in fig1 a production fluid line 10 from a petroleum well contains four phases of material . these phases are gas , oil , water and solids . the production fluid is pumped to the surface and into line 10 by a deep well pump located in the petroleum recovery zone of the well . some of the oil from the production fluid provides the power fluid for the pump located down in the well . this power fluid oil should be comparatively pure and free of the other phases of production fluid . a separator 12 forms the clean oil power fluid by centrifugal separation . centrifugal separation relies on the density differences between the phases . gas leaves the separator by line 14 . clean oil leaves the separator by line 16 . clean water leaves the separator by line 18 . dirty water and solids leave the separator by line 20 . a reservoir 22 receives the flow from lines 18 and 20 . the reservoir has a compartment 24 and a compartment 26 , both formed by a partition 28 which separates dirty water from clean water . dirty water effluent discharges into compartment 24 and clean water effluent discharges into compartment 26 . dirty water has that character because of the presence in it of the solid phase . this phase comes out of suspension by gravity and accumulates at the bottom of compartment 24 , a hole 25 communicates the clean and dirty water compartments so that water in both is at the same level . the hole is above the level of normal solid accumulation in compartment 24 . dirty water exits from compartment 24 through a line 30 and a normally open valve 32 in the line . this valve is controlled by a float switch 34 which senses the level of water in reservoir 22 . line 36 from compartment 26 contains a normally closed valve 38 to control the flow of clean water through the line . a check valve 40 in line 36 prevents backflow of fluid through this line into compartment 26 . valve 38 is also controlled by float switch 34 . when the liquid level in compartment 26 drops sufficiently , switch 34 closes , activating a circuit to normally open valve 32 , which closes that valve , and a circuit to normally closed valve 38 , which opens that valve . the closing of switch 34 also energizes an electric motor to pump 42 so that the pump operates . water then can accumulate in dirty water compartment 24 and can be drawn from compartment 26 through line 36 . line 36 is in series circuit with a pump 42 which , through a line 44 , can supply production fluid intake line 10 with water from reservoir 22 . pump 42 , then , provides water to production fluid intake line 10 from compartment 26 . as will become apparent , the interfaces between the water and oil phase in separator 12 can move radially depending entirely on the quantity of water in a rotor of the separator radially outside the oil in the rotor . the water is denser than the oil and prevents the oil from moving radially outward . the oil pickup is stationary and therefore whether it sees oil can depend on the quantity of water in the rotor chamber radially outside the oil . when insufficient water in the rotor exists , the level of water in reservoir 22 drops and makeup water to the separator input flows from the reservoir to maintain the correct amount of water in the rotor . any overflow of oil from separator 12 , that is , oil in addition to the oil flowing in line 16 , leaves the separator through a line 46 . a sensor 48 in line 46 senses the flow of oil in that line of control downstream equipment in a manner to be described . overflow oil in line 46 discharges into holding vessel 50 . any gas in the overflow oil occupies an upper gas zone of the vessel and the oil itself occupies the lower zone , as indicated . a float switch 52 gauges the level of oil in this vessel . when the level of oil increases beyond a prescribed level , switch 52 closes a circuit to a valve 54 on an excess flow line 56 . the valve responds by opening , and oil flowing through line 56 drops the level of oil in vessel 50 until switch 52 senses a set low level and closes valve 54 . a line 58 having a normally closed valve 60 draws excess oil from vessel 50 in response to a signal from sensor 48 that the flow of oil in line 46 has dropped to a prescribed low level , say zero . the signal energizes a circuit to valve 60 which opens the valve . a pump 62 in line 58 also operates in response to the same signal from sensor 48 to pump oil through the line and into line 16 . a check valve 64 in line 58 prevents flow from line 16 through line 58 . a normally open valve 66 in line 16 responds to the signal of sensor 48 that the flow in line 46 is at the prescribed low level and closes . this forces an increase in the oil in the separator and a discharge of oil out through line 46 . sensor 48 will sense the flow of the oil in line 46 . as a consequence , valve 60 will close , pump 62 will stop , and valve 66 will again open . fig2 illustrates the preferred centrifugal separator 12 . an outer housing 70 of multipiece construction defines an inner space 72 for a rotor 74 . the rotor is driven in rotation by a prime mover ( not shown ). the rotor has a horizontal axis of rotation 75 . a chamber 76 within the rotor provides a volume for centrifugal separation of the phases of the production fluid and a place for the fluid to receive the energy necessary to effect the centrifugal action on it . rotor 74 has a deeply hollowed drum 77 closed by a radially oriented lid 79 which meets the drum at a circular joint 81 . the lid and drum fasten together through male fasteners 83 and together define chamber 76 . production fluid intake 78 from line 10 ( fig1 ) communicates with chamber 76 through an annular passage 80 outside of an annular wall 82 of a clean oil exit passage 84 . passage 84 couples directly with line 16 ( fig1 ). passage 80 feeds radial passage 86 which exits into chamber 76 at horizontal port 88 . thus production fluid reaches the interior of rotor 74 . a plurality of orifices 90 in the outer radial wall of rotor 74 provide for exits of water dirtied by contaminant solid phase material . a circumferentially opening inlet port 92 supplies a radial passage 94 from a point of relatively large radius in chamber 76 to an annular exit port 96 . discharge from port 96 enters annulus 98 and leaves the separator through port 100 , which connects with line 18 ( fig1 ). annulus 98 seals from the balance of the interior of the housing at annular , rotary seal 102 . this seal defines from radially overlapping circular flanges 103 and 105 of the housing and rotor , respectively . in use , passage 94 passes clean water from the rotor . water in passage 94 is resisted by centrifuged force , which resistance provides positive constraint or control on water flow . a pitot tube 104 has a mouth which faces fluid rotating about axis 75 in chamber 76 and is in a zone to be occupied by oil . the pitot tube receives oil from this zone and passes the oil into passage 84 . passage 84 connects to line 16 ( fig1 ). pitot tube 104 caps a pitot assembly which includes the pitot tube and the passage . this assembly also has an exterior seal 106 between it and a hub 108 of the rotor . seal 106 prevents production fluid in passage 80 from entering chamber 76 in a zone reserved for gas . the assembly attaches to a cover 110 of the housing remote from the chamber and proximate the entrance of the production fluid inlet . attachment is at a flange 112 of the assembly . this flange receives male fasteners 114 , which in turn bear through a washer 116 on an external side of an annular flange 118 of cover 110 . flange 112 bears on the interior side of flange 118 . on the right side of rotor 74 as viewed in fig2 hub 108 of rotor 74 receives an annular nose 120 of a ring 122 , which is concentric with axis 75 . the ring axially engages a seal ring 124 located in a bearing recess of cover 110 . hub 108 defines a plurality of passages 130 parallel to axis 75 between chamber 76 and a cavity 140 . cavity 140 feeds line 14 with gas separated from incoming mixture . passages 130 also pass any overflow oil forced there by accumulation of oil and water in chamber 76 of sufficient quantity to place the oil - gas interface at a radius within the entrance to passage 130 . this overflow condition represents the normal condition of the system . overflow oil collects in a base 142 of the cavity for overflow out exit 144 and into line 46 ( fig2 ). a rotary seal 146 between hub 108 and a radially extending annular wall 148 bounding cavities 98 and 140 keeps the fluids within these cavities separate . chamber 76 contains an agitator vane 150 affixed through male fasteners 152 to the pitot tube assembly . this agitator creates turbulence local to exit orifices 90 which prevent their clogging with the sand , sediment and the like , which material constitutes the solid phase of the production fluid inlet mixture . a stack of straighteners 154 have radial interstices between them to flow fluid and solids radially of axis 75 . the stack reduces fluid motion to purely radial and circular and promotes separation of constituent phases of input mixture . the stack attaches to a cover 156 of the rotor as by male fasteners 158 . a cover 160 affixed to the stack forms an annular shroud 162 for pitot tube 104 . rotor 74 secures to a drive axle 164 at a hub 166 of the latter through male fasteners 168 acting against a circular mounting flange 170 of the rotor . axle 164 journals in bearings 172 . a seal ring 174 receives a step 176 of the axle and is retained in place by clip retainer 178 . seal 180 blocks leakage through the axle &# 39 ; s passage . weep holes 182 relieve the seal and drain externally of the housing . bearing housing 184 secures to a housing wall 186 as through fasteners 188 . housing wall 186 is formed of radial segments 189 , 190 and 192 . an external cavity 194 has an annular rubber bumper 196 against which centrifuged solid matter impinges . this reduces the eroding effect of solid material against the wall of the housing of the separator . the annular bumper rings the inside wall of a circular cover ring 198 of the housing which itself secures to radial segment 192 . segment 192 extends axially to cover 110 to which it attaches through male fasteners 200 . solid material falls to the bottom of cavity 194 by gravity after its kinetic energy has been exhausted and it leaves the cavity with water also exited through orifices 90 for collection in compartment 24 of reservoir 22 . during normal operation , production fluid flows through line 10 from a petroleum well . this fluid is fluid from a producing zone of the well , and , possibly , exhausted power fluid of the deep well pump . the fluid is a mixture of solids , oil , water and gas . this mixture enters separator 12 at port 78 . port 78 , through passage 80 and radial passage 86 , empties into chamber 76 of the rotor of the separator . the constituents of the mixture all have different densities and , accordingly will separate into radial zones in accordance with their density under the influence of a centrifugal force field . the zonation places the most dense material in the outer radial zone with progressively less dense materials occupying zones of smaller radius . the outer zone will be for dirty water and sand . the next zone will be for water . the water of these two zones obviously intermix , the solids tending to stay to the radial outside zone because of their greater density . oil and water form an annular interface at , say , 210 which defines the outer radial boundary of a doughnut - like annular zone of clean oil . the inner boundary of the oil is determined by axial passages 130 . the inner boundary is indicated by reference numeral 212 . within this inner boundary , a zone for the lightest of the phases , gas , is defined . gas exits through passages 130 , cavity 140 , and out of the separator for collection . clean oil enters the mouth of pitot tube 104 and passes out axial passage 84 and into line 16 . the flow of production fluid in line 10 will be determined by the deep well pump and not by separator 12 . the flow rate of each constituent of the stream can vary , however , so long as the flow rate of all the constituents remains the same . with drop in the water content of stream 10 , the content of oil , or gas , or both of the stream increases . in response interface 210 would move radially away from axis 75 , absent correction . assuming no correction , interface 210 moves to larger and larger radii . eventually interface 212 follows and moves into the radial limits of the mouth of pitot tube 104 . discharge from the pitot tube will then be gas as well as oil , and this is not desired , for the oil forms the power fluid of the deep well pump and only a modest amount of gas is acceptable in this application . the occurrence of an inadequate amount of water in the chamber to maintain the oil in the bounds of the desired annular zone is determined by reservoir 22 . flow rate out of the reservoir and the flow rate into the reservoir are normally the same , so the level of the tank is constant . this flow rate into and out of the reservoir corresponds to the expected input of water into the separator from the petroleum well . if the separator does not separate out enough water , however , the flow rate into the tank will drop and float level switch 34 will open valve 38 and close valve 32 . the opening of valve 38 introduces water from the reservoir into the separator through lines 36 , 44 and line 10 , with a boost in head by pump 42 . with this augmented supply , water in the outer radial zone of chamber 76 will be sufficient to force interface 212 between the oil zone and the gas zone into the desired region to present essentially oil to the mouth of the pitot tube . more in particular , when the water fraction in line 10 decreases and the gas content in the line makes up the decrease , interface 212 moves radially outward and away from passages 130 . this stops the flow of oil out passages 130 and stops the flow of oil through line 46 . sensor 48 senses this fact and sends a signal which closes valve 66 , opens valve 60 and energizes pump 62 . the closing of valve 66 stops oil flow through line 16 and permits the build up of the oil fraction in separator 12 from what oil is provided from line 10 . with valve 60 open and pump 62 energized , oil from vessel 50 supplies line 16 downstream of valve 66 and , thus , the deep well pump continues to be supplied with power fluid . when the water fraction in line 10 decreases and the oil content in the line makes up the decrease , oil flows in line 46 and oil to the production pump will continue to be supplied by the separator through line 16 . when the water fraction in line 10 increases , the head of the water in the separator increases . with increase in head , water will flow out of the separator faster , adjusting the position of interface 212 . when the oil in line 10 increases and the water content remains the same , oil continues to flow through lines 16 and 46 and no external adjustments to the content of the separator are made . when the oil in line 10 decreases and the water content remains the same , the flow in overflow line 46 stops initiating the closing of valve 66 and the supply of the deep well pump by oil from vessel 50 . the closure of valve 66 permits the build up of the fraction of oil in the separator . when the level of oil in vessel 50 gets too large , float switch 52 opens valve 54 and oil is drained from the vessel . the present invention has been described with reference to a certain preferred embodiment . the spirit and scope of the appended claims should not , however , necessarily be limited to the foregoing description .