A mechanical seal is a vital device employed to effectively prevent the leakage of liquid from the inside casing to the outside, specifically by creating a seal between the static and rotating components. These seals find extensive usage in centrifugal pumps and various other equipment. The classification of mechanical seals is based on their design, operation, and arrangement.
Among the components comprising a mechanical seal, the seal face holds utmost significance. This face material is typically the most costly part of the seal as it facilitates the formation of a seal between the static and rotating parts of the equipment. The following is a description of different types of mechanical seals:
Types of seal
- Type 1: This type of mechanical seal consists of two flat seal faces perpendicular to the shaft. It is widely used in pumps handling non-abrasive fluids.
- Type 2: With a rotating face, type 2 mechanical seals possess a single spring and are employed in pumps handling moderate to high-pressure applications.
- Type 3: These seals comprise two independent seals with a barrier fluid between them. They are commonly used in applications involving hazardous or toxic fluids.
- Type 4: Type 4 mechanical seals are designed for high-temperature and high-pressure applications, featuring multiple springs and seal faces.
- Type 5: This seal type utilizes an O-ring as the secondary seal and is commonly found in pumps handling low-pressure applications.
- Cartridge Seal: A cartridge seal is a pre-assembled unit containing all the necessary components, making it easy to install and maintain.
- Split Seal: Split seals are designed for equipment with limited access, as they can be installed without dismantling the entire assembly.
- Balanced Seal: A balanced mechanical seal incorporates a mechanism to counterbalance the hydraulic forces acting on the seal faces, thereby enhancing its performance.
- Unbalanced Seal: Unlike the balanced seal, an unbalanced seal does not possess a balancing mechanism and is typically used for low-pressure applications.
These are some of the various types of mechanical seals available, each serving different purposes based on the specific requirements of the equipment and the nature of the fluid being handled.
A mechanical seal is a dynamic type of seal designed to prevent leakage between the static and rotating parts of equipment that rotate in relation to each other. This type of seal finds applications in various equipment such as pumps, blowers, compressors, turbines, valves, and more.
There are several types of dynamic seals available:
- Gland Packing: This type of seal involves using packing material, such as braided or molded fibers, which are compressed within a gland to create a seal.
- Mechanical Seal: Mechanical seals consist of two flat faces, one stationary and the other rotating, pressed together to create a seal. They often utilize a combination of elastomers, metal components, and springs for effective sealing.
- Labyrinth Seal: Labyrinth seals use a series of interlocking ridges and grooves to create a tortuous path for the fluid, preventing leakage through the seal.
- Oil Seal: Oil seals, also known as rotary shaft seals, are used to prevent oil leakage from rotary shafts. They typically consist of a flexible lip that makes contact with the shaft, creating a barrier against fluid leakage.
- Water Seal: Water seals are specifically designed to prevent water leakage in applications such as water pumps or underwater equipment. They often utilize similar principles as mechanical seals.
- Lip Seal: Lip seals are radial shaft seals that consist of a flexible lip that makes contact with the shaft surface, creating a barrier against fluid leakage.
- Hydrodynamic Seal: Hydrodynamic seals use the fluid pressure generated by the rotating shaft to maintain a sealing effect. They rely on the hydrodynamic force to create a barrier against leakage.
These different types of dynamic seals offer varying levels of sealing performance and are selected based on the specific requirements of the equipment and operating conditions.
Classification of mechanical seal
- Inside mounted single seal
- Outside mounted single seal
Inside Mounted Single Seal:
The inside mounted single seal, as the name suggests, is installed inside the pump or equipment housing. It operates by maintaining a seal between the rotating shaft and the stationary housing from within the system. This type of seal is commonly used when the pumped fluid poses challenges such as high temperature, hazardous nature, or corrosive properties. By positioning the seal inside the equipment, it benefits from the cooling effect of the pumped fluid, which aids in dissipating heat generated during operation. Additionally, an inside mounted single seal provides increased protection against contamination from external factors, such as dirt or environmental debris.
Outside Mounted Single Seal:
In contrast, the outside mounted single seal is installed externally to the equipment housing. It forms a barrier between the rotating shaft and the stationary housing, acting as a mechanical barrier to prevent leakage. This type of seal is typically used when the pumped fluid is clean and poses no significant challenges in terms of temperature, chemical compatibility, or other factors. The outside mounted single seal is relatively easier to install and maintain since it can be accessed without the need to dismantle the equipment. It also offers the advantage of visual inspection, enabling quick detection of any potential leaks or performance issues.
Double mechanical seal
A double mechanical seal, also known as a dual mechanical seal, consists of two separate seals designed to provide enhanced reliability and leakage prevention in mechanical systems. When the primary seal fails, the secondary seal serves as a backup to maintain the integrity of the system. In some cases, an O-ring is also utilized as a secondary seal.
There are two common types of double mechanical seals: the dual pressurized seal and the pressurized seal.
In a dual pressurized seal configuration, both seals are subjected to pressurization, which helps prevent the entry of external contaminants and minimizes the risk of leakage. This arrangement ensures greater reliability and increased safety.
Alternatively, a pressurized seal setup utilizes pressurization only on the primary seal, while the secondary seal functions without external pressure. This arrangement is suitable for applications where the risk of leakage is lower, but still requires the backup of a secondary seal.
Furthermore, there are two main arrangements for double mechanical seals: back-to-back and face-to-face.
- In a back-to-back arrangement, the two seals are positioned in opposite directions, facing away from each other. This configuration offers improved resistance to fluid leakage and provides effective containment of hazardous substances.
- On the other hand, a face-to-face arrangement involves positioning the two seals in direct contact with each other. This arrangement is commonly used when the process fluid is abrasive or contains solid particles, as the face-to-face design reduces the likelihood of debris buildup between the seals.
- Lastly, a tandem arrangement refers to a configuration where one seal is installed behind the other in a series, with the fluid passing through both seals consecutively. This arrangement is often employed when the operating conditions demand a higher level of sealing reliability.
Dual pressurized mechanical seal
Dual non pressurized mechanical seal
The dual seal arrangement required an external seal flushing plan. Its pressure approx around the atmospheric pressure of the liquid, the face lubrication is also done by the external sources of liquid. Unpressurized seals are used for low pressure, hazardous liquids.
- Inappropriate NPSHA
- Cavitation by air aspirations
- On very expensive services
- For explosive services
- With toxic and vaporized liquid
- For volatile liquids that have a tendency to vaporize, convert the gaseous form
Balance and unbalance
A mechanical seal can be classified as either a balanced seal or an unbalanced seal based on the ratio between the closing area and the opening area of the seal.
- A balanced seal is characterized by a closing area to opening area ratio that is less than 1. This type of seal requires less closing forces and is beneficial in reducing power consumption. It is typically utilized for pressure levels up to 3000 PSI and is commonly employed in applications involving volatile liquids.
- On the other hand, an unbalanced seal has a closing area to opening area ratio greater than 1. Such a seal necessitates high closing forces and exhibits a very low leakage rate. Unbalanced seals are suitable for services with pressure up to 200 PSI but are not recommended for use with volatile liquids.
Cartridge seal and split seal
A mechanical seal cartridge is a fully assembled unit consisting of seals, flanges, and other components. It offers easy installation without the need for measurements during the installation process. The spring load is fixed during assembly and has already been tested by technicians using air and equipment. However, it is important to note that cartridge seals tend to be more expensive compared to other types of seals.
Another type of mechanical seal is the split seal, which is typically axially split into two parts. The primary advantage of a split seal is its ease of installation, requiring less time and reducing downtime. However, similar to cartridge seals, split seals also tend to be more costly than other seal options.
What factors consider selection of mechanical seal applications?
Factors for the Selection of Mechanical Seals Considerations:
- Temperature and Pressure: The mechanical seal’s selection should take into account the temperature and pressure conditions of the service liquid inside the casing. The seal material and design should be capable of withstanding the specific operating conditions.
- RPM (Rotations Per Minute): The rotational speed of the driver and associated equipment, as well as any potential misalignment, should be considered. The mechanical seal should be designed to operate effectively within the specified RPM range and accommodate any misalignment to ensure optimal performance.
- Tolerance Runs Out: The tolerance runs out of the shafts within the seal chambers must be considered. Proper alignment and tolerance specifications should be adhered to in order to prevent excessive wear and ensure a reliable seal.
- Clearance: The radial and axial clearance in the seal chambers should be evaluated. Sufficient clearance should be provided to accommodate the mechanical seal without causing interference or excessive friction.
- Flushing and Cooling Plan: The selection process should also consider the need for flushing and cooling. Depending on the application, a proper flushing and cooling plan may be required to maintain the desired operating temperature and prevent the accumulation of contaminants or heat-related issues.
- Barrier and Buffer Fluid: The purpose of the mechanical seal, whether it’s to provide a barrier or buffer, should be considered. The type of fluid required for sealing, such as barrier fluid or buffer fluid, should be determined based on the specific application and the characteristics of the process fluid.
- Availability of Power: The availability of power for driving the mechanical seal, such as electricity or other power sources, should be assessed. This includes ensuring that the necessary power supply is accessible and compatible with the seal’s requirements.
What are the factor requirements for smooth operation of the mechanical seal?
- Service fluid at the pump.
- Seal leakage rate should be minimum
- The fluid is in the pump seal area appropriate with seal face materials.
- Effective force in mechanical seal
- liquid lubricant for mechanical seal face
- Internal seal face material compatibility with service liquid.
- Power consumption and power losses
What is the purpose of providing seal plans?
The API standard used is 682 for the pump. There are many types of seal plans used, and dry gas seals used in centrifugal compressors. There are many components of mechanical seal like springs, O rings, Rotary seal’s face, static seal’s face, retainer rings, seal’s face material, below, spring.
What should be the gap between seal face?
The gap between seal faces is very small. When it is running the hydrodynamic gap is created and that gap is approx around 3 to 5 microns. There are two types of material used for seal face. For seal face, one is a soft material used like carbon, and Teflon, another one is a hard material that is tungsten carbide, silicon carbide, PTFE and
Where was the mechanical seal used?
The life of a mechanical seal is very long as compared to gland packing its maintenance time is much less. But it is highly expensive and very sensitive, because of the use of mechanical seals to save the downtime of the equipment. Mechanical seals are used in many applications like pumps, compressors, blowers, liquid ring compressors, agitators, submersibles, pumps and others.
What is the reason for the mechanical seal being used?
- To prevent and minimize the leakage of service fluid.
- Prevent the hazards and toxic fluids and gases that come out of casings of equipment.
- Reduce the maintenance downtime of equipment and power losses.
What is the advantage of a mechanical seal?
The primary advantage of a mechanical seal is to use any equipment for a good deal of safe operation.
- Reduction in power losses
- Save the equipment downtime and
- Reliability of equipment
- Safe operations.
Why is the cartridge seal mostly used?
- Fewer number of skilled workmen required
- Increase the reliability of equipment
- Reduce the time for maintenance
- Reduce the downtime of equipment
- It is easy to fix
Why did the mechanical seal fail?
- Lubrication failure and dry run
- Contamination in flushing fluid
- Foreign materials inside the seal face
- Rubbing seal faces against each other’s system
- High Temperature
- Pressure Variations in the Seal Chamber and Misalignment
- Overloading and not operating at Designs
- High vibrations and not installed properly.
What type of force is acting on a mechanical seal face?
- Radial and axial force
- Seal faces closing and Opening Force
- Hydrodynamic and hydrostatic types of force.
Manufacturer names of Mechanical Seals.
Top seal manufacturers are John Crane, Flowserve, Eagle Bergmann, Grundfos and SanMar.
What is the type of mechanical seal face materials?
Seal face materials are carbon, graphite, aluminum oxide, tungsten carbide, silicon carbide, diamond coated, glass coated and others.
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