In the case of failure, the fact that the pump comes to a standstill is often the least of one’s problems. In order to select
the ideal sealing system for each centrifugal pump application, one aspect (among others) to be determined is the general suitability of the pump for the medium to be conveyed. Similarly, the potential hazards associated with the medium have to be taken into account. Statutory regulations and provisions – starting with protection against health hazards, through mandatory emission values, to instructions for explosive environments (ATEX, zone classification) – have rightly been stipulated and must be complied with without fail.
By Dipl.-Ing. Frank Bungartz, Director, Paul Bungartz GmbH & Co.
A wide variety of sealing systems, such as a stuffing box (also in combination with a hydrodynamic seal), magnetic coupling, canned motor or mechanical seal, are available for selection. Single mechanical seals are most frequently used for non-hazardous media. Additionally, labyrinth seals which are open to the surroundings and which are equipped with conveying and locking devices, stuffing box packings made of various materials that are lubricated using water, grease or graphite, or shaft seal rings are found. In the case of media with a medium to high hazard potential, there are three sealing systems which have been approved for the sealing of pump shafts between the atmospheric surroundings and the working fluids: double mechanical seals, magnetic couplings and the canned motor. In the case of the mechanical seals, a distinction is made between liquid-barrier and gas-barrier double mechanical seals. Liquid-lubricated seals are usually used in double mechanical seals. The barrier systems required are complex and the maintenance costs high. In order to avoid high maintenance costs, it can be a good idea to use magnetic-coupled pumps.
A further version has become popular for more difficult pumping applications. The gas-lubricated mechanical seal is preferred in particular for vertical pumps. This is also approved for these applications and is characterized by low operating and maintenance costs due to the simple gas supply system.
Mechanical seal versus magnetic coupling
In contrast to the double-acting mechanical seal, magnetic coupling and canned motor pumps do not work independently of the pumping medium. The medium must be suitable for flowing inside the containment can and slide bearings, whereby magnetizable media or media which contain solids or gases can only be pumped by these devices through the use of additional measures such as external flushing. If centrifugal pumps are used with a magnetic coupling, the motor shaft is fitted with an external magnetic rotor in the so-called block design. It transfers the magneticforces through a containment can to the internal magnetic rotor. The pumping medium flows around the containment can and slide bearings, which must not be allowed to run dry. The fluid is hermetically shielded from the surroundings by the containment can. The greatest hazard potential for the magnetic coupling is if the containment can is fractured. In this case, the liquid which is subject to the pumping pressure passes in an uncontrolled manner into the atmosphere. Further disadvantages: Solids in the medium can block the cooling channels to the containment can or destroy the bearing. This increases the wear of the containment can. If the medium contains gases, boiling or outgassing substances or low-viscosity liquids, the lubrication of the slide bearing tends to be interrupted. The consequence: stalling and process downtime!
Furthermore, heating of the partial flow (by the induced eddy currents within the containment can of the coupling) can result in undesirable or even dangerous heating of the medium. Only through the use of complex and expensive additional measures would it be possible in this case to achieve dry-running safety.
Hermetically sealed and permanently dry-running
For the pumping of hazardous liquids and/or solid-laden, gas-containing media, permanently dry-running magnetic coupling pumps can prove their value.
Horizontal pumps that are hermetically sealed and combine outstanding properties with a ceramic containment can and roller bearings, which are lubricated with grease for their entire service life, can lead to extremely low-maintenance. As the containment can runs without coming into contact with the product, ATEX-compliant use is guaranteed. Pumps with a dry-running and depressurized magnetic coupling have an intrinsically safe design which impresses even in the case of incorrect operation: Dry-running lip seals secure the system with an arrangement of three seals. This completely non-wearing sealing and bearing technology operates independently of the pumping medium. The feed pressure of the pumping medium is applied only to the labyrinth seal, which operates with zero friction. The sealing gas atmosphere does not contain any oxygen, moisture or solids.
If you would like to read the full article by Frank Bungartz, please email Pump Engineer Editor Deirdre Morgan