Structurally speaking, the twin-screw pump is an externally engaged screw pump. It uses two screw rods that mesh with each other and do not touch each other to pump liquid. In terms of structural type, the twin-screw pump is also very complete, including various types such as horizontal, vertical, and heating jackets.
The unique structure of the twin-screw pump enables it to transport various media without stirring, without pulsation, and smoothly; because the structure of the pump body ensures that there is always pumped liquid in the working components of the pump as the sealing liquid, the twin-screw pump has a strong excellent self-priming ability, and can be mixed with gas and liquid. The special design of the twin-screw pump also ensures that the pump has high suction performance, that is, a small NPSHr value.
This article will introduce the different types of twin screw pumps available on the market bellow
1. Sealed twin-screw pump and non-sealed twin-screw pump
2. Single-suction twin-screw pump and double-suction twin-screw pump
3. Twin-screw pumps with built-in bearing structure and external bearing structure
4. Vertical and horizontal twin screw pumps
1. Sealed twin-screw pump and non-sealed twin-screw pump
What is the difference between a sealed twin-screw pump and an unsealed twin-screw pump?
The sealed twin-screw pump has only two screws, but it meets all sealing conditions. Its composition and characteristics are similar to those of the three-screw pump. It is suitable for transporting clean lubricating liquid without impurities and can be used in high-pressure situations.
The volumetric efficiency and mechanical efficiency of non-closed (rectangular or trapezoidal thread) twin-screw pumps are relatively low, and they are only suitable for working at low pressure when transporting low-viscosity liquids. However, it is used to transport high-viscosity liquids and work under high flow and low pressure, so the volumetric efficiency will not be too low. The viscosity of the transported liquid can reach hundreds of thousands of centipoise.
In addition to the common characteristics of screw pumps, the characteristics of non-hermetic twin-screw pumps are:
1) Using external bearings
The screw is in contact with the bearing bush (bearing) (not in contact with the conveying medium), and the bearing is well-lubricated. This can be used to transfer non-lubricating high-viscosity liquids.
2) Synchronous gear drive
Since the tooth profile does not meet the meshing conditions, a synchronous gear must be used to drive the driven screw. At this time, the screw and the screw and the screw and the bushing are not in contact. Therefore, the wear and power consumption of the non-hermetic twin-screw pump is small. In addition, the use of synchronous gears can change the tooth depth, pitch, and tooth profile in a wide range (not limited by meshing conditions), which is beneficial to unified standardization.
3) Suitable for low-pressure, high-viscosity liquid delivery
Since the separation between the suction chamber and the discharge chamber is not sealed, the volumetric efficiency is lower than that of the closed Wu screw pump under the same conditions (obviously under high pressure), but the difference is not obvious under low pressure and high viscosity. Therefore, hermetic twin-screw pumps are suitable for low-pressure, high-viscosity liquid delivery.
4) Small vibration, no noise
Because the screws do not touch each other, the vibration is small and there is no noise when rotating at high speed.
2. Structurally, twin-screw pumps are divided into single-suction twin-screw pumps and double-suction twin-screw pumps
1) Double-suction twin-screw pump
The product feature of the double-suction twin-screw pump is that there are two pairs of left and right spiral sleeves in the pump unit, as shown in the figure. The product structure is more complicated than the former, and the use and maintenance are more difficult than the former. It is mainly used for large flow or (and) high pressure. occasions. The double-suction twin-screw pump has been carefully built by our company’s R&D personnel for many years and has realized effective measures such as quick replacement of seals and quick adjustment of the helical surface clearance. Its structural form is divided into three types: bearing built-in type, bearing external type and hydrodynamic type;
2) Single-suction twin-screw pump
Our single-suction twin-screw pump includes three structural types
a. Single suction cantilever structure.
Its characteristic is that a pair of screw sleeves cantilever out of the transmission mechanism. It is convenient to install, disassemble, replace, and adjust the screw rod. It is mainly used in occasions where the flow rate is not large and the pressure is not high; its structural form is external bearing.
b. Single-suction high-efficiency structure.
It is characterized in that the screw sleeve and the shaft are made as a whole, and the length of the screw sleeve is extended, which has very high volumetric efficiency and is mainly used in the occasion of conveying low viscosity and high pressure; its structure is uniformly built into the bearing.
c. Single suction A-line construction.
Generally, there is no need for gear transmission in the structure, the efficiency of conveying medium is high, and the structure is simple. It is mainly used for conveying media with high viscosity, low flow rate, low pressure, and certain lubricity. Its structural form is a built-in sliding bearing.
In recent years, the spiral surface with independent intellectual property rights developed by our company is at the domestic leading level.
3. Twin-screw pumps can be divided into two types: built-in bearings and external bearings.
In the version with built-in bearings, the bearings are lubricated by the conveyed material.
In the twin-screw pump with an external bearing structure, the working chamber is separated from the bearing. The structure of this pump has side clearance between the screw and the independently lubricated outboard bearings allowing it to deliver a variety of 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 at the same time. Like all screw pumps, the external bearing twin-screw pump also has the self-priming ability, and most of the pump delivery components themselves are double-suction symmetrically arranged, which can eliminate axial force and have a large suction height.
The advantages of the twin-screw pump structure with external bearings greatly broaden the scope of use of the twin-screw pump, that is: in addition to conveying medium with good lubricity, it can also convey a large amount of non-lubricating medium, various viscosities (high viscosity up to 3 *106mm/s) medium and corrosive (acid, alkali, etc.), abrasive liquid;
In addition, the twin-screw pump is a non-sealed volumetric pump due to the existence of a constant gap and the characteristics of the molded line. Therefore, in addition to transporting pure liquid, it can also transport a mixture of gas and liquid, that is, vapor-liquid mixed transport. This is also a double-screw pump. One of the very unique advantages of the screw pump;
The twin-screw pump can also run dry: because the moving parts do not touch each other during work, the short-term dry running will not damage the pump components. This feature provides great convenience for the automatic control process, but the dry running time is limited. Restricted by many factors, it is generally very short. In addition, the twin-screw pump has no shearing and emulsification during the conveying process, so it will not damage the molecular chain structure and the specific fluid properties formed in the working process, and because the transmission relies on synchronous gears, the pump runs with low noise and vibration, work smoothly.
The various characteristics of the external bearing twin-screw pump make it widely used in the oilfield chemical industry and the shipbuilding industry. It can also be made of different materials such as ordinary cast iron and stainless steel according to various usage conditions; the conveying temperature can reach 250°C; it has different heating structures, and the theoretical flow rate can reach 2000 cubic meters per hour.
4. Vertical and horizontal twin screw pumps
As the name implies, vertical pumps and horizontal pumps are two types of appearance, and the appearance characteristics of vertical pumps and horizontal pumps also determine some characteristics of the corresponding twin-screw pumps.
1) Different connection forms
The vertical pumps are superimposed and connected from bottom to top, and the horizontal pumps are arranged longitudinally on the base. The vertical pumps are generally called pipeline pumps. The motor and the pump body are connected by a sub-port; the horizontal pump is connected to the motor by a coupling.
2) Occupy space
Vertical pumps occupy a small area while horizontal pumps occupy a large area. The vertical pump occupies a smaller area than the horizontal pump, and it is not necessary to lay a foundation; the horizontal pump set has a base, which requires a foundation.
3) Maintenance Difficulty
It is difficult to overhaul the vertical pump, for example, the upper part needs to be removed before overhauling the impeller; while the horizontal pump is relatively easy.
4) Installation form
After the horizontal pump is installed, precision adjustment is required, and the vertical pump is easier to connect and install as a whole.
Therefore, horizontal pumps have an economic advantage over vertical pumps. However, for situations with special requirements or scene restrictions, vertical pumps still have a large application market.
