Patent Application
20070170657
This invention relates to an improved sealing system for a stuffing box of a rotary mechanical device, and more particularly to a one-piece shaft sealing bushing to be placed about a mounted shaft in a seal cavity or stuffing box of the mechanical device, and a method for fabricating and installing the one-piece bushing.
Rotary mechanical devices, such as mixers and centrifugal pumps include an impeller mounted on a shaft which is driven by an electrical motor. The shaft passes through a seal cavity or a stuffing box defined by a cylindrical cavity in the device housing. Typically, the shaft is supported by bearings at the motor end and seals are placed in the stuffing box to engage the shaft to prevent fluid from passing through the seal cavity and reaching the bearing and the motor to avoid causing damage to both.
In order to eliminate the need to disassemble the equipment to allow insertion of a solid bearing or bushing over the shaft, stuffing box bearings and bushings are usually molded or machined in two halves. This allows for easier installation, but requires mechanical registration of both halves. This is usually done with steel pins prior to insertion so that both halves will be installed and sealed together in the bottom of the stuffing box. In mechanical pumps, the seal cavity passes chemical fluids, solids or solvents being pumped, many of which are corrosive or erosive. Accordingly, it is important that appropriate packing material is placed within the seal cavity. Seal water is pumped into the seal cavity through a flush port to prevent the fluid being pumped or mixed from traveling along the shaft to the bearings and motor and to provide lubricant to the packing. Over extended use, the pump shaft may develop a whip as the bearing or bushing wears. Thus, it is highly desirable to provide a sealing system with a non-compressive member that provides an effective seal to reduce the amount of pumped fluid from entering the seal cavity.
A suitable stuffing box sealing system is shown in U.S. Pat. No. 6,834,862 issued to the applicant herein, the contents of which are incorporated by reference. This patent describes a seal system with a bearing surface. This shaft sealing system works well particularly when an additional bearing surface is required.
While there are a wide variety of devices available for providing seals in rotary devices, it is desirable to provide a improved sealing system for the impeller end of the shaft and allow a reduction in fluid entering the seal cavity that can be easily installed.
Generally speaking, in accordance with the invention, a one-piece semi-rigid bushing to be positioned over a shaft in the seal cavity of a rotary mechanical device and moved to the impeller end is provided. The bushing is formed with a lantern ring at the motor end. The one-piece bushing is formed from a substantially planar piece of suitable material with milling slots or other types of relief formed on one surface and shaped into a cylinder and restrained to provide easy installation over an installed device shaft.
The semi-rigid bushing is manufactured of a non-ferrous metal, thermoplastic polymer, filled thermoplastic material, or other man-made non-compressible material for providing an improved seal and resistance to industrial solutions. The inner bore of the one-piece bushing closely approximates the outer diameter of the pump shaft and the outer wall forms a cylinder approximating the diameter of the stuffing box.
The bushing is manufactured by cutting a plurality of milling slots in a sheet of bushing material of appropriate thickness to fill the stuffing box annulus. The sheet is cut to a length corresponding to the diameter of the stuffing box annulus. The milled piece is then formed into a cylinder, restrained and heated and coined to form the one piece bushing with a single slit with the milling slots on the side of the bushing facing the pump shaft.
The motor end of the bushing may include an internal groove and a corresponding outer groove with seal water openings to provide a lantern ring on the motor side of the bushing. Generally, two or three packing rings are added on the motor side of the bushing to complete the assembly. An O-ring groove may be formed on the outer surface for improved isolation of pumped product within the cavity.
Accordingly, it is an object of the invention to provide an improved shaft sealing system to be inserted into a seal cavity of a rotary mechanical device.
Another object of the invention is to provide an improved shaft sealing system for a rotary mechanical device including a one piece semi-rigid bushing member to surround the impeller end of the device shaft.
A further object of the invention is to provide an improved shaft sealing system for a rotary mechanical device including a semi-rigid bushing member having an integral lantern ring grove for controlling the amount of seal water entering the seal cavity.
Another object of the invention is to provide an improved shaft sealing system for a rotary mechanical device including a one piece semi-rigid bushing member having a plurality of milling slots or other types of relief on the inner bore to allow for shaping in cylindrical form with the milled side facing the rotary shaft.
Yet another object of the invention is to provide an improved shaft sealing system for a rotary mechanical device including a one piece semi-rigid bushing member having a single slit to allow for easy installation about an installed shaft.
Yet a further object of the invention is to provide a method of manufacture of an improved semi-rigid bushing member particularly well adapted for installation about a mounted motor shaft.
Still another object of the invention is to provide an improved sealing system for a rotary mechanical device having an outer O-ring for improved isolation of the product being handled and seal water in the seal cavity.
Yet another object of the invention is an improved semi-rigid seal bushing for use with complementary flexible packing rings for use in a shaft sealing system to be inserted into the seal cavity of rotary mechanical devices.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention comprises a product possessing the features, properties, and the relation of components which will be exemplified in the product hereinafter described and the scope of the invention will be indicated in the claims.
For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawings, in which:
Pump 11 operates by drawing a fluid to be pumped into inlet 17 and forced to exit at outlet 18. During pumping, fluid tends to migrate and be forced into seal cavity 22 through opening 23. A wide variety of seals and venting configurations are available to be placed in seal cavity 22 abutting opening 23 in order to restrict and limit entry of pumped fluid into seal cavity 22. If fluid enters seal cavity 22 and migrates to bearings at the motor end, the bearings may be subject to substantial degradation due to the corrosive action of the pumped fluid.
As shown in
Bushing seal element 31 is a substantially cylindrical semi-rigid member with an outer surface 33 dimensioned to be slightly smaller than and fit within an internal bore 21 of pump housing 19. The dimension of inner bore 34 of bushing 31 is slightly larger than the outer dimension of pump shaft 14. Bushing seal element 31 has a top end 31a at the impeller end and a bottom end 31b at the gland end. Outer surface 33 of bushing seal element 31 is formed with an outer seal water groove 36 at the gland end thereof. Inner bore 34 of seal element 31 provides an additional bearing surface for shaft 14 when in position as shown in
Bushing element 31 is formed from a flat rectangular piece of material 41 having an upper surface 42 and a lower surface 43 as shown in
An inner seal water groove 48 is formed in bushing seal element 31 at a position corresponding to the location of outer seal water groove 47 to create a lantern ring at the gland end of bushing 31. A plurality of seal water holes 49 are formed between outer seal water groove 47 and inner seal water groove 48. Bushing seal element 31 is fabricated so that outer surface 33 at the impeller end is longer than the width of outer seal water groove 47 as shown in
Bushing seal element 31 is semi-rigid and formed of materials which will not be attacked or destroyed by corrosive fluids being transported by pump 11. The material of construction includes a wide variety of material. These include non-ferrous metals, silica-based materials, carbonaceous materials, polymeric materials, such as nylon, polyacetals, polyvinylchloride, polyethylene, polypropylene and fluorine containing polymers, such as (polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), or other suitable plastic material). The polymeric materials may be filled or unfilled. Filled materials may include a molybdenum/carbon or glass filled thermoplastic material, such as a PTFE, PFA and nylon.
In order to be able to place bushing seal element 31 in position in seal cavity 22, bushing seal element 31 is formed by deforming milled piece 41 into a cylinder and restrained in position. Thus, it will have only a single slit 39. A single slit allows bushing seal element 31 to be pried apart and placed around installed shaft 14 and pushed into position at the impeller end of seal cavity 22. Slit 39 may be substantially axial and parallel to shaft 14 or vary in any degree up to but not including 90 degrees depending on the specific application.
In another embodiment, outer surface 72 of bushing seal element 71 may also contain milled slots to help assist in properly centering the one-piece shaft sealing bushing assembly if the application bore is elliptical in shape rather than cylindrical in shape. Also, inner bore 34 may be fitted with contact devices such as O-rings or hydraulic seals that would provide directional dams against materials entering the stuffing box area.
Bushing seal elements in accordance with the invention, a milled sheet of bushing seal element with or without the lantern ring groove may be milled for a standard size shaft. Once a particular shaft is measured for its diameter, the milled sheet may be cut to the correct size to fit with the cavity annulus about the shaft. For example, a milled sheet made of PTFE is first cut to the dimension needed and is then coined by restraining the cut milled sheet in a steel form. The milled sheet is also wrapped around a inner metallic sleeve to form a cylinder. The coined milled sheet in then heated for about one hour at 350° F. The temperature time of the heat varies depending on the material and the thickness of the material.
The dimensions of a typical bushing seal element 31 made of filled PTFE where the shaft is 3″ in diameter is as follows:
(1) Width of each of the milling slots are 0.093″;
(2) Length of the lantern ring groove is 0.375″;
(3) Depth of the lantern ring groove is 0.1″;
(4) Length of the bushing element between each milling slot is ⅛″; and
(5) Length of the bushing element from the top to the bushing to the lantern ring is 1.5″.
It can readily be seen that the bushing including a cylindrical seal and seal system constructed in accordance with the invention can be easily installed in a conventional rotary impeller pump without the need to use pins necessary to guarantee alignment due to the one-piece bushing construction of the seal upon installation. Generally only two packing rings are added to complete installation. When in place, the seal will temporarily support the impeller end of the pump shaft, providing an additional bearing surface to aid in eliminating the whip commonly found in pump shafts. Since the clearance between the pump shaft and the bushing is relatively small, seal water entering the lantern ring groove would be considerably throttled, thereby minimizing the quantity of seal water flushing and lubricating the bearing and finally entering into and diluting the product being pumped.
Due to the close clearances available, improved support of the impeller is assured, resulting in longer life of the main bearings and packing materials as well as reduced wear of the throat of the rotary device. Minimum seal water is required with less product contamination because of this throttling effect. Minimum external leakage also results from the installation of the sealing system constructed and arranged in accordance with the invention.
It will thus be seen that the object set forth above, among those made apparent from the preceding description are efficiently attained and, since certain changes may be made in the device set forth without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention, which, as a matter of language, may be said to fall there between.
