Useful Information on Pump Magnetic Couplings
What are magnetic couplings?
In a magnetic drive pump, the pumping element and fluid are contained within a hermetically sealed housing. The drive shaft from the motor rotates an assembly of magnets on the outside of the housing. Opposing this, on the inside of the housing, is a matching ring of magnets on the pump shaft (Figure 1). Torque is transferred through the housing as a result of the coupled magnets. The most important advantage of a magnetic coupling is that it allows the transmission of torque through a barrier without the use of rotating seals, along with the associated problems of leakage, potential contamination and continual maintenance.
Magnetic drives are available for rotodynamic centrifugal, turbine and side-channel pumps and positive displacement pumps including vane pumps, internal gear pumps, and external gear pumps and the basic principles and advantages are the same: the pumped fluid is contained within a hermetically sealed housing – the containment shell - eliminating the risk of leakage.
How do magnetic couplings work?
The coupled magnets are attached to two concentric rings on either side of the containment shell on the pump housing (Figure 2). The outer ring is attached to the motor’s drive shaft; the inner ring to the pump shaft. Each ring contains the same number of matched and opposing magnets, arranged with alternating poles around each ring.
What are the magnets made of?
The magnets are often made of rare earth metals such as samarium or neodymium alloyed with other metals. The most common combinations are Samarium-Cobalt and Neodymium-Iron-Boron. These complex alloys have two main advantages over traditional magnets:
- Lower mass required to maintain a specific torque – hence smaller and less complex pumps.
- Greater temperature stability – magnetic torque reduces with increases in temperatures but less so with rare earth alloy magnets than with traditional iron magnets (see Figure 3).
The magnets in a magnetic drive pump can demagnetize if exposed to temperatures above their upper limit. In high temperature applications, pumps should not be run dry or in any other conditions that could cause heat build-up within the pump.
The use of rare earth metals is a major factor in the cost of magnetic drive pumps. They are mined in only a few places around the World (notably China) and prices can be volatile. For example, China manufactures 76% of the World’s neodymium magnets. Apart from their cost, another disadvantage of these alloys is their poor resistance to corrosion. In a magnetic drive, it is necessary to coat the magnets on the inner ring (which are exposed to the pumped fluid) with some form of protective resin or enclose them in a corrosion resistant casing. Common magnet casing materials include polypropylene, PVDF (polyvinylidene fluoride), PTFE, PFA, stainless steel and Hastelloy-C.
What is the maximum torque?
The maximum torque that can be achieved in a magnetic drive pump is determined by the gap between the magnets: the smaller the gap, the larger the torque transfer. However, there is a limit to how small this can be engineered, since the gap must include the containment shell and any protective materials coating the inner magnets (see Figure 2). For the safe operation of the pump, it is important that there is a reasonable gap between the rotating parts and the containment shell, especially if the pumped fluid is viscous. All parts must therefore be machined to high tolerances for greatest efficiency. In addition to these engineering concerns, the material used in the construction of the containment shell is important in maintaining a high coupling efficiency between the two sets of magnets and in reducing power losses and temperature increases.
The inner magnet ring, the pump shaft, and bearings are immersed in and lubricated by the pumped fluid. It is important that these parts are designed to operate efficiently in the environment. With highly viscous liquids, friction losses can be high; in an abrasive or chemically aggressive medium, bearing wear can be a problem. However, with the right choice of wetted materials - including silicon carbide, thermoplastics, stainless steel and high nickel alloys - magnet drive pumps are ideal for handling aggressive, corrosive and hazardous liquids.
What is decoupling?
When selecting a magnetic drive, it is necessary to determine whether its coupling has sufficient torque transmission capability to deliver the required flow. Normally, the coupling works synchronously, that is the motor speed is equal to the pump speed.
Magnetic drives are sensitive to extreme operating conditions that result in excessive torque. All magnetic couplings are rated for a maximum torque capability. When this is exceeded, the magnet rings may decouple and the pump shaft, will slip and may stop rotating completely. If this happens, the load cannot be picked up again unless the motor is stopped and then restarted. Decoupling can occur during start up when the torque is significantly higher than that expected under normal operating conditions. It is therefore important to take start-up conditions into account when sizing a pump and magnetic coupling for an application.
Decoupling can be used as a safety feature allowing the pump to cut out automatically if an extreme condition occurs. However, the magnets may be permanently demagnetised if the pump operates in this state for a prolonged period. The use of power monitors is recommended to detect the onset of decoupling.
What is an eddy current?
When a magnetic field moves across a conductor, a current loop is induced in the conductor. This is called an eddy current. Eddy currents can produce significant drag on the motion, called magnetic damping, and also generate heating in the conductor. The rotating magnets in a magnetic drive can induce eddy currents in the containment shell if it is conductive, resulting in power loss and heating.
What causes power loss in a magnetic coupling?
Eddy currents in a magnetic coupling reduce pump efficiency and heat up the fluid around the inner magnets. Power losses arising from eddy currents can be described by the following relationship:
For maximum coupling efficiency, the magnet assemblies should have small diameters and low rotational speed since power losses are proportional to both the square of the speed (n) and the square of the drive radius (r). Torque can be increased by using a greater mass of magnets but this is difficult to achieve without also increasing the radius of the coupling. The decrease of coupling efficiency that would be expected with big, powerful pumps therefore sets an economic limit on the application of magnetic couplings.
The field flux density (B) rotates with the magnets and flows through the containment shell. The containment shell thickness and material of construction, the gap between the inner and outer magnets, and the rotating speed all affect the field flux density. Power losses due to eddy currents are inversely proportional to the resistivity (ρ) of the containment shell and are significantly higher if it is made from a conductive material. Manufacturers usually offer non-metallic containment shells which may have a composite construction for improved chemical resistance and strength. These will not result in eddy currents from the magnetic flux passing across it and hence have lower power losses.
The inner, driven magnets rotate within the pumped liquid and this generates a frictional torque and power loss, particularly when handling viscous liquids. Frictional losses also increase with magnet size, the square of rotational speed, and are inversely proportional to the size of the gap between the inner magnets and the containment shell. In extreme cases, frictional resistance can cause decoupling.
How are magnetic couplings cooled?
Magnetic losses in the containment shell generate heat inside the pump. In order to dissipate this heat (and avoid the potential of flashing (vaporisation of fluid) a certain flow of the pumped fluid is required through the gap between the inner magnets and containment shell. There are a number of ways this can be achieved:
- Circulation from the pump discharge around the inner magnets and back to the pumping element.
- An outer jacket can maintain the pump at a reduced temperature.
- A double-layered containment shell allows a cooling fluid to be applied directly. This is ideal for applications requiring precise and uniform temperature control.
Sensors can also be used to detect the first symptoms of overheating in the containment shell allowing conditions to be assessed and modified. Power monitors on the motor can also detect a low-load condition from, for example, dry running which could result in overheating.
Summary
Magnetic drives offer the key benefit of containment and zero leakage of the pumped medium. Rare earth alloy magnets with high field strength allow compact design. New materials for containment shell construction lower power losses due to eddy currents.
In selecting a magnetic drive it is important to consider the following to minimise power losses:
- The maximum torque required by the pump
- The speed (rpm) at which the torque is to be transmitted
- The gap and material through which the torque is transferred
- The operating temperature
- Any physical size limitations of the magnetic coupling
Magnetic drives are available for many pumps, including centrifugal pumps, side channel pumps, turbine pumps, vane pumps and internal and external gear pumps.
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