of which
passes through front and back sets of packing rings, a method of
flushing particles from said plunger, said method comprising
introducing flush liquid to said plunger at a location in
front of said front set of packing rings, introducing a smaller
volume of flush liquid at a location between said front and back
sets of packing rings, and initially maintaining the liquid
introduced at the two locations at approximately the same pressure
and relying on said back set of packing rings to provide a seal for
said Plun~er, wherebv when the ~ackinq rinqs of said back set wear
and commence to leak a differential in the pressure of the flus~
liquid introduced at the two locations is created and said front
set of packing rings provides the seal.
The invention also provides a positive displacement pump
for handling a suspension of particles, comprising at least one
cylinder, a plunger reciprocable in said cylinder, a stuffing box
carried by said cylinder, front and back sets of packing rings
within said stuffing box surrounding said plunger, and a flushing
apparatus comprising means for introducing flush liquid to said
stuffing box at a first location in front of said front set of
packing rings to wash particles from said plunger and prevent
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the particles from being drawn into said packing rings, and means
for introducing a smaller volume of ~lush liquid to said stuffing
box at a second location between said front and back sets of packing
rings initially at substantially the same pressure as the liquid
introduced at the first location, whereby said back set of packing
rings initially provides a seal for said plunger, and said front
set of packing rings provides a seal for said plunger after the
packing rings of said back set wear and commence to leak and
thereby create a differential in the pressure of the flushing
liquid introduced at the two locations.
The invention is further described, by way of example,
with reference to the accompanying drawings, in which:-
Figure 1 is a vertical sectional view through onecylinder of a positive displacement pump equipped with our
flushing apparatus; and
Figure 2 is a diagrammatic view of a modified
arrangement for supplying flush liquid to the pump.
Figure 1 shows one cylinder 10 of a positive displacement
pump, such as may be used for pumpin~ slurries of abrasive
particles suspended in water. The pump includes a plunger 12
arranged for reciprocable movement within the cylinder, suction
and discharge manifolds 13 and 14, and suction and discharge
valves 15 and 16. A ~tuffing box 17 surrounds the plunger, and
contains front and back sets of packing rings 18 and 19/ an
intermediate lantern ring 20 between the two sets of packing
rings, a front lantern ring 21, and spreader rings 22 in front of
the lantern ring 21. A suitable drive (not shown) is connected
to the plunger 12 to reciprocate i~ out of and into the cylinder 10
through suction and disch~rge strokes. The pump illustrated is
conventional, apart from ~he flushing apparatu~ and some of the
packing details, and hence ~s not described in detail.
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i2
The flushing apparatus of the present invention includes
a flush manifold 26 from which extends a pipe 27 preferably
containing a check valve 28. ~wo branch pipes 29 and 30 extend
from pipe 27 beyond the check valve. The larger branch 29 is
c~nnected int~ the stuffing box 17 in line with the front lantern
ring 21, and contains a relatively large orifice 31 and a check
valve 32. The smaller branch 30 is connected in~o the ~tuffing
box in line with the intermediate lantern ring 20 and contains
a relatively small orifice 33. Preferably we connect a pressure
measuring device 34, illustrated as a gauge, to the stuffing box
17 also in line with the intermediate lantern ring. The flush
manifold 26 is connected to a suitable source of flush liquid at
a pressure higher than the discharge pressure of the pump.
Figure ~ shows a modification in which we locate the
large orifice 31 in pipe 27 upstream of the branch pipes 29 and
30. This modification assures that the flow through the small
orifice 32 is relatively low without need for the small orifice
to be so small that it readily clogs.
According to our flushing method, the larger branch
pipe 29 and orifice 31 carry a relatively large volume of flush
liquid from the manifold 26 to the stuffing box ~7. This liquid
serves to wash particles from the plunger 12 during its suction
strokes and thus acts in the usual manner to prevent the
particles from being drawn ints the front set of packing rings
18. The smaller branch pipe 30 and orifice 33 carry a relatively
small volume of flush liquid to the intermediate lantern ring ~0.
The pressure measuring device 34 indica~es the pressure at this
ring. The check ~alves 28 and 32 prevent flush liquid from
backing up during discharge stro~es ~ thc plunger. The check
valve 32 also prevent~ ~lurry from entering the back section o~
the flush ~ystem if the pac~ing fail~ or if there i5 a ~alfunction
in ~he flu~h ~y~te~.
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Initially there is approximately equal pressure at
~pposite si~es ~f the fr~nt set of packing rings 18, while there
is a pressure differential at opposite sides of the back set
of packing rings 19, since the latter are subject only to
atmospheric pressure at their back end. Under these conditions
the back packing rings provide the seal around the plunger. Hence
they are the first t~ wear and eventually begin t~ leak.
Nevertheless they continue t~ provide the seal until they leak at
a greater rate than flush liquid is delivered t~ the intermediate
lantern ring 20. At this stage the pressure at the intermediate
lantern ring drops a~d there i5 a pressure differential at
opposite sides of the front set of packing rings la, The fr~nt
packing rings now provide a seal for the plunger 12.
We can detect the point at which the back packing rings
19 cease to pr~ide a seal in several ways. We may detect it by
observing an increase in flow through the branch pipe 30
supplying flush liquid to the intermediate lantern ring 20. We
may detect it by observing a pressure drop at the intermediate
ring, as indicated by the pressure gauge 34. We may also detect
it by observing the volume of liquid leaking past the back packing
rings. By observing the deterioration of the back packing rings,
we can schedule packing replacement while the front packing rings
are providing a seal before they wear out.
~ rom the foregoing description it is seen that our
inventi~n provides a flushing method and apparatus which
effectively wash particles from the plunger of a p~si~ive
displacement pump and at the same time facilitate ~etermining
the ~tate ~f wear of the plunger paclcing. The invention enables
the packing t~ be replaced ~nly when necessary with~ut waiting
for a total failure to take place.
