Mechanical seal assembly with improved fluid circulation
Mechanical seal assembly with improved fluid circulation
A seal assembly for a mechanical seal includes a rotatable shaft with at least one mechanical seal disposed about the shaft, the seal having a rotatable face coupled to the shaft and a stationary face, wherein the respective faces of the seal are in contact with one another, and a chamber for holding a cooling fluid, disposed about the shaft and in communication with the faces of the seal, and a closed loop fluid path disposed about the outer diameter of the shaft, preferably or in a non linear manner, in fluid communication with the chamber, for circulating fluid about the seal faces.
A variety of mechanical seals have been developed for use along a shaft, often in the context of pumps. One typical configuration is a mechanical seal with one stationary face and one rotating face. The rotating face of the seal rotates with the shaft of the pump, while the stationary face of the seal is generally coupled to the housing of the pump. In order to provide a tight seal, the two faces are typically in contact with each other. The frictional contact between the faces generates heat.
In order to dissipate heat, a fluid may be added to help transfer the heat away from the seal faces. Typically, a small fluid chamber is disposed about the shaft, so that the fluid is in communication with the seal face. As these mechanical seals are frequently used in a double or tandem configuration, the chamber may be disposed along the shaft, between and including the two mechanical seals. Often, a cooling fluid reservoir is added, with an auxiliary pump to circulate the fluid between the reservoir and the chamber. However, the addition of an auxiliary pump adds cost, requires additional space, and adds another component that is subject to failure, thereby reducing reliability.
There are many applications where a mechanical seal is subjected to fluid at the ID of the face. One of the most common is that of an unpressurized tandem seal where the barrier fluid is in contact with the ID of the primary seal and at the OD of the secondary seal. There is circulation of the barrier fluid into and out of the seal chamber by means of some type of pumping device that is usually part of the secondary seal rotating element. This circulation is adequate for cooling the secondary seal but is less than satisfactory for cooling the primary seal. This lack of cooling performance for the primary seal is due to the inability of the fluid to circulate to the ID of the seal.
Another application where cooling is needed at the seal face is in a vertical pump gear box seal oriented with the gear box oil at the ID of the seal. Gravity ensures that oil is at the ID of the face. However, during dynamic operation this fluid can not circulate with the bulk fluid in the gear box. This leads to increased seal temperature and possibly coking of the oil at one or both of the seal faces. Coking leads to increased leakage and damage to seal faces.
Rather than use an auxiliary pump, other configurations have built a “pumping rotor” into the system. See, e.g., U.S. Pat. No. 4,466,619 to Adams U.S. Pat. No. 4,560,173 to Adams et al. A slotted sleeve is fitted concentrically about the shaft, whereby the rotational movement of the shaft aids in circulating fluid along a fairly linear path, drawing the fluid from the reservoir into the chamber through an inlet, moving it radially around the shaft, and pushing it out of the chamber through an outlet and back into the reservoir. Another type of seal uses screw-type threads on the shaft to move the fluid between an inlet and an outlet. However, the fluid may only be moved in one direction in the chamber, between the inlet and the outlet, and away from the mechanical seal.
Because of the seal mechanisms themselves, it is generally not possible to position an inlet or outlet directly adjacent a seal face. Thus there is a space in the chamber between the inlet and outlet, which define the path of circulation, and the seal face, where the heat is generated and where the fluid will be heated the most. This causes a “dead end” space in the chamber between the seal faces and the respective inlet and outlet, where the cooling fluid is substantially stagnant, and does not circulate with the rest of the fluid. In the “pumping rotor” configuration discussed above, the radial circulating action occurs in a “band” that is aligned with the inlet and outlet; fluid outside this band remains substantially uncirculated. In the screw type circulator discussed above, these dead spots occur on either side of the inlet and outlet, as the fluid is substantially circulated only between the inlet and outlet. In the double or tandem configuration, there is generally a band or path of circulation between the seals, but there is inadequate circulation directly at the seal faces, where circulation is most necessary.
Thus there exists the need for a circulation device for mechanical seals which provides circulation to the seal faces, preferably without the use of auxiliary pumps, which can circulate fluid about the seal faces beyond the respective inlet and outlet locations, and which operates under rotation of the shaft in either direction.
An important feature of the present invention is the ability to circulate the fluid within the “dead end” space at the opposed faces of the seal. Although fluid cannot be pumped beyond this point, the present invention provides both radial and axial circulation, lifting the heated fluid away from the opposed seal faces and replacing it with cooler fluid.
Another important feature of the present invention is that it operates effectively upon rotation of the shaft in either direction. Thus the mechanical seal remains cooled regardless of the direction of shaft rotation.
