Mechanical Seals

Mechanical Seal

Mechanical Seal

Mechanical seal concept

Called mechanical seal is means least 1 pairs perpendicular rotating axis end face in fluid pressure and compensation institutions elastic (or magnetic) role under and auxiliary sealed tie to maintain paste merger relative sliding consisting prevent fluid leak devices.
Elastic loading mechanism and the secondary seal is a metallic bellows mechanical seal which we call metal bellows seal.
In light seal there use rubber Bellows make auxiliary sealed, rubber bellows elastic limited generally need supplemented spring to meet load elastic.

Mechanical seal composition

Mainly following four categories parts.
·Main components: Activity ring and static ring.
·Auxiliary seals: seals (there O shaped, X shaped, U type, wedge, rectangular flexible graphite, PTFE coated rubber O ring etc.).
·Elastic compensation institutions: spring, push ring.
·Transmission pieces: shells Kei Block and key or various screw.

Mechanical seal is quite soft packing seal

Advantages:
·cope Canton mechanical seal can used Cold, heat, vacuum, high pressure, different speed and various corrosive medium and containing abrasive medium etc. sealing.
·Friction power consumption small mechanical seal friction power only soft packing seal 10% ~ 50%;
·Sealed reliable long cycle operations, sealing state very stable leakage small, press rough statistics, its leakage general only soft filler sealed 1 / 100;
·Long life in oil, water class medium general reach 1 ~ 2 years or more, in chemical medium usually can more than six months;

* Shaft or bushings are basically free from wear;
· Maintenance cycle long end face worn automatically compensation general, without recurrent maintenance;
· Vibration good on rotating axis vibration, partial pendulum and axis on sealed cavity skew insensitive;

* About the current number of factories “zero leakage” needs, packing not meet this requirement; not adapt to a wide range of arbitrary bigger, but in factories, often replacing or maintenance will plant great loss.

The following shortcomings:
· Disposable high investment.
·Structure was complex manufacturing processing demanding;
·Occurs contingency accident, handling more difficult;
·Installation and replacement troublesome and requested workers certain installation technical level;

Mechanical seal with cylindrical balance sleeve

Mechanical seal with cylindrical balance sleeve

A mechanical seal assembly comprising rotatable and stationary seal rings with faces opposing one another. Means are provided to insure a pressure balance on the outside surface and on the inside surface of the softer seal ring, so that this seal ring does not deflect and distort, and wear in an undesirable manner.

Mechanical seal assemblies usually comprise the combination of a rotatable seal ring connected to a rotatable shaft for rotation therewith, and a non-rotatable or stationary seal ring connected to the flange of a housing. Each seal ring has aradial seal face and the seal faces oppose one another. Whether or not the seal faces engage one another is debatable because there is usually a film of fluid therebetween providing lubrication and cooling for the relative rotation between the faces. In some mechanical seals leakage across the seal face is controlled. In many seal assemblies, one or more coil springs urge one of the rings toward the other, so that in reality, one or both of the seal rings are capable of limited axial movement, eventhough they are commonly referred to as “rotatable” or “stationary”. Many conventional mechanical seals can be used as single stage seals or in a multiple stage seal assembly.

In a common type of mechanical seal, one of the seal rings is constructed of a relatively brittle, soft material, such as carbon, whereas the opposing ring is constructed of a harder material, such as titanium carbide, silicon carbide, and thelike. In many of these seal assemblies, the carbon ring is “backed up” by a back-up ring constructed of a harder material, such as a stainless steel. The mating faces of the relatively brittle, soft seal ring and the back-up ring are lapped, so thatthe carbon ring becomes stuck or “married” to the back-up ring. Because of the difference in the modulus of elasticity between the two materials of the seal ring and the back-up ring, i.e., carbon with a modulus of about 2×106 to about4×106 and 18-8 stainless steel with a modulus of about 30×106, a compressive load on the mated rings will cause the carbon ring to shrink more diametrically than the back-up ring. The carbon ring, being married to the back-up ring,will shrink more at its seal or running face, so that this face becomes concave which seriously affects the sealing area of the distorted face, leading perhaps to failure of the seal. The compressive load is mainly due to excessive differentials of thepressures on the inside and outside surfaces of the married rings, which frequently exist in the before-enumerated pumps.

Zobens, U.S. Pat. No. 4,174,844, describes a mechanical seal for high pressure sealing applications having a carbon seal ring, supported on a rigid backing ring, in sliding contact with a seal ring of dissimilar material. A barrier is providedin overlying relation to the outer circumferential surface of the carbon ring to separate this surface from exposure to the pressure exerted by the sealed fluid. There is no communication between the inner and outer circumferential surfaces of thecarbon ring to equalize the pressures on these surfaces, nor is there any attempt to equate the axial pressures.

Martinson, U.S. Pat. No. 4,272,084, describes and claims a multi-stage mechanical seal assembly for pumps of the kind before enumerated. However, the problem of seal ring distortion is not discussed, no back-up ring of a high modulus ofelasticity axially abutting a seal ring of a lower modulus of elasticity is used.

Wiese, the applicant herein, in earlier U.S. Pat. No. 3,813,103, discloses a mechanical seal assembly in which the back-up ring has a marginal portion exposed in a pressure chamber in the seal housing, and the nonrotatable seal ring (backed upby the back-up ring) is ported to allow flow of fluid into the pressure chamber from between the sealing faces to reduce the distortion of the back-up ring and the stationary seal ring.

While the latter may be effective for some installations, it has not been found to be effective where the pressure differentials are as experienced in pumps of the type above described.

Wiese, in U.S. Pat. No. 4,114,900, teaches a mechanical seal in which a rotatable seal ring is provided with an internal radial, annular chamber exposed to low pressure fluid via a radial passage, such that high pressure on the seal ring at aseal face distorts the ring and causes it to be convex. The degree of convexity determines the leak rate across the seal faces. There is no attempt to eliminate distortion of one seal ring; distortion of the seal ring is actually caused by theconstruction.

The mechanical seal assembly of this invention comprises the combination of a rotatable seal ring and a stationary seal ring, the seal rings having seal faces which oppose one another and across which the flow of a fluid from a high pressure zoneto a low pressure zone along the rotatable shaft is substantially prevented. The rotatable seal ring, in the preferred embodiment, is made of a carbide material, and the stationary seal ring, or at least its seal face, is made of a softer material, suchas carbon. The stationary seal ring has a rear face mating with a face of back-up ring and the back-up ring is supported for limited axial movement on a cylindrical balance sleeve surrounding the shaft. The balance sleeve has a stepped outer surfaceand there is provided an annular surface exposed to high pressure fluid. The balance sleeve is received in a cylindrical cavity in a seal flange of a housing and is biased by fluid pressure on its annular surface to insure its seating in the cavity. The back-up ring and the stationary seal ring are resiliently urged toward the rotating seal ring by a plurality of coil springs.

The mechanical seal assembly of this invention is constructed in such a manner to substantially, if not totally, eliminate compressive loads on the outer periphery of the softer seal ring which causes distortions and deflections of this seal ringand leads to failure of the seal assembly. This is accomplished by encircling the softer seal ring and its back-up ring with a cylindrical member, and by providing one or more passages in the seal ring and back-up ring subassembly to insure equalizationof fluid pressures on their inner and outer surfaces. The fit of the cylindrical member around the seal ring and its back-up ring is such to permit fluid to exist therebetween and the outer surface of the cylindrical member is exposed to the highpressure fluid. Also, the seal ring and back-up ring subassembly is constructed so that axial fluid pressure on the back-up ring is sufficient to insure sealing of the mating faces of the soft seal ring and the back-up ring, and to limit thetransmissions of deflections of the back-up ring to the softer seal ring. Axial fluid pressures on the opposite sides of the stationary seal ring are substantially balanced, which insures little, if any, deflections of the softer seal ring which couldbe caused by axial pressure differentials.

Mechanical seal with embedded lubrication

Mechanical seal with embedded lubrication

Mechanical seal for providing a seal around the drive shaft of a variable displacement swash plate type compressor used in an air conditioning system for a vehicle comprising a rotating bearing surface, an associated stationary bearing surface and a lubricant embedded in the stationary bearing surface.

During operation of the compressor, the rotating bearing surface is caused to move against the stationary bearing surface causing lubricant to be released between the bearing surfaces, thereby providing efficient lubrication of the bearing surfaces.

The present invention relates to a mechanical seal, and more particularly to a mechanical seal disposed between two relatively moving bearing surfaces such as, for example, between a rotating bearing surface and an associated stationary bearing surface for providing a seal around the drive shaft of a variable displacement swash plate type compressor used in an air conditioning system for a vehicle.

A mechanical face seal is frequently used in an automotive cooling pump or refrigeration compressor. Generally, such seals include a stationary annular bearing surface integral with the compressor housing and an associated sealing ring disposed on a rotating drive shaft. A means are provided for urging the facing surfaces of the stationary bearing surface and the sealing ring together. The rotating surface of the sealing ring contacts the stationary bearing surface to form a sealing face which is perpendicular to the shaft. The stationary sealing surface is typically formed of cast iron, stainless steel, ceramic, hard chromium-plated steel or hardened bearing steel; and the associated rotating ring is formed of sintered carbon-graphite, resin-bonded carbon-graphite, resin impregnated carbon-graphite or ceramic.

Refrigeration compressors are used to compress refrigerants, such as carbon dioxide, as part of a standard vapor-compression refrigeration cycle. Typically, a gaseous refrigerant is mixed with a liquid lubricating medium, such as oil, before entering the compressor. The oil is employed to lubricate the compressor components, such as bearings and seals, to reduce component wear. Refrigeration compressors typically include a shaft rotatably supported by bearings within a compressor housing. Mechanical seals are typically employed in such refrigeration compressors to inhibit leakage of lubricating oil between the compressor housing and the shaft.

When a mechanical seal is mounted in a conventional variable displacement swash plate type compressor of an air-conditioner for a vehicle in which carbon dioxide refrigerant is used, the operational conditions of the mechanical seal become severe. The pressure within such a compressor is greater than within a compressor using a different refrigerant, resulting in a greater axial sealing force on the mechanical seal. Additionally, conventional lubricating oil is not soluble in carbon dioxide and therefore the lubricating oil cannot be efficiently distributed within the compressor. Such inefficient distribution of lubricating oil can cause the sealing face of the mechanical seal to receive an insufficient amount of lubricating oil. Insufficient lubrication will cause excessive friction in the sealing face, resulting in over-heating and failure of the mechanical seal.

The mechanical seals of the prior art rely primarily on the flow of oil mixed with refrigerant gas to effect proper lubrication. Therefore, ineffective lubrication of the sealing face occurs due the lack of consistent flow of refrigerant gas within the compressor.

It would be desirable to produce a mechanical seal wherein a constant supply of lubrication is released into the sealing face to result in improved lubrication of the mechanical seal.

Consistent and consonant with the present invention, a mechanical seal wherein a constant supply of lubrication is released into the sealing face to result in improved lubrication of the mechanical seal has surprisingly been discovered.

Mechanical Seals

Mechanical Seals

In general a mechanical seal is a piece of equipment which enables you to connect the systems or mechanisms collectively to stop the outflow or the leakage in a plumbing structure which contains pressure.

In the fluid business, the meaning is much narrower. By a mechanical seal we mean a device which is used to seal shafts of pumps, mixers or agitators.

The older technology, in many cases still used now, is mechanical packing. Mechanical packing is compressed in a stuffing box by a gland so it effectively fills the space between the shaft or stem and the casing of the equipment. Certain contact pressure on the shaft or stem is needed to effect the sealing function. We could call it a radial sealing device. While mechanical packing is indispensable to seal valve stems where the speed is very low, it is not as good to seal pump shafts due to a number of problems associated with shaft rotation:

Friction consumes power;
Frictional heat requires cooling by allowing certain leakage of the sealed product, which may be expensive or unsafe;
Friction leads to shaft sleeve wear;
Friction leads to mechanical packing wear which calls for frequent packing adjustments.

While waste of power is an issue in every industry, in many industries use of mechanical packing is prohibited where even small, controlled amount of leakage from a pump is not acceptable due to environmental or fire safety reasons.

In mechanical seals main friction happens in the axial direction, between faces (seal rings) lapped to a high precision. Wear is compensated in axial direction by springs or bellows. Many mechanical seals do not wear shaft at all. Contact pressure and thus the friction force is kept to a minimum by a balanced design. A balanced seal consumes much less power and it does not require product leakage to cool the seal faces. Actually when a mechanical seal leaks (visually seen leakage) usually it means that the seal needs to be repaired. Sometimes a perfectly normal mechanical seal leak. This is true if the sealed pressure is high. Good seals do not have visual leakage up to 50 bar and above.

The technology of mechanical seals is by no means new. It has been accepted in industry for decades. In most cases it has proven to be effective and reliable. The range of designs and materials extends from home heating circulating pumps to 380C hot hydrocarbons pumps at oil refineries or to 100 bar crude oil transfer pumps, costing from 10 dollars to 20,000 dollars and even more, depending on the industry and country.

Construction:

Mechanical seals consist of the following components:

Seal rings or faces, rotating and stationary;
Secondary seals such as o-rings, PTFE V-Rings or wedges, rubber boots, PTFE or flexible graphite gaskets;
Spring action elements such as springs, metal bellows;
Drive elements such as pins, set screws, etc.;
Metal parts such as shaft sleeves, glands, collars, set plates or clips, holders, etc.
Additional elements such as safety bushings which, if a seal malfunctions, will direct leakage to a specialized drain.

Mechanical Seals can be classified by two ways.

1: Classification on the basis of Arrangement

Single – inside and outside;

Multiple (usually, dual) – tandem or double. Such a seal will require a seal support system to supply fluid or gas between the inboard and outboard faces. If the pressure of the supplied fluid is lower than the pressure in the seal chamber, it is called buffer fluid or gas, and the seal is called a tandem mechanical seal. And if the pressure of the supplied fluid is higher than the pressure in the seal chamber, it is called barrier fluid or gas, and the seal is called a double mechanical seal.

2: Classification on the basis of Design

Balanced and Unbalanced. Unbalanced seals are the cheapest and used only for low linear speed and pressure;

Pusher type – O-Ring, V-Ring and Wedge Ring, single spring or multiple spring;

Non-Pusher type such as a metal bellows seal.

There are competent companies that provide mechanical seals for process pumps with high dependability and good value for money. Some mechanical seal manufacturers are strong in providing seals in huge quantities for automotive industry or home heating at very low cost, others are best in pulp and paper industry, third ones are very strong in making mechanical seals for the oil and gas industry.

Seals for each industry will have certain commonalities driven from field experience. Some industries may even accept a standard which will specify minimum requirements for seal design, testing and documentation. The API 682 Standard for making seals for petroleum industry is a good example. End-users specifying such standards in their purchasing requirements greatly benefit from this as they are protected from low quality or inexperienced manufacturers.

Mechanical Seals Introduction

Mechanical Seals Introduction

Mechanical seal is being used increasingly on fluid pumps to replace packed glands and lip seals.Pumps with mechanical seals perform more efficiently and generally perform more reliably for extended periods of time.

Mechanical seals are provided to prevent pumped fluids from leaking out along the drive shafts.The controlled leakage path is between two flat surfaces associated with the rotating shaft and the housing respectively.The leakage path gap varies as the faces are subject to varying external loads which tend the move the faces relative to each other.

The mechanical seal requires a different shaft housing design arrangement compared to that for the other type of seals because the seal is a more complicated arrangement and the mechanical seal does not provide any support to the shaft.

In order for the mechanical seal to perform over an extended time period with low frictional the faces are generally hydrodynamically lubricated.The fluid film will need to carry substantial load.If the load becomes to high for the film surface contact will take place with consequent bearing failure. This lubricating film is generally of the order of 3 micrometres thick , or less. This thickness is critical to the required sealing function.Mechanical seals often have one face of a suitable solid lubricant such that the seal can still operate for a period without the fluid film.

Pressure Balance Mechanical Seals
It is possible to reduce the seal contact pressure by using a pressure balanced seal design of off-set a proportion of the force generated by the pumped fluid pressure.This principle is illustrated in the sketch below.

Design Features
The mechanical seal generally includes a three static seals.

The sleeve seal – this is usually an O-Ring
The seal between the moving seal member and the shaft or sleeve.- This is often an o-ring but can be a wedge or vee seal. This seal may not be used for bellows type mechanical seals
The housing seal is generally an o-ring of a gasket.
All of these seal must be compatible with the fluid being contained and the associated environment.These seals may limit the design for high temperature applications. In this case the bellows type alternative may be the best option.

The sealing faces are generally pressed together using some form of spring loading. Several different spring loading systems are available.

Single spring
Multiple springs distributed around seal body
Disc Springs
Disc Springs
Bellows
Magnetic
For conventional mechanical seals the single spring arrangements is used.The other spring arrangements are used in the space is restricted.

It is vitally important that the sealing surfaces perfectly flat and are parallel.

The seal faces are usually dissimilar materials with the softer face being the narrower surface. For abrasive applications similar hard materials are used e.g tungsten carbide. The seal surfaces must have sufficient strength to withstand the hydrostatic fluid forces and must be able to remove the heat generated by sliding action.Carbon is often used against bronze, cast iron, stainless steel etc.

The seal surface must be flat, smooth and square to the shaft.  Both surfaces a normally lapped to a high quality finish. The harder surface is most important because the softer surface is designed to run-in over the initial operating period.

The shaft design is critical. It must be rigid enough to support the seal in the correct position and the shaft surface finish must be suitable to ensure good sealing on the static seals (0.4 micrometers CLA or better). The shaft Total Indicated Runout (TIR) should not exceed 0.125mm.There should be minimum shaft vibration.The shaft may be subject to fretting corrosion as a result of micro-movements of the seal and is is often desireable to have locally hardened surfaces or to use sleeves.

Assembly Options
There are a number of mechanical seal options

External Seal.. This design is installed on the outside of the stuffing box with the sealed pressure inside. This provides good access allowing the seal components to be be cleaned.
Internal Seal.. Generally mechanical seals are mounted inside the stuffing box with the sealed pressure outside the seal.
Double Seals.. Mechanical seals mounted in pairs are used for sealing hazardous, toxic or abrasiv fluids and are often provided with clean flushing fluid between the seals.Double seals also provide an additional degree of safety were the pressure differentials are likely to reverse and/or there is a high risk of the sealing failing.  There are a number of double seal assembly options as listed below
In Series – Used primarily to overcome the risk of failure of a single seal.

Face to Face – Used when a cooling fluid interface is required . One seal is used for the process fluid the other seal is used for the coolant.

Back to Back – Used when an abrasive fluid is being contained and both seals are flushed with a clean buffer fluid.The flushing fluid is introduced at a higher pressure the process fluid

The are a large number of variant mechanical seals e.g split seals.Improved systems are constantly being introduced onto the market

Additional Equipment
The use of mechanical seal generally involve the use of additional equipment primarily for the flushing /coolant systems.This includes pumps, coolers, strainers, filters etc.