Introduction to different types of mechanical seals used in centrifugal pumps

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=== Introduction Mechanical seals are key components in pump systems that maintain the integrity of the pump system by preventing leakage of liquid or gas and preventing the entry of contaminants. Mechanical seal systems are used in various seal designs to detect leaks, control the seal environment and lubricate secondary seals. There are a variety of mechanical seal types to choose from depending on the pump type and process and pressure variables. Each seal type has its own unique design and features, making it suitable for the working conditions on site. This article highlights the advantages and disadvantages of different types of mechanical seals in pumps. === Common Seal Types Mechanical seal types vary in design, installation, and how they determine the fluid pressure acting on their surfaces. The most common seal types include: - Balanced seal - Unbalanced seal - Pushing seal - Non-pushing seal - Non-cartridge seal - Cartridge seal === Balanced and unbalanced seals A balanced mechanical seal refers to a system in which the forces acting on the sealing surface are balanced. Because the sealing surface load is lower, the sealing surface is lubricated more evenly and the sealing life is longer. Balanced mechanical seals are particularly suitable for higher operating pressures, typically above 200 Psig (1.4 MPa·G). They are also a good choice when working with low lubricity and highly volatile liquids. For more complex balanced seals, unbalanced mechanical seal types are often used (instead of balanced ones) as a more economical option. Unbalanced seals may also exhibit less leakage due to tighter control of the sealing surface fluid film, but may therefore exhibit a lower mean time between failures. Unbalanced mechanical seals are not recommended for use in high pressure or most hydrocarbon applications. === Pusher and non-pusher seals Pusher seals use one or more springs to maintain seal closing force. The spring can be located in the rotating or stationary element of the mechanical seal. Pusher seals can maintain a sealing effect under high pressures, but have drawbacks because the elastomer beneath the main sealing surface is subject to wear as it moves along the shaft/sleeve during operation. Non-push seals use metal or bellows to maintain sealing force. This seal is ideal for dirty and high-temperature (or low-temperature) applications. Bellows seals are limited to medium/low pressure applications. Both propelled and non-propelled designs are available in balanced and unbalanced configurations. === Non-cartridge mechanical seals Traditional seals generally cost less and are usually installed on general-purpose equipment. When these seals are installed as separate components, they require higher operational skills to assemble the repair parts and require the installer to adjust themselves. === Cartridge Mechanical Seals Cartridge mechanical seals combine all sealing elements into a single assembly. this * * Reduces the possibility of improper installation and the time required to replace seals. === Non-cartridge vs. cartridge seal comparison When selecting a type of mechanical seal, the user must consider installation costs, long-term operating costs, pump characteristics, and expected operating conditions (temperature, vibration, and pressure). Also be aware of the technical capabilities of your maintenance/repair personnel, as having skilled technicians on-site to perform regular maintenance can make a significant difference versus having to outsource maintenance and repairs. These factors will determine whether a non-cartridge seal is better than a cartridge seal. === Cartridge seal A cartridge mechanical seal is a fully enclosed seal with pre-assembled components. Typically, this seal type consists of a sleeve, gland, sealing element and other accessories and can be pre-assembled. advantage: - Simple and easy to install (does not require high technical skills) - Higher functional safety due to pre-assembled seals with fixed axial settings, eliminating measurement errors - Eliminates the possibility of axial misalignment and resulting sealing performance problems - Prevents dust from entering or damaging the sealing surface - Reduces installation costs by shortening installation time (reduced downtime during maintenance) - Reduces excessive disassembly of the pump (set) for seal replacement - Cartridge assembly is easy to repair - Protective effect on the customer's pump shaft - Since the cartridge seal comes with its own sleeve, there is no need to adjust the shaft to adjust the balance of the seal. shortcoming: - The disadvantages of cartridge mechanical seals are the higher initial cost and the larger space required in the pump chamber. - To design a cartridge seal, a large amount of data is required as support. === Non-cartridge seals Non-cartridge (component) seals are composed of separate dynamic and stationary parts. Unlike cartridge seals, component seals are not pre-assembled and require a skilled technician to install. If the sealing surface, O-ring and seal axial arrangement on the pump shaft are not installed correctly, it will lead to shortened seal life and, in some cases, even cause immediate seal failure. advantage: - Reduced cost of first purchase of seals - Possibility to reduce spare parts costs (replacement of only wearing parts) - Suitable for very small pumps Disadvantages: - The main disadvantages of non-cartridge mechanical seals are that they are easily damaged and have a short service life. Non-cartridge seals often have exposed parts that are more susceptible to damage during shipping or installation. - Non-cartridge seals require precise measurements during installation to prevent leakage due to seal failure. - This type of seal requires more repair time to properly install, so it is more expensive to install or reinstall after repair. === Mechanical Seal Selection Considerations When selecting the type of mechanical seal and its auxiliary system for a pump, the designer must make a selection based on its unique application. Failure to select the correct seal type can result in loss of pump (pack) integrity, failure and expensive repairs. To avoid these undesirable results, designers must consider the following factors before designing. Type of Fluid Pumped The fluid being pumped is the most important factor when deciding on the type of seal. Factors such as cleanliness, lubricity and volatility will significantly affect the design of mechanical seals and sealing systems. The amount of pressure exerted by the pumped fluid pressure on the mechanical seal face has a significant impact on its performance. If the pump is to operate at low pressure, an unbalanced mechanical seal will be suitable. However, where higher pressures are expected, a balanced seal will prove to be a more reliable solution. Temperature Considerations Balanced mechanical seals perform better than unbalanced mechanical seals when the operating temperature is higher than normal. Heat-sensitive components are better protected in balanced mechanical seals than in other seal types. Operator Safety Issues With all types of rotating machinery, design is a top priority. The use of double seal mechanical seals in any pump will provide additional protection as they increase the sealing effect and are generally more reliable.