Centrifugal pumps are amongst the most frequently used pumps in the chemical and petrochemical industries, accounting for 80 to 90 % of the total. Their functional principle is based on hydraulics and consists of an impeller, a casing and the seal and bearing unit. The selection and combination of an appropriate seal and bearing unit is of great importance, especially when it comes to dangerous media. This is the speciality of Bungartz centrifugal pumps with a magnetic drive.
Author Annette van Dorp Freelance Journalist
The task of the sealing systems is to seal the rotating shaft from its environment. Before selecting the optimum sealing system for each centrifugal pump application, the general suitability for the medium being pumped must first be ascertained. Depending on the hazard potential that this medium or mixture represents to the environment under Germany’s Technical Instructions on Air Quality Control (TA Luft), various statutory provisions need to be taken into consideration, ranging from emission values and health hazards to explosive environments (Atex, zone classification).
Centrifugal pumps with magnetic drives are often used when different types of material must be conveyed. A motor shaft in ‘block’ design supports an outer magnet rotor. This transmits the magnetic forces through a containment cup to the inner magnet rotor. The pumped fluid flows around the containment cup and the slide bearings, which must not be permitted to run dry. The containment cup hermetically seals the fluid from the environment. The greatest potential hazard for the magnetic drive arises from a containment cup fracture. In this situation, there is an uncontrolled escape of liquid into the atmosphere under pumping pressure. The fact that solids in the medium can block the cooling channels to the containment cup or destroy the bearing is a further disadvantage since it increases the wear on the cup. In the case of gas fractions, boiling or outgassing materials or low-viscosity liquids, the lubrication of the slide bearing can be interrupted. This results in flow stalling and process stoppages. Moreover, heating of the partial flow (by the coupling) can cause undesirable or hazardous heating of the fluid. Dry-running safety can only be achieved here through complex and cost-intensive additional measures.
Dry-running magnetic drive
Dry-running safety is the capability of a pump to master operating situations in which there is little or no pumping medium in the pump casing, or where gas fractions occur in the pumping process, without damage. Dry operation of a magnetic drive is only possible if the containment cup between the inner and outer magnet rotors facilitates vortex-free magnetic field transmission. A metallic containment cup would very quickly be heated to high temperatures. This would result in demagnetisation – and after a few minutes the total destruction of the coupling and bearing. Ceramic rolling bearings only allow dry running for a short time. They cannot withstand the high mechanical loads from the hydraulic pump forces during continuous dry running. The solution is to use grease-lubricated rolling bearings, which run in a sealed gas atmosphere. The advantage of this is that the bearings achieve a long service life in a very clean environment.
Intelligent combinations
Bungartz is known for its ‘problem-solving pumps’ with unique characteristics that provide a starting point for selecting the optimal magnetic drive pump. The special MPCHDryRun horizontal pump has proven effective at pumping hazardous liquids and/or media carrying solids or gases (Figure 1). This permanently dry-running magnetic drive pump combines exceptional characteristics: it is hermetically sealed with a ceramic containment cup and lifetime grease-lubricated rolling bearings that mean virtually no maintenance is required. The containment cup is manufactured from ceramic or high-performance Peek plastic to facilitate vortex-free magnetic field transmission. This in turn permits high energy efficiency. The sealing gas atmosphere inside is ensured by an inert gas which is introduced in similar amounts to gas-lubricated double mechanical seals. Dry-running lip seal rings protect the system in the event of operating errors. Since the containment cup runs without product contact, operation in conformity with Atex regulations is guaranteed. The intrinsically safe design makes this special pump insensitive to operator mistakes. One positive effect of this is that, thanks to the shaft with grease-lubricated rolling bearings, the pump’s efficiency is higher than that of conventional magnetic pumps with slide bearings.
If this permanently dry-running magnetic drive pump is functionally expanded with the properties of the self-regulating centrifugal pumps in the V-AN series, new potential is opened up for various applications. Numerous pumping problems can be permanently and economically solved with these special property combinations. The pumps in the Bungartz V-AN series offer several advantages:
The NPSH value is < 0 m, in other words the pumps are cavitation-free. They are self-regulating, self-ventilating, pump directly from a vacuum and require only minimal suction head. They are ideally suited for emptying tanks or tanker trucks completely, even when unloading liquefied petroleum gas, etc. The MPCV type vertical pump with magnetic drive demonstrates how these properties can be combined intelligently (Figure 2).
Highly hazardous medium
The pump works without a bearing in the pumping liquid. A gas barrier consisting of a frictionless throttle bush protects the rolling bearings against product vapours. The efficient shaft sealing concept consists of several components: the effective hydrodynamic relief of the pumping pressure on the bearing and sealing unit through the use of back vanes and a gas barrier, which prevents the penetration of product vapours into the bearing unit. The vortex-free magnetic drive hermetically seals the pump from its environment. Thanks to the pump’s vertical orientation, the bearing and sealing unit runs entirely without product contact, even in the event of a sealing gas fault.
The range of applications extends from pumping toxic waste water or hazardous or outgas-sing liquids to solids-laden, sludgy, crystallising or corrosive media. Even high volumes of solids and gases are permitted. Bungartz already has decades of positive experience with an especially critical, toxic medium.
Hydrochloric acid vapour forms on contact with the moisture in the air. Previously, pumps were fitted with double mechanical seals. However, the process is made more difficult by free chlorine ions, which diffuse through the sliding clearance of these seals, which last for a maximum of two years. Today, Bungartz MPCV magnet-coupled pumps are used to convey this toxic medium. In continuous operation, the rolling bearings achieve a service life of more than five years. These intelligently combined pumps have proven effective around the world, from China to the USA.
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V-AN self-regulating centrifugal pump
Advantages at a glance
- NPSH value 0 m, no cavitation
- Minimal suction head
- Self-regulating and self-ventilating
- Impervious to gas bubbles
- Dry-running and reliable
- Savings on investment costs due to reduced construction dimensions
- No risk of dry-running for magnet-coupled pumps
- In the reservoir, the suction pressure can be reduced to boiling condition (large suction head for emptying tanker trucks from above)
- Robust and durable construction
- Dry-running, depressurised magnetic drive
- Completely wear-free sealing and bearing technology works independently of pumping medium
- Grease-lubricated rolling bearings with a minimum service life of 32,000 hours require no maintenance during this period.
- The labyrinth zone between the impeller rear side and the shaft bearing is free of product and filled with sealing gas, ensuring a thermal barrier with a high temperature gradient, and thus a low bearing temperature.
- Very high safety standard thanks to triple seal system
- Frictionless lip seal ring in the sealing gas atmosphere at the labyrinth seal, pumping pressure only
- Hydraulic discharge
- Containment cup is depressurised and without product contact
- High operating safety – even in the event of an interruption in the flow of the medium
- Low life-cycle costs
- Efficient energy consumption
Advantages of the dry-running MPCHDryRun
magnetic drive pump
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