- PD pumps are inherently self-priming. Centrifugal pumps must be pre-primed. Industrial facilities can design their process around this by including pre-fill cycles, minimum tank levels or expensive below-grade installations. On the other hand, industrial facilities can reduce or eliminate these considerations by using a PD pump that is inherently self-priming. This capability allows the PD pump to draw a suction vacuum and compress air into the discharge piping, all while dry running. This enables the pump to be used in top-offload suction-lift installations or with long sections of suction pipe.
- PD pumps can line strip. Product recovery is critical for both safety and cost savings. PD pumps left on after a batch cycle can evacuate the suction and discharge piping, preventing product spillage when the pumps are maintained or when an operator disconnects a hose. This is a critical safety advantage. Further, PD pumps can extract costly products from the bottom of supply tanks (known as the liquid “heel”). The site may purchase an entire tanker, but a portion of the liquid may be left behind by centrifugal pumps, which creates the risk of product cross-contamination, as well as increased tank-cleaning costs. PD pumps can also recover liquid from within the piping system.
- PD pumps are insensitive to the system’s pressure fluctuations. Some operations feature a batch process, have high backpressure or struggle with operating near a centrifugal pump’s best efficiency point (BEP). PD pumps do not have a BEP and are immune to any system pressure fluctuations.
- PD pumps are viscosity flexible. There is no reduced performance at increasing viscosity (or decreasing viscosity for ultrathin multiphase liquids). PD pumps can operate continuously at 0.2 centipoise (cP) or 200,000 cP viscosities. In fact, many PD-style pumps can be used on a thin solvent and on thick crude oil, which makes them ideal for liquid-terminal or bulk-transfer applications.
- PD pumps operate at reduced speeds. This capability reduces the surface velocity of rotating seal faces and improves seal life. PD pumps often require less than 500 revolutions per minute (rpm), which yields slower sealing velocities, friction-heat buildup and the cooling-lubrication needs of the seal. The result is that seals are more robust, resulting in longer component life and reduced maintenance costs over the life of the pump.
PD pumps are available in two categories—reciprocating and rotary, with rotary pumps consisting of single- or multiple-rotor configurations.
This article will address rotary PD pumps. By their design and operation, rotary pumps displace a fixed quantity of liquid for every rotation of the pump shaft. Again, vanes, lobes, gears and screws are among the pumping elements that can be used to facilitate the transfer of the liquid.
Within the single-rotor rotary-pump family tree resides sliding vane pumps. The design of the sliding vane pump places a series of metal or plastic “vanes” in dedicated slots in an offset rotor in the pump casing. As the rotor spins past the suction port, the vanes are forced out of their slots and ride against the inner bore of the pump casing, forming pumping chambers.
The pumping chambers trap the liquid and transport it around the pump casing to the discharge port, where it flows into the discharge piping. This design virtually eliminates product “slip” (the movement of the fluid being handled against the direction it is being pumped), meaning that the pump’s high volumetric efficiency is maintained.
The method of operation also makes sliding vane pumps ideal for use with thin, low-viscosity liquids, such as propane, ammonia, solvents, alcohol, gasoline, fuel oil, petroleum-based chemicals, refrigerants, and multiphase and high vapor-pressure liquids with zero net positive suction head available (NPSHa).
Based on their self-compensating design that accounts for pumping element wear through the extension of the vanes during operation, PD sliding vane pumps are energy and mechanically efficient. Other operational advantages of sliding vane pumps include:
- dry-priming capability that allows them to run dry for extended periods
- superior suction-lift capabilities
- ideal operation in low-flow, high-head applications
- low-pulsation, low-vibration pumping for shear-sensitive liquids
- suitability for use in metered-flow applications
- optional sealless construction for elimination of potential leak paths
- easy vane replacement
Sliding Vane vs. Gear Pumps
Rotary self-compensating sliding vane pump technology was developed in the early 1900s by an engineer who was looking for a solution to ill-performing gear pumps that were consistently wearing and losing volumetric efficiency, leading to product slip that hampered flow rates.
At the time, gear pumps were far and away the most common type of PD pump used in industrial liquid-handling applications. There are two typical types of gear pumps: internal and external gear, both of which transport liquids through the action of a series of gears coming into and out of mesh. While the pumping action for both is comparable, external gear pumps use two similar rotating gears to mesh, whereas the internal gear pump uses a drive, or rotor, gear operating against a smaller internal, or idler, gear.