Sanitary Twin Screw Pump
The twin-screw pump is composed of a sealed chamber with a constant volume formed between the meshing spiral sleeve and the pump body or bushing on the driving and driven shafts. The medium is sent to the middle of the pump body as the screw shaft rotates, and the two merge together and finally reach the pump outlet, thus achieving the purpose of pump delivery.
type
Twin-screw pumps are divided into two types: sealed and unsealed. Depending on whether the medium enters the meshing space from one end or from both ends, the twin-screw is divided into two structures: double-suction and single-suction.
The twin-screw pump is an externally meshing screw pump that uses two screws that mesh with each other but do not contact each other to pump liquid.
The twin screw pump is a double suction non-sealed twin screw pump. The active screw extending out of the pump at one end is driven by the prime mover. The active screw and the driven screw have threads with different rotation directions. The screw fits tightly with the pump body. The driven screw is driven by the active screw through a synchronous gear.
As a positive displacement pump, the suction chamber of the twin-screw pump should be tightly separated from the discharge chamber. Therefore, the gap between the pump body and the outer surface of the screw and between the screws should be as small as possible. At the same time, the screw and the pump body, and between the screws form a sealed cavity to ensure airtightness, otherwise the liquid may flow back from the gap.
Twin screw pumps can be divided into two types: internal bearing and external bearing. In the internal bearing structure, the bearing is lubricated by the conveying material. The working chamber of the twin screw pump with external bearing structure is separated from the bearing. Due to the structure of this pump and the side clearance between the screws, it can convey non-lubricating media. In addition, the synchronous gear is adjusted so that the screws do not touch, and half of the output torque is transmitted to the driven screw. Like all screw pumps, the external bearing twin screw pump also has self-priming ability, and most pump conveying elements themselves are double-suction symmetrically arranged to eliminate axial forces and have a large suction height.
The twin screw pump series products are divided into two categories: single suction twin screw pump and double suction twin screw pump. This series of products has a special screw profile and a matching special structure. It is widely used in various industries such as petroleum, chemical industry, metallurgy, steel, electricity, shipbuilding, pharmaceuticals, food, building materials, etc. Its wide range of applicable media, wide range of conveying, and complex application conditions are unmatched by other pump products. These characteristics also make this series of pumps have potential development prospects.
Structural features
This type of pump adopts a double-suction structure. The two ends of the screw are in the same pressure chamber, and the axial force can be balanced by itself. The bearings at both ends are externally mounted and lubricated with lubricating oil (grease) separately, so they are not affected by the conveying medium. The two screws are driven by a pair of synchronous gears. The screw tooth surfaces do not contact each other, but leave a small gap. Impurities in the medium cannot cause direct wear on the screw tooth surfaces (except for scouring). Except for some small-displacement pumps (2W.W4.0 and below), the pump body is generally equipped with an internal flow safety valve, which has a certain protective effect when the discharge pressure exceeds the rated value. There are two inlet and outlet directions on the pump body, one is horizontal inlet and horizontal outlet; the other is horizontal inlet and vertical outlet; users can choose according to their needs.
Features and advantages
1. The liquid is transported smoothly, without pulsation, stirring, small vibration and low noise.
2. It has strong self-priming performance. When multi-phase mixed transmission is carried out, the gas content is not higher than 80% and the sand content is not higher than 500g/m3.
3. External bearing structure, with independent lubrication, can transport various non-lubricating media.
4. Synchronous gear drive is adopted, and there is no contact between the two rotors, so it is OK even if they idle for a short time.
5. The pump body is equipped with a heating jacket, which can transport various clean or low-viscosity or high-viscosity media containing small solid particles (generally the particle diameter is less than 0.12-0.2mm)
6. Correct selection of materials can even transport many corrosive media.
7. Double suction structure, no axial force on the rotor.
8. The shaft end adopts mechanical seal or bellows mechanical seal, which has the characteristics of long life, less leakage and wide application range.
Performance range
1. Maximum working pressure 4.0MPa
2. Flow range: 1～1000m3/h
3. Temperature range: -20～120℃
4. Medium viscosity is 1-3000mm2/s, and the speed can be reduced to 106mm2/s. The medium viscosity has a great influence on the performance of the twin-screw pump. The nominal displacement of the pump refers to the displacement under specific viscosity conditions. To ensure that the pump can work at a higher efficiency, it is recommended to select the speed according to the instructions when the test is insufficient.
Application Examples
1. Oil field: used as multiphase mixed delivery pump for oil, gas, water, trace fine solid particles, etc. and crude oil delivery pump.
2. Shipbuilding industry: used as ship loading pump, ship bottom sweeping and sewage treatment, main engine lubrication pump, fuel oil pump.
3. Petrochemical industry: used as loading and transport pumps for various resins, pigments, paraffin, paints, inks, latex, various oil products, crude oil, heavy oil, etc.
4. Thermal power plant: used as heavy oil, crude oil delivery pump, main engine lubricating oil pump.
5. Food industry: used as alcohol, honey, syrup, fruit juice, animal and vegetable oil, milk, and pulp oil delivery pump.
Selection Tips
The selection of twin-screw pumps (high-temperature twin-screw pumps, large-flow twin-screw pumps) includes the selection of performance parameters and the selection of pump structure types. For the selection of pump structure types, please refer to the introduction to the structure types of twin-screw pumps.
1. Flow Q:
As a positive displacement pump, the factors that affect the flow rate of a twin-screw pump mainly include the speed n, pressure p, and the viscosity v of the medium.
1.1 Influence of speed n:
When the screw pump (high-temperature twin-screw pump, large-flow twin-screw pump) is working, a sealed cavity is formed between the two screws and the bushing. Every time the screw rotates one circle, a sealed cavity is moved from the inlet to the outlet, that is, the volume of liquid in a sealed cavity is discharged. Under ideal conditions, there is no leakage inside the pump, so the flow rate of the pump is proportional to the speed. That is: Qth=n*q. n- speed; q-theoretical displacement, that is, the volume of liquid discharged by the pump for each rotation; Qth-theoretical displacement.
1.2 Influence of pressure △ P:
During the actual working process of the twin-screw pump (high-temperature twin-screw pump, large-flow twin-screw pump), there is leakage inside it, also known as slippage. Since there is a certain gap in the sealing chamber of the pump and there is a pressure difference △ P before and after the sealing chamber, a part of the liquid flows back, that is, there is leakage. The leakage is represented by △ Q, then Q=Qth- △ Q
Obviously, as the pressure difference △ P before and after the seal chamber increases, the leakage △ Q gradually increases. For different profiles and structures, the magnitude of the impact is also different.
1.3 Effect of viscosity v:
Imagine: let the same volume of clean water and viscous paste leak out of a funnel-shaped container. Obviously, water leaks faster than paste.
Similarly, for twin-screw pumps, fluids with high viscosity leak less than fluids with low viscosity, and the amount of leakage has a certain proportional relationship with the viscosity of the medium.
To sum up, it is necessary to comprehensively consider the above factors and conduct a series of calculations to accurately know whether the actual flow rate of the pump meets the working requirements.
2. Pressure △ P:
Unlike centrifugal pumps, the working pressure △ P of twin-screw pumps is determined by the outlet load, that is, the outlet resistance. The outlet resistance matches the pressure at the pump outlet. The greater the outlet resistance, the greater the working pressure. If you want to know the pressure, you need to use the knowledge of fluid mechanics to accurately calculate the outlet resistance.
3. Shaft power N:
The shaft power of a twin-screw pump (high-temperature twin-screw pump, large-flow twin-screw pump) is divided into two parts, namely: Nth----hydraulic power, that is, the energy of the pressure liquid; Nr----friction power.
For a certain pressure and flow, the hydraulic power is certain, so the factor affecting the shaft power is the friction rate Nr.
Friction power is the power consumed by the friction of moving parts. These friction powers obviously increase with the increase of working pressure difference, and the increase of medium viscosity will also cause the increase of liquid friction power.
Therefore, the shaft power of the pump includes not only hydraulic power, but also friction power which increases with the viscosity of the medium and the working pressure. Therefore, when selecting a matching motor, the viscosity of the medium is also a very important reference data. Especially when conveying high-viscosity media , a more accurate calculation is required.
After calculating the power, the matching motor should be selected in accordance with the relevant regulations specified in the sample table.
4. Calculation and selection of suction performance:
The operation of the pump (high temperature twin screw pump, large flow twin screw pump) is divided into the following stages:
4.1 Suction, when the liquid moves continuously along the suction pipe;
4.2 The rotating screw transfers energy to the working fluid;
4.3 Extrusion, at which point the liquid is discharged from the pump with the pressure necessary to overcome all resistance in the extrusion piping system.
Among the above three stages, the most important stage is to ensure the suction conditions of the pump so that the pump can work normally. This is an important condition for the operation of the pump. Otherwise, cavitation will occur, which will cause vibration, noise and other problems.
5. Calculation of NPSH:
The NPSHr of the pump is related to the pump speed n, lead h and the viscosity v of the medium transported by the pump. The Bornemann twin-screw pump introduced by our factory is calculated using the following formula: NPSHr=(1.5+0.253VF 1.84345+0.0572VF 1.55)*v 0.4146 VF- axial flow velocity, VF=n*h/60(m/s); n- speed (r/min); h- lead (m); v- working viscosity (°E). It can be seen that the NPSHr of the pump increases with the increase of VF and v. Therefore, when the suction conditions are not good, it is advisable to choose a twin-screw pump with a small lead. This is very important when selecting.
5.1 The calculation of the NPSHa of the device will not be elaborated here.
5.2 To keep the pump working properly, that is, to avoid cavitation, vibration and other problems, the following conditions must be met: NPSHa > NPSHr This is the suction condition of the pump.
6. Speed selection of twin screw pump (high temperature twin screw pump, large flow twin screw pump):
Choosing different speeds often involves the following issues:
6.1 By selecting the appropriate pump speed, appropriate performance parameters such as flow rate can be achieved.
6.2 The pump speed should also change with different viscosities.
For Boremann twin-screw pumps, the change in viscosity is the main factor determining the speed. As the viscosity increases, the allowable speed becomes lower.
The speed selection is also a question of suction performance, especially in the case of high viscosity. If the speed is too high, it will cause insufficient suction, resulting in noise and vibration problems. Therefore, it is necessary to select the speed according to the relevant principles.
