Mechanical Seal vs Sealless Centrifugal Pumps

Approximately 88% of industrial process centrifugal pumps in the USA use mechanical seals which have been in commercial use since the 1920s. The concept of sealless magnetic drive centrifugal pumps has been around since the 1940s. In the past 30 years increased market demand for sealless pumps have driven pump manufacturers to continue to develop and evolve sealless pump designs. There are two primary types of sealless pumps; magnetic drive and canned motor pumps. This article will focus on magnetic drive pumps.

What key points should you consider in deciding whether your application is best suited for a pump with a mechanical seal or a magnetic drive sealless pump?

Process Fluid

To determine what type of pump to use in a process plant it’s critical to understand not only the service conditions, but other factors that may be important to the user such as initial cost versus lifetime cost of ownership, spare parts commonality with existing pumps, etc. Certain applications are ideal for magnetic drive pumps, while others are definitely not appropriate for them. For example, applications with a high concentration of solids are inappropriate for magnetic drive pumps unless an external or filtered and recirculated flush is used. Services where the liquid will solidify in upset conditions or on shutdown which will plug the cooling/lubrication passageways inside the pump are also inappropriate for magnetic drive pumps.

Cost

Initial cost is a consideration although not necessarily the most important one in critical services. However in most cases it is very important to go beyond the initial cost. Single mechanical seal pumps typically have lower initial cost BUT a much higher cost over the life of the pump. A properly selected and operated mag-drive pump offers years of trouble-free service; some users have run magnetic drive pumps for decades with no maintenance whatsoever except for routine oil changes or motor bearing grease requirements. For this reason many pump users are slowly retiring all their mechanical seal pumps and replacing them with magnetic drive pumps.

To increase the reliability of a mechanical seal pump double mechanical seals can be used, however they are expensive and generally require an expensive and elaborate barrier fluid system which are a challenge to operate and maintain. Typically a double mechanical seal pump with a seal pot and auxiliary equipment will have a higher initial cost than a sealless magnetic drive pump which makes the choice of which type of pump to buy a no brainer.

Furthermore, a close-coupled configuration can be readily used on mag-drive pumps up to 40 hp, depending on the manufacturer. The close coupled configuration not only eliminates the coupling cost, but the need for motor/pump shaft alignment, which if not done correctly, can result in mechanical seal failures as well as ball bearing failures of the pump power frame and/or electric motor, both of which result in costly downtime. Additionally a close coupled pump package has a much smaller foot print than a long coupled pump, making mag-drive close coupled pumps ideal for applications where space is limited.

Mode of Failure

Mechanical seals are the weak point in a mechanical seal centrifugal pump whereas the internal ceramic inner magnet bearing system is the weak point for magnetic drive process pumps.

The process fluid is circulated within the magnetic drive pump to cool and lubricate the pump’s internal bearing system, just as the mechanical seal in a standard centrifugal pump is cooled and lubricated by the pumped fluid or an external flush. However, unlike a pump with a mechanical seal that allows the pumped fluid to escape the pump when it fails, the mag-drive pump’s bearings and the shaft that supports them are located within a hermetically sealed rear casing/containment shell, so that potential fluid leakage into the atmosphere in the event of a catastrophic failure is virtually eliminated.

Running any centrifugal pump dry will result in a failure. However, the failure mode is different for a pump with mechanical seals versus a magnetic drive pump. Mechanical seals require a small amount of liquid to pass across their faces to provide cooling and lubrication otherwise the seal will run dry and fail. A small amount of that liquid will escape the confines of the seal faces and leak into the atmosphere as either a liquid or vapor. This is why mechanical seal pumps must be monitored for fugitive emissions in VOC (volatile organic compounds) as well as toxic or noxious applications.

Magnetic drive pumps require internal circulation of the pumped liquid to cool and lubricate the ceramic bearing system that supports the inner magnet carrier. To prevent magnetic drive pump bearing systems from running dry many manufacturers recommend a simple power monitoring device that shuts the pump down in the event of upset conditions such as low flow or no flow on the suction or discharge side of the pump. Some mag drive pump manufacturers have developed bearing materials/coatings that are more forgiving of upset conditions and can run dry for limited periods of time depending on the size and weight of the inner magnet assembly. Several companies are working on alternate bearing technologies which will eventually allow dry running for extended periods of time. The typical magnetic drive pump bearing system is manufactured from silicon carbide which while extremely hard is susceptible to thermal shock if the pump is run dry for a period of time followed by a slug of cold fluid entering the pump suction. In addition, different thermal expansion rates between pump components can cause mechanical damage and failures during run-dry conditions, commonly seen on start-up with no liquid in the pump. Another concern is that dry running can increase the temperature to the point that it will deteriorate the strength of the rare earth magnets.

The most common process upset condition involves a temporary loss of liquid entering the pump suction which will quickly result in a bearing system failure. Inexpensive power monitoring devices easily eliminate these problems by shutting the pump down when the power draw of the pump drops below a preset level because of a loss of suction or because the pump is run against a closed valve on the discharge side. These power monitoring devices can also shut the pump down if the power increases beyond a preset limit caused by internal damage to the pump or if the pump is operated too far to the right of the BEP.

A major advantage of some magnetic drive pump designs is reduced radial loading compared with standard, seal-type, overhung models. For example, as illustrated below, a straddle-mounted design with bearings on either side of the inner magnet assembly provides excellent stability, reduces radial loading and allows the pump to operate smoothly when run too far to the left or right of BEP. This is even more critical with high specific gravity liquids such as sulfuric acid.

The straddle mounted inner magnet bearing design illustrated above commonly offers many years of operation without maintenance except for routine oil changes or motor bearing grease requirements. Few if any mechanical sealed pumps in process applications will operate this well.

Magnet drive pumps are an excellent, cost-effective option for providing years of trouble-free operation in many industrial process applications, potentially saving pump users tens of thousands of dollars in maintenance and repair costs over the lifetime of the equipment. Magnet drive pumps are definitely not the answer to every pump application however, they can be an ideal solution across a broad range of applications where eliminating mechanical seals is desired for cost reduction or safety concerns. Magnetic drive pumps are excellent for applications involving toxic, explosive, corrosive, noxious, high purity or expensive fluids. In one particular hot oil application that the author is familiar with, over $100,000 of heat transfer fluid was being wasted each year just to cool and lubricate a number of mechanical seal pumps because liquid was lost across the seal faces in normal operation and from additional leakage when seals were damaged or broken. Less “dangerous” fluids can also be added to the list, including liquids that, when leaked onto a plant floor, might cause employees to slip and injure themselves. Similarly, using magnetic drive pumps to move process fluids with an objectionable odor can provide a much more pleasant working environment.

For more information please call Magnatex Pumps, Inc. @ 713-972-8666 or toll free 866-624-9867 to speak with one of our pump engineers.

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