Do you know about screw pumps? What selection techniques does it have?
Screw pump is a positive displacement rotor pump, was born in 1931. Unique structure, self – absorption capacity, high efficiency, small size, reliable work, can transport a wide range of viscosity of various media. Screw pump is divided into single screw pump, double screw pump, three screw pump, what is their working principle? What are the characteristics? The selection methods and techniques are also different. Do you know all these?
Screw pump working principle is the use of screw rotation to suction and drainage of liquid. The middle screw is an active screw driven by the prime mover, and the screw on both sides is a driven screw that rotates in reverse with the active screw.
Each screw meshing with each other, the screw and the inner wall of the liner closely match, between the suction and discharge of the pump, will be separated into one or more sealing space. With the rotation and engagement of the screw, these sealing Spaces are continuously formed at the suction end of the pump to seal the liquid in the suction chamber into it, and the self-suction chamber continuously moves along the axial direction of the screw to the discharge end, and the liquid in the enclosed space is continuously discharged.
As can be seen from the above working principle, the advantages of screw pump can be more clearly understood:
Wide range of pressure and flow;
The type and viscosity of the liquid transported are wide;
Because the rotary part of the pump inertia force is low, it can use a high speed;
It has good absorption ability and self-imbibition ability.
Uniform and continuous flow, small vibration, low noise;
Compared with other rotary pumps, less sensitive to incoming gas and dirt;
Solid structure, easy installation and maintenance.
Accordingly, the screw pump is also more obvious shortcomings: the processing and assembly of screw higher requirements; Pump performance is sensitive to the change of liquid viscosity.
Although the family of screw pump is not huge, but according to the standard of screw, it can be divided into different types. This paper introduces the most commonly used single screw pump, double screw pump.
Single screw pump
Single screw pump is a new type of internal engagement rotary positive displacement pump. The main working parts are the eccentric screw (rotor) and the fixed bushing (stator).
Compared with other pumps, single screw pump has its own unique advantages:
Compared with centrifugal pump, single screw pump does not need to install a valve, and the flow rate is stable linear flow.
Compared with plunger pump, single screw pump has better self-priming capacity.
Compared with diaphragm pump, single screw pump can transport a variety of mixed impurities, containing gas and solid particles or fiber medium, also can transport a variety of corrosive substances.
And gear pump, single screw pump can transport high viscosity of the material.
Different from plunger pump, diaphragm pump and gear pump, single screw pump can be used for filling and metering agents.
Double screw pump
Single from the structure, twin screw pump is the external meshing screw pump. It USES two screws that engage with each other and do not touch each other to pump liquid.
In the structure of double screw pump is also complete, horizontal, vertical, with heating sleeve and other types, can transport particles of low viscosity or high viscosity medium, according to the size of the particle adjustment screw spacing, choose the right material, even can transport a lot of corrosive media.
Features: twin screw pump as a positive displacement pump, pump suction room should be closely separated from the discharge room. This requires the pump body and screw cylindrical surface and screw and screw clearance should be as small as possible. At the same time, the screw and pump body, screw and screw between each other and form a sealing chamber, to ensure airtight, otherwise there may be liquid from the gap backflow back.
Double screw pump unique structure so that it can achieve no stirring, no pulsation, smooth transmission of various media; Due to the pump body structure to ensure that there is always pumping liquid as a sealed liquid in the working elements of the pump, so the twin screw pump has a strong self-suction capacity, and can be mixed with liquid. The special design of twin screw pump also ensures that the pump has a high suction energy that is a small NPSHr value.
Twin screw pump can be divided into two types of internal bearing and external bearing.
The bearing is lubricated by a conveyor in the form of a built-in bearing. External bearing structure of double screw pump is the work cavity with the bearing separate.
External bearing double screw pump features: the pump structure and the screw between the existence of side clearance, independent lubrication of the external bearing allows it to transport a variety of non-lubricating media. In addition, the synchronous gear is adjusted so that the screw is not in contact and half of the output torque is transferred to the driven screw.
Like all screw pump, external bearing type double screw pump also has self-suction capacity, and most of the pump transmission elements themselves are double suction symmetrical layout, can eliminate the axial force, also has a very high suction.
The advantages of external bearing double screw pump structure greatly expand the scope of use of double screw pump, that is: in addition to conveying lubricity good medium, but also can transport a large number of non-lubricating medium, a variety of viscosity (z* high viscosity up to 3*106mm/s) medium and corrosive (acid, alkali and other properties), abrasive liquid;
In addition, double screw pump due to the existence of a constant gap and the characteristics of the mold line, it belongs to the non-sealed positive displacement pump, so in addition to conveying pure liquid, but also can transport the mixture of gas and liquid, that is, the mixture of vapor and liquid transport, which is also one of the unique advantages of double screw pump;
Twin-screw pump can also dry rotation: because the moving parts in the work of non-contact, so a short time of dry rotation will not destroy the pump elements, this feature to the flow of automatic control provides a great convenience, but dry operation time is limited by a variety of factors, generally very short.
In addition, the twin-screw pump has no shear and no emulsification in the conveying process, so it will not destroy the molecular chain structure and the specific fluid properties formed in the working process. Moreover, because the transmission relies on synchronous gear, the pump has low running noise, small vibration and smooth operation.
Application: the characteristics of external bearing twin screw pump make it widely used in oilfield chemical industry and shipbuilding industry. It can also be used according to a variety of different USES of ordinary cast iron, stainless steel and other materials manufacturing; Conveying temperature up to 250℃; With different heating structures, the theoretical flow rate can reach 2000 m3 / h.
Selection of screw pump
Screw pump widely used, there are “screw pump can transport any medium”. But this is not to say that a certain kind of screw pump can transport all the medium, but according to the characteristics of the medium and the number of performance parameters can choose different types of screw pump.
If inadvertently picked out the wrong pump screw pump, is likely to bring unnecessary trouble. Single screw pump, double, three and five screw pump, each has its advantages, in the promotion of the application of screw pump must have a choice, only to make full use of their respective characteristics, in order to better achieve energy saving, material saving, increase efficiency or meet some special requirements.
The following common single screw pump, double screw pump and three screw pump as an example to introduce some skills in the selection of screw pump.
Single screw pump selection points
The selection of single screw pump mainly focuses on the following parameters:
1. Pressure determination of single screw pump:
The large output pressure of single screw pump z* is determined according to the bushing series, namely the lead number of bushing (stator) :
Grade 1: z* high working pressure is 0.6mpa;
Grade 2: z* high working pressure is 1.2mpa;
Grade 4: z* high working pressure is 2.4mpa;
Due to the different conditions of the conveying medium, the proper stator series should be selected according to the requirements of conveying pressure for the medium with serious abrasion.
2. Single screw pump speed selection:
Single screw pump due to its structural characteristics, most of the use in the transmission of higher viscosity of liquid and liquid containing particles, so the choice of its revolution is very critical. For the choice of single screw pump speed is mainly based on the wear of the medium and the medium viscosity.
3. Selection of rubber material for lining of single screw pump:
Single screw pump liner for rubber products, but also a single screw pump wearing parts, its choice, directly affect the life of the liner, generally under normal circumstances, the life of the liner is 3-6 months, if the choice is not appropriate, the liner may fall off from the steel pipe or rubber off.
4. Material combination selection:
Transmission of different properties of the medium, the need for different material combinations.
General single screw pump performance table or characteristic curve are 20℃ water as the medium (viscosity is 1cst) when the data, for the transmission of different viscosity of the flow and shaft power is different.
Shaft seal: 6.
According to the need and conveying medium, mechanical seal and packing seal can be used, and the two structures have interchangeability.
7. Driving mode of the pump:
Due to the single screw pump for the low speed pump, the pump driving mode is more, generally has the low speed motor direct connection (6, 8), gear reduction motor drive, stepless speed change motor drive.
Double screw pump selection skills
Double screw pump, three screw pump collectively referred to as multi – screw pump. Since the invention of multi-screw pump, it has been widely used to pump crude oil, fuel oil, lubricating oil, power fluid, food, chemical and synthetic fiber pulp. There are about one million multi-screw pumps in operation. The application of such a wide range of multi-screw pump in the choice of course is also very cautious.
The choice of twin screw pump is also the choice of its performance parameters:
1. The flow:
As a positive displacement pump, the factors that affect the flow rate of twin-screw pump are mainly speed, pressure and viscosity of the medium.
Screw pump in the work, two screw and bushing between the formation of a sealing cavity, screw every rotation week from the inlet to the outlet to move out of a sealing cavity, that is, a sealed cavity volume of liquid is discharged. Under the ideal condition of no leakage inside the pump, the flow rate of the pump is proportional to the speed.
But in the actual working process, there is a certain gap in the sealing cavity of the pump, and the pressure difference exists before and after the sealing cavity, therefore, there is a part of the liquid reflux, that is, there is leakage. With the increase of pressure difference between front and back of the sealing chamber, the leakage increases gradually. For different lines and structures, the size of the impact is also different.
For twin screw pump, the viscosity of the fluid than the viscosity of small liquid leakage is small, leakage and medium viscosity has a certain proportion.
The working pressure of twin screw pump is determined by the outlet load, that is, the outlet resistance. Outlet resistance and pump outlet pressure is matched, 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 calculate the outlet resistance accurately.
3. Shaft power:
Pump shaft power is divided into two parts, namely: hydraulic power (pressure liquid energy) and friction power. For the determined pressure and flow, the hydraulic power is certain, so the factor that affects the shaft power is the friction rate.
Friction power is the part of power consumed by friction of moving parts. Obviously, friction power increases with the increase of working pressure difference, and the increase of medium viscosity will also cause the increase of liquid friction power. Therefore, when choosing matching motors, the viscosity of the medium is also a very important reference data. Especially in the transmission of high viscosity medium, need to make a more accurate calculation.
4. Suction performance:
Pump work is divided into the following stages: inhalation, at this time the liquid continuously along the suction pipe movement; The rotating screw transfers energy to the working liquid; Out, at which point the liquid is discharged from the pump with the pressure necessary to overcome all resistance of the out piping system.
In the above three stages, z* is an important stage is to ensure the suction conditions of the pump, pump can work normally, this is an important condition of pump work, otherwise cavitation will occur, that is, cause vibration, noise and other problems.
5. Cavitation allowance:
The cavitation of the pump and the speed of the pump, the lead and the pump transmission medium viscosity and other factors are related, it increases with the axial velocity, the increase in the viscosity. In the case of bad suction conditions, the appropriate choice of small lead double screw pump. This is important when choosing the type.
6 double screw pump speed selection often involves the following issues:
By choosing the appropriate pump speed, to achieve the appropriate performance parameters such as flow rate.
With the different viscosity, the speed of the pump should also be changed.
The choice of speed is essentially a matter of suction performance, especially in the case of high viscosity, if the speed is selected too high, it will cause insufficient suction, resulting in noise and vibration and other problems. Therefore, according to the relevant principles to choose the speed.
To sum up, to comprehensively consider the above factors, through a series of calculations to accurately know whether the actual flow of the pump meets the requirements of the working condition.