What is a Rotary Pump?
Rotary pumps are positive displacement pumps, which mean they pump a fixed amount of liquid after each revolution and are mostly used to treat viscous fluids. Regardless of the resistance to which the pump is pushing, a fixed amount of fluid is pushed with each revolution of the pump. It self-primes and maintains a nearly constant delivered power independent of pressure.
In terms of numbers, rotary pumps are the second most common kind of pump. They are also the second most cost-effective choice after centrifugal pumps.
Functions of Rotary Pumps
They can work on fluids that have a very high viscosity. However, as viscosity rises above a certain threshold, the most effective speed decreases. This is determined by clearance and shear action. The clearance is usually opened up by the production company with high viscosity fluids to minimise power consumption and retain low shear effects on the component.
Their capacities vary with speed but their effect on the slip in the low viscosity ranges is somewhat impacted by pressure but this effect tends to be reduced to a point as the viscosity increases.
Rotary Pump Parts
A fixed case comprising gears, cams, vanes, screws, plungers or related components, which are actuated by a rotation of the driving shaft, which are some of the types of the rotary pump.
Normally, rotary pumps are driven by electric motors, geared motors or motor belt drives. Only a steam turbine and in extremely unusual circumstances an internal combustion engine drive a rotary pump.
Instrumentation and control
Capacity regulation is achieved either by changing the pump speed or by recirculating a part of the discharge. The recirculation of fluid is a typical control mechanism, since rotary pumps are mostly powered with electric drives at a constant speed.
To safeguard rotary pumps from exposure to foreign materials, a continuous suction strainers should be mounted. This extends to most rotary pump projects except in comparatively clean plants where initial costs do not warrant a strainer to cover the pump.
A dirty or clogged strainer can affect the pump by raising the friction loss in the suction piping.
Rotary pump design
These pumps have been developed to minimise leakage from the discharge side to the suction side with very narrow gaps between their revolving and fixed sections. As they run at comparatively slow speeds, they are vulnerable to corrosion and unnecessary wear at higher speeds, which lead to greater clearances and lower pumping power.
Features of Rotary Pumps
• Adapted to transmit viscous liquids and liquid slurry (screw pumps)
• Movement of pulse less metering (gear pumps)
• Easy to use and maintain (rotary pumps, lube pumps)
• Ability to move highly viscous fluids and solids (hose pumps)
Types of Rotary pumps
Rotary pumps are broadly classified based on the type of their rotating element.
- Gear type pump
- External gear pump
- Internal gear pump
- Vane type pump
- Screw pump
- Lobular pump
- Cam and piston pumps
Internal Gear pumps
Internal gear pumps carry fluid from the inlet to the outlet ports between the gear teeth. On a stationary pin, the outer gear (rotor) drives the inner or idler gear. When the gears emerge from the mesh, voids form, and liquid flows into the cavities. The gear transmissions return to a mesh, reducing the volume and forcing the liquid out of the port of discharge. Liquid cannot flow backwards from the outlet to the inlet port because of the crescent.
External Gear Pumps
Gears that come in and out of mesh are often seen in external gear pumps. Water runs towards the pump as the teeth emerge from the mesh, which is brought to the discharge side of the pump between the teeth and the casing. The liquid is pulled out the discharge port while the teeth return to mesh. Two equal gears are rotated against each other by external gear pumps. Both gears are mounted on a shaft with bearings on both sides.
The vanes, which are blades, buckets, rollers, or slippers, act with a cam to pull fluid into the pump chamber and drive it out. Vanes can be used in either the rotor or the stator. Pumping components for vane-in rotor pumps may be either fixed or variable displacement.
Between the rotor teeth and the pumping chamber, fluid is transported. Continuous sealing is created by the rotor surfaces. Timing gears drive all gears and synchronise them. Biwing, trilobe, and multilobe rotors are available configurations.
Fluid is carried in the gaps between the screw threads by screw pumps. When the screws mesh, the fluid is displaced axially. Progressive cavity pumps are a type of single screw pump. They have an externally threaded rotor and an internally threaded stator. The rotor threads are offset from the rotating axis.
Fluid travels from the inlet to the outlet through the gaps between the piston surfaces. Each rotor can have one or more piston parts, and each rotor must be timed separately.
Applications of Rotary pumps
Rotary pumps are ideal for pumping oil and other viscous liquids. Rotary pumps are used in the engine room to handle lube oil and fuel oil, and they can handle liquids with a wide variety of viscosities.
Advantages of Rotary Pumps
To minimise leakage (Slippage) from the discharge side back to the suction side, rotary pumps are equipped with very limited clearance between moving parts and stationary parts. To ensure these clearances, rotary pumps are configured to run at low speeds.
Disadvantage of Rotary Pumps
Higher speed activity induces corrosion and unnecessary wear, resulting in increased clearances and a reduction in pumping capability.