Oct. 07, 2024
Agriculture
In a standard centrifugal pump, the drive shaft from the motor is connected, normally via a flexible coupling, to the impeller through the pump housing. This necessitates some form of seal where the drive shaft enters the housing (Figure 1).
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In a magnetic drive centrifugal pump, the impeller and the pumped 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 a shaft attached to the impeller (Figure 2). Torque is transferred through the housing as a result of the coupled magnets.
There are also magnetic drive positive displacement pumps, 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 sealed housing eliminating the risk of leakage. For the purposes of this article, we shall continue to describe magnetic drives in the context of centrifugal pump design.
With a standard centrifugal pump, some form of seal is necessary to stop the pumped medium from leaking out around the pump shaft, especially if it is at high pressure. There are three main options:
A soft packing material compressed around the pump shaft in a &#;stuffing box&#; Packing material is held in place within the opening in the casing (the stuffing box) and compressed by a gland nut which can be progressively tightened as the packing material wears or settles (Figure 1).
Lip seal or O-ring
A rubber or plastic ring fits around the drive shaft and is held in place in a recess in the pump housing.
Mechanical seal
A mechanical seal consists of two parts: a stationary component attached to the pump housing and a rotating component on the pump shaft. The faces of the two components are machined to be flat and smooth and are spring-loaded to keep them pressed together. This is the most effective option for reducing leaks but can be expensive and difficult to set up.
Leakage cannot be eliminated completely with any of these solutions and, in fact, it is important to maintain a small leakage to lubricate and cool the seal and pump shaft. In some cases, it is necessary to inject a lubricant to avoid overheating and this introduces the possibility of contamination of the pumped fluid.
A pump seal requires monitoring and frequent maintenance to avoid excessive leakage, particularly when the pumped fluid contains abrasives. All leaked fluids have to be contained and disposed of safely. If the fluid is also toxic, flammable, radioactive or environmentally damaging, the possibility of leaks, even minor ones, can be extremely dangerous. Leakages are one of the main causes of pump failures or shutdowns and the maintenance of seals and packing materials is expensive and time-consuming.
Environmental concerns and legislation has driven industry to implement cleaner pumping technology. A magnetic drive pump contains the pumped fluid completely within the pump housing. It only has a hermetic seal &#; a stationary gasket or O-ring - that is not subject to wear from moving parts and is therefore ideally suited to applications where no leakage can be tolerated either on safety grounds or because the costs of recovery and treatment are prohibitive.
The coupled magnets are attached to two concentric rings on either side of the containment shell on the pump housing (Figures2&3). The outer ring is attached to the motor&#;s drive shaft; the inner ring to the driven shaft of the impeller. Each ring contains about the same number of identical, matched and opposing magnets, arranged with alternating poles around each ring. 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
The use of these materials is a major factor in the cost of magnetic drive pumps. Most rare earth metals 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. 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, for example polypropylene, PVDF (polyvinylidene fluoride), ETFE or stainless steel.
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 magnets. 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 highly viscous or contains solids. 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 due to the generation of eddy currents.
The inner magnet ring, the pump shaft and its bearing are immersed in the pumped fluid and lubricated by it. 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.
In a canned motor pump, the motor&#;s rotor winding is encapsulated in a &#;can&#; and it, and the entire drive shaft up to the impeller is immersed in the pumped fluid (Figure 4).
Canned motor pumps tend to be smaller, have fewer bearings and can be more efficient. They also offer secondary containment as standard: if the &#;can&#; is ruptured then the pumped medium is contained within the stator housing. This can be a particular advantage if the pumped medium is so dangerous or expensive that secondary containment is essential. Magnetic pump manufacturers offer secondary containment on some designs but this is usually an additional cost.
The main disadvantage of canned motor pumps is that a motor failure requires replacement of the whole unit. A magnetic drive pump can be repaired or upgraded because the motor is not an integral part of the pump. Both canned and magnetic drive pump designs have versions available to handle slurries, liquids at both high and low temperature, and volatile fluids.
Often, the choice of pump depends only on site standards or preferences.
Advances in pump technology have enabled engineers to reduce the size of magnetic drive pumps whilst increasing their power and efficiency. Rare earth alloy magnets with high field strength allow compact design. New pump bearing designs and a wide range of wetted material options have enhanced pump lifetime, lowered power losses due to eddy currents and reduced maintenance downtime.
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Different designs of magnetic drive pumps are available including centrifugal pumps, side channel pumps, turbine pumps, vane pumps and internal and external gear pumps. Magnetic drive pumps can be specified for applications where self-priming or handling solids or running dry is required and are often specified for pumping difficult liquids such as abrasive liquids, acids, bases, corrosives, flammables, inorganic solutions, liquids with entrained gases, organic liquids, slurries, solvents, toxic liquids, viscous and volatile liquids. Magnetic drive pumps are available for a wide range of viscosities at high and low temperatures at capacities from ml/hr to m³/min and for high system pressures and high differential pressures.
All these different designs of magnet drive pumps offer the key benefit of zero leakage of the pumped medium.
Mag-drive pumps are a design of magnetically-driven chemical-process pump which eliminate the need for shaft sealing. This greatly reduces the initial cost of the pump as well as day-to-day operational costs as there aren&#;t any mechanical seals, seal-fluid pots, or cooling lines fitted.
The connection between the motor drive and the pump drive is made via a magnetic connection which works through the isolation shroud. This means that there is no direct or indirect path through which any fluids or gases can escape which can pose a risk to both operators and the environment.
Mag-drive pumps comes in a variety of materials of construction including cast iron / ductile iron, 316 stainless steel, and Hastelloy. This type of pump contributes to the technical standardisation in process engineering thanks to its universal applications.
Mag-drive pumps are available in close-coupled and long-coupled (frame mounted) configurations.
Generally when the product being pumped is of a corrosive nature, or the leakage of that product has the potential to harm operators or cause damage to the environment.
Examples of such products would be:
Most pump types, apart from magnetic drive pumps, have a shaft which passes through the pump casing. The shaft is surrounded by a seal which has the potential to leak. This leakage can be minimised firstly through seal quality, and secondly by the use of a seal-fluid pressurisation system &#; commonly referred to as &#;seal pots&#;. The adjacent image shows a discpac Discflo long-coupled pumping system, fitted with a seal-fluid pressurisation system.
The seal-fluid pressure is higher than the suction pressure in the pump. It prevents, or at least minimises the potential for, the product escaping past the seal.
For applications where the potential of even the smallest quantity of product escaping could be hazardous, however, the mag-drive pump is a solution as there is no shaft seal for the the product to escape past.
This can be seen clearly in the graphic below, where the product being pumped (shown in red) has no connection directly or indirectly to the pump shaft:
Arrangement eliminates need to perform pump / motor alignment. Single piece, dual-bolt circle adapter accommodates required motor sizes for maximum application flexibility.
For ease of dis-assembly for preventative maintenance, overhaul or general periodic inspection.
Minimizes axial thrust for extended thrust bearing life. Unlike two-piece designs, one-piece construction eliminates possibility of front shroud failure.
Fully supported stationary design maximizes radial bearing life and reduces shaft deflection. Straight geometry eliminates stress concentrations and possibility of failure during pump operation
High strength neodymium iron, provide high torque and hard-start capability without slip. Drive is synchronous.
Another option available with a mag-drive pump is an in-line version, where the pump is installed vertically. In-line mag-drive pumps are generally close-coupled, although a long-coupled configuration is possible. In-line configuration can be useful where space is at a premium as the pump is installed directly in the line, just like a valve:
Alternatively, send us a product enquiry for further information on using Mag-drive pumps for specific applications (or information on other pump technologies) or speak to John Scott on Ireland: +353 21 451 ; or UK: +44 .
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