Offering a combination of functional, reliable and flexible seal-less performance, magnetic-drive sliding vane pumps deliver zero leakage and numerous operational benefits, explains Geoff VanLeeuwen
There are some things you just can’t avoid: death, taxes and a pump’s common operational pain points (leaks, dry run, solids handling, cavitation-causing NPSH imbalance operation off the BEP and incomplete performance curves) that are inherent in chemical-transfer applications.
It was in the 1980s that the first viable pump technology was created that could ostensibly deliver leak-free performance – a crucial front-of-mind concern in applications where the handling and transfer of high-value hazardous chemicals was required – with a design that could also mediate or eliminate the negative effects of the common operational pain points.
This technology was called seal-less, zero-leak or leak-free, and it had one main task: prevent the leakage of hazardous materials better than sealed pumps through the incorporation of a seal-less design.
There are three reasons why leak-free performance in chemical-transfer applications is paramount. Firstly, most raw chemicals are expensive, meaning that they are too valuable to leak, get flushed down a drain during a cleaning process after a product run and or get left behind in a storage tank, railcar, tank truck or piping system after an unloading event. Secondly, many chemicals are too hazardous for humans to handle or be exposed to, which requires leak-free operation. Finally, many chemicals are caustic or corrosive, which makes them hard to seal and can lead to cracks, breaks or crystallisation on the seal faces, resulting in leak-causing failures.
As an unquestioned step forward in pump design, the first seal-less pumps appeared to possess the ability to meet those three challenges, but they were not readily embraced at the outset because they were more expensive than traditional sealed pump technologies, and there was broad scepticism regarding whether or not the seal-less design could truly deliver on its leak-free promise.
It wasn’t until the early 2000s that seal-less pump technology had evolved to the point where it could claim to be both a pain-point resolver and totally leak-free. A change in the mindset of the user also helped – leak-free pumps were not just being used when handling hazardous materials, they were being used for the handling of basic liquids such as water. This mindset changed because users had become less willing, or capable, of accepting or dealing with seal failures than they were 50 years ago. In other words, even a so-called “nuisance” water leak would require the pump to be taken offline, which would result in expensive downtime and maintenance costs. The solution: pivot to a seal-less leak-free pump that would never (theoretically) need to be maintained or repaired because of a leak incident.
This combination of change in mindset and the now field-proven capabilities of seal-less pumps has turned the pump industry on its head. Today, after steady growth over the past 15 years, the global seal-less centrifugal pump market is valued in excess of US$4.5 billion. In fact, sales of seal-less ANSI centrifugal pumps have the potential to eclipse sealed-pump sales in the coming years. But despite all that, there remain challenges that seal-less pumps must overcome.
The challenges of pumping systems
At this point – while acknowledging that seal-less pumps have essentially cured the chemical-leak conundrum – let’s delve deeper into the causes and effects of the common pump paint points and highlight how centrifugal pumps and internal gear pumps have risen as the leading technologies in the ongoing battle to optimise pump performance.
The dominant technology for seal-less pumps in chemical-transfer applications has been centrifugal pumps because their manufacturers were the first to embrace the technology and develop what came to be recognised as the preferred leak-free pump style. The result is a class of pumps that offers leak-free performance with reasonable reliability when handling a wide range of chemical products. The success of centrifugal pumps also brought a rash of copycat models to the market, to the extent that there are now hundreds of “me too” brands available for purchase, which translates to increased market share.
Similarly, the manufacturers of internal gear pumps quickly adapted their legacy sealed and packed designs to accommodate a seal-less option. Keeping the internal pumping elements unchanged, the early seal-less gear pumps provided a basic leak-free technology that was attractive during the early development of seal-less pumps.
Despite their undoubted success in penetrating the seal-less market in chemical-processing applications, centrifugal and gear pumps have inherent design and operational attributes that create challenges for the user. These users should be aware of these characteristics when designing systems and selecting pump technologies. In short, chemical-transfer applications are rarely pristine and can be very unpredictable, often leading to pervasive system outages and equipment failures if the proper pumping solution is not deployed.
Specifically, we have the aforementioned common pain points in pump operation. Here’s a closer look at each and how centrifugal or gear pumps (or both) can fall short in satisfying these pump conditions.
Dry run is defined as “operating a pump without any liquid,” but although the definition may be simple, the consequences of doing it can be anything but. When a typical seal-less pump rotates without liquid inside, it generates heat that leads to catastrophic failure of internal components. Gear pumps struggle because the internal components are constantly in contact, making them unable to handle dry-run operation without damaging internal gears, idlers, bushings and pins. Looking at centrifugal pumps, sleeve bearings are the weakest component. Various material options exist, but each leaves the pump vulnerable to dry-run failure:
Silicon carbide cracks within seconds of beginning dry-run operation and requires use of power-monitoring systems to turn the pump off in the seconds before failure.
Protective coatings applied to traditional bushings provide a short cumulative wear allowance that is depleted over time, is not renewable and eventually leads to failure, though when that failure is liable to occur is impossible to know succinctly.
Composite blends have self-lubricating benefits, but wear quickly and sacrifice useful life at the fast rotational speeds of centrifugal pumps, leading to failure.
Solids handling: most chemicals contain some level of suspended solids or particulates that come from either the process or supply tanks. The solids will rub against the pump’s casing and other internal components, causing wear. Both centrifugal and gear pumps rely on internal circulation paths, which can be clogged by solids and cause failure. In the case of centrifugal pumps, the solids are thrown around at such high velocities that pitting and premature wear will occur even quicker. For gear pumps, the result of solids handling is the same as dry run: failure. Solids cause gear pumps to lock and fail. At best, solids wear down contacting gear components, resulting in reduced pump capacity.
NPSH imbalance/cavitation: every pump consumes net positive suction head (NPSH). If a pump consumes more NPSH than the system provides, vapour forms and cavitation will result. Cavitation is the violent implosion of entrained vapour bubbles that sends shock waves and vibrations through the fluid. Depending on the intensity and frequency of the cavitation, the pump’s internals degrade, leading to breakdowns, leaks and costly downtime and repairs or replacement. Gear pumps fail fast with vapour and thin liquids because of the galling of internal parts and failed bushings. Centrifugal pumps rely on converting velocity head to pressure head, which is not possible with compressible entrained vapour. In short, both centrifugal and gear pumps fail when operating under sustained cavitation and poor NPSH applications.
Inflexible operating range: pumps are typically built to operate at a single specified design point. Meanwhile, these pumps are installed in dynamic systems that operate across a wide range of operating points. This is impactful to centrifugal pumps, which have a Best Efficiency Point (BEP), or the single operating point where they are most efficient. Maintaining operation close to BEP is critical to a centrifugal pump’s reliability because a centrifugal pump that operates outside of its BEP will see amplified loads that result in excess stress on the bushings and shaft. This stress leads to deflection, rubbing contact, premature wear, leak-path development and compromised product containment. Unlike centrifugal pumps, positive displacement (PD) pumps do not require BEP-reliant operation because they function consistently with changing environmental and liquid conditions. In this way, centrifugal pumps have a narrow operating range, whereas PD pumps offer flexibility to operate across the full system range.
Pump technology as the solution
Sliding vane pumps have proven their standing as a first-choice alternative to centrifugal and gear pumps in chemical-transfer applications because they are simple to use, reliable and flexible. They don’t require tuning to a single BEP; the vanes self-compensate for wear, sustaining like-new performance throughout the system’s operational life; and they can easily handle pumping conditions that feature varying system pressure, zero NPSHa, liquid/vapour mix, suspended solids and regular dry-run operation.
The latest advancement in seal-less magnetic drive technology is the Magnes series magnetic-drive sliding vane pumps from Blackmer. Offering a combination of functionality, reliability and flexibility, these magnetic-drive sliding vane pumps deliver performance in severe-duty liquid transfer for high-value chemical-processing applications.
Magnetic-drive sliding vane pumps solve legacy pain points with technically superior functionality that’s designed for chemical and severe-duty applications. They eliminate sensitivity to intermittent, extended and unexpected dry-run conditions. The dry-run capability is orders of magnitude better than competing technologies that rely on bushings made from sensitive ceramics, temporary coatings or soft composites. In contrast to pumps that boast of cumulative allowances, sliding vane pumps have an indefinite dry-run range. Imagine the flexibility of being unaffected by poor operating conditions and operator error. Consider the increased functionality of self-priming, suction-lift, product-recovery and line-stripping operations.
Unlike most competing technologies that self-destruct when confronted with contaminants, magnetic-drive sliding vane pumps can effectively process liquids with suspended-solids levels of up to 20%.
Performing as a zero-NPSHr solution, magnetic-drive sliding vane pumps are ideal for challenging pump inlet conditions, offering sustained performance with liquids featuring up to 20% vapour or air content.
Unlike alternate options that must be tuned to a single BEP, magnetic-drive sliding vane pumps have the robustness and flexibility to handle multiple changing fluid and system conditions. In other words, vane technology provides the flexibility to operate across the full system range of chemical-transfer systems.
They also boast zero leakage. The containment shell is the foundation of a pump’s leak-free capabilities. This magnetic-drive sliding vane pump’s shell is unlike any currently available, as it offers dry run, unmatched pressure containment, maximum coupling rating and leak-free operation.
The metallic shells in competitive technologies have thin walls that maximise coupling strength; however, these competing metallic shells produce eddy-currents that generate heat and limit pump reliability. Some competing non-metallic shells do avoid eddy-current heat issues; however, these competing non-metallic shells use unreinforced polymers or short carbon-fibre enforcement. As such, these competing shells require greater wall thickness that reduces coupling strength. In contrast, this magnetic-drive sliding vane pump’s containment shells use a proprietary design and production process. The shell is constructed of long carbon fibres embedded in polyether ether ketone (PEEK). This gives the containment shell the coupling strength of a thin-wall shell and the reliability of being eddy-current-free.
The optional close-coupled drive design enables quick and easy system set-up, eliminating time-consuming alignment processes And with suction-lift capability exceeding 25ft, sliding vane pumps offer new functionality, reduce additional system operating costs and enhance safety for all operators by eliminating the need to pre-prime the system.
The evolution of pump technology
The evolution of the seal-less leak-free sliding vane pump has reached its next level with the development of the Magnes series. This advanced pump technology eliminates the pervasive pain points of legacy centrifugal and gear pumps in chemical-transfer applications. Namely, it has indefinite dry-run capability, solids handling, operates well with cavitation and vapour mixtures, and offers full curve and system performance. Combine these features with a new, seal-less magnetic-drive design and today’s chemical manufacturers have an innovative new option when searching for the best pump to deploy in their critical and severe-duty operations.
Geoff VanLeeuwen is with PSG Dover.