1. General Information
Mechanical face seals are devices used to prevent the leakage of fluid contained in machines through which a rotating shaft passes.
Mechanical face seals consist of two parts: one fixed to the rotating shaft and the other to the stationary part of the machine containing the fluid. The rotating ring and the stationary ring stay in contact due to the mechanical load provided by springs or a metal bellows and the hydraulic thrust generated by the pressure of the fluid to be sealed.
The size of the sealing rings allows the process fluid to get between the sealing surfaces, creating a fluid film that cools and lubricates the sealing rings.
Mechanical seals are preferred for rotary pumps and agitators because they do not require regular adjustment, do not cause machine wear, have negligible leakage, offer superior reliability over time compared with other seals and can operate at higher limits. They are standardized by the API682 standard for the oil and gas industry and the UNI standard.
2. Applications and types of mechanical seals
Mechanical seals are primarily installed on the shafts of centrifugal or positive displacement rotary pumps and agitators to prevent process fluid leakage.
Component seals are mechanical face seals comprising rotary and stationary units. Their housings must have specific dimensions and tolerances.
Cartridge seals function like component seals but come complete with a shaft sleeve and locking flange, making installation easier and reducing errors.
Stationary seals have springs in the stationary part of the seal and are used when rotational speeds are high.
Balanced seals have a thrust surface to sliding surface ratio of less than 1, generating less friction and heat, which limits fluid film evaporation.
3. Selection and configuration
Pressure, temperature, the nature of the process fluid and the rotational speed of the shaft affect the operation of mechanical seals.
Mechanical seal configuration refers to whether the seal is single or double, internally or externally mounted, back-to-back or tandem, with rotary or stationary springs, depending on operating conditions, the nature of the fluid and the type of machine on which they are installed.
Within certain limits of pressure, temperature and shaft speed, Fluiten manufactures seals suitable for dry operation. Beyond these limits, liquid flushing from an external source is required. To prevent the flushing liquid from leaking into the atmosphere, a second mechanical seal is installed, forming a double seal.
The basic choice is graphite against silicon carbide. Different types of graphite and silicon carbide, with different characteristics and limits, are selected based on operating conditions, type of seal and cost.
First, determine the most suitable configuration (single or double seal) based on the fluid to be sealed. Next, check the operating parameters: pressure, temperature, and shaft speed guide the choice of the most suitable model and materials. Finally, check the available dimensions and choose between a component or cartridge seal based on customer requirements or reference standards. Seal selection is typically performed by trained technicians.
4. Maintenance and common issues
Yes, the fluid film needed for lubrication and cooling can cause minor drips, often not visible since they evaporate into the atmosphere.
A double seal is necessary to allow flushing with an auxiliary liquid from an external source at higher pressure than the process fluid. This creates a fluid film with the auxiliary liquid (known as a barrier fluid), ensuring that any leakage into the atmosphere or the process consists only of this barrier fluid.
The temperature between the sealing surfaces rises rapidly, causing sealing surface deformation and loss of elasticity in the secondary gaskets. This results in leakage beyond normal limits.
The main cause is the absence or instability of the fluid film between the sealing surfaces, which may occur for several reasons (see bulletin…). Other reasons, related to the main one, include materials incompatible with the process, unsuitable sealing for operating conditions, assembly errors, machine startup errors or excessive mechanical stress from the machine or plant.
Signs include greater than normal leakage (over 2 drips/min), excessive overheating in the seal installation area and excessive graphite release.
If not severely damaged, seals can be repaired by replacing or lapping the sealing surfaces and replacing secondary gaskets and springs or other worn metal parts. Repairing can be cost-effective for expensive seals.
5. Technical specifications and standards
Balanced seals are needed at high PxV limits, with liquids close to vapour pressure, and for uses in the oil and gas industry as specified by API682.
PxV is the product of the fluid pressure and the rotational speed of the shaft (bar x m/sec). As PxV increases, the heat generated by the sealing surfaces increases.
This standard defines the overall dimensions of component seals for shafts from 10 to 100 mm, making seals from different manufacturers interchangeable if they comply with the standard.
No. To ensure a perfect fit between the contact surfaces for fluid film formation, the surfaces must be perfectly flat with very low roughness. Coupling materials of different hardness, where one has self-lubricating characteristics, like graphite, ensures a perfect fit. Graphite also allows tolerance for transients when the fluid film is unstable or absent for short periods for various machine- or plant-related reasons. Dry operation is not tolerated with two hard materials.
Flush plans are support configurations that keep the sealing surfaces clean, cool and lubricated, which is essential for the operation and longevity of seals.
