fig1 of the drawings is an illustration of a typical prior art oil well drilling ring mud circulation system . each part in the system is labeled and needs no further description . fig2 and 3 of the drawings are labeled &# 34 ; prior art &# 34 ; and are , respectively , a half - section view taken through a swivel 10 , and an enlarged half - section view illustrating the washpipe 12 , the upper seal 14 and the packing 16 in the swivel 10 . the swivel 10 ( fig2 ) comprises an outer housing 18 to which a housing cap 20 is connected by cap screws 22 . the swivel 10 is suspended from a rig ( not shown ) by a bail 24 , only a part of which is illustrated . a goose neck 26 is connected to the housing cap 20 for the passage of drilling mud from a supply to and through the washpipe 12 , to and through a stem 28 , and to and through the kelly ( see fig1 ) to the drill string ( also see fig1 ). the washpipe 12 is stationary while the stem is arranged to rotate with the kelly and the drill string . conventional means , not shown , are used to rotate the stem and the kelly . the stem 28 is journaled in spaced bearings 30 and 32 and the rotating assembly of the stem , kelly and drill pipe is supported by a thrust bearing 34 . the outer housing 18 contains lubricating oil for the bearings which is supplied through a plug 36 with a connected oil level dipstick 38 , so that the oil level can be periodically checked . the upper seal 14 surrounds the washpipe 12 at the juncture with the gooseneck 26 and the packing 16 surrounds the washpipe 12 adjacent to the juncture thereof with the stem 28 . the packing 16 is surrounded by a packing housing 40 threadably connected to the upper end of the stem 28 , so that the housing 40 rotates with the stem 28 . the packing 16 comprises a bottom adaptor ring 42 , alternate elastomeric packing rings 44 , alternate spacer rings 46 , and an upper packing adapter 48 . the housing 40 can be tightened down on the stem to adjust the packing 16 , at least until parts thereof become worn and inefficient . after this , the assembly must be disassembled and rebuilt . lubricating oil is supplied to the juncture of the packing 16 and the washpipe 12 through the fitting 50 . the present invention is directed to a pressure balanced mechanical seal structure 61 ( see fig4 ) to replace the packing 16 . the mechanical seal 61 comprises a pair or seal rings 63 and 65 constructed of silicon carbide or tungsten carbide which are exposed at their inner cylindrical surfaces 67 and 69 , respectively , to the drilling mud flowing through the washpipe 72 and seal housings 73 and 75 . the housing 73 has a flange 76 extending into and sealed by an o - ring seal 78 to washpipe 71 . the washpipe 71 is connected to the gooseneck ( not shown in fig4 ) in the same manner as the washpipe 12 illustrated in fig3 and 4 and a similar upper seal is used . because the washpipe and gooseneck connection is the same as in the prior art , no further description is deemed necessary . the seal rings 63 and 65 have relatively large internal diameters , as for example , on the order of 3 to 4 inches . the lower seal housing 75 , is connected to a stem ( not shown ) by means of a cylindrical safety shield and nut 77 shown schematically . the nut replaces the part 40 in fig2 and 3 . upper seal ring 63 has a relatively narrow seal face 79 with inner and outer dimensions less than the inner and outer dimensions of the ring 63 . thus there are defined three radially oriented surface areas , namely , seal face 79 flanked by inner radial surface 81 and outer radial surface 83 . ring 63 is received in an open - ended cavity 85 defined by a cylindrical wall and a radial wall in upper housing 73 . the cavity 85 is radially deeper than the ring 63 , and a portion of the housing 73 axially extends over the axial dimension of the ring 63 . studs 89 , only one of which is shown , carried by washpipe 71 , extend into upper housing 73 and eliminate relative rotation between the washpipe 71 and the upper housing 73 . a plurality of springs 91 , only one of which is shown , carried in pockets 93 in the washpipe 71 , resiliently urge upper housing 73 in a downward direction , thus providing the spring force to urge seal ring 63 toward seal ring 65 . the outer cylindrical surface 97 of the seal ring 63 defines with the housing 73 a narrow cylindrical cavity 99 , and o - rings 101 and 103 adjacent the ends of ring 63 and between ring 63 and housing 73 further define the cavity 99 . a passageway 105 extending radially through the ring 63 communicates the inner and outer surfaces 67 and 97 of ring 63 and thus permits the passage of mud flowing through the washpipe to the stem into the cavity 99 . the end surface 107 of the upper seal ring 63 is supported by the sealed to the housing 73 by an o - ring 109 , and the housing is grooved to provide a shallow , circular cavity 111 with the surface 107 . the cavity 111 is vented to the atmosphere by a passage 113 through the upper housing 73 . the lower seal ring 65 is a mirror image of the upper seal ring 63 and has a relatively narrow seal face 115 with inner and outer dimensions less than the inner and outer dimensions of the seal ring 65 . the face 115 is directly opposed to the seal face 79 of the seal ring 63 . three radial surfaces are defined , namely the seal face 115 flanked by inner radial surface 117 and outer radial surface 119 . the seal ring 65 is received in an open - ended cavity 121 defined by a cylindrical wall and a radial wall in lower housing 75 ; the lower housing being connected to the stem by the safety shield and the nut 77 so as to rotate therewith . the cavity 121 is radially deeper than the seal ring 65 , and portion of the housing 75 extends over the axial dimension of the seal ring 65 . thus , the seal ring 65 is arranged to rotate relative to the seal ring 63 . the outer cylindrical surface 123 of the seal ring 65 defines with the housing 75 a narrow cylindrical cavity 125 , and o - rings 127 and 129 adjacent the ends of the seal ring 65 further define the cavity 125 . a passageway 131 extending radially through the ring 65 communicates the inner and outer surfaces 69 and 123 of seal ring 65 and thus permits the passage of mud flowing through the washpipe to the stem into the cavity 125 . the end surface 133 of the seal ring 65 is supported by and sealed to the housing 75 by an o - ring 135 and the housing is grooved to provide a shallow circular cavity 137 with the surface 133 . the cavity 137 is vented to the atmosphere by a passage 139 through the housing 75 . as will be apparent from fig4 of the drawings , the seal rings 63 and 65 are recessed by making the id of the rings larger than the id of the washpipe 71 and the stem , so that their inner surfaces 67 and 69 are protected from erosion by avoiding direct inpingement of the mud thereon which would occur if the ids were the same . the flow of the drilling mud will be substantial laminar along the surfaces 67 and 69 . the radial surfaces 83 and 119 of the seal rings 63 and 65 , respectively , are exposed to the atmosphere because the housings 73 and 75 are slightly spaced from each other . areas defined by the surfaces 83 and 119 are substantially the same as the circular areas defining the cavities 111 and 133 . because the cavities 111 and 133 are vented to the atmosphere , the net axial pressure effect on the seal rings 63 and 65 is substantially zero . the inner surfaces 67 , 69 and outer surfaces 97 , 123 of the two seal rings 63 and 65 are exposed to the product fluid , i . e ., drilling mud , and its pressure and and temperature because of the cavities 99 and 125 and the passageways 105 and 131 . thus the radial pressure differentials are extremely low and the rings are thus not subjected to extreme radial pressure differentials . the axial pressure urging the rings 63 and 65 toward one another is also low when compared to the pressure of the drilling mud . these axial pressures are supplied by the springs 91 and reduced by the product pressure on the radial face surfaces 81 and 117 . seal rings as used in the described structure and constructed of silicon carbide or tungsten carbide are strong when placed in compression but are relatively weak when placed in tension . the arrangement described is such to substantially minimize tensional forces on the seal rings . unbalanced radial and axial force differentials on the inner and outer surfaces and also on the radial surfaces of the seal rings are substantially minimized . the appended claims are intended to cover all reasonable alternative constructions of the seal above described . for example , the housings can be drilled to provide passages 145 and 147 ( fig5 ) to provide communication between the cavities 99 and 125 surrounding the outer surfaces 97 and 123 if found desirable without departing from the spirit of the invention . the passages 145 and 147 function in the same manner as passageways 105 and 131 in the seal rings 63 and 65 , respectively , fig4 .