A rotary pump refers to a pump that changes the working volume by the relative motion between the rotor and the pump body, thereby increasing the energy of the liquid. It is a rotating positive displacement pump that has unparalleled advantages over other pumps for transporting liquids of different viscosities. So, under what circumstances should a rotary pump be selected?
1. Viscous liquid
The viscosity range of the medium that a rotary pump can transport is very wide. It can handle media with very low viscosity, such as liquefied petroleum and liquid ammonia, as well as very viscous media, such as extremely viscous media with a viscosity of 1000000mm2/s.
For centrifugal pumps, as viscosity increases, their flow rate and head decrease, and power consumption increases. When the viscosity of the medium is greater than 20mm2/s, the centrifugal pump needs to undergo performance conversion When NPSH is much smaller than NPSH, if efficiency and energy consumption are not considered, centrifugal pumps can sometimes be used in situations where viscosity reaches 550mm2/s; When NPSH is close to NPSH, if the viscosity is high, the centrifugal pump is likely to experience cavitation.
For rotary pumps, on the one hand, as the viscosity increases, the internal leakage decreases, the volumetric efficiency increases, and the flow rate increases, thereby improving the efficiency of the pump and reducing power consumption; On the other hand, an increase in viscosity will increase the frictional resistance of the liquid, which in turn will reduce the efficiency of the pump and slightly increase power consumption. However, overall, as the viscosity increases, the efficiency of the rotary pump improves.
Non flowing liquids, such as oils, cellulose mixtures, paints, starch pastes, slurries, and sugar materials, will experience changes in viscosity when disturbed under constant temperature conditions. If a centrifugal pump is used, when the viscosity increases, the flow rate and head cannot meet the needs, and the motor will also be overloaded. Therefore, when the viscosity of the medium is high or when transporting non flowing liquids, it is advisable to use a rotary pump.
2. In situations where self-priming is required
Except for specially designed self-priming centrifugal pumps, general centrifugal pumps do not have self-priming function, but rotary pumps have self-priming function. When the rotor is wet (referring to not completely submerged or filled), a vacuum of 635-710mmHg can be formed to suck in liquid.
3. Environments containing gases
Centrifugal pumps generally do not allow the gas content in the liquid to exceed 5%. When the gas content is high, in addition to a decrease in flow rate, head, and efficiency, vibration, noise, and corrosion may occur, and corrosion may intensify or flow or shaft breakage may occur.
Due to its self-priming function, the rotary pump allows for a high gas content in the liquid, usually around 20%. When the gas content is high, the flow rate and efficiency will decrease, and vibration and noise will be generated. But generally, there will be no exacerbation of corrosion or phenomena such as cutting off or shaft breakage.
4. Small flow, high pressure situations
When the flow rate is small and the outlet pressure is high, if a suitable centrifugal pump cannot be selected, a rotary pump can be considered. The outlet pressure of the rotary pump is relatively high. The outlet pressure of ordinary series produced rotary pumps can reach 1.4~2.1Pa, and some special models can reach 3.5~7MPa. The outlet pressure of specially designed rotary pumps can even reach 14~21MPa.
5. Pump with low shear force
Some media that are sensitive to shear forces, and media that do not require the suspension or solid matter in their solution to be damaged during pumping, require the selected pump to have a very gentle impact on the medium during operation. At this point, low shear rotor pumps such as cam pumps, single screw pumps, etc. can be selected.
6. Pumps that need to be reversed
Some production facilities require liquid to be transported from location A to location B during a certain period of time, and from location B to location A during another period of time; Some production facilities have inlet pipes that are prone to blockage and require backwashing. This often requires the pump to have a reverse function. Centrifugal pumps cannot reverse, and many rotary pumps have a reverse function. Of course, in order to meet the reverse function of the pump, a series of problems caused by pump reverse must be considered, such as safety valves. Not only should the outlet pipeline be set, but also the inlet pipeline should be set.
