Mechanical seal is an important part of the centrifugal pump. It helps to maintain the integrity of the pump system by preventing the leakage of fluid and gas. It keeps the contaminants out between a rotating shaft and the housing under extreme conditions such as high pressure, high shaft speed, and high/low temperature. It can be single or double-acting. Mechanical seal is available in a wide range of designs and arrangements for detecting leakage, controlling different seal environments, and lubricating secondary seals. Each type of mechanical seal has its own characteristic and design to capture a specific application. In this article, we will introduce the features, applications, and drawbacks of different types of mechanical seal that used in the centrifugal pump system.
Balanced Mechanical Seals vs Unbalanced Mechanical Seals
The mechanical seal balance refers to the load that acts across the seal faces. In a balanced mechanical seal, the forces acting at the seal faces are balanced which results in a low face loading and more lubrication of the seal faces. It is suitable for operating under higher pressure and good at handling fluid with high volatility, low lubricity, and hydrocarbons that have a high vapor pressure. In contrast, an unbalanced mechanical seal is an economic choice that is inexpensive. It is used for drastic conditions such as misalignment of the shaft, vibrations, and problems with cavitation of the fluid. It is stable, has tighter control of the face film, and less leakage under a low-pressure range. However, an unbalanced mechanical seal is not recommended to be used for high pressure and most hydrocarbon applications as the lubricating film between faces may squeeze out to cause the failure of the seal.
Pusher Mechanical Seals vs Non-Pusher Mechanical Seals
The pusher mechanical seal uses a secondary seal that moves axially along the rotating shaft to have contact with the seal faces. This arrangement will help to compensate for the wearing that may occur at the seal face and absorb the wobbling caused by misalignment. Pusher mechanical seal is less expensive and available in a wide range of sizes and designs. However, these seals tend to hang up or wear on the shaft. Non-Pusher mechanical seal utilizes the elastomeric bellow to keep the seal closing force. This bellow type of mechanical seals does not have to move axially to maintain contact with seal faces. The key advantage of a non-pusher mechanical seal is that it is able to operate under low and high-temperature conditions due to its metal bellow, and without the secondary seal that may cause wearing of the shaft. Few things we need to consider when selecting a non-pusher mechanical seal, which is the thin bellow cross-sections have to be upgraded when using under a corrosive environment and the cost of metal bellows seals is relatively high.
Single Mechanical Seals vs Double Mechanical Seals
The single seal is the most common type of mechanical seat used for sealing the pump head. It has two flat seal faces that are forced together by the spring. There is a thin fluid film in between the two seal faces that are created by the process fluid. The fluid film lubricates and cools the mechanical seal to prevent the seals from destructing due to the frictional heat. The sealed fluid travel across the seal faces from a high-pressure side to the low-pressure side and vaporizes into the atmosphere. Therefore, a single seal is only suitable for processing the fluid that has little or no risk to the environment and operator.
Double Seals has two mechanical seals that are arranged either back to back or face to face. The primary seal ensures the product is contained in the pump housing while the secondary seal prevents the flush fluid from leaking to the atmosphere. Double mechanical seals are popular to be used for high pressure, high-temperature application or for fluid that is hazardous to the environment or operator.
Cartridge Type Mechanical Seals
It is a single assembly that houses all types of sealing components such as a gland plate, shaft sleeve, seal, and hardware. The design of cartridge seals has eliminated the issue incurred during the installation such as the need for seal setting measurement and spring compression. A careful evaluation of the dimensional aspect prior to the installation can ensure a proper fit into the existing seal housing. In this case, cartridge mechanical seals have lower seal setting errors and require less maintenance that brings dramatic benefits for long-term application. Along with these advantages, this type of mechanical seal has a relatively high up-front cost that may not favorable for short-term applications.
How To Choose The Right Mechanical Seal For The Pump System
When considering the proper seal system for the pump, we must choose according to the application and requirements. Failure to choose the right seal type may result in lossing of pump integrity, machinery breakout, or high repair cost. There are few factors that we need to take into account when selecting the right seal type. The most important point is the type of fluid to be pumped into the system. Because the cleanliness, volatility, and lubricity of the fluid will affect the mechanical seal and the overall seal support system design. Another factor is the pump fluid press which is the amount of pressure exerted at the mechanical seal’s faces. If the pump is operated under a low-pressure condition, an unbalanced mechanical seal is suitable for this application. On the other hand, if the pump is operated under a high-pressure condition, we should choose a balanced mechanical seal that can provide a reliable solution for the application. The operation temperature also a factor that will affect the performance of the mechanical seal. Balance mechanical seals perform better than unbalanced parts in a higher operating temperature condition as the balanced mechanical seal is better in preserving the heat-sensitive components than other types of seals. In terms of safety concerns, a double mechanical seal can provide additional protection as it is more reliable with a higher sealing capacity. Many factors will influence our selection of mechanical seals. We must understand the work requirements, pros, and cons of the different types of mechanical seals in order to choose the right types for the system.
MecHero Malaysia provides engineering and fabrication services for different types of mechanical seals. You can always contact us at +6011-7000 7687 or email us at my.mechero@gmail.com for an inquiry about the supply of mechanical seal and fabrication services in Malaysia for industrial usage. As a steel and online hardware store Malaysia, we also supply a wide range of steel materials, general hardware, or mechanical components. Tell us your requirements now and we can advise you accordingly.
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