Magnetic Drive Pump Problems and Concerns

8 Common Concerns about Mag-Drive Pumps

I want to address some common magnetic drive pump problems and concerns. Mag-drive pumps are very different from typical sealed pumps. Therefore, it is natural for people to have concerns. I narrowed the list to 8 questions that are commonly asked with regard to mag-drive pumps.

1. Won’t Dry Running Cause Catastrophic Damage to Mag Drive Pumps?

When a magnetic drive pump experiences an upset condition, specifically when it is starved of product, it does not necessarily mean that the pump is completely devoid of liquid. Most of the time, liquid remains in the pump to aid bearing lubrication during brief upset episodes and to prevent the bearings from breaking during brief dry-run periods. Taking it a step further, upgrading standard silicon carbide (SiC) bearings to enhanced SiC bearings with diamond-like coatings vastly increases bearing strength and decreases the coefficient of friction to 1/4 of that of standard SiC bearings.

2. Will Temperature Spikes Impact the Magnets of the Pump?

It is important to select proper magnet material relative to operating temperature considerations. Upset conditions can result in temperature excursions that impact the strength of the magnets, but a power monitoring device can minimize or eliminate this concern. When a mag-drive pump runs dry or against a closed discharge valve, the lower power drawn by the pump trips the monitor, shutting off the pump.

3. What Happens When a Mag-Drive Pump Decouples?

If a mag-drive pump operates for a long time in a decoupled state, the magnets will be permanently demagnetized. However, the same power monitoring device that monitors the pump for temperature excursions also monitors for this condition as well. The design of mag-drive pumps do not make them particularly vulnerable to abnormal operating conditions. The power monitor addresses the concerns about increased capacity, specific gravity, and viscosity. Proper use of a power monitor eliminates many concerns.

4. Is Radial Loading Common with Mag Drive Pumps?

A major advantage of some mag-drive pumps is reduced radial loading compared with standard, seal-type overhung models. The straddle-mounted design with bearings on either side of the inner magnet provides excellent stability, reduces radial loading, and enables the pump to be more tolerant of off peak-operation.

5. What Does the Maintenance Schedule Look Like with a Mag-Drive Pump?

Mag-drive pump designs commonly offer an 8 to 10 year time table, sometimes more, without maintenance. One user discovered his mag-drive pump had operated 16 years without an outage. It did not fail at that point either — he took it down for preventative maintenance. Few sealed pumps in process applications operate so well.

6. Are Mag-Drive Pump Repairs more Expensive Than Sealed Pump Repairs?

The Sealed versus Sealless discussion has been one of great debate for a long time. But In failures of major proportions, this has some potential to be true. However, I submit that the frequent repairs, including replacement of often expensive seals over the life of a traditional sealed pump, would be more than the total cost of a major mag-drive repair. It should be added that major mag-drive failures are not particularly commonplace if some of the noted precautions (upgraded SiC bearings, proper magnet material, power monitors, etc) are implemented. Mag-drive pumps are good, cost-effective options for providing years of trouble-free operation in many pump applications, potentially saving pump users tens of thousands of dollars over the life of the equipment.

7. Can Mag-Drive Pumps Handle Toxic or Dangerous Process Fluids?

Mag-drive pumps can be very reliable in certain applications involving toxic and other dangerous process fluids including those that are corrosive, noxious, and high purity liquids. A mag-drive pump can definitely be solution to handling expensive fluids too; one sealed pump user reported that he was losing over $100,000 per year due to issues including leakage when handing a high-cost thermic oil used for heat transfer.

Less “dangerous” fluids can also be added to the list— including liquids that, when leaked onto the floor, could cause an employee to slip. A risk management department might find mag-drive pumps very attractive. Similarly, using mag-drive pumps to move process fluids with an objectionable odor can provide a much more pleasant working environment. 

8. Will Caustics Damage Internal Rotating Components in a Mag Drive Pump?

Mag-drive pumps can be configured to withstand cleaning from caustic agents. This is a powerful advantage in applications that use process food grade oils due to the need to subject the pump to regular cleaning. If your application requires regular cleaning, be sure to mention it to your pump manufacturer to ensure proper outfitting for this need.

Obviously, magnetic drive pump problems can easily be avoided. employing the preventative steps I discussed will almost certainly help keep your mag-drive pump running without issue. Although magnetic drive pumps are not the answer to every application, they can be an ideal solution across a broad range of applications when eliminating mechanical seals is desired. Magnetic drive pumps, when properly applied and operated, are the obvious choice for many applications.

How Long Can I Dry Run My Pump and Will it Survive? | Enhancing SiC Bearings

In a previous article, we explored why, ideally, a pump should not endure dry running. Let’s now examine if a pump CAN survive dry running under system upset conditions.

SiC bearing material does not tolerate mechanical shock, which is what occurs when there is insufficient lubricating liquid between the bearing component surfaces, leading to contact between the components. There are several situations during system operation that can produce such conditions.

What are Some Common Upset Conditions?

The following are considered system upset conditions, during which the bearing lubricating film barrier is compromised:

• operation of the pump with valves closed

• insufficient pressure at the pump suction (inadequate net positive suction head available [NPSHa]) causing cavitation

• extended periods of operation at extremely low flow conditions

• extended periods of operation at high flow conditions with low pressure resistance at discharge—operation “off the curve” or beyond the maximum flow range

• excessive cycling of system flow from one flow rate to another

• excessive temperature that causes pumped liquid (lubricating flow) to vaporize at the bearing component surfaces—or not enough internal system pressure to keep the pumped fluid in liquid state as it passes through the pump

• energizing motor without liquid in the pump, including briefly to check rotation

• plugged impeller vane inlet at suction eye

Enhancing SiC Bearings

SiC-X Bearing (enhanced)Specially treated sintered SiC-X bearings are available from Magnatex to provide enhanced dry-running capabilities in the case of a system upset condition. All of these are called diamondlike coatings (DLC). Some manufacturers use coatings that adhere to the substrate SiC, while Magnatex uses infusion methods that mechanically integrate the coating into the base material. Integrating the diamond-like substance into the bearing ensure that the bearing will last longer verses those that simply coat the bearings’ surface. These treatments substantially reduce the surface coefficient of friction compared to that of standard sintered SiC. Less friction during system upset and other dry run conditions generate less heat, reducing the potential for bearing component breakage from mechanical contact or thermal shock.

In reference to pumping equipment and product lubricated bearings, no one recommends that the pump be operated without liquid passing through it. There is some residual liquid that remains when the unit loses suction but that will quickly be pressed out of the sides of the bearing components with continued operation. Vaporization of the liquid can occur quickly depending on the size of the pump.

How Long Can Enhanced SiC Bearings Last?

Definitive dry run times would require specific testing of the pump size and conditions of service in question. For small pumps under 2 horsepower (hp), testing has shown that the dry run time is minutes and can be stretched to more than an hour for the smallest pumps with the DLC coated bearings. The greater the power input to the pump, the shorter the time period before damage occurs in the bearing system. For larger pumps in the 50-plus-hp range, while DLC coatings will still help, the time before damage occurs remains quite short. Energizing the motor to check rotation of a pump without DLC coated bearings or liquid in the unit will likely result in some bearing damage. Damage from dry running will initially appear as cracked or chipped SiC components. As the liquid vanishes and the rotating ceramic components come in contact with stationary components, the SiC can shatter, breaking into small pieces. Continued operation damages more pump components such as the shaft, rear casing, casing cover and impeller—all of which are costly parts. This type of damage to the pump is preventable.

What About Other Bearing Types and Seals?

Lined, mag drive pumps use either SiC or nonmetallic bearing materials. Some of the latter may have enhanced lubricity but are still subject to the same axioms of running with compromised lubrication.

Canned motor pumps, the other sealless centrifugal type, typically use softer bearing materials including carbon that are more lubricious, but susceptible to wear. These require a bearing monitor system to avoid component damage.

With mechanically sealed pumps, seal faces operate on a similar hydrodynamic film and require the same type of lubrication. They will not run dry long without damage either.

How to Prevent Dry Running

Again, it is recommended that the pump be protected by a power monitor and that the low power trip point be set at a level representing the first indication that suction has been lost. Setting the low trip point time delay at less than 2 to 3 seconds can prevent tripping of the pump when a large bubble of vapor/air vents through the pump without losing suction. Trying to run longer to empty the suction line of product will result in damage to the pump that is more costly than the small amount of product that may remain in the suction line. A properly set power monitor will save money for the pump owner in several enterprise accounts.

Conclusion

Most damage to mag drive pumps can be avoided. So in revisiting our original title question, “How long can I run my pump dry?” we should instead ask, “Can I or Should I run my pump dry?” Our qualified, “yes,” means that while you CAN run your pump dry, it should be avoided.

How Long Can I Run My Pump Dry? | Can SiC Bearing Hold Up?

Originally by Neal Gunn – Technical Manger

Anyone familiar with industrial magnetic drive (mag drive) pumps has been asked about dry running the pump. After all, a dishwasher and washing machine at home do that all the time. There are bearing materials that allow that, right? The answer is a qualified, “Yes,” but let’s take a closer look at dry running.

Dry running, except in the literal sense, is a misnomer. Industrial pumps are mechanically sealed or sealless and can suffer from the results of what the industry has dubbed “dry run operation.”

Silicone Carbide (SiC) Bearings

Let’s examine the mag drive pump configuration with silicone carbine (SiC) bearing components, one of the two sealless types of rotodynamic centrifugal pumps.

Mag drive pumps have product lubricated bearings that require liquid to be present for proper operation. These hydrodynamic (plain) bearings depend on a fluid barrier separating the bearing components that becomes a wedge when there is relative motion between the two surfaces. When this fluid barrier is disrupted, the two surfaces will touch and can lead to component damage. This effect can be cumulative, resulting in component failure when limits are reached.

If the SiC surfaces touch during operation, the components can, depending on several factors, break in a short time. This does not mean the pump immediately stops operating. The broken SiC pieces may still support the shaft well enough to allow rotation of the pump rotor. Broken SiC can be detected by increased vibration and often a power increase. The condition of the pump will only get worse if operation is continued.

Conclusion

When the pump loses suction, it should be shut down immediately.  Damage to the SiC components can occur in a matter of seconds. Using a power monitor to detect when the motor load has suddenly changed, such as when the pump loses suction, will protect major pump components. Continued operation without adequate liquid at the suction can quickly lead to severe damage of the SiC components.

For long life, the pump requires that there be liquid available and that it be operated within the pump flow range of the curve and within the pump design limits. Please note that it is best to start the pump against a partially closed (about a quarter open) discharge valve. The suction valve should always be open when operating and only closed when the pump must be isolated for maintenance reasons.

So SHOULD a pump be dry run? Ideally, no… but CAN it survive under upset conditions?

Medium to High Head Magnetic Drive Pumps

Close coupled, compact, MPL Series pumps are an efficient and dependable choice for medium to high head applications. The process side of the MPL pump conforms to ANSI B73.3 dimensions. Affordable, high-performance Magnatex pumps give you higher efficiency with lower first cost, lower horsepower and a lower total cost of ownership.

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Is Permeation Putting You at Risk?

Fluoropolymer linings or coatings for centrifugal pumps are a great solution for highly corrosive applications. However, simply selecting the right fluoropolymer coating is not enough to ensure that you will get a cost effective product that will perform maintenance-free for years.

The primary mechanism that reduces the life of fluoropolymer coated metal products (such as centrifugal pumps) is permeation. Permeation is the transfer of liquids or gases into and through the fluoropolymer lining.

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Benefits of Using a Power Monitor to Protect Centrifugal Process Pumps

All centrifugal pumps will fail if they are run dry. One of the most cost-effective ways to prevent this from happening is a simple, inexpensive POWER MONITOR.

How Does a Power Monitor Work?

A power monitor detects low electric power draw caused by loss of flow to the suction or discharge of the pump and immediately shuts the pump down before any serious damage can occur. Power monitors are also set to shut pumps down if they detect high power draw which can be caused by running the pump too far to the right of the best efficiency point or by metal-to-metal contact of some kind inside the pump. 

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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?

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MAGNETIC DRIVE SUB-ANSI PUMPS

Close-coupled, compact, MP Series pumps are the efficient and dependable choice for medium-flow, medium-head applications. The MP Series pump features sub-ANSI sizes for efficient lower flow applications. Affordable, high-performance Magnatex® pumps give you higher efficiency with lower horsepower and a lower total cost of ownership.

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