Believe it or not, the pump is a truly ancient piece of technology. Industry journal Pumps & Systems highlights how the first known pump originated in Egypt around 2,000 BC. Called the shadoof, it aided in irrigation by using a crane-like apparatus to move water. Pumps have advanced a lot since then, but their basics remain the same: Using some kind of mechanical action, they move liquids, gasses, and/or slurries from one place to another. Magnetic drive pumps — such as the kinds we craft here at March Pumps — are particularly interesting since they replace several standard pieces of pump equipment with a magnetic coupling. This leads to greater endurance and longevity.
Or at least that’s how it’s supposed to work. Truth be told, mag drive pump problems still crop up, and pump failure still occurs with magnetic drive pumps. Fortunately, mag drive pump limitations are relatively easy to understand and manage. When these kinds of pumps fail, it usually owes to dry running, overheating, unplanned for transfer of solid matter, and cavitation (i.e., an issue related to rapid pressure changes). We’ll discuss each of these problems in more detail and describe the steps you should take to avoid them.
To understand some of the ways in which magnetic drive centrifugal pumps can fail, it helps to know how pumps generally work. Pumps are comprised of three basic parts, namely the casing, impellers, and a motor. The impeller creates centrifugal force, which moves material through the pump, and the casing contains and guides that force.
With mag drive pumps, magnetic force replaces the traditional shaft that connects the motor to the impeller, reducing the chance of breakage due to friction — reducing, not eliminating. That matters when dry running occurs. Dry running is when a pump operates without any liquid, and it matters because the pumped liquid provides lubrication for the pump. Because the pump’s only moving part that comes into contact with the liquid is the impeller and because the impeller spins around a shaft at 3450 rpm, mag drive pumps can experience significant friction while dry running.
This friction produces heat, and it can cause the impeller to melt onto the shaft. Sometimes the impeller will melt onto the rear housing. This will cause the pump to cease working and may even lead to leaks as holes develop in the rear housing. (This tends to happen more often with plastic pumps rather than metal pumps.) In order to avoid this, you should never run your pumps while they’re dry.
Temperatures Causing Demagnetizing and Decoupling
Speaking of metal mag drive pumps, they face unique risks when managing high-temperature liquids. The magnets in mag drive pumps are usually crafted from ferrite, neodymium, or samarium cobalt. All of them face declining strength as temperatures rise, but neodymium suffers the most under high temperatures. If operators allow the magnets to cool off, they will regain their strength. However, if they continue to run them, these magnets will permanently lose strength. At a certain material-specific temperature dubbed the Curie temperature, all magnetism will vanish forever. This will cause your pump to stop working.
Additionally, high-temperature liquids may damage the binders that hold magnets in place. Sometimes ferrite magnets use plastic binders, and these binders can warp, causing the magnets to lose their shape. This also will break your pump.
Solving this problem requires some forethought. You should always make sure that the materials which comprise your pumps can handle whatever material you’re pumping. Additionally, don’t pump any new hot matter prior to making sure it’s compatible with your pump.
Transferring Unexpected Solid Material
While certain pumps can handle slurries or particulate matter, mag drive pumps usually don’t excel with such applications. As Processing Magazine notes, “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.”
Another thing to consider is the kind of impeller your mag drive has. Pumps designed to handle “chunky” liquids typically possess flexible impellers that can bend as they encounter solids. Without one, your pump will likely jam or break.
As we’ve mentioned in previous sections, the most important thing you can do to ensure your pump’s longevity is to match sure its construction matches your intended use.
Another potential cause of pump failure is cavitation. Cavitation for pumps occurs when voids form in the body of the pump as a result of rapid pressure changes, and several causes can lead to it. The most of common of these involves insufficient liquid supply. All pumps have certain requirements in order to work correctly, the most important of which being that it has access to at least as much inlet flow as its running speed requires. To put it more simply, the pump needs to have the same rate of material coming in as it does going out. Otherwise, cavitation occurs, and your pump might break.
Other causes of cavitation may include:
- The pipe diameter feeding the inlet of the pump being too small
- Some kind of obstruction
- Human error (e.g., a valve has been shut completely or partially on the pipeline going into the pump)
- The pump is oversized, requiring too much flow
- A variable pump running to fast
- The pump’s net positive suction head requirement (NPSHr) or minimum inlet liquid pressure requirement not being met
As you can see, pump failures and flow issues tend to arise from mismatches between a pump and other elements of a system or an end-use application. Rarely is the problem inherent with the pump. In the future, we will address the ideal mag drive pump setup, but if you’re having trouble with your pump or need a mag drive pump for a specific application, reach out to us at (847) 725-0580. We have more than six decades of experience and are happy to help! Contact us today.