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What are the advantages of seal-less pumps

FlowEd

New member
Hi,

I've had this question sent to me as editor of Flow magazine, I'd be interested in the thoughts of the forum users.

With todays modern technology, am I best moving to seal-less magnetic drive technology versus Mechanical seal?
 

TonyCrest

New member
As a manufacturer and distributor of both magnetically driven and mechanically sealed pumps I feel I can answer this in an unbiased way, but it is really a case of horses for courses.

Magnetic drive pumps have the obvious and significant advantage in that they do not leak so are ideal for handling aggressive or toxic chemicals. However there are limits to their application. Generally they are manufactured with closed impellers that rotate on a fixed shaft with internal bearings, this makes them only suitable for very clean liquids or with absolute minimal solids content. Another potential disadvantage of magnetic drives is that they have limited torque and power transmission, not a problem when handling liquids with a low specific gravity but when higher SG (generally 1.3 and above) liquids need to be pumped then the impeller often needs trimming to accommodate the magnetic couplings power requirements. Often this means using a larger and more expensive pump than if it was directly driven.

Mechanically sealed pumps have been in use for very many years with seal manufacturers constantly updating their designs and material technology. Whilst any seal can leak over time they can also give a long and trouble free life if care is taken with initial selection and the pump manufacturer is furnished with full details of the application. Advantages of mechanically sealed pumps is that they can handle a greater degree of solids in suspension (particularly if fitted with an open or semi-open impeller) and that there is a direct connection between the motor and the impeller.

I hope this helps.
 
Hmmm, an extremely interesting question that provides years of debate for process, reliability, production, and mechanical engineers alike.... Echoing the view of Tony above;

Magnetically Driven pumps primarily fit with Toxic, Corrosive, Hot media, and Chemicals that are typically harmful to people and our environment. They have some solids handling capabilities, within the same boundaries that one would expect from a single mechanical seal. Free-flow solids filtration is often an integral feature of magnetically-driven pumps for this reason. Double containment options also provide similar levels of security for dangerous media - like Chlorine derivatives, Thermal oils, Acids, Carcinogens, etc.

Fundamentally basic technology mag-drive machines struggle to handle (a) high/low/fluctuating temperatures because of the difference in thermal expansion (ca 6x) between the ceramic bearing components and metallic parent materials, and (b) Dry-running, as is also the problem with single mechanical seals. Power losses can also be higher than sealed pumps because of the Eddy-Current losses across metallic containment shells. Consequently, modern designs with ceramic containment shells, internal filtration, and thermal stability features are essential for higher-level applications where reliability and running power are important.

Single mechanically sealed pumps offer a perfect fit-for-purpose pumping solution; also for relatively clean liquids. When the mechanical seal eventually fails, it is a low-cost simple task for the maintenance department. Care needs to be taken for spills in the event of failure, as toxic/hazardous media can leak to the environment. Pungent media can also pass single mechanical seals and leave surrounding odours. Seal-face, and elastomer materials need careful consideration and advice taken if the specifier is unsure about potential erosion, corrosion, or thermal shock...
Double mechanically sealed pumps would ordinarily be used for dangerous liquids, potential dry running, and/or presence of solids. Along with the essential ongoing maintenance of the barrier fluid system, the seal materials need careful consideration. Selection of barrier fluid is also important with pressurised double seal systems to avoid process contamination. Often the double seal & its support system can be more expensive than the pump itself. It is this Whole-Life-Cost balance that underpins the decision between a double mechanically sealed pump & support system versus a stand-alone seal-less magnetic drive machine.

Shaft deflection (stiffness ratio L3/d4) at the seal face, and seal chamber design, play critical roles in mechanical seal life & reliability. But that's another topic...
 
Just thought I would add my comments also as I also work for a manufacturer that produces both Mag-drive and Mechanically sealed pumps. I would again agree with the comments above.

One thing that should be considered - Is there a danger to the environment/life if there is a leak? If the answer is yes due to the medium being pumped, then a mag-drive is possibly the best solution. I have come across an instance many years ago (Not with my current employer) where a double mechanical seal was used. It was fine under normal operating conditions, but the plant had a shut-down for a period over summer. The pump was in direct sunlight and the isolation vales on the suction and discharge were both closed. As a result of this, as the medium expanded in the heat, the weakest point was the mechanical seals. They leaked. The medium ignited on contact with air and this resulted in a fire. Fortunately because the isolation valves were both closed, the fire was limited. Note: It is not good practice to shut both valves on a pump unless you are removing it. If they had not both been closed the medium could have expanded throughout the system and there would not have been an issue.

In normal applications with a non toxic/hazardous medium I would always use a mechanically sealed pump. Where the medium is more aggressive, speak with the pump manufacturers and they will be able to advise on an application by application basis. There are many reputable pump manufacturers that manufacture chemical process pumps with double mechanical seals. The designs are tried and tested and are reliable if operated and maintained correctly.
 
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