Michelle Knott takes a look at the strengths and weaknesses of sealed and sealless pumps and which applications they suit.
Should you choose a sealed pump or opt for a sealless alternative in your particular application? While the right choice can be straightforward in some instances, it may be far from clear in others.
Upfront costs versus ongoing maintenance, the ability to handle different fluids, temperatures and pressures, the likelihood of dry running and the acceptability or otherwise of any fugitive emissions must all be considered.
If in doubt, it’s worth talking to an expert (say the experts) but a quick rundown of the pros and cons may at least help users begin that conversation from a more informed position.
A sealless rotary pump uses a magnetically coupled drive (or mag drive) to link the motor and the pump impeller.
A mag drive does not need a conventional drive shaft, so it also eliminates the gland packing or mechanical seals that would otherwise be needed to seal around the shaft.
Instead, rotating magnets outside the pump casing are coupled to magnets on the inside to drive the impeller.
Since it’s fully contained, a mag drive pump should reduce the risk of leaks, as well as eliminating the need for ongoing seal maintenance. But these advantages come at a price – in terms of both upfront cost and versatility.
It all depends on the type of fluid you’re asking it to handle. If [the process fluid] is nice and clean and low viscosity, a mag drive may be the right option
Peter Staddon, managing director with The Pump Company
“Nobody wants their pump to leak and no one wants to stop operations to change the seal,” says Malcolm Walker, marketing manager with Axflow.
“If you took cost out of the equation [mag drives are typically between 20 and 40% more expensive upfront than standard mechanical seal-based alternatives], you’d nearly always use a mag drive pump, where possible,” he adds.
As is so often the case, the devil is in the detail, and it’s the “where possible” that needs careful interpretation.
Peter Staddon, managing director with The Pump Company, says: “Mag drive pumps don’t have the flexibility that mechanical seal pumps have. It all depends on the type of fluid you’re asking it to handle. If [the process fluid] is nice and clean and low viscosity, a mag drive may be the right option.”
The biggest group of fluids for which a mag drive will certainly not be suitable are those containing suspended solids. That’s because the internal and external magnets need to be as close as possible in order to generate a secure magnetic coupling and minimise slippage.
This in turn means that the internal magnets must be very close to the wall of the pump chamber, as Walker explains: “You have a tight tolerance inside the wall of the chamber, so any solid bits [suspended in the process fluid] can jam the pump really easily.”
Accidents can happen
That rules out obvious slurry-based applications such as sewage or some types of food, for example. But it’s also worth bearing in mind for any applications where there’s a realistic possibility that particles might be expected to find their way into the process stream by accident.
That is especially important if the particles could be iron-based, according to Walker, since such particles would be actively attracted to the drive magnets, turning the possibility of a jam into a virtual certainty.
“Magnetically coupled pumps don’t need routine maintenance. They’ll run and run. But if they do break down a repair can be quite costly,” confirms Staddon.
High viscosity can sometimes pose another challenge, because a highly viscous fluid may not flow as freely between the magnets and casing, where it may be needed to provide cooling.
“You’ve got tight clearances. Coupled with that, [mag drives] do generate a bit of heat,” warns Staddon.
What’s more, if overheating leads the process fluid to vaporise, this can result in dry running, which happens to be the other main issue that can knock mag drives out of contention.
Walker explains: “Mag drives have very close tolerances so they can overheat really quickly. In normal operation, the whole assembly is floating in the pumped liquid. If there’s no liquid, bits will be touching the side.”
In reality, most people don’t care that much about a pump’s electricity consumption – it’s just not that important compared to systems breaking down and scheduled maintenance costs
Malcolm Walker, marketing manager with Axflow
Pump protection systems can monitor the situation and shut the pump down if they detect dry running, but add-on extras such as these will add to the upfront cost.
Apart from the potential deal-breakers of suspended solids and dry running, the choice between sealed and sealless technologies will depend largely on how far users are prepared to go to minimise the risk of process fluids leaking into the environment (in liquid or vapour form).
This will obviously vary hugely between applications. In the water industry, for instance, a little leakage may not present much cause for concern, but if a pump is handling something toxic, environmentally damaging or very high-value, it’s a different story.
Unlike packing-based seals, modern mechanical seals should be leak-free when they are installed. However, constant friction between the static and moving faces within the seal inevitably leads to wear over time.
This means that seals must be maintained and/or replaced regularly. It also increases the chances of fugitive emissions and unscheduled downtime owing to seal failure.
Some sealed systems are designed to mitigate the risk of emissions. For example, the seal might be flushed with a buffer at a higher pressure than the process fluid, so that any leaks are in the direction of the process, rather than the environment.
However, any extra kit will add to the costs and the system may even end up being more expensive than opting for a mag drive alternative.
Watching the costs
Energy consumption is another black mark against mag drives. Slippage between the two sets of magnets means they typically consume 10 to 15% more power than sealed equivalents.
Yet Walker stresses that added energy costs are usually more than offset by the extra costs of regular seal maintenance.
What’s more, he argues that additional energy consumption is a minor consideration in most industries compared with the possibility of unscheduled stoppages owing to seal failure.
“In most companies, an engineer only gets in trouble if they can’t get liquid from A to B. In reality, most people don’t care that much about a pump’s electricity consumption – it’s just not that important compared to systems breaking down and scheduled maintenance costs,” he says.
If a magnetically coupled pump is the right choice for the application, users then face another decision – whether to opt for a conventional mag drive or a canned pump.
Canned pumps also use a mag drive, but the motor and pump are closely integrated into a single unit. According to Walker, they offer four specific advantages over more conventional mag drives.
The first is double containment, which provides even more security against fugitive emissions. “If the pump is handling something very, very toxic, canned makes sense,” says Walker.
The second reason is somewhat related to this and that’s the ability to monitor virtually everything that’s happening on a canned pump. Axflow was recently asked to supply canned pumps for a big nuclear project, for instance.
A more integrated set-up also means that canned pumps are a good option where space is tight because they’re much shorter than other mag drive units.
Finally, containing everything within the motor chamber means that canned pumps generate very low noise levels compared with the alternatives.
Equipment – Stuck together
There is a common misconception among pump users that mag drives are only an option for centrifugal pumps. In fact, any rotary pump can be magnetically coupled to its motor, including gear pumps, vane pumps and turbines.
“A lot of companies have one or two types of pump in their factories and not much awareness beyond that,” says Laurence Fletcher, sales manager with specialist gear pump supplier Suurmond.
Suurmond offers a good example of why mag drives can be useful in specialised applications.
The company recently designed a blending rig to enable a leading adhesives manufacturer to mix chemical components together to form the finished adhesives immediately before they were applied to labels.
The manufacturer wanted to offer the mixing station to its customers as part of an all-round package alongside its adhesives.
The highly-concentrated adhesive consists of an aqueous component that’s mixed with an additive to form a highly- concentrated adhesive that can then be applied to labels.
Accurate dosing and mixing of the components is vital in determining the properties of the resulting adhesive, so Suurmond chose gear pumps from Gather to provide the required precision and pulseless flow.
The two Gather dosing pumps each delivered a precise volume into a static mixer to create a homogeneous product.
More specifically, the chosen pumps were magnetically coupled because it was essential that they be suitable for cleaning-in-place (CIP).
“With a pump with a seal, they’re not as easy to CIP. There are lots of areas that you can’t get to with the cleaning solutions. The cavities around the seal zone can be very difficult to access,” notes Fletcher.
