Advantages and Disadvantages of Magnetic Drive Pumps

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Magnetic Drive Pumps are designed to be driven by the force of magnets. Because of this, the shaft of this pump does not come out of the pump casing and therefore does not require a shaft sealing device such as a gland packing or mechanical seal.

For magnet-driven pumps, an external magnet is placed on the drive shaft and is responsible for transferring motion to the magnet attached to the impeller. The impeller rotates and allows fluid to pass through the pump. The two magnets are separated by a backshell, creating what is known as a seal for the fluid, which cannot come into contact with the outside.

Magnetic Drive Pumps

We find magnetic drives on various types of pumps, such as Centrifugal Pumps, self-priming multistage pumps, vacuum pumps, etc. The obvious advantages and disadvantages of conventional drive pumps and magnetic drive pumps should be fully understood before selecting them.

Advantages

Based on the design of magnetic drive pumps, the risk of liquid and vapor leakage is extremely low. As a result, people working on or near the pump are not exposed to the dangers of dangerous, corrosive, flammable or explosive liquids and other toxic chemicals. In addition, expensive liquids are not wasted.

This type of pump requires very little maintenance. The main reason for this is that its design is actually quite simple. In fact, when used for normal operation, it is common for a magnetic drive pump to be used for 10 years or more before it needs any kind of repair. Even then, maintenance costs are often very low.

Another huge benefit associated with magnetic drive pumps is that there is no need to calibrate the pump or motor due to the ease of coupling.

Disadvantages

For applications involving solids, even in small proportions, magnetic drive pumps are not suitable, as they are only suitable for clean liquids that do not contain solids.

Magnetic drive pumps require higher power absorption than conventional pumps. From a cost perspective, magnetic drive pumps tend to be more expensive, however, quick payback can also be achieved due to very low maintenance costs.

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Magnetic Drive Pump (Canned Pump) technology has been commercially available for more than a quarter of a century, yet this technology has not been widely adopted by Industry because of the technology&#;s inherent limitations.

Some end users approach the magnetic drive pump as a cure-all for all pump issues. Unfortunately, this isn&#;t the case. It is fact that magnetic pumps have less tolerance for misapplication and process upsets than conventional pumps and their incorrect specification can therefore cost plants dearly over the life time of the pump.

Magnetic Drive Pumps do have a place in certain industry/application sectors, however this technology is not generally favoured by reliability and commercially aware engineers who operate multi-purpose plants with standardised equipment. Below are some of the pumps limitations, helping the reader to understand why most engineers and plant operatives prefer conventional pump and mechanical seal technology.

Pump Bearings

A major difference between magnetic drive and conventional pumps is the location and type of bearings. In a conventional pump design, the bearings are usually located well away from the pumped liquid in a well-controlled environment. This offers the operator a wide choice of bearing lubricants which can be used to keep the bearings operating cool and reliably.

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However, with magnetic drive pumps, the bearings on the impeller shaft are lubricated by the process fluid and often this is not an appropriate lubricant for bearing life optimisation. In addition, when the pump runs dry or operates at very low flows, the lubricant tends to disappear and the bearings will overheat. More importantly, as the bearings are usually of the sleeve type with slots or grooves to supply the lubricant to the bearing running surface, any solids in the process fluid will be detrimental to the bearing reliability.

Consequently, the bearings in a magnetic drive pump tend to be the first failure point with often catastrophic health and safety consequences or at the very least significant cost implications.

Temperature Considerations

Magnets are temperature sensitive and will demagnetize if exposed to temperatures exceeding their upper limit. To provide some degree of protection against this problem, the material of the magnets should be selected to be able to handle at least 30 degrees C (50 Deg F) above the expected maximum operating temperature.

When processing fluids at elevated temperatures, or in multi-purpose plants with varying batch conditions, this means that the magnets and hence the pump, needs to be specified withstand very high temperatures, which increases the procurement cost.

Operators must also avoid any process upset condition that would cause the generation of heat with this type of pump. Such conditions would include running the pump dry or against a closed discharge valve.

Experience plant engineers will understand that such process upset and operating conditions will be encountered from time to time. Whereas a double mechanical seal and seal support system can protect the pump against such conditions, a Magnetic Drive Pump has no protection and therefore is prone to frequently and prematurely failing.

In summary, failure to correctly specify Magnetic Drive Pumps and anticipate what the duties the pump will encounter in the plant over the full life time of the pump, will result in reduced equipment Mean Time Between Failure (MTBF).

Decoupling

All magnetic couplings are rated for a maximum torque capability beyond which the magnets no longer operate at the same speed. This is referred to as &#;decoupling&#; and, if the pump operates in this state for very long, the magnets will be permanently demagnetized.

Consequently, the magnetic drive pump is particularly vulnerable to any abnormal operating conditions that might result in an excessively high torque demand. The use of power monitors is recommended for all applications in which magnetic drive pumps are used, which increases equipment cost but will not prevent failure.

Commercial availability & Inventory (Stock) implications

Correctly selected Magnetic Drive pumps are typically much more expensive that conventional pumps in a like for like environment.

Refurbishment costs are also significantly more expensive with Mag Drive Pumps and therefore over the life of the pump, despite conventional pumps requiring mechanical seals, mag drive pumps are often much more expensive to operate.

Furthermore, given the limited application base for mag drive pumps, they cannot be readily used across applications, batches and/or in multi-purpose plants. This increases the plants equipment stock costs and increases pump downtime, as Mag Drive pump parts are typically on a longer lead time.

Summary

The paper is designed to help inform operators and procurement engineers about the limitations of Magnetic Drive Pumps. It can be used for training purposes

Unfortunately, there are many reasons why magnetic drive pump technology has not widely taken off as a preferred solution for pumping process fluids in industrial applications.

As such, conventional pumps which use mechanical seal technology with seal support systems, is by far the most commercially and technically attractive long term reliable solution for modern, best practice process plant operations. For further information, please do not hesitate to contact the undersigned

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