Twin screw pumps and triple screw pumps are collectively referred to as multi screw pumps. Since the invention of the multi screw pump, it has been widely used for pumping crude oil, fuel oil, lubricating oil, power fluid, food, chemical agents, and synthetic fiber slurry. Currently, there are over a million multi screw pumps in operation. The selection of such a widely used multi screw pump is certainly very cautious.
The selection of twin-screw pumps is also the selection of their performance parameters:
1. Traffic:
As a positive displacement pump, the main factors affecting the flow rate of a twin-screw pump are speed, pressure, and the viscosity of the medium.
When the screw pump is working, a sealing chamber is formed between the two screws and the liner. After each rotation of the screw, a sealing chamber is moved from the inlet to the outlet, and the liquid in one sealing chamber volume is discharged. In an ideal state where there is no leakage inside the pump, the flow rate of the pump is proportional to the speed.
However, in the actual working process, there is a certain gap in the sealing chamber of the pump, and there is a pressure difference between the front and back of the sealing chamber. Therefore, some liquid flows back, which means there is leakage. As the pressure difference between the front and back of the sealing chamber increases, the leakage gradually increases. The impact size varies for different profiles and structures.
For twin-screw pumps, the leakage of fluids with high viscosity is smaller than that of liquids with low viscosity, and the leakage amount is proportional to the viscosity of the medium.
2. Pressure:
The working pressure of a twin-screw pump is determined by the outlet load, that is, the outlet resistance. The outlet resistance matches the pressure at the pump outlet, and the greater the outlet resistance, the greater the working pressure. If you want to know the pressure, you need to use knowledge of fluid mechanics to accurately calculate the outlet resistance.
3. Shaft power:
The shaft power of the pump is divided into two parts, namely hydraulic power (the energy of the pressure liquid) and frictional power. For a certain pressure and flow rate, the hydraulic power is constant, so the factor that affects shaft power is the friction rate.
Friction power is the portion of power consumed due to the friction of moving parts. Obviously, the friction power increases with the increase of working pressure difference, and the increase of medium viscosity will also cause an increase in liquid friction power. Therefore, when choosing a matching motor, the viscosity of the medium is also a very important reference data. Especially when transporting high viscosity media, more accurate calculations are required.
4. Absorption performance:
The pump operation is divided into the following stages: suction, during which the liquid continuously moves along the suction pipeline; The rotating screw transfers energy to the working liquid; Press out, at this point, the liquid is discharged from the pump with the pressure necessary to overcome all the resistance of the pressure pipeline system.
Among the above three stages, the most important stage is to ensure the suction conditions of the pump in order for it to operate normally. This is an important condition for the pump to work, otherwise cavitation will occur, causing vibration, noise and other problems.
5. NPSH:
The cavitation margin of a pump is related to factors such as pump speed, lead, and viscosity of the medium being transported by the pump. It increases with the increase of axial flow rate and working viscosity. In case of poor suction conditions, it is advisable to choose a small lead twin-screw pump. This is very important in the selection process.
6. The speed selection of twin-screw pumps often involves the following issues:
By selecting the appropriate pump speed, appropriate performance parameters such as flow rate can be achieved.
As the viscosity varies, the pump speed should also change.
The choice of rotational speed is essentially a matter of suction performance, especially in high viscosity situations. If the rotational speed is chosen too high, it will cause insufficient suction, resulting in problems such as noise and vibration. Therefore, it is necessary to choose the rotational speed according to relevant principles.
In summary, it is necessary to comprehensively consider the above factors and accurately determine whether the actual flow rate of the pump meets the operating requirements through a series of calculations.
Common knowledge for selecting a three screw pump:
Although three screw pumps have many advantages, if not selected properly, it will not only fail to achieve satisfactory operating results, but also cause noise and vibration of the pump, and even seriously damage the internal parts of the pump, making the pump system unable to work properly. Therefore, the selection personnel should carefully consider multiple factors and choose a reasonable one.
When selecting a three screw pump, it is necessary to have a detailed understanding of the pump's operating conditions. In addition to clear operating parameters such as flow rate and pressure, the characteristics of the conveying medium, such as the corrosiveness of the medium, steam content, ratio of solid solution and solid particle size, as well as the working temperature, viscosity, specific gravity, corrosiveness to materials, and the suction and installation conditions of the pump device, should also be understood.
Similar to single screw pumps and double screw pumps, the selection of triple screw pumps is mainly based on performance parameters.
The selection of pump speed is mainly based on the viscosity and specifications of the medium being transported to determine its range.
When conveying high viscosity media, the pump should choose a low speed. If the viscosity is low, a high speed can be selected accordingly. When the viscosity of the medium is greater than 20 ° E, for large-sized pumps (with an outer diameter of the main rod of 60mm or more), the speed should be 970rpm or 720rpm. If the viscosity is higher (viscosity>80 ° E), such as viscous liquids, the speed can be reduced for use. It is recommended to use at 200-500rpm; For small-sized pumps, when the medium viscosity is greater than 20 ° E, it is advisable to use a speed of 1450rpm or 970rpm. If the viscosity is higher (viscosity>80 ° E), the speed can be reduced for use, and 300-600rpm is recommended;
Due to the higher speed of the pump, the smaller the volume of the pump under the same performance parameters. However, due to the higher speed and friction power, the wear of the pump is greater and its lifespan is shorter. If the lubrication of the conveying medium is poor or contains trace impurities, a lower speed should be selected to maintain a longer lifespan of the pump, recommended below 1450rpm.
The selection of structure can be based on the installation and usage conditions of the pump, referring to the series and types of three screw pumps, among which there are four principles that can be referred to:
Transporting lubricating oils, select pumps with built-in bearing structures for temperatures below 80 ℃;
When the temperature exceeds 80 ℃ or the lubrication of the conveying medium is poor, choose a pump with an external bearing structure;
When transporting media with poor fluidity and high viscosity, or when heating or insulation is required for the transported media, choose a pump with a double-layer heating pump body structure;
When transporting at high temperatures, pumps made of high-temperature resistant materials should be selected.
In addition, according to different situations, factors such as the material combination of the pump, suction capacity, and the selection of matching motors should be comprehensively considered. As long as the selection is appropriate and maintenance is reasonable, the three screw pump can ensure satisfactory operation.
