Mechanical seal with high pressure high temperature secondary seal
A secondary seal of a mechanical seal comprises an annular sealing gasket and backing plate surrounding a cylindrical sealing surface, all of which can be made of the same metallic material. The gasket is C-shaped in cross section, and is axially pressed during operation into a C-shaped groove formed in the backing plate, the groove being shallower and larger in inner diameter than the gasket, so that the gasket, when pressed into the groove, deforms radially inward and seals against both the backing plate and the sealing surface. The contact area between the gasket and sealing surface increases with pressure. In embodiments, when the pressure is released, the gasket retracts from the sealing surface, allowing easy disassembly and allowing axial movement of the gasket and backing plate. Highly polished surfaces of the gasket and sealing surface can allow axial motion thereof during operation to maintain the seal face gap.
1. A mechanical seal comprising:
first and second seal faces sealed to corresponding first and second structures, one of the seal faces being a rotating seal face surrounding, fixed to, and coaxial with a rotating shaft, the other of the seal faces being a static seal face coaxial with the rotating seal face and fixed to a static housing, the seal faces having opposing sealing surfaces in close proximity to each other, thereby forming a non-contacting primary seal therebetween;
the first seal face being sealed to the first structure by a secondary seal comprising:
a cylindrical sealing surface surrounding and coaxial with the rotating shaft, and sealed to the first structure;
an annular backing plate surrounding and coaxial with the rotating shaft and sealed to the first seal face;
an annular, metallic sealing gasket surrounding and coaxial with the sealing surface and proximal to the backing plate, the sealing gasket having an arcuate cross section that approximates a half-arc of a circle, curved axially toward the backing plate and terminated at an inner edge proximal to the sealing surface and an outer edge distal from the sealing surface; and
an arcuate groove formed in the backing plate proximal to the sealing gasket, the arcuate groove having an inner radius that is larger than a radius of the inner edge of the sealing gasket and a cross sectional curvature that is shallower than a curvature of the sealing gasket's cross sectional shape, so that when the sealing gasket is axially pressed into the arcuate groove, it forms a seal with the backing plate while, at the same time, the inner edge of the sealing gasket is forced radially inward beyond the arcuate groove of the backing plate and against the sealing surface, thereby forming a seal with the sealing surface.
2. The mechanical seal of claim 1 , wherein the backing plate, sealing gasket, and sealing surface are configured such that the inner edge of the sealing gasket initially forms a press-fit with the sealing surface before the sealing gasket is pressed into the arcuate groove.
3. The mechanical seal of claim 2 , wherein before the sealing gasket is pressed into the arcuate groove, the diameter of the inner edge of the sealing gasket is between 0.0005 and 0.001 inches smaller than the diameter of the sealing surface.
4. The mechanical seal of claim 1 , wherein the backing plate, sealing gasket, and sealing surface are configured such that, after the sealing gasket is pressed into the arcuate groove, the inner edge thereof forms a region of contact with the sealing surface that is shaped as a cylindrical band.
5. The mechanical seal of claim 1 , wherein the backing plate, sealing gasket, and sealing surface are configured such that, after the sealing gasket is forced by applied pressure into the arcuate groove, and subsequently said pressure is released, the inner edge of the sealing gasket is retracted from the sealing surface, forming a clearance fit gap therebetween.
6. The mechanical seal of claim 5 , wherein the clearance fit gap is between 0.0005 and 0.001 inches wide.
7. The mechanical seal of claim 1 , wherein the sealing gasket is made from Alloy 718.
8. The mechanical seal of claim 1 , wherein the sealing surface, the backing plate, and the sealing gasket are all made from the same metallic material.
9. The mechanical seal of claim 8 , wherein the sealing surface, the backing plate, and the sealing gasket are all made from Alloy 718.
10. The mechanical seal of claim 1 , wherein the first seal face, backing plate, sealing gasket, and sealing surface are axially movable relative to the second seal face.
11. The mechanical seal of claim 1 , wherein the arcuate cross sectional shape of the sealing gasket is uniform in thickness between the inner and outer edges thereof.
12. The mechanical seal of claim 8 , wherein the uniform thickness of the sealing gasket is approximately 0.005 inches.
13. The mechanical seal of claim 1 , wherein an outer surface of the sealing gasket, an inner surface of the arcuate groove, and the sealing surface are all prepared with a finish having an Ra of 16 micro-inches or less.
14. The mechanical seal of claim 1 , wherein an outer edge of the arcuate groove is between 0.001 and 0.002 inches smaller in diameter than the outer edge of the sealing gasket.