Frequently Asked Questions

A mechanical seal is a device that helps join systems or mechanisms together by preventing leakage, containing pressure, or excluding contamination. In pumps, it serves to prevent the leakage of the fluids being pumped, thereby maintaining the efficiency and safety of the system. It typically consists of a rotating part attached to the pump shaft and a stationary part attached to the pump housing.
There are several types of mechanical seals, including single spring, cartridge, and double mechanical seals. Each type is suited to different applications depending on factors like the fluid, operating pressure, temperature, and pump speed. Single spring seals are common for general purposes, cartridge seals are pre-assembled and easy to install, while double seals are used for high-risk applications.
The correct mechanical seal selection depends on the pump application, the fluid being pumped, the operating conditions, and the environment. Factors to consider include the fluid's chemical properties, temperature, pressure, and the pump's operating speed. Consulting with seal manufacturers or specialists is advisable to ensure the optimal selection.
Mechanical seal failures can be attributed to several factors, such as incorrect installation, unsuitable seal material, operational errors, thermal shocks, and chemical attacks. Misalignment, improper start-up procedures, and inadequate lubrication can also lead to premature seal failure.
Extending the life of a mechanical seal involves regular maintenance, proper installation, and ensuring the pump operates within its intended parameters. Using the correct seal material and design for your specific application is crucial. Regular inspections and following the manufacturer’s maintenance recommendations can also significantly extend seal life.
Indications of wear or failure include an increase in leakage, unusual noises, vibration, overheating, or a drop in pump performance. These signs suggest that the seal may need inspection, repair, or replacement to prevent pump downtime or more significant damage.
Whether a mechanical seal can be repaired or must be replaced depends on the extent of the damage and the specific circumstances. Some seals can be refurbished with replacement parts, but others require complete replacement, especially if the damage is extensive or affects critical components.
Proper installation is crucial for the performance and longevity of a mechanical seal. This includes ensuring clean and correct installation surfaces, following the manufacturer's instructions, checking for correct alignment, and avoiding contamination. It's often recommended to have a professional or experienced technician perform the installation.
Safety precautions include wearing appropriate personal protective equipment, following all installation guidelines, and being aware of the handling requirements of the specific seal and pumped fluid. Ensuring a clean work environment and using the correct tools are also vital to prevent injury and equipment damage.
For further assistance or information, you can consult the mechanical seal manufacturer’s documentation or contact us on +44 7767 842601.

In many instances, the preferred combination for seal face materials is carbon graphite versus silicon carbide. However, in certain scenarios, carbon graphite may not be suitable as a face material. In such cases, a blend of tough materials like silicon carbide or tungsten carbide becomes necessary. This determination is typically based on the properties of the substance being sealed or to adhere to stringent contamination standards. Reasons for opting for two hard faces include:

  • The sealed substance contains abrasives, leading to rapid wear of the carbon face, as seen in materials like gypsum slurry, seawater, and crude oil.
  • Substances with high viscosity tend to bond faces together, making carbon graphite unsuitable, as seen in bitumen.
  • Certain applications, such as pharmaceuticals, food, and cosmetics, cannot tolerate any contamination from carbon wear debris.
  • In de-ionized water applications, carbon wear can occur rapidly.
  • Operating temperatures exceed the tolerance of carbon graphite.
  • When the product is corrosive and can attack the carbon face, such as acids.

Typically, liquid-lubricated seals should avoid regular or prolonged dry running. However, certain mechanical seals featuring a carbon face can withstand minimal lubrication and brief periods of dry operation intermittently.

When the temperature at the mechanical seal faces rises excessively due to inadequate heat dissipation, the liquid film between the faces may vaporize, leading to instability. Consequently, the seal face materials may come into excessive contact, resulting in rapid wear during dry running conditions. While many wet seals employing general-purpose carbon cannot endure this, some mechanical seal designs utilize face materials more tolerant to sporadic dry running, especially during upset conditions.

Certain types of mechanical seals are engineered for continuous dry operation, utilizing seal face materials tailored for this purpose. For instance, certain top entry mixers integrate single mechanical seals that effectively seal a nitrogen gas blanket, with the seal faces solely exposed to vapors. In industries like chemical and pharmaceuticals, double mechanical seals are crucial, operating on pressurized dry nitrogen barrier gas.

To enable carbon graphite to dry lubricate effectively, moisture presence is essential; otherwise, rapid wear occurs. However, there exist grades of carbon graphite specifically formulated to function on dry nitrogen with minimal face wear.

Every mechanical seal undergoes hydraulic balancing to regulate the forces that open and close the seal rings. This hydraulic equilibrium is quantified using the balance ratio, which compares the area of the seal face to the net hydraulic closing area. If a mechanical seal has a balance ratio of one or higher, it's termed an unbalanced seal, whereas a seal with a balance ratio below one is considered balanced.

The extent of hydraulic balance is determined by adjusting the dimensions of the radially disposed areas of the seal rings. Typically, an unbalanced seal maintains a balance ratio of approximately 1.2. Consequently, the pressure on the seal face equates to 1.2 times the sealed pressure, in addition to the pressure generated by the spring force. As a result, the face pressure on an unbalanced seal consistently exceeds the pressure being sealed. Consequently, unbalanced seals are suitable for applications with limited to low-pressure scenarios.

Contrastingly, a balanced seal typically exhibits a balance ratio ranging from 0.6 to 0.9. This design ensures that the seal face pressure remains lower than the sealed pressure. This characteristic enables the seal to generate an adequate fluid film between the seal faces, facilitating operation at high pressures.

A single mechanical seal represents the simplest and often preferred solution across various applications. However, in cases where the sealed product is unsuitable for lubricating a single seal or where enhanced reliability and safety are necessary, a multiple-seal arrangement becomes essential.

In a single mechanical seal setup, a sole set of seal faces is present, pressurized, and lubricated by the sealed product. Consequently, the seal allows for the leakage of the sealed product into the atmosphere.

Conversely, a face-to-back (tandem) mechanical seal incorporates two mechanical seals arranged consecutively, with an unpressurized buffer fluid circulating between them. The primary seal, also known as the product side seal or inboard seal, operates under pressure and is lubricated by the sealed product, similar to a single seal. Meanwhile, the secondary seal termed the atmosphere side seal or outboard seal, is lubricated by a clean buffer fluid. In the event of primary seal failure, the secondary seal prevents product leakage into the atmosphere, thereby enhancing safety and reliability.

A back-to-back mechanical seal comprises two mechanical seals arranged opposite to each other with a pressurized barrier fluid circulating between them. The barrier fluid pressure consistently exceeds that of the sealed product. Both the inboard and outboard seal faces receive pressurization and lubrication from the clean barrier fluid. Should the inboard seal fail, the pressurized barrier fluid leaks into the product, while the failure of the outboard seal leads to leakage of the barrier fluid into the atmosphere. In either scenario, the sealed product is safeguarded against leakage into the atmosphere. Double seals find application in situations demanding a high level of safety, particularly when the sealed product is challenging to lubricate effectively due to its abrasive, volatile, viscous, or hazardous nature.

Mechanical seal failures are generally categorized into three primary groups:

  • Incorrect operation and process upsets or changes.
  • Improper fitting and alignment.
  • Inaccurate (or slightly erroneous) seal or system selection.

The predominant cause of mechanical seal failures lies within the first category, with over 60% of failures being caused by some kind of process upset or a change in operating conditions.

The determination of a mechanical seal failure varies depending on the seal type and its application. However, a general indication of a failed mechanical seal includes excessive leakage and the inability to uphold the designated pressure. A mechanical seal may be deemed to have failed if:

  • There is a notable loss of sealed product.
  • The product or barrier fluid pressure falls below the required level.
  • There is a significant loss of barrier fluid.
  • An excessive flow of barrier gas occurs.
  • There is an abundance of process contamination (barrier fluid, wear debris, or other impurities).

As a general guideline, a mechanical seal is considered failed if its leakage exceeds 250 times its theoretical average rate.

Mechanical seals inherently experience leakage as part of their operation. This leakage serves the crucial purpose of maintaining a lubricating fluid film between the sealed faces, minimizing friction, heat, and wear.

  • Assessing the hazards and risks associated with leakage.
  • Compliance with legislation governing emission levels.
  • Evaluating the operating environment and measures for cleanliness control.

Consider the significant costs linked to lost product or barrier fluid. The actual leakage rate is contingent on various factors, including the mechanical seal type and size, fluid properties, shaft speed, sealed pressure, and operating temperature.

The lifespan of a mechanical seal is primarily determined by the rate at which the carbon face wears down if no other factors are at play. However, it's rare for mechanical seals to deteriorate solely due to age; instead, they typically fail due to other causes long before the carbon wears out significantly.

Numerous unknown variables complicate the prediction of a mechanical seal's service life, making it challenging to estimate accurately. Generally, comparisons can only be made based on empirical data gathered from similar applications.

For instance, a well-operated seal handling a clean and stable medium like a light hydrocarbon may endure for over five years. Conversely, a seal tasked with managing highly viscous or abrasive substances might only last a few months under similar conditions.