There are some things that cannot be avoided, including a pump’s common operational pain points such as: leaking, solids handling, dry run, cavitation-causing NPSH imbalance operation off the BEP, and incomplete performance curves, all of which are inherent in chemical-transfer applications.
In the 1980s the first viable pump technology was created that could ostensibly deliver a leak-free performance, a crucial concern in applications where the handling and transfer of high-value hazardous chemicals was required. The pumps were made 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.
By Geoff VanLeeuwen, PE, Director of Product Management for PSG® Grand Rapids
The seal-less, zero-leak, or leak-free technology had one important task: prevent the leakage of hazardous materials through the incorporation of a seal-less design. There are three reasons why leak-free performance in chemical-transfer applications is paramount:
1. 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.
2. Many chemicals are too hazardous for humans to handle or be exposed to, which requires leak-free operation.
3. Many chemicals are caustic or corrosive, which makes them hard to seal and can lead to cracks, breaks or crystallization on the seal faces, resulting in leak-causing failures.
The first seal-less pumps appeared to possess the ability to meet these three challenges, but they were not readily embraced at the outset for two reasons: they were more expensive than traditional sealed pump technologies, and there was broad skepticism regarding whether or not the seal-less design could truly deliver on its leak-free promise.
It was not 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 like water. This combination of change in mindset and the field-proven capabilities of seal-less pumps have turned the pump industry on its head. Now, after steady growth over the past 15 years, the global seal-less centrifugal pump market is valued in excess of USD $4.5 billion. In fact, sales of seal-less ANSI centrifugal pumps have the potential to eclipse sealed-pump sales in the coming years. Despite all the success however, there remain challenges that seal-less pumps must overcome.
Pump Pain Points
While acknowledging that seal-less pumps have made a giant leap towards curing the chemical-leak conundrum, it is still important to delve deeper into the causes and effects of the common pump pain points and highlight how centrifugal pumps and internal gear pumps have risen as leading technologies in the ongoing battle to optimize pump performance.
The dominant technology for seal-less pumps in chemical-transfer applications has been centrifugal pumps, as centrifugal pump manufacturers were the first to embrace the technology and develop what has come to be recognized as the preferred leak-free pump style when handling a wide range of chemical products.
Similarly, the manufacturers of internal gear pumps quickly adapted their 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 success in penetrating the seal-less market in chemical-processing applications, centrifugal and gear pumps have design and operational attributes that could create challenges for the user. Therefore, users should be aware of these pain points and 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.
Dry run is defined as ‘operating a pump without any liquid,’ but while 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 potentially 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, such as silicon carbide, protective coatings, and composite blends, but each leaves the pump vulnerable to dry run failure.
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.
Every pump consumes Net Positive Suction Head (NPSH). If a pump consumes more NPSH than the system provides, vapor forms and cavitation will result. Cavitation is the violent implosion of entrained vapor 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 vapor 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 vapor. 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, specific pumps are installed in dynamic systems that operate across a wide range of operating points. This has an impact on 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. 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.
Sliding Vane Pumps
Sliding vane pumps have been proven as an alternative to centrifugal and gear pumps in chemical-transfer applications because they are simple to use, reliable and flexible. These pumps do not require tuning to a single BEP; the vanes self-compensate for wear, sustaining like-new performance throughout the system’s operational life. They can also easily handle pumping conditions that feature varying system pressure, zero NPSHa, liquid/vapor mix, suspended solids and regular dry-run operation. An example of an advancement in seal-less magnetic drive technology is the MAGNES Series Magnetic-Drive Sliding Vane Pumps from Blackmer®.
Sliding vane magnetic-drive pumps are able to solve pump pain points with functionality that’s designed for chemical and severe-duty applications. Key attributes of this type of pump include:
Indefinite Dry-Run Capability: Magnetic-drive sliding vane pumps eliminate sensitivity to intermittent, extended and unexpected dry-run conditions.
Solids Handling: The sliding vane pump can effectively process liquids with suspended-solids levels of up to 20%.
Low Required NPSH: Performing as a zero-NPSHr solution, these pumps can handle challenging pump inlet conditions, and offer sustained performance with liquids featuring up to 20% vapor or air content.
Full-Curve Performance: Unlike alternate options that must be tuned to a single BEP, these pumps can handle multiple changing fluid and system conditions.
Zero Leakage: The containment shell is the foundation of a pump’s leak-free capabilities. The sliding vane pump shell offers dry run, high pressure containment, maximum coupling rating and leak-free operation.
Zero Alignment: The optional close-coupled drive design enables quick and easy system setup, eliminating time-consuming alignment processes.
The evolution of the seal-less leak-free pump design has been advancing for decades. Innovative new pump technologies, such as the MAGNES Series Magnetic-Drive Sliding Vane Pumps from Blackmer®, can eliminate the pervasive pain points of legacy centrifugal and gear pumps in chemical-transfer applications. Some of the most important aspects are the indefinite dry-run capabilities, solids handling, and operating well with cavitation and vapor mixtures. Combining these features with a next-generation seal-less magnetic-drive design, chemical manufacturers have new options when searching for the best pump to deploy in their critical and severe-duty operations.
ABOUT THE AUTHOR:
Geoff VanLeeuwen is the Director of Product Management for PSG® Grand Rapids, and can be reached at email@example.com.
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