Pumping abrasive and shear-sensitive liquids
Today, many processes in the food industry rely upon the transfer of products in a liquid state and the study of the ‘fluids in motion’ is the key to understanding how they will behave within a given process.
A balance between pump efficiency and product integrity always ensues when designing a pump solution for a given process application and, in many instances, a rotary lobe pump is the best solution when considering factors such as cost of ownership for the life of the unit, availability of hours for production, product integrity and flexibility for process variations.
Rotary lobe pumps lend themselves to pumping shear-sensitive and abrasive media that include, but are not limited, to some of the common industry media such as emulsions, yoghurt, surfactants, creams, acids, dyes, waxes, cheese, ice cream, starch, glycerin, chocolate, wastewater sludges, glues and adhesives,
Why a rotary lobe pump?
Rotary lobe pumps are a type of positive displacement pump and, as such, offer some key advantages when compared to other pump types available in the market:
- When designed correctly, they have an ability to run dry for prolonged periods of time.
- Rotary lobe pumps generally operate at slow speed and have a gentle pumping action.
- Rotary lobe pumps give a non-pulsating flow.
- The pumps do not rely on contact-to-contact parts which wear.
- Liquid media with large inclusions can be pumped.
- Only a low net positive suction head is required.
- They provide a constant flow.
- They can achieve higher flow rates, due to less slippage as viscosity increases.
- They are easily cleaned, and can be used in sanitary applications and cleaned in place.
- They are available in various materials of construction to suit application and process requirements, including stainless steel for hygienic applications.
- They are suitable for acid applications.
- Many mechanical seal material options are available to suit application and process requirements.
- They usually have a compact pump housing and gearbox.
- They provide reliable operation with a low cost of ownership.
- They are self-priming and offer robust construction.
Principle of operation
By definition, rotary lobe positive displacement pumps displace a defined quantity of liquid media for each revolution of the lobes/rotors within the pump casing. This is achieved by trapping the liquid media between the dynamic lobes and the pump's stationary casing.
During the pumping process, the liquid medium flows into the cavity formed on the inlet side of the pump as the lobes pass the suction port. The liquid is trapped between the dynamic lobes as they rotate within the stationary pump casing, thus being transported around the interior of the pump casing to the discharge port. The meshing of the lobes forces the liquid media out under pressure through the discharge port of the pump housing. The liquid does not pass between the lobes although, depending upon viscosity and lobe tip speeds, there is some requirement to allow for slippage past the rotors when the pump is in operation.
Rotary lobe versus other positive displacement pumps
The other typical positive displacement type pump used in industry today is the progressive cavity or helical rotor pump that typically has two rotating elements (rotor and stator) used to transfer the pumped media from the inlet to the discharge port, while also transmitting torque. The shafts and bushings of this type of pump are running in the pumped media, potentially increasing the wear and maintenance requirements of the pump.
The biggest threat to this type of pump is running dry while in operation. These types of pumps generally run or rotate quickly to achieve the same discharge rate as a rotary lobe pump, which itself leads to greater pump component wear. Typically, the higher the speed, the higher the mechanical wear of pump components, which can be quantified with known physics. In practical terms, if the speed is doubled, the evident wear rate increases by as much as a factor five, depending on viscosity and other media friction properties.
In comparison, rotary lobe pumps, when designed correctly, have the ability to run dry continuously and with the shafts, bearings, gears and seals all outside the pumped media chamber, abrasive media is able to be pumped with little or no wear to the pumps’ components. Furthermore, the lobes do not touch each other in operation and the nature of the rotating lobes is very gentle on the pumped media. They provide a non-pulsating flow, while also allowing the passage of particles contained within the pumped media. Torque transmission is accommodated through a set of timing gears, in a separate gearbox outside of the pumped media.
Rotary lobe pumps are also suitable for shear-sensitive materials and offer advantages in relation to other pumps available to industry. An important factor affecting shear rate is the amount of pumped media exposed to the shear forces within the operating pump assembly. In rotary lobe pumps, only a small percentage of the pumped media is exposed to shear forces – couple this with the typical slow operation of rotary lobe pumps and you have the perfect solution to maintain product integrity of shear-sensitive and abrasive pumped media.
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