Sliding vane pumps offer a potent processing option for low-viscosity liquids
Multiple industries rely on process pumps for handling a variety of substances that help the world function as it does today. Those substances come in a wide variety of forms, from very thick materials like asphalt and adhesives to thinner liquids like acetone and water.
Low-viscosity solvents, such as acetone, benzene, ethanol, glycerin, hexane, methanol, toluene and water present several challenges to process pumps. Not all pumps can handle thin liquids well, which can lead to process-performance and efficiency problems. One of the primary concerns is keeping solvents contained within the pump and the associated systems. Some solvents pose health risks, especially with long-term exposure, making containment essential for safety. Solvents can also be expensive, meaning that operations cannot afford leakage — even in small amounts. Operators must also find a way to avoid wasting product that could be left over in the pump and transfer lines. If there isn’t a way to retrieve and salvage a solvent, it must be flushed out and the material is lost altogether. Line-stripping becomes a key feature that operators need from their equipment when processing and transferring solvents. Without it, they also run the risk of product contamination if they are processing more than one solvent through the same pump.
Another challenge is the low lubricity on top of a solvent’s low viscosity. Without lubricating properties, pumps that can’t run dry will struggle and suffer when trying to process solvents. Running a solvent through a pump that requires lubrication means the internal components will sustain damage during the process cycle. Repeated processing will eventually lead to failure.
A variety of pump options are available for handling solvents. For example, one type that is often considered for handling low-viscosity liquids is the centrifugal pump. There is, however, another pump type that is very well suited for handling a variety of solvents and has several features that make it stand out as a choice for this task — the sliding vane pump (Figure 1). These pumps are designed with versatility in mind, making them an appropriate option for handling solvents. This article looks at sliding vane pumps and why they are one of the best-suited process technologies for solvents.
FIGURE 1. Sliding vane pumps are designed with versatility in mind, and are well-suited for handling solvents
How sliding vane pumps work
Sliding vane pumps use a set of sliding vanes guided by a rotor inside the pump. When the rotor turns, the vanes slide in and out of their respective cavities. This motion creates variable chambers, allowing the liquid to flow into one chamber after the other. Each vane provides a positive mechanical push to the liquid in front of it. Once the liquid reaches the outlet, the pumping chamber compresses and discharges the liquid (Figure 2).
FIGURE 2. Sliding vane pumps use a set of sliding vanes guided by a rotor inside the pump. When the rotor turns, the vanes slide in and out of their respective cavities
Vane contact with the chamber wall is maintained by three forces: centrifugal force from the rotor’s rotation; push rods moving between opposing pairs of vanes; and liquid pressure entering through the vane grooves and acting on the rear of the vanes.
Each revolution of a sliding vane pump displaces a constant volume of fluid. Variance in pressure has minimal effect. Energy-wasting turbulence and slippage are minimized, and high volumetric efficiency is maintained. Variances in pumping pressure have little effect on the sliding vane pump’s flowrate, and the open-flow profile provides a gentle and low-shear environment within the pump.
Sliding vane pumps’ operating principles allow them to offer volumetric consistency throughout their operational life, as well as the ability to handle a wide range of liquids, including ultra-thin liquids (0.2 cP) up to 22,500 cP, without compromising their performance.
Tight seals for solvents
Many pumps struggle with solvents due to the fact that they are thin liquids with low viscosities and will find leak points easily. Most of the time, those leak points occur around or near a pump’s mechanical seals.
Sliding vane pumps combat leak points with their design, offering few, if any, areas for solvents to seep through. The magnetic-coupled model, for example, offers zero shaft leakage, meaning even the most hazardous solvents will be contained within the chambers of the pump.
Even if a sliding vane pump experiences a leak, likely from a seal or O-ring, those components are easily replaceable without taking the pump offline. The sliding vane pump’s design allows for quick access, removal and replacement of those components.
Dynamic seals aren’t an issue for the seal-less version of the sliding vane pump. This variety offers even fewer leak points, creating a safe environment for operators processing or mixing dangerous solvents. It also means one less wear part to worry about.
Lubrication not needed
Several processing pumps require a self-lubricating fluid to handle substances like solvents, which have low lubricity. Those pumps will suffer premature component wear and eventual failure if they repeatedly run those types of liquids through them.
Sliding vane pumps, meanwhile, don’t have metal-on-metal contact, which allows them to run dry, meaning processing liquids with or without lubricating properties isn’t an issue. Being able to run dry also means sliding vane pumps don’t suffer from galling or related effects that impact pump life.
No waste product allowed
While sliding vane pumps aren’t the only pumps that can process solvents, some pumps run into the problem of full product extraction. Some pump technologies require flushing the pump out — sometimes with an expensive cleaning chemical — before running a different solvent or other material through the pump.
Sliding vane pumps, however, are ideal at product recovery. Because sliding vane pumps can run dry without detrimental effects, operators can line-strip solvents from the pump, ensuring that the costly material being pumped doesn’t go to waste. This affects more than just the pump. Sliding vane pumps can start empty and still draw a vacuum. This suction force not only clears out product from the pump but also from piping and hoses. The suction lift capability of sliding vane pumps exceeds 25 ft (8.3 m).
These features also allow sliding vane pumps to self-prime. After drawing a vacuum, the pump compresses air in the piping, pushes it through the discharge piping and then draws liquid from the suction source. This process occurs until the pump is primed, all without damaging it.
Because of these features, operators don’t have to worry about running an expensive cleaning chemical to clear out the pump, while also losing the contained material. Additionally, there are no concerns of the detriments that come from an accidental dry-run, because the pump is designed to operate in this capacity.
Thin-liquid processing
Thin-liquid processing is a specialty of sliding vane pumps. The sliding vanes are designed to self-adjust for wear to sustain flowrate. The edges of each vane that contact the pump’s cylinder eventually wear out over time. But the vanes, held within the rotor, slide out as the edges wear, maintaining the same contact as a fresh variant. This means that over the years, even with expected wear on the vanes, the performance of the pump will not suffer because the vanes continue to maintain their contact as they did out of the box. The pump will function optimally over its lifetime, with the vanes ensuring volumetric consistency and preventing internal slip.
When the vanes eventually wear out, an operator can replace them seamlessly without removing the pump from the piping system, ensuring less downtime for the pump. The vanes, along with other wear parts on sliding vane pumps, are easily replaced, allowing operators to enjoy shorter maintenance periods and longer times between intervals.
The low-viscosity nature of solvents plays well in sliding vane pumps, especially given their viscosity range. Along with viscosity flexibility (0.2 to 22,500 cP), sliding vane pumps also have pressure flexibility, handling pressures up to 100 psi (6.9 bars) without sensitivity to a narrow best efficiency point (BEP) common to centrifugal pump designs.
Additionally, this technology is effective with liquid variances as well. From ultra-thin liquids to fluids with suspended solids, to those with seasonal viscosity to vapors, sliding vane pumps are versatile when it comes to the substances they can process.
Additional benefits and features
Functionality is a key attribute for sliding vane pumps. Some pump technologies operate optimally at a specific performance point. Because of a sliding vane pump’s wide and consistent flow range, the technology doesn’t have to reach a particular point to run at its best; it is always at its BEP.
Other pump technologies struggle to reach this point when certain conditions occur, such as extreme weather, but sliding vane pumps always achieve this peak efficiency regardless of the changes in their operating environment.
Sliding vane pumps also function in conditions that would hurt similar pump technologies, such as continuous cavitation in low net positive suction head (NPSH) applications. This functionality plays a key role in solvent processing as vapor pressure — high or low — proves troublesome for other processing pumps.
For example, higher vapor pressure will cause other pump technologies to consume more energy as they process solvents. Sliding vane pumps often handle multi-phase vapor/liquid mixtures well because the flowrate isn’t as harsh as it is in comparable pumps.
The design of sliding vane pumps also assists with component life. These pumps have a between-the-bearing support design, meaning the rotor is supported equally on both sides, resulting in minimal shaft deflection and the prevention of cyclical deflection and fatigue. The sealing surfaces are also immediately adjacent to the bearings, making them stationary and ensuring longer seal life. The ultimate benefit of the between-the-bearing design is that no uneven loads occur, which means even wearing of all components. n
Edited by Dorothy Lozowski
Acknowledgements
All pictures are supplied courtesy of Blackmer
Author
Chris Hordyk is a product manager for Blackmer, a product brand of PSG, a Dover company (1809 Century Avenue SW, Grand Rapids, MI 49503; Email: [email protected]; Website: www.psgdover.com). He joined PSG Dover in 2010 and has enjoyed fitting products for customers ever since. Hordyk initially worked as an application engineer and has recently taken the position of product manager. He holds a B.S.M.E. degree from Michigan Technological University and has prior experience with the hydraulically powered equipment industry. Blackmer is a leading global brand of sliding vane, gear, regenerative turbine and centrifugal pump, and reciprocating compressor technologies for the transfer of liquids and gases.