Noncontact mechanical seal

Noncontact mechanical seal

A noncontact mechanical seal has a seal ring which includes a sealing face portion formed by ventilative porous materials and a ventilating portion to pass a gas of a high pressure side to the sealing face portion from the back side of said sealing face portion. The sealing face portion is preferably a porous ring independently formed of said seal ring and mounted on the seal ring providing a space at the back side thereof. The seal ring preferably is provided with a small diameter thin portion at the portion forming the bottom of the space.

Noncontact seals are utilized to seal gas or the like. Noncontact seals include seal faces where high pressure gas is induced to keep the seal faces out of contact with each other. Such noncontact seals are provided at the sealing face with a very slightly tapered portion, a circle groove, a thin air flow hole or a spirally radial groove to induce high pressure gas to the sealing face thereof.

However it is hard (it increases the cost) to form the tapered portion, the groove or the hole with a high accuracy. In noncontact seals the seal faces are not completely noncontact; they contact each other slightly owing to vibration and/or poor accuracy of machining. Such contact causes wear of the sealing faces and deformation of the grooves follows. Thus the sealing ability of noncontact seals become worse as the sealing faces wear and therefore noncontact seals cannot keep providing stable and reliable sealing.

Furthermore the hole and the groove can sometimes be bigger than the required size because of difficulty of forming of small grooves, holes or the like and because of the need to make an allowance for the wear of sealing faces caused by vibration. Such a bigger groove and hole permits entry of more air, that separates the sealing faces too much and makes fine adjustment of the gap between the sealing faces difficult.

In such mechanical seal shown in FIG. 1, the high pressure gas in the high pressure side X flows through the ventilating groove 70, the porous ring 6 and the porous sealing face 11 toward the rotary seal ring 2. The gas in the relief space 60 maintains some gas pressure. The distribution of the pressure at the sealing face of the nonrotary seal ring 1 is shown in FIG. 2 as lines a, b and c. A line d represents the pressure which the back end of the nonrotary seal ring 1 receives. When the sealing face of the nonrotary seal ring 1 is closest to the sealing face of the rotary seal ring 2, as shown in FIG. 1 the pressure on the face is high as shown by line c. When the nonrotary seal ring 1 is more distant from the rotary seal ring 2, as shown in FIG. 5, the pressure decreases in the gap therebetween as shown by line a and this creates a vacuum force. Thus the pressure in the gap will then increase the balance with the force pushing the nonrotary seal ring 1 toward the rotary seal ring 2 at the line b, and the operation is carried out at this stable condition.

Since ventilation (or flowing the gas) through the porous ring 6 can be made very small, the gap between the nonrotary seal ring 1 and the rotary seal ring 2 can be very small. The gap is adjustable by controlling the balance factor of the nonrotary seal ring 1 and/or the ventilation of the porous materials of the porous ring 6. The ventilation may be decreased by reducing the number or the diameter of pores, so that the gap between the nonrotary seal ring 1 and the rotary seal ring 2 can be reduced. On the other hand the gap can be enlarged by increasing the number or the diameter of the pores. Such adjustment of the gap can be controlled by adjusting the thickness in the axial direction of the porous ring 6.

It is easier to mount the porous ring 6 on the nonosmotic ring 5 than to form holes on sealing faces in the prior art. The small diameter thin portion 76 of the nonosmotic ring 5 gives elasticity to the nonosmotic ring 5 to bend the portion 76 so as to move the outer fringe 62 apart from the rotary seal ring 2, which prevents improper contact between the outer fringe 62 and the rotary seal ring 2 and the extraordinary torque generated at the rotary seal ring 2 by such contact. Furthermore the relief groove 60 provides the porous ring 6 with the thin portion 61 which also gives elasticity to the porous ring 6 per se and the unwanted contact between the outer fringe 62 and the rotary seal ring 2 is further effectively prevented thereby.

FIG. 3 shows a graph indicating the difference of the values of leakage and contact torque between the noncontact mechanical seal of FIG. 1 and the noncontact mechanical seal of the prior art which include holes for ventilation and cannot provide elasticity. White circles indicate the rotation torque of the embodiment of FIG. 1 and white triangles indicate the leakage of the embodiment. Black circles indicate the rotation torque of the prior art and black triangles indicate the leakage of the prior art. The abscissa expresses the gas pressure of the sealed gas in the high pressure side X.

Since the mechanical seal of the invention provides an elasticity effect at the outer fringe 62, unwanted contact does not happen and therefore the rotation torque is small. Furthermore it is obvious that the leakage is larger at higher pressure than the prior art because of the small gap of the sealing faces. On the other hand, the torque of the prior mechanical seal increase as the pressure in the high pressure side X increases as shown by the black circles in FIG. 33, which is caused by the bending of the outer fringe of the sealing ring of the prior art toward the opposite counter sealing ring due to the pressure in the high pressure side X.

The ventilation of the porous ring 6 may not substantially change as the face of the porous ring 6 wears, which is contrary to prior noncontact mechanical seal. This provides stable and reliable sealing.