What are the disadvantages of magnetic drive pumps?

In applications where chemicals, acids and corrosive liquids are involved it&#;s necessary to pay a great attention to the kind of pumps used and sometimes it&#;s difficult to choose between seal or sealless pumps. So which are the advantages and disadvantages of mag drive sealless pumps in comparison a with mechanical seal pumps? This is the neverending debate for pump manufacturers and pump users and in this article I would like to point out some aspects to keep in mind when choosing the pump design.

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MAG DRIVE PUMPS

Mag drive pumps have a particular sealless design where the pump is closed coupled to the motor. The external magnet placed on the drive shaft transmits the motion to the internal magnet connected to the impeller which rotates and moves the fluid through the pump. The external magnet and inner magnet are separated by a rear casing that creates a hermetic containment of the liquid that has no access to the outside

Advantages

1. This special hermetic pump design prevents any leakage of fluid and fugitive emissions, that in case of chemicals, corrosive liquids, explosive and flammable fluids could be very dangerous for people dealing with the pump and especially for the environment. So mag drive pumps allow to follow strict environmental and safety objectives required by many regulations.  We shouldn&#;t forget also that some liquids could be very expensive and their loss due to a seal failure may cause high unnecessary extra costs.
2. Mag drive pumps are very reliable and need very low maintenance thanks to their simple design. With normal working conditions these pumps can work without any kind of repair for more than a decade so their life cost is highly reduced  even if it&#;s always better to check o-rings and bearings every one/two years just to be sure that there is no wearing.
3. The coupling is very easy because there is no need for a motor/pump alignment.

Disadvantages

1. In applications involving even a small percentage of solids the mag drive system is not the suitable solution. Magnetic drive pumps, in fact, can work only with clean liquids without solids in suspension.
2. Magnetic drive pumps are usually more expensive than mechanical seal pumps. However, as written above, the maintenance costs are very reduced during the life of the pump and these long-term financial advantages should be considered when choosing the pump design.

MECHANICAL SEAL PUMPS

The seal in mechanical seal pumps is composed by a static ring and a rotating ring placed on the pump shaft which is directly coupled to the motor shaft. The two surfaces sliding together need to be lubricated and the seal lubricant is the liquid itself that is being pumped.

Advantages

1. Mechanical seal pumps are the perfect solution for applications involving solids in the liquid (for example waste water treatments) because their design with open impeller allows to pump very dirty liquids and fluids with high viscosity.
2. The cost of this kind of pumps is lower in comparison with mag drive pumps so, if the financial aspect is critical when choosing the pump, sealed pump may be the right selection.

Disadvantages

1. Seals are often the weak point in standard pumps because they are the first cause of failure in a chemical process. When a mechanical seals fails it allows the liquids to escape causing dangerous leakage.
2. Considering the point above, to avoid harmful events and dangerous leakage it&#;s necessary to plan a periodical maintenance of mechanical seal pumps to check the status of the seal and replace worn parts. In this case the costs of maintenance of these pumps could be higher than those of mag drive pumps.

In some applications it&#;s possible to install both mechanical seal or mag drive pumps and technicians can decide according to the advantages and disadvantages of each pump which is the best for them.

Pumps are usually categorized in two major groups: conventional-sealed pumps and seal-less pumps. The first group contains pumps that need a seal system, usually in the form of a mechanical seal. The second group includes all types and models of seal-less pumps; these pumps do not require a seal. Magnetic-driven pumps are an important class of seal-less pumps. They are used for services in which leaks cannot be tolerated or liquids are difficult to seal. Magnetic drive pumps, also known as magnetically coupled pumps, vary from the traditional pumping style because the electric motor (the driver) is coupled to the pump by magnetic means rather than by a direct mechanical shaft. The pump works via a drive magnet that eliminates any shaft sealing required by the pump. This is a major advantage. On the other hand, magnetic drive pumps cannot be used in some applications and do have limits for power ratings. This article discusses magnetic drive pumps in processing facilities and their characteristics, advantages and disadvantages. 

Magnetic drive pumps

Magnetic drive pumps are typically used where leakage of the pumped liquid poses a great risk such as with aggressive or risky liquids, exotic materials, acids, alkalis, corrosives, pollutants and toxics. They are also used for ultra-pure liquids and hard-to-seal liquids. Sealed pumps applied to these types of services may leak over time or require complex, expensive double seals to prevent hazardous/challenging liquids from escaping to the atmosphere, which can lead to safety hazards, downtime and increased maintenance requirements. 

Another important application for magnetic drive pumps is for difficult liquids; for example, some liquids can crystallize on seal faces, which then can cause seal failures. To avoid this, a permanent flush system should be run to the seal. This, however, can increase the cost of maintenance, seal flushing liquids and energy consumption. A better solution for these difficult services is to use a magnetic drive pump.

Magnetic drive pumps have no direct connection between the electric motor shaft and the impeller; therefore, no seal is needed. There is no risk of leakage unless the pump casing is broken. Seals are known as a major cause of pump trips and unscheduled shutdowns. Obviously, the elimination of seals is a great improvement for performance, reliability and availability of pumps. The risk of leakage is completely eliminated; this means that liquids can be pumped without spillages. Eliminating the seals also gets rid of associated friction loss, wear, costs and noise. This provides complete separation of liquid from the pump drive and better transfer of motor power to the pump. There is practically no heat transfer from the electric motor because the pump chamber is completely separated from the electric motor by a large air gap; this provides an effective barrier between the two. The magnetic coupling will soften any shock or spike torques. Even in extreme situations, it will act as a fuse. Magnetic couplings can be broken temporarily if the load of the pump is too great. In practice, it means the pump does not overload and get damaged. 

Magnetic drive pumps are available in most metallurgies and materials, metallic and nonmetallic. Polymer lining pumps are also used because they offer improved corrosion resistance. Polymer coating options include polytetrafluoroethylene (PTFE), perfluoroalkoxy alkanes (PFA) and polyvinylidene fluoride (PVDF). These lined or non-metallic options are usually employed for ordinary temperatures, below 90°C as a rough indication. Metallic magnetic drive pumps have been used for even higher temperatures. 

Bearings for magnetic drive pumps

Because a magnetic drive pump is an enclosed piece of equipment, lubrication oil or grease cannot be used for bearings. Therefore, pumped liquid is used for the lubrication of bearings as well as for cooling needs. A portion of pumped liquid is usually taken from the discharge of the pump; this flow is often known as recirculation flow and is usually employed for the cooling of the magnetic system, lubrication of radial and thrust bearings and for other similar applications where lubricant or cooling fluid is needed. 

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Magnet drive pumps are usually equipped with "sleeve bearings," which use the pumped liquid for hydrodynamic lubrication. A sleeve bearing is one of the simplest types of bearing, comprising just bearing surfaces and no rolling elements. Therefore, the sleeve or journal slides over the bearing surface where pumped liquid is used for its lubrication. These bearings are also compact and lightweight and have a high load-carrying capacity and longer life compared to many rolling-element bearings. The materials of construction of these and the required clearances of the sliding surfaces may restrict the liquids and services for which this kind of pump may be used.

Traditionally, magnetic drive pump bearings have been made from silicon carbide, which has been an excellent bearing material under normal operation; however, it can suffer during some abnormal conditions such as dry running, dynamic loading and potential impact. Some modern materials such as advanced ceramics have been used in these with great success. Another proposed material is a fiber-reinforced silicon carbide ceramic composite with bearing properties similar to silicon carbide, but with high impact resistance and excellent tolerance of dry running. These modern options offer great benefits such as better impact resistance, excellent wear resistance and generally longer life and better performance. 

Modern sleeve bearings for magnetic drive pump applications should feature smooth surfaces whether by a dedicated surface treatment on sleeve surfaces or a coating to sufficiently reduce the coefficient of friction, which enables longer periods of safe operation under dry run conditions. Dry running can occur from a wide range of malfunctions and maloperations. The heat generation from dry running conditions is the number one mode of failure for magnetic drive pumps. Modern bearing materials are usually up to two times harder than conventional silicon carbide, ensuring they won&#;t be compromised in even the harshest operational environment.

To successfully perform its task of lubrication and cooling, the recirculated flow should remain in the liquid phase at all points at all times. An area for concern is the magnetic coupling area where the intense heat could boil the liquid. Therefore, a robust design and sufficient flow rate are needed for these areas. The determining factors for preventing phase change are the mass flow rate, heat capacity and localized pressure at any point within the system. The details related to recirculation flow will affect the variables of mass flow rate and localized pressure.

To act as a lubricant, the pumped liquid should maintain a proper film thickness (at least a few microns) at operating temperature and load or else the sleeve bearings will experience severe wear. Many liquids such as hot water and most solvents do not have this property and cannot act as lubricant. On the other hand, if sufficient pressure or flow rate is not provided for the liquid through the bearings and other critical areas, it leads to overheating of the area, which can become a serious problem. The result can be flashing of the liquid and a potential loss of lubricating ability as the liquid increases in temperature and decreases in viscosity. 

Disadvantages and limitations

Because magnetic drive pumps utilize magnets to transfer the torque and power from the drive assembly to the impeller assembly, there are some limitations with these pumps. For example, when magnet materials are exposed to temperatures above their threshold, they can begin to lose their magnetism. Therefore, temperature characteristics of each service are a critical factor. Some energy is lost in the magnetic coupling. This is primarily due to some magnetic resistance. As the result of this and some other effects, a typical magnetic drive pump is usually less efficient than conventional centrifugal pumps.

There are some limitations in power rating as a very large or powerful magnetic coupling is neither feasible, nor cost-effective. Power and torque limitations should always be considered for the selection of these pumps.

One of major limitations of magnetic drive pumps is the risk associated with running dry. Because the pumped liquid will act as lubricant and coolant, in case of dry run, the bearing and some other parts could overheat and eventually become damaged. In services and applications with risks of dry running, magnetic drive pumps should not be used. 

Magnets should be sized correctly so magnetic-coupling breakaway torque is not exceeded during startup or in all other potential transient operational cases. If breakaway torque is exceeded, the magnetic coupling between the drive and the driven assemblies is lost, and the impeller will cease to spin, which means the pump should be tripped to allow the magnets to recouple. These pumps are also extremely sensitive to transient cases such as low flows and operation near shutoff head conditions. 

Magnetic drive pumps for liquids containing solids

Magnetic drive pumps have very limited solids handling capability. The pumped liquid should be clean or else the solids will collect in the passages surrounding the magnet and in the close tolerances of bearings such as between the sleeves. This will interfere with the pump&#;s performance and cause problems, sometimes even failures. In addition, solids will wear out the bearings and other components.

Liquids containing ferrous particles are problematic in a magnetic drive pump because the particles collect on the magnet and can cause the pump to weaken and even stop working over time. Theoretically, sleeve bearings might be able to grind up some particles of a lesser hardness; this has resulted in some claims for solids handling capabilities of magnetic drive pumps. These "wild" claims by some manufacturers have been extraordinary, exaggerated and even false. This solids handling capability depends on details, like the size of the solids as well as the materials and characteristics of the bearings &#; the presence of some solids can be less problematic with hard and strong bearing materials than with weaker materials used in some low-cost bearings. 

As a rough indication, the upper percentage and size limits of solids in the pumped liquid might be noted as 1.5 percent and up to 70 microns. For some special pumps, the latter might be up to 100 microns. There was a case of very strong and robust magnetic drive pumps that successfully handled 2 percent or 2.5 percent of solids to 150 microns. The limit on particle size is usually dictated by bearing clearances. Higher particle size may be screened out with a screen, strainer or filter in the way of liquid to bearings. There can be pumps operated for 5 percent solids (or sometimes more) up to 200 microns (or even more) if a clean flush is provided to the magnetic drive or if a closed-loop system is fitted; such a system is a special one available only from a few manufacturers.

Indeed, the above-mentioned figures are some very simplified and rough limits. For a reliable and optimal selection of pump for each service, comprehensive analyses and studies should take into account characteristics and the nature of solids. For instance, sticky or fibrous solids may block bearings or flow channels. Highly abrasive solids will eventually wear parts, and in these services, strong wear-resistant materials should be selected for the maximum pump life. All potential operating ranges should be considered for a proper pump selection. At low flows, the recirculation will accelerate wear, and at higher flow rates, the high rates of solids will increase the wear rate.  

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