WO2002093046A1

WO2002093046A1 - Mechanical seal

Info

Publication number: WO2002093046A1
Application number: PCT/GB2002/002162
Authority: WO
Inventors: Alan Roddis
Original assignee: AES Engineering Ltd
Current assignee: AES Engineering Ltd
Priority date: 2001-05-15
Filing date: 2002-05-15
Publication date: 2002-11-21
Anticipated expiration: 2003-11-15

Abstract

A mechanical seal, for providing sealing between a rotatable shaft and a housing, includes a seal face (3) which is provided with a textured portion (17) and a non-textured portion (18). The non-textured portion acts as a sealing dam.

Description

Mechanical Seal

Field of the Invention This invention relates to mechanical seals used to seal equipment with rotating components.

Background to the Invention

A mechanical seal comprises a "floating" component which is mounted so as to be axially movable around the rotary shaft of, for example, a pump and a "static" component which is axially fixed, typically being secured to a housing. The floating component has a flat annular end face, i.e. its seal face, directed towards a complementary seal face of the static component. The floating component is urged towards the static component to close the seal faces together to form a sliding face seal, usually by means of one or more spring members. In use, one of the floating and static components rotates; this component is referred to as the rotary component. The other of the floating and static components does not rotate and is referred to as the stationary component.

Those seals whose floating component is rotary are described as rotary seals. If the floating component is stationary, the seal is referred to as a stationary seal.

If the sliding seal between the rotary and stationary components are assembled and pre-set prior to despatch from the mechanical seal manufacturing premises, the industry terminology for this is "cartridge seal". If the rotary and stationary components are despatched individually (unassembled) from the mechanical seal manufacturing premises, the industry terminology for this is "component seal".

The sliding interface between the rotary and stationary components is particularly important to the operational performance of the mechanical seal. The varying conditions at the sliding interface between a pair of mechanical seal faces, are well documented. The term "fluid film" is often used to describe the lubrication characteristics at the sliding interface. It is commonly understood that the action of a rotating component sliding against a stationary component can create undesirable operational conditions. Such conditions include the generation of heat, increased equipment power consumption, increased component wear and ultimately reduced component operational life.

US 6,046,430 (Etsion) describes an invention which changes the surface texture of at least one of the two components at the sliding interface. Etsion defines a series of micropores across the entire radial width of a mechanical seal face, as shown in Figure 1. According to Etsion, such a textured surface helps to reduce some of the prementioned undesirable conditions.

An etched surface across the entire radial seal face width can, in some circumstances, lead to static seal face leakage. This is due to the respective distance between any two micropores and the hydrostatic effects of the fluid film under

media pressure.

This leakage could be reduced by increasing the radial cross section of the seal face, effectively increasing the distance between any two micropores. This, however, can have a counter-effect to the invention and increase heat generation in dynamic operation. Furthermore this can dramatically limit where the seal can be physically applied or installed. This approach has massive commercial implications and in most applications is not practical.

This leakage could also be replaced by reducing the number of micropores on the seal face. This reduces the aforementioned benefits of the invention accordingly and therefore, once again, is counter-productive.

In addition to the possibility of static leakage, experiments have proven that micropores across the entire radial seal face width can create an effect which "rings" or sticks the two mechanical seal faces together. This has a detrimental and potentially disastrous effect on brittle seal face materials particularly on equipment start up conditions. This is due to the high torque required to overcome the seal face ringing effect.

It is considered to be particularly advantageous to create a mechanical seal face which, in dynamic operation, reduces heat generation, equipment power consumption and seal face wear, yet under static operation provides a reliable seal interface with minimal potential for the seal faces to "ring" together.

Statements of Invention According to the present invention there is provided a mechanical seal to provide sealing between a rotatable shaft and a housing, the seal having a stationary part for connection to the housing and a rotary part for rotation with the shaft, mating sealing faces being carried by said stationary and rotary parts, said rotary parts being for mounting on the drive shaft for rotation therewith, at least one seal face having a textured portion and a non-textured portion, the non-textured portion acting as a sealing dam

This helps to extend the seal life whilst reducing the possibility of seal face impact damage due to high torque requirements on equipment start-up conditions.

Preferably the edge of the sealing dam is positioned at or close to the seal face balance diameter.

The sealing dam may be radially positioned on the lower pressure side of the seal face relative to the textured portion. Alternatively the sealing dam may be radially positioned on the higher pressure side of the seal face relative to the textured portion.

The sealing dam may cover up to 99 per cent of the seal face area and correspondingly the textured surface may cover up to 99 per cent of the seal face area. Preferably the textured surface consists of micropores or indentations in the surface of the seal face.

The micropores or indentations may be evenly distributed in the area of the seal face adjacent to the sealing dam. Alternatively they may be randomly distributed in the area of the seal face adjacent to the sealing dam.

There may be two or more areas of a textured surface which are separated by at least one area of a non-textured surface. Additionally or alternatively, there may be two or more areas of a non-textured surface separated by at least one area of textured surface.

Description of the Drawings

The drawings accompanying this application are as follows:

Figure 1 illustrates an end view of a prior art seal face design extracted from US 6,046,430 (Etsion).

Figure 2 is a graph which compares the typical start up torque required to overcome the ringing or sticking effect of a conventional "non-textured" seal face to that of a micropore "textured" seal face which is textured across its entire radial length, over a range of process pressures using water as a process media.

Figure 3 is a graph which corresponds to Figure 2 but uses a nitrogen gas as the process media.

Figure 4 is a partial longitudinal cross section through a single cartridge mechanical seal of the invention.

Figure 5 is a sectional view showing the invention and corresponding to

Figure 4. Figure 6 is a partial longitudinal cross section through a double cartridge mechanical seal of the invention.

Figure 7 is a cross section through a seal face showing a sealing dam and typical micropore indentations in its surface.

Figure 8a is an end view of a seal face with a surface texture across its entire radial cross section.

Figure 8b corresponds to Figure 8a and part of Figure 6 and is an end view of a seal face of the invention with a portion of its seal face encompassing a sealing dam and a surface textured portion.

Detailed Description of the Invention The present invention will now be described by way of examples only and with reference to Figures 2 to 8 of the accompanying drawings. Figure 1 shows a prior art arrangement as previously mentioned.

From Figure 2 it is clear that over identical operating conditions, a seal face with a texture surface across the entire radial width of the seal face, requires significantly more start up torque to that of an equivalent non-textured seal face.

Figure 3 clearly shows a similar torque difference between a fully textured seal face and a conventional seal face when starting a mechanical seal operating in nitrogen gas.

From Figure 4, of the invention, the rotary and axially floating seal face (1) is spring biased towards a static stationary seal face (2). The rotary seal face (1) is allowed to slide on the static seal face (2). The interface between the rotary seal face (1) and stationary seal face (2) forms sealing area (3). This sealing area (3) is the primary seal that prevents the process media (4) from escaping from the process chamber (5). In addition to the sliding seal face (3), the process media (4) is sealed by a sleeve elastomer (6) in contact with the shaft (7) and sleeve (8). This has been termed the first secondary sealing area (9).

The second secondary sealing area (10) is formed between stationary seal face (2) and stationary gland (11) using elastomer (12).

The third secondary sealing area (13) is formed between the rotary seal face (1) and the sleeve (8) using elastomer (14).

The fourth secondary sealing area (15) is formed between the gland (11) and the process chamber (5) using gasket (16).

The four secondary sealing devices and the primary sliding sealing interface prevent the process media (4) from escaping. The remaining components shown in Figure 4 are typically found in any mechanical seal design of this nature and therefore are not further described.

At least one of the two sealing faces, rotary (1) and stationary (2) has a partly micropore textured surface (17). Said micropore textured surface, in single seal operation, preferably extends radially inwardly from the outside diameter terminating at the sealing dam.

The invention illustrated in Figure 4 therefore offers a cartridge mechanical seal with at least one set of mechanical seal faces which are textured across only part of the radial cross section.

Figure 5 shows the section at the mechanical seal faces which corresponds to Figure 4. Figure 5 illustrates an inwardly positioned sealing dam (18) since the micropore textured surface (17) is positioned radially outwardly to the sealing dam (18). This seal dam (18) invention provides an adequate, non- textured sealing surface for static applications.

It is considered advantageous to locate the edge of the sealing dam (18) at or close to the seal balance diameter (19). It is further advantageous, although not essential, for the radially largest part of the sealing dam (18) to coincide with the mechanical seal face balance line (19). The sealing dam (18) therefore protrudes radially inwardly allowing the micropore surface (17) to be lubricated by the process fluid (4). From the single seal design in Figure 4, the process fluid (4) is often at a higher pressure than the inner most part of the seal face (3). This radially inner most part of a single seal design is often termed the atmospheric side of the seal face (3).

The sealing dam to balance diameter ratio can be changed to suit specific applications. Likewise the percentage seal face area coverage of the sealing dam and textured surface can be changed to create optimised conditions at the fluid film, for given process conditions.

The radially smallest part of the sealing dam (18) can be designed to coincide with the mechanical seal face balance diameter (19). The sealing dam (18) therefore protrudes radially outwardly allowing the micropore surface (17) adjacent to the lower pressure or atmospheric side of the seal faces (3). This is shown in Figure 6, assuming the barrier pressure (20) is lower than the process pressure (4).

The majority of seal faces found in industry have a hydrostatic fluid film between the mating faces. This supports the seal faces and in dynamic conditions this fluid film helps to prevent seal face burnout. Hydrostatic fluid film conditions between the mechanical seal faces (3) are a known and well-documented fact of mechanical seal design. The hydrostatic fluid film conditions between any two mating seal faces (3) may be approximated as a linear pressure drop. The linear pressure drop creates an axially thicker fluid film at the higher pressure outer or inner most radial side of the seal faces (3). The fluid film then reduces in a linear axially manner to the radially lower pressure side of the seal faces (3). At the lower pressure side of the seal faces the fluid film is said to be "thin".

It is this "thin" fluid film that can significantly and substantially increase the breakout torque of the seal faces. By positioning the micropore surface (17) adjacent to the lower pressure or atmospheric side of the seal faces (3), the fluid film conditions at the "thin" part are changed. It has been surprisingly found that this reduces the breakout torque of the mechanical seal faces, therefore offering significant advantages when the invention is used with brittle seal face materials.

Figure 6 illustrates the invention applied to a double cartridge mechanical seal. From Figure 6 the stationary and axially floating seal face (2) is spring biased towards an axially static rotary seal face (1). The rotary face (1) is allowed to slide on the stationary seal face (2). The interface between the rotary seal face (1) and stationary seal face (2) forms sealing area (3). This sealing area (3) is the primary seal that prevents the process media (4) from escaping from the process chamber (5).

The second primary seal is made in the barrier media chamber (20) on the outboard side of the mechanical seal. The stationary and axially floating seal face (21) is spring biased towards an axially static rotary seal face (22). The rotary face (22) is allowed to slide on the stationary seal face (21). The interface between the rotary seal face (22) and stationary seal face (21) forms sealing area (23). This sealing area (23) is the primary seal that prevents the barrier media from escaping from the barrier media chamber (20).

The sealing dam (24) is positioned approximately radially outwardly of the seal face balance diameter (25), whilst the textured surface (26) is positioned adjacent to the barrier media in the barrier media chamber (20).

Figure 7 shows a cross section through a seal face of the invention. By way of example only the micropore indentations (27) have been shown as spherical indentations which penetrate the surface in depth up to, but preferably less than, the radius of the spherical indentation. It is considered self evident that the surface indentations could be of any physical shape, but ideally be of a symmetrical nature (viewed on the end of the seal face) and of any physical depth.

As the seal face comprises of a sealing dam portion (24) and a textured portion (26), the spacing of the micropore indentations (27) has minimal influence on the static sealability of the seal face. It is therefore possible to distribute the micropore indentations (27) so that they are positioned as close to each other as physically possible. By way of example only the micropore indentations (27) could cover up to 95% of the designated textured surface area (26). This invention therefore allows the optimum dynamic performance of the mechanical seal without increasing the physical radial cross section (28) of the seal face. This reduces heat generation, power consumption and seal face wear in comparison to a conventional mechanical seal.

From Figure 7, when the process fluid (4) acts on the outside diameter of the seal faces (1) and (2), it is considered preferable, although not essential, to position the sealing dam on the inner most radial portion of the seal face. This allows the process fluid (4) to act effectively in the micropore indentations (27). However, for brittle materials, if the sealing dam (24) is positioned on the outer most radial portion of the seal faces (3) and the micropore indentations positioned on the outer most radial portion of the seal faces, the mean radius of the micropore section, reduces. This reduces the torque required to overcome the frictional ringing effect of the seal faces.

It has been shown from Figures-2 and 3 that additional torque is required to overcome the ringing force acting on a fully textured seal face. In such a design, the mean ringing force may be approximated to lie on the radius midway between the outside radius (Rmax) of the seal face and inside radius (Rmin) of the seal face. This is mean radius (Rmean) is shown in Figure 8a. Figure 8b shows the mean radius (Rmean), where the ringing force acts, on a seal face of the invention when the micropore surface (17) is on the radially most inner part of the seal face (3). Comparing Figures 8a and 8b, for the same seal face radial cross section, the mean radius is smaller in that of the invention (Figure 8b). Torque is derived from Force and Radius. If we assume the same amount of surface indentations in the two designs, the force will be constant. If the mean radius of the invention reduces, then the torque required to overcome the ringing effect in the invention will be proportionally smaller.

This reduction in torque, of the invention, is significant, and reduces the likelihood of damaging the seal face in equipment start up conditions.

As shown in Figure 5, if the process fluid is positioned on the radially outer part of the seal face, and the sealing dam is on the inner radial part of the seal face, fluid will more readily enter the surface indentations and thereby help to contribute to the hydrodynamic fluid film conditions.

It has been surprisingly found that the invention considerably reduces the initial torque required to slide or rotate one seal face against the other. The width of the sealing dam is also sufficient to provide acceptable static sealing.

The invention may be employed for at least one seal face, for both rotary seals and stationary seals, single, double or triple mechanical seals, whether designed in a cartridge or component seal format. It is also considered self evident that the invention may be used with metallic components as well as non-metallic components. Some types of equipment rotate the housing and have a stationary shaft. It is considered that the invention can be similarly applied to such designs.

Furthermore the invention may be employed for seal designs with hydraulically balanced seal faces as well as hydraulically unbalanced seal faces. Furthermore, for seal designs like double or triple designs, with more than one set of seal faces, the textured micropore portion of the seal face may differ between each set of faces. For example the micropore portion may be on the inner radial portion of at least one seal face in contact with the process media (4) and on the outer radial portion of at least one seal face in contact with the barrier media (20).

Claims

1. A mechanical seal to provide sealing between a rotatable shaft and a housing, the seal having a stationary part for connection to the housing and a rotary part for rotation with the shaft, mating sealing faces being carried by said stationary and rotary parts, said rotary parts being for mounting on the drive shaft for rotation therewith, at least one seal face having a textured portion and a non-textured portion, the non-textured portion acting as a sealing dam.

2. A mechanical seal according to claim 1 in which the edge of the sealing dam is positioned at or close to the seal face balance diameter.

3. A mechanical seal according to claim 2 in which the sealing dam is radially positioned on the lower pressure side of the seal face relative to the textured portion.

4. A mechanical seal according to claim 2 in which the sealing dam is radially positioned on the higher pressure side of the seal face relative to the textured portion.

5. A mechanical seal according to any of the preceding claims in which the sealing dam covers up to 99% of the seal face area.

6. A mechanical seal according to any of the preceding claims in which the textured surface covers up to 99% of the seal face area.

7. A mechanical seal according to any of the preceding claims in which the textured surface consists of micropores or indentations in the surface of the seal face.

8. A mechanical seal according to claim 7 in which the textured surface consists of micropores or indentations in the surface of the seal face which are evenly distributed in the area of the seal face adjacent to the sealing dam.

9. A mechanical seal according to claim 7 in which the textured surface consists of micropores or indentations in the surface of the seal face which are randomly distributed in the area of the seal face, adjacent to the sealing dam.

10. A mechanical seal according to any of the preceding claims in which two or more areas of a textured surface are separated by at least one area of a non-textured surface.

11. A mechanical seal according to any of the preceding claims in which two or more areas of a non-textured surface are separated by at least one area of textured surface.