Centrifugal separator

This invention relates to centrifugal separators of the kind having sludge ports at the rotor periphery to be opened during operation and separate outlets for separated heavier liquid phase and separated lighter liquid phase. To reduce the losses of valuable lighter liquid phase during total discharge or partial discharge of the separator by opening the sludge ports, the centrifuge is provided with an overflow device connected to the heavy phase outlet, said overflow device having an inner annular overflow and also having a number of passages at a larger diameter than the inner overflow border. The passages are dimensioned to let through all heavier liquid phase in normal operation but not to let through an increased amount of heavier liquid phase fed to the separator just before opening the sludge ports for the purpose of displacing the interface between the heavier and the lighter liquid phase towards the rotor axis to a level that corresponds to said inner overflow border. In order to push the interface between the liquid phases inwards before discharge, no other action is therefore needed than an additional supply of the heavier liquid phase.

THE DISCLOSURE 
The present invention relates to a modification of a centrifugal separator 
of the kind having two separate liquid outlets for a relatively lighter 
separated liquid phase and a relatively heavier separated liquid phase and 
also having sludge ports at the rotor periphery to be opened during 
operation for ejecting separated heavier components. 
More particularly, the invention relates to a modification of said 
separator for the purpose of reducing the losses of the light phase being 
unavoidably connected with total discharge and often also in connection 
with partial discharge. 
In a common separator of the kind noted above, an interface is formed 
during operation between the separated light liquid phase and the heavy 
liquid phase. A usual method for controlling the radial position of the 
interface is to use overflows in the outlet chambers for the two liquid 
phases, said overflows being easily controlled by means of replaceable 
elements, namely, so-called level rings for the light liquid phase and 
gravity discs for the heavy liquid phase. 
Liquid level control will be discussed below in context with the common 
separation process which consists in cleaning a valuable lighter liquid 
phase from possibly intermixed heavier liquid phase and sludge. One 
example is the cleaning of lubricating oils from water and solid 
particles. 
In separators used for this purpose, so-called total discharge is a usual 
way of operating, meaning that the entire liquid content in the rotor is 
ejected together with collected sludge when the sludge ports are opened. 
Thus, all light phase inside the interphase is obviously lost with the 
heavier contaminations. One way to reduce the losses of light phase would 
be to arrange said level rings and gravity discs so that the interphase is 
located far towards the center (axis) of the rotor. However, it is well 
known that this will impair the separation quality with respect to the 
light phase, and consequently the perfect approach is to locate the 
interface relatively far out from the center during operation and to 
displace the same radially inwards before discharge. 
In a method already practiced for displacing the interface inwards before 
the opening of the sludge ports, the outlet for the heavier liquid phase 
is closed and an additional amount of heavy phase is added to accelerate 
the inward displacement of the interface. However, this method requires a 
control system which, in the discharge operations, controls valves on both 
the inlet side and the outlet side. 
One object of the present invention is to provide a centrifugal separator 
of the above-mentioned kind in which the interface between the lighter and 
the heavier liquid phase at total discharge or partial discharge can be 
displaced inward toward the rotor axis without the need for any valve 
action on the outlet side. 
According to the invention, this object has been achieved in a centrifugal 
separator having a rotor with a central inlet for a mixture to be 
centrifuged, sludge ports at the periphery of the separation chamber to be 
opened during operation to intermittently eject heavier components 
separated from said mixture, a first outlet chamber communicating with the 
separation chamber and including outlet means, such as paring means, to 
remove separated relatively lighter liquid phase, and a second outlet 
chamber communicating with the separation chamber through an annular 
overflow device, said second outlet chamber including an outlet device, 
such as paring means, to remove separated relatively heavier liquid phase. 
The invention is characterized in that said annular overflow device 
comprises passages located at an outer level with respect to the rotor 
axis and also an inner overflow located closer to the rotor axis, said 
passages being dimensioned to let through all heavier liquid phase in 
normal operation but not to let through an increase in heavier liquid 
phase that is added to the separator before the opening of said sludge 
ports for the purpose of displacing the interface between the heavier and 
the lighter liquid phase towards the rotor axis to a level corresponding 
to said inner overflow. 
The modification according to the invention can easily be achieved in 
existing separators by replacing the above-mentioned gravity disc having 
only one central aperture by a gravity disc in which the central aperture 
has a radius corresponding to the desired level of the interface between 
light phase and heavy phase immediately before discharge, said gravity 
disc further being provided with a number of apertures located on a larger 
radius corresponding to the desired level for the interface during the 
separation operation between the discharges. 
The invention can be applied to any centrifuge having separate outlets for 
lighter liquid phase and heavier liquid phase. If in such a centrifuge the 
outlet device for the light phase (for example, a paring disc) is designed 
to remove light phase at a radius larger than the level to which the heavy 
phase is brought before discharge and being determined by the inner 
annular overflow for the heavy phase, it is obvious that an unlimited 
additional supply of heavy phase for discharge may cause the heavy phase 
to reach the light phase outlet and contaminate the light phase separated 
out. Thus, in this general case the amount of additional heavy phase to be 
supplied must be limited so that the heavy phase never reaches the light 
phase outlet. 
In order to completely eliminate the risk of heavy phase reaching the light 
phase outlet before discharge, the centrifuge according to an advantageous 
embodiment of the invention is provided with an overflow border known per 
se (a so-called level ring) between the outlet chamber for light phase and 
the separation chamber. Then such a radius for the inner overflow of the 
gravity disc can be selected that the interface between light phase and 
heavy phase can never reach said overflow border, i.e., so that a certain 
layer of light liquid phase will always remain in the inner part of the 
separation chamber irrespective of the amount of additional heavy phase 
supplied.

The centrifuge rotor shown in FIG. 1 has a central inlet 1 for the mixture 
to be separated into three components, i.e., one sludge phase 
intermittently ejected through sludge ports 2 at the rotor periphery, one 
lighter liquid component discharged through an outlet 3 and one heavier 
liquid component discharged through an outlet 4. The rotor is further 
provided with a number of conical discs 5 having apertures 6 at a certain 
diameter to form axial distribution conduits for the liquid mixture added 
from the central inlet 1 through corresponding apertures in the 
distributor 7. 
An outlet chamber 8 for separated lighter liquid phase communicates with 
the inner part of the separation chamber and is provided with a paring 
means 9 communicating with the outlet 3. Another outlet chamber 10 is 
provided with paring means 11 to take out heavier liquid phase to the 
outlet 4 and communicates with the separation chamber through passages 12 
formed between the so-called top disc 12a and the rotor wall at a level 
c-c determined by the outer diameter of said top disc. 
Between the passages 12 and the outlet chamber 10 an overflow 13 is 
provided for separated heavier liquid phase. The overflow 13 comprises 
both a central opening 14 and a number of apertures 15 located at a larger 
diameter than said central opening, the apertures 15 being dimensioned to 
let through all separated heavier liquid phase during normal operation. 
The overflow device 13 can simply be an easily exchangeable disc so that 
its overflow openings can be quickly changed with respect to size as well 
as radial position. 
The repressing of the light phase before the discharge through the sludge 
ports 2 is carried out as follows. During the separating operation the 
interface between light phase and heavy phase is supposed to take the 
radial position a-a shown in FIG. 1. The apertures 15 in the overflow 
device for the heavy phase are sufficient only to drain all the separated 
heavy phase being fed to the separator in the form of an unseparated 
mixture through the inlet 1. Before a discharge operation is initiated by 
opening the sludge ports 2, an additional amount of heavy liquid is added, 
as by shifting the inlet valve 17 so that the supply of unseparated 
mixture is interrupted and a supply line for heavy phase is opened. Thus, 
inlet valve 17 serves as a means for intermittently supplying said 
additional amount of heavy liquid phase to the separating chamber. In the 
case where oil is cleaned from water and sludge, pure water can thus be 
added. 
When the discharge operation is initiated, the interface is supposed to 
take the radial position b-b in FIG. 2. The overflow border 16 in the 
separation chamber and the inner overflow 14 to the outlet chamber 10 for 
heavy phase have such interrelated radial positions that the interface b-b 
cannot reach the border 16. 
The gravity disc shown in FIG. 3 can be used in a centrifuge to modify the 
same according to the invention. The apertures 18 at the radius R and the 
central opening 19 at the radius r correspond to the apertures 15 and 
inner overflow 14 in FIG. 1. 
Although the invention can be applied with special advantage in cases where 
there is to be total discharge of the centrifuge, the invention can also 
be used to advantage where there is only a partial discharge. As soon as 
the liquid-sludge amount ejected at each discharge operation is large 
enought to bring the interface between light phase and heavy phase after 
discharge to at least the same radius c-c as the outer diameter of the top 
disc, a so-called liquid trap break will occur, which means that the light 
phase will flow through the passage 12 to the heavy phase outlet 10. The 
light phase losses and contamination of outgoing heavy liquid phase 
connected with such liquid trap break can be effectively avoided in a 
centrifuge modified according to the invention by means of exactly the 
same interface displacement procedure described above in a total discharge 
operation.