Useful information on Vane pumps
What is a Vane Pump?
Vane pumps are a type of rotary positive displacement pump. A set of paddle-like vanes, mounted radially on a cylindrical rotor, create a number of compartments in which fluid can be trapped and transported through the system. The vanes maintain a close seal against the wall of the pumping chamber preventing fluid from leaking back across the pump. Vane pumps are particularly useful for pumping thin liquids at high pressures. The pumps give low pulsation, accurate flows and have hardened components to resist wear and extend pump life.
How does a vane pump work?
During a rotation cycle, the volume between adjacent vanes changes because of the rotor’s eccentric mounting position. This creates the pumping action. There are two main types of vane pumps: sliding vane and flexible vane (see Figure 1):
In a sliding vane pump, the vanes are fitted into radial slots in a cylindrical rotor. When the pump is stationary, the vanes rest in their slots. However, when the shaft rotates at sufficient speed (about 700 rpm) the vanes travel outwards under centrifugal force, maintaining close contact with a perforated cam ring around the casing wall. The vanes may also be spring-loaded to ensure contact even when the pump is stopped. Any wear occurring at the edge of the vanes is compensated by further extension.
In a flexible vane pump, the rotor, or impeller, is made from a flexible material, shaped with several supple lobes that maintain contact with the perforated cam ring and pump casing. The rotor is offset and slightly larger than the pump casing so the vanes are compressed at the ‘short’ side of the cycle and expand again at the opposite side as they conform to the internal shape of the pumping chamber. This action creates separate compartments between the vanes, expanding at the pump inlet to create suction and compressed at the outlet causing discharge. Some wear on the rotor’s lobes can be tolerated since it is larger than the pump casing.
Vane pump casings can be balanced, unbalanced or variable. The pumps shown in Figure 1 are unbalanced designs since the rotor is offset: the centre of the drive shaft and the centre of the pump casing are not aligned. Pressure differences between the inlet and outlet can cause vibrations and increased wear on the drive shaft bearings. In a balanced design, the centre of the pump casing and the rotor coincide (see Figure 2). To achieve this, and still create the same functionality, the pumping cavity is elliptical rather than circular. Pairs of inlets and outlets on opposite sides of the pump also cancel out any pressure imbalances. In a variable design, the dimensions of the pumping chamber can be varied. This feature allows the discharge rate of the pump to be adjusted.
There are several other designs including external vane pumps. In these, the vanes (flexible or rigid) are fitted into the pump casing instead of the rotor. Pumps of this type are preferred for handling fluids containing solids.
What are the advantages of a vane pump?
Vane pumps are ideal for pumping low to medium viscosity liquids, including those with entrained gases, and can give an accurate, smooth, low pulsation output. With a varying feed pressure, a vane pump will continue to provide a constant flow. They are especially noted for their dry priming, ease of maintenance, and good suction characteristics over the lifetime of the pump. There is no internal metal-to-metal contact and the pumps self-compensate for wear through vane extension. Vane pumps can handle thin liquids at relatively high pressures and can run dry for short periods. They are also reversible so can be used to load and unload a vessel and also ensure liquid is fully recovered from delivery hoses.
What are the disadvantages of a vane pump?
The efficiency of a vane pump decreases with increasing fluid viscosity so they are not suitable for high viscosity liquids. The maximum differential pressure for vane pump operation is about 15 bar. Flexible vane pumps can be used to pump liquids containing solids but sliding vane pumps are more suitable for clean liquids. Flexible vane pumps can be used with slurries but it may be necessary to limit pump speed to restrict wear. Vane pumps are prone to damaging wear when used with feeds containing abrasives and usually require protection by a suction-side filter.
Unless the pump uses a magnetic coupling, the drive will require a shaft seal of some type and this can be a source of leaks. Mechanically, vane pumps are complex with many parts but any maintenance to replace worn vanes or shaft seals is generally straightforward and inexpensive. As with most positive displacement pumps, some form of pressure relief is necessary in case of a downstream blockage. Vane pumps with multiple vanes will give essentially pulse-free flow but pulsation may be problem at low speeds with pumps fitted with only one or two vanes.
What materials are vanes made of?
Sliding vanes are often made of materials capable of self-lubrication so they can slide easily inside their rotor slots and against the casing. They are available in carbon, PEEK for chemical resistance, glass-fibre reinforced PTFE or bronze.
The rotors in flexible vane pumps are available in a wide range of materials for chemical compatibility with pumped fluids. For example: natural rubber (for water-based fluids), Neoprene, Viton®, Nitrile rubber (for foods, fats, fuels and oils, EPDM (for foods, hot fluids, acidic and alkaline fluids) or silicon carbide (for very high temperatures).
What applications are vane pumps used for?
Typical applications in which vane pumps are used are:
- Transfer of LPG
- Fuel loading and transmission in automotive and aviation systems
- Refrigeration coolants: ammonia, freons
- Chemical industry: transfer of acids, solvents, aqueous solutions
- Solvents, aqueous solutions
- Drinks dispensers
Summary
A vane pump is a type of rotary positive displacement pump. It consists of vanes mounted radially on a cylindrical rotor, which is eccentrically located in the pump casing. The vanes maintain a close seal against the casing wall. During a rotation cycle, the volume between adjacent vanes changes because of the eccentric mounting position, creating suction and drawing in fluid at the pump inlet, and compressing and discharging the enclosed fluid at the outlet.
In a sliding vane pump, rigid vanes are fitted into radial slots in a cylindrical rotor. When the shaft rotates at sufficient speed, the vanes travel outwards under centrifugal force, maintaining close contact with the casing wall. The vanes may also be spring-loaded to ensure contact even when the pump is stopped. In a flexible vane pump, the rotor is made from a flexible material, shaped with several supple lobes that maintain contact with the pump casing.
Vane pumps are ideal for pumping low to medium viscosity liquids, including LPG and those with entrained gases, and can give a smooth, low pulsation output. Pumping speed and efficiency decrease with increasing fluid viscosity. Vane pumps can be used to pump liquids containing solids. It may be necessary to limit pump speed to reduce wear although vane pumps can tolerate and compensate for this to some extent through vane extension. Wetted components are available in a wide range of materials for chemical compatibility with pumped fluids.
