Seal for a joint or juncture – Seal between relatively movable parts – Relatively rotatable radially extending sealing face member
Patent
1998-02-23
2000-10-17
Browne, Lynne H.
Seal for a joint or juncture
Seal between relatively movable parts
Relatively rotatable radially extending sealing face member
277390, 277431, 277910, F16J 1534
Patent
active
061319139
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a shaft seal for rotating shafts in turbo-machines, in particular but not exclusively to non-contacting shaft seals.
This type of shaft seal is often used with machinery for the pumping of gas (nitrogen, argon, hydrogen, natural gas, air, etc.) where the transmission of gas along the shaft needs to be prevented. Due to the high-pressure, high-speed machinery which is normally used, the shaft seals may be non-contact type seals, in order to reduce heat build up in the seals and the wear of the sealing parts.
Non-contacting operation avoids this undesirable face contact when the shaft is rotating above a certain minimum speed, which is often called a lift-off speed.
Non-contacting shaft seals provide advantages over seals where the sealing surfaces contact one another due to reduction in wear and the lower heat generation. Articles entitled "Fundamentals of Spiral Groove Non-contacting Face Seals" by Gabriel, Ralph P. (Journal of American Society of Lubrication Engineers Volume 35, 7, pages 367-375, and "Improved Performance of Film-Riding Gas Seals Through Enhancement of Hydrodynamic Effects" by Sedy, Joseph (Transactions of the American Society of Lubrication Engineers, Volume 23, 1 pages 35-44) describe non-contacting seal technology and design criteria and are incorporated herein by reference.
As with ordinary mechanical seals, a non-contacting face seal consists of two sealing rings, each of which is provided with a very precisely finished sealing surface.
These surfaces are tapered-shape perpendicular to and concentric with the axis of rotation. Both rings are positioned adjacent to each other with the sealing surfaces in contact at conditions of zero pressure differential and zero speed of rotation. One of the rings is normally fixed to the rotatable shaft by means of a shaft sleeve, the other is located within the seal housing structure and allowed to move axially. To enable axial movement of the sealing ring and yet prevent leakage of the sealed fluid, a sealing member is placed between the ring and the housing. This sealing member must permit some sliding motion while under pressure, therefore normally a top quality O-ring is selected for that duty. This O-ring is often called the secondary seal.
As mentioned above, to achieve non-contacting operation of the seal, one of the two sealing surfaces in contact is provided with shallow surface recesses, which act to generate pressure fields that force two sealing surfaces apart. When the magnitude of the forces resulting from these pressure fields is large enough to overcome the forces that urge seal faces closed, the sealing surfaces will separate and form a clearance, resulting in non-contacting operation. As explained in detail in the above-referenced articles, the character of the separation forces is such that their magnitude decreases with the increase of face separation. Opposing or closing forces, on the other hand, depend on sealed pressure level and as such are independent of face separation. They result from the sealed pressure and the spring force acting on the back surface of the axially movable sealing ring. Since the separation or opening force depends on the separation distance between sealing surfaces, during the operation of the seal or on imposition of sufficient pressure differential equilibrium separation between both surfaces will establish itself. This occurs when closing and opening forces are in equilibrium and equal to each other. Equilibrium separation constantly changes within the range of gaps. The goal is to have the low limit of this range above zero. Another goal is to make this range as narrow as possible, because on its high end the separation between the faces will lead to increased seal leakage. Since non-contacting seals operate by definition with a clearance between sealing surfaces, their leakage will be higher then that of a contacting seal of similar geometry. Yet, the absence of contact will mean zero wear on the sealing surfaces and therefore a relatively low amount of heat generated be
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Auber Philippe Jacques
Rabuteau Michel Emile Marie
Binda Greg
Browne Lynne H.
Dresser Rand Company
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