Mechanical seal for steam. Double mechanical seal. Pump covers and chambers
In modern petrochemical, oil refining and other industries, in technological and production processes, liquids and gases are often used, the leakage of which, as a result of a malfunction or in an accident, can pose a serious danger to both the environment and human life.
For these reasons, when discussing issues related to the mechanical seal of the pump shaft, they imply the development of a reliable sealing complex.
About 20 years ago, there was a sharp decline in the use of asbestos materials, most of which were sealed. This was due to the fact that the directive on the elimination of asbestos from the economy as a material was extremely harmful to health. In addition, the increased interest in mechanical seals makes traditional soft seals less applicable. The aim of the work was to find the right combination of wear-resistant materials that meet certain temperature and chemical parameters and have a long service life.
In addition, the expected cost of manufacturing soft sealants from these materials was the same as for asbestos-containing materials. In just a few years of work to refine these materials, new sealing products have completely replaced asbestos.
Mechanical seals for pumps
The sealing complex of pumps is a complex system, which consists of two main components: a support system and a sealing unit of modular type.
If at least one of the component of the mechanical seal of the shaft is unreliable, then there can be no question of the tight and safe operation of the rotating shaft.
Comparison of gaskets and mechanical seals. General principles of pump sealing. Mechanical seals are usually considered “empty and forgotten” prior to periodic inspection of the device. The use of packing glands requires periodic adjustment to track wear and leakage of the sealed medium above an acceptable value, usually determined by the rate of fall.
Correctly controlled stuffing box equipment requires properly trained personnel. Typically, adjustments are made while the unit is operating, and the package itself can be easily compressed, while losing fluid lubrication. Compliance and familiarity with its characteristics should significantly reduce the possibility of improper maintenance of the device. The bag should be replaced when tightening the gland does not reduce leakage to the desired level. At this stage, as a rule, it is still possible to work with a slight excessive leak, until there is a good time to immobilize the device for replacement.
In turn, the modular-type sealing unit consists of the main and auxiliary mechanical seals. By the way, there can be several auxiliary sealants at once.
Basic provides the task of sealing the working environment.
In cases where a double version is used, the main one is the internal one, which is in contact with the working environment. The most commonly used two single in tandem scheme - it is economically justified and recommended by the requirements of API 682.
Auxiliary, this seal is a contact or gap type, which are selected based on their purpose.
In the case of installation and installation, it can be a longer and more complex task requiring proper installation conditions and fine adjustment of the pump. In addition, in the case of mechanical seals, it is rarely possible to observe warning signals that need to be replaced. A sharp failure can lead to significant leakage of the sealed medium, damage to the pump, shutdown of the entire installation and subsequent expensive repairs. Periodic maintenance usually involves the complete dismantling of the seal to replace the front rings and elastomeric components such as o-rings or bellows.
The support system serves to create optimal operating conditions for the system and includes:
- means of control;
- heat exchanger;
- piping system piping.
Automation and control tools control the temperature, pressure, level of the barrier fluid in the system.
The heat exchanger performs the function of cooling a shutter or buffer fluid, which removes frictional heat from friction pairs.
Packing is generally more economical than mechanical seals operating under the same conditions. Especially in high-performance industrial pumps, where the cost difference can reach 10 times in favor of traditional sealant. For small sizes - less than 25 mm, the cost of mechanical seals is usually lower, and, in addition, it ensures trouble-free operation in the initial period of operation.
In the case of high working pressures or high speeds of rotation, mechanical seals have a definite advantage over packaging materials. They can work almost without wear, but wearing soft sealant under the same conditions can be large. In a broad sense, wear is proportional to the product of speed and pressure.
The piping system of the piping is responsible for the circulation of cooling water and sealant.
The basis for the development of the most effective and safe mechanical seal for pumps are the provisions of GOST 26-06-2028-96 "Pumps for general industrial use. Safety requirements".
This standard applies to centrifugal pumps that are installed in fire hazardous and explosive areas. However, many decades have passed since the development of this GOST; therefore, it requires refinement and some expansion in order to guarantee the protection of the environment and human health.
If we talk about seals for pumps, then terms such as “systems for providing a sealing assembly” and “sealing complex” have appeared.
Chemically aggressive or abrasive media can be sealed with both methods, as well as with food and drinking water. The operating temperature range of gaskets and mechanical seals is almost entirely dependent on the materials used. With the exception of the construction of steel bellows, the limiting factor in mechanical seals is usually the strength of their elastomeric or polymer components - usually o-rings and bellows.
Modern printed stuffing is made of synthetic and natural materials. At such low temperatures, mechanical seals can only work effectively when their springs are made from expensive special alloys. The main technical parameters of the main types of packaging.
Stages of developing mechanical seal for pumps
- Learning the conditions of use is a crucial step. Knowledge of international and Russian standards, the study of specific operating conditions, analysis of the application of previous designs, the study of the specifics of a particular industry - all this is the basis for development.
- The choice of the type of product, where the main criteria are the parameters and properties of the pumped liquid: pressure and temperature in the chamber.
The mechanical seal can be divided into three main blocks:
- with pinch rotating unit;
- with metal rotating bellows;
- with metal fixed bellows.
The first type is used in pumps that pump liquids with temperatures up to 200 degrees. This design is the most common and most often used in world practice.
The second and third types are used in pumps that pump liquids with temperatures ranging from 150 to 400 degrees.
- The layout of the sealing assembly.
There are three main types of layout of the sealing assembly:
- Layout 1 is a seal along with an auxiliary seal. It is an internal single seal of a modular type with a rotating pinch assembly.
- Layout 2 is a double seal without pressure buffer fluid. It is a unit of two single units, which can be used as internal sequential seals with two mating rings of a friction pair and two compressive rotating units.
- Layout 3 is a double mechanical seal with hydraulic unloading (double) under pressure of the barrier fluid.
The choice of a particular arrangement may depend on various factors - the pressure of the pumped liquid, its temperature, the requirements of safety standards, the presence of solid inclusions in the working environment, and so on.
Face seals for pumps
We suggest you familiarize yourself with the various brands of products, their purpose and the scope of their possible use. To go to the page with a detailed description of each brand, click on its name in the table below. You can buy a mechanical seal of any brand in our "Sealant" company.
These impellers are attached to the bearing seats with additional epoxy paint in the bearing seats. Teeth with high tooth quality and shaft alignment were applied. The chamber separating the working part of the pump and the bearing housing allows the use of a mechanical seal, which can work in both directions.
The above characteristics show the fields of operation of various pumps and allow you to pre-select the size of the pump. The reliability of the drive belt depends on the continuous operation of the equipment and industrial equipment. So what can be the damage to the belts and can they be prevented? Are you using and supporting the tape correctly to increase its durability?
The mechanical seal is a device that forms a rotating seal between the moving and stationary parts. They were designed to eliminate the disadvantages of stuffing box packing. Leakage can be reduced to the level of compliance with environmental standards of state regulatory authorities and maintenance and repair costs can also be reduced.
Pump users estimate that 80% pump failure caused seal damage. Consequently, the desire of designers to limit or distribute seals where possible. The best seal is the lack of sealing. Taking into account the possibility of failures, it is worth paying attention to structural solutions that eliminate the source of such problems, for example, in the case of centrifugal magnetic pumps. Their principle is to transfer the drive from the engine to the pump section using a magnetic coupling.
There is no mechanical connection between the pump section and the motor, which eliminates the need to seal the axle, and this is a source of leakage in conventional pumps. Magnetic clutch pumps are well suited for continuous operation; they have a sealed design that can seal hazardous and crystalline liquids in contact with air. Limiting the potential for failure is a significant factor here.
Advantages of the mechanical seal compared to conventional stuffing box packing:
- No or limited leakage of pumped fluid.
- Reduced friction and loss of pump power.
- Elimination of shaft or wear sleeve.
- Reduce maintenance costs.
- Possibility of use at higher pressures and more aggressive environments.
- A wide variety of designs allows the use of mechanical seals in almost all pumps.
Figure 1. Single mechanical seal
However, we cannot always use such solutions. When buying a pump beyond all technical aspects, attention should be paid to the manufacturing instructions, especially to the design of the seal. When it comes to efficient replacement, it’s important to have elements such as gasket seals and ease of interchangeability with other types of seals.
There are also solutions that help determine seal wear. In centrifugal pumps - one of the most commonly used in practice - rings are used for radial shaft seals. A typical construction consists of a steel sheet or an elastomeric housing connected to a sealing lip. It should always indicate the environment that needs to be stopped. Depending on the needs, rubber lips or other special materials that are resistant to aggressive chemicals are used.
How the mechanical seal works.
The primary sealing seal is achieved with two very flat polished surfaces that create a difficult path perpendicular to the leak path (prevent leakage). The rubbing contact between these two flat surfaces minimizes leakage. As with all seals, one surface is fixedly mounted in the housing, and the other is fixed and rotates with the shaft. One of the surfaces is usually made of wear-resistant material such as carbon graphite. The other, as a rule, is made of a relatively resistant material of solid material like silicon carbide. Heterogeneous materials are typically used for stationary gaskets and rotating O-rings to prevent these two surfaces from sticking. The soft side usually has less mating surfaces and is usually called an abrasive edge.
If the pump uses radial rings to maintain pressure, the lips should have a profile that is resistant to deformation due to the pressure applied by the pressure. Pay attention to the rotation speed. The special shape of the lips is especially important at relatively low shaft speeds. This provides the best clamp for the shaft. Where the speed is higher, the pressure difference between the sealing surfaces is less, and the same increase in the shaft rotation speed more accurately pushes the seal.
A larger shaft clamp is at the same time more friction and a shorter shaft and shaft. Therefore, the parameters of pressure and shaft rotation speed should be selected in such a way as to ensure maximum performance and durability. It characterizes the maximum possible use of the material at a given load and speed without taking into account friction wear. The most important selection of seals for this type of pump is the selection of radial o-rings so as not to exceed the maximum shaft speed specified by the manufacturer.
There are four main seal points at the end of the mechanical seal (Figure 2). The primary seal is the end face of the seal, point A. The creepage path to point B is blocked by either O-rings, a V-ring, or a wedge. Creepage distances to points C and D are blocked by gaskets or O-rings.
There are also factors such as sealing torque, energy consumption, temperature, and dynamic rhythm. Only by taking these factors into account will you achieve optimum durability for this type of seal. It should be noted that standard radial shaft sealing rings are designed for a specific pressure value. It is also important to finish the shaft surface. For labyrinth seals, surface cleanliness does not matter.
Their principle is to create a dynamic inner track that removes contaminants such as liquids and solids and maintains lubrication. They can be made of metal or plastic if necessary for resistance to chemicals. When installed correctly, they will also provide a long service life. In addition to these values, it should be noted whether they are special embodiments that allow such use of labyrinth seals. Usually these are special designs.
Surfaces in typical mechanical seals are lubricated with a boundary layer of gas or liquid. When developing seals with the desired leakage parameters, seal life, energy consumption, the designer must consider how the surfaces will be lubricated and the principle of their lubrication.
To select the best seal design, you must have as much information as possible about the operating conditions and the fluid handled. Complete product and environmental information allows you to choose the best seal for your application.
Remember that each sealing material has a specific temperature range in which it can operate. In the case of nitrile rubber, the aging process also affects the aging process, changing its properties. Mechanical seals as precision equipment require a different approach than the sealing approach for repair and installation work. Other problems are already encountered in pre-assembled and pre-assembled pumps, compressors, mixers or reactors, as well as when problems arise when processing from the seal system to mechanical seals.
Figure 2. Mechanical seal points
Mechanical seal designs
Single internal
This is the most common type of mechanical seal. These seals are easily modified to provide a buffer flushing system for seals and can be balanced to withstand high pressure media. It is recommended for non-aggressive and aggressive fluids with satisfactory lubricating properties when the cost does not exceed the cost of a double mechanical seal.
According to Latala, the most common errors are made either by their choice for a working device, or by installing them - few people read the manufacturer's instructions, and also launches and exploits users. The technical condition of the entire device has a decisive influence on its further proper operation. All the time, new technical innovations are introduced to extend the service life and reliability of both the sliding pair and the materials used and their structural features. In many companies, the principle of cheap product purchase and, therefore, greater unreliability, Latala emphasizes.
Single external
If a very aggressive fluid has good lubricating properties, an external seal is an economical alternative to the expensive metals necessary to provide corrosion resistance in internal seals. The disadvantage is the exposure to shock and hydraulic pressures, therefore, seals have low pressure limits (balanced and unbalanced).
The seals produced are single, double, compact, etc. The price of which increases depending on their reliability. Since seals are an important part of pump reliability, unpainted products from unspecified materials should generally be avoided. This can be dangerous even if you can interact with cleaning products used to flush the line. The unused type of elastomer is also a lack of information about what temperatures can work and what is its compatibility with chemicals, shelf life, sensitivity to steam cleaning, or the reaction to certain lubricants used when installing the seal.
Double (double under pressure)
This design is recommended for liquids that are not compatible with a single seal (i.e. liquids that are toxic, hazardous (environmental protection), containing abrasive or caustic substances requiring expensive metals). The advantages of double seals are that they have a five times longer service life than single seals in harsh conditions. In addition, the metal inside the seal is never exposed to pumped fluids, viscous, abrasive and thermosetting fluids are easily sealed without the need for high costs. In addition, recent tests have shown that the service life of a double mechanical seal practically does not change with a change in the technological parameters of the pump. This is a significant advantage in favor of a double mechanical seal.
The final decision when choosing between double or single seals is determined by the cost of the seal, the cost of operation, as well as the norms of industrial emissions into the environment during leakage of the mechanical seal.
Double gas barrier (under double pressure)
Very similar to a double seal cartridge, an inert turf seal, such as nitrogen, is used as a surface lubricant and coolant instead of a coolant system or a jet of liquid, like conventional or double seal cartridges. This concept has been developed since many barrier fluids are commonly used in double mechanical seals and can no longer be used according to new emission standards. As a gas barrier, seals use nitrogen or air as harmless and inexpensive, which helps prevent the release of products into the atmosphere and fully complies with emission standards. Double gas barrier seals must be used when pumping toxic or hazardous fluids that are regulated or in situations where increased reliability is required.
Tandem (double without pressure)
In accordance with health, safety and environmental standards, tandem seals are used to pump products such as vinyl chloride, carbon monoxide, light hydrocarbons, as well as a wide range of other volatile, toxic, carcinogenic or hazardous liquids.
Tandem seals prevent icing of light hydrocarbons and other liquids, the temperature of which can drop below the atmospheric freezing point of water in the air (32F or 0C). (Typical buffering fluids are ethylene glycol, methanol and propanol). Tandem seals also increase reliability. If a conventional seal fails, the outer seal may take over the maintenance function.
Mechanical seal selection
The right choice of mechanical seal can only be made when there is complete information on operating conditions.
- Liquid
- Pressure
- Temperature
- Fluid characteristics
- Reliability and emission standards
1. Liquid
First of all, the working fluid must be determined. Metal parts must be corrosion resistant, usually steel, bronze, stainless steel or Hastelloy. The mating surfaces must also resist corrosion and wear. For example carbon, ceramic, silicon carbide or tungsten carbide. Buna, EPR, Viton and Teflon stationary seal elements are the most commonly used.
2. Pressure
The actual type of seal, balanced or unbalanced, is based on the seal pressure and seal dimensions.
3. Temperature.
In particular, it determines the use of seal parts.
Materials should be selected according to fluid temperature.
4. Fluid characteristics
Abrasive fluids cause excessive wear and shorten seal life. Double seals or flushing system from
external sources allow the use of mechanical seals on these
difficult liquids. When pumping light hydrocarbons, balanced seals are often used to increase seal life, although pressure is low.
5. Reliability and emission standards
The type of seal chosen and its design must meet the desired reliability and emission standards. Double seal and double gas barrier seal are preferred.
Large diameter seal chambers
Developed in the mid-80s, an enlarged seal chamber with an increased radial clearance between the mechanical seal and the wall of the seal chamber provides better fluid circulation. Improved lubrication and heat dissipation from the seal (cooling) extends seal life and reduces operating costs.
Enlarged compaction chamber
Improves fluid circulation without using external flushing. It provides benefits such as reduced operating costs, no need to use a strapping system, reduced energy costs (associated with the seal washing system) and increases the reliability of the seal. The conical bores of the seal chamber are usually compatible with ANSI chemical pumps. API pumps use conventional oversized seal chambers. The pulp transfer pumps use both conventional enlarged compaction chambers and compaction chambers with large conical openings. Only conical seal chambers with flow modifiers provide reliable operation with or without particulate matter, air and steam.
Conical seal chamber with conical bores
The mechanical seal is broken when solid particles or vapors are present in the fluid. A conventional conical seal chamber is used to extend the seal life when pumping liquids containing solid particles or vapors. Seals in such a working medium fail prematurely due to the content of solid particles or vapors. As a result, severe erosion of the seal and pump parts, damage to the seal surfaces and dry running.
Modified seal chamber with axial ribs:
It is good at pumping liquids containing air, and small solid particles.
This type of seal chamber provides longer seal life when air or vapor is present in the fluid. Axial ribs prevent vapor trapping by improving flow circulation in the seal chamber. The likelihood of dry running is excluded. In addition, solids of less than 1% are also not a problem.
A new flow regime arises on the path of solids / liquids. Pumping liquids with a significant solids content (more than 1%) may cause the sticking of the seal spring or bellows, the ingress of solid particles on the seal surface and the final breakdown of the seal.
Environmental Monitoring
Environmental monitoring is essential for reliable operation of the mechanical seal. The seal manufacturers offer various measures to deal with the following problems:
- Corrosion
- Temperature control
Figure 3 and 4
Corrosion
Corrosion can be avoided by choosing the corrosion-resistant materials of the mechanical seal. When this is difficult, it is possible to perform external injection into the formation of liquid or gas from stainless materials. Single and double seals can be used when the customer is mistaken about the properties of the pumped medium.
Temperature control
When the seal rotates, the friction surfaces are in contact. Heat is generated in such images, and if this heat is not removed, the temperature in the seal chamber may increase and cause the seal to fail. A simple bypass tube removes the heat produced by the contact of the friction surfaces. (figure 3). For higher temperatures, the bypass pipe must pass through the cooler (Figure 4). An external fluid supply can also be used.
Dirty or incompatible media
Mechanical seals generally do not work well with liquids that contain solids or crystallize upon contact with the atmosphere. In this case, passing the bypass tube through a filter, cyclone separator or filter provides cleaning of the fluid to lubricate the friction surfaces.
Filters are effective for particles 40 mesh larger than the filter opening.
Zilcon separators are effective for solids with a diameter of 10 microns or more, if they have a specific gravity of 2.7 and the pump provides a pressure drop of 30-40 atmospheres. Filters trap particles from 2 microns or more.
If external flushing with clean liquid is available, then this is the most trouble-free system. The lip seal or throttle have the ability to control the flow of injected fluid up to 1/8 GPM. The cooling type of seal is used when working with liquids that tend to crystallize on contact with air. Water or steam flowed in this way will solve this problem. Other systems are supplied in accordance with service requirements.
Maximum sealing flexibility - Dynamic sealing.
To eliminate mechanical seal problems and reduce repair and maintenance costs.
Dynamic seals are designed to work with harsh environments where conventional mechanical seals or stuffing boxes require constant expensive maintenance. The main advantage is that, thanks to the patented design, an external water seal is not required, thereby eliminating leaks, contamination of the pumped medium, dilution of the product and problems associated with routine monitoring of the seals
