Screw pump working principle and characteristics

Screw pump is a single screw pump, its main working parts are eccentric screw spiral (known as the rotor) and the inner surface of the double helical screw bush (called stator). Its working principle is that when the motor drives the pump shaft to rotate, the screw rotates on its own axis on the one hand, and on the other hand, it rolls along the inner surface of the liner to form a sealed chamber of the pump. Each revolution of the screw, the liquid in the sealed cavity is pushed forward by one pitch. As the screw rotates continuously, the liquid spirally presses from one sealed cavity to the other sealed cavity, and finally the pump body is extruded. Screw pump is a new type of liquid conveying machinery, with the advantages of simple structure, safe and reliable operation, convenient use and maintenance, continuous and uniform liquid discharge, and stable pressure.

A pump that uses the rotation of a screw to pump liquid, which is best suited for absorbing viscous liquids.

1 screw pump basic working principle

Screw pump is the use of screw rotation to suction and discharge liquid. Figure 1 shows a cross-sectional view of a three-screw pump. In the picture, the middle screw is the active screw which is driven by the prime mover. The screw on both sides is the driven screw, which rotates reversely with the active screw. The main, driven screw thread are double-headed thread.

Due to the mutual engagement of the screws and the tight fit of the screw with the inner wall of the liner, one or more sealed spaces are separated between the suction and discharge of the pump. As the screw is rotated and engaged, these sealed spaces are constantly formed at the suction side of the pump, the liquid in the suction chamber is enclosed therein and continuously displaced in the axial direction of the screw from the suction chamber to the discharge end, to be enclosed in the spaces Continued discharge of liquid, like a nut in the screw rotation is constantly moving forward, as the case, which is the basic working principle of screw pump.

The sealing surface between the screw and the housing is a curved surface. There are unsealed areas such as ab or de on this surface and a number of triangular notches abc, def with the groove portion of the screw. The notches of these triangles form a passage for the liquid, allowing the drive screw groove A to communicate with the grooves B, C in the driven screw. The grooves B, C, along with their spiral around the back, respectively, and the back of the grooves D, E communicated. Since there are also triangular notches a'b'c 'similar to the front face on the sealing surfaces where the grooves D, E engage with the groove F (which belongs to the other thread), D, F, E will also communicate. In this way, the groove ABCDEA also form a "∞" -shaped sealing space (such as the use of single-head thread, the groove will be along the axial disk Rao screw, the suction port through, can not form a seal). Not hard to imagine, in such a screw, will form a number of independent "∞" -shaped sealed space, each sealed space occupies the axial length exactly equal to the lead of the tired rod t. Therefore, in order to make the suction and discharge of the screw, the length of the screw thread should be longer than one lead.

As can be seen from the above working principle, screw pump has the following advantages:

1) Wide range of pressure and flow. Pressure is about 3.4-340 kgf / cm2, flow up to 18600cm3 / min;

2) Wide range of liquid types and viscosities;

3) Because of the lower inertial force of rotating components in the pump, it can use high speed;

4) Inhalation performance, self-priming capability;

5) uniform and continuous flow, small vibration, low noise;

6) less sensitive to incoming gas and dirt compared to other rotary pumps;

7) solid structure, easy installation and maintenance.

The disadvantage of the screw pump is that the screw machining and assembly requirements are higher; the performance of the pump is sensitive to changes in the viscosity of the liquid.

2 three screw pump structure

It is mainly composed of a bushing (pump cylinder) fixed in the pump body and an active screw rod inserted in the pump cylinder and two driven screws engaged with the pump rod. Three intermeshing screws, in the pump cylinder by each lead to form a sealed chamber, resulting in suction and discharge between the seal.

Pump work, due to the two driven screw symmetrical with the active screw meshing, so the role of the radial force on the active screw completely balanced, the active screw does not bear the bending load. The radial force exerted on the follower screw is supported by the pump cylinder bushing along its entire length. Therefore, it is not necessary to provide a separate bearing on the outer end and substantially not to bear the bending load. In operation, a layer of oil film formed between the outer surface of the screw and the inner wall of the pump cylinder can prevent the direct contact between the metal and greatly reduce the wear of the screw tooth surface.

Screw pump work, both ends of the role of the liquid suction pressure, so the screw to produce axial thrust. For small pumps with differential pressure less than 10 kgf / cm2, thrust bearings can be used. In addition, high pressure oil is introduced into the bottom of each screw sleeve through the center bore of the active screw to create a balanced thrust at the lower end of the screw that is counter to axial thrust.

Screw Pump Like other positive displacement pumps, when the pump outlet is completely closed, the pressure inside the pump rises to the point of damaging the pump or overloading the motor. Therefore, the suction pump outlet, you must set the safety valve.

Screw pump shaft seals, usually mechanical shaft seal, and can work in accordance with the level of pressure to take different forms.

3 screw pump performance

3.1 displacement

The theoretical displacement of the screw pump can be calculated by the following formula:

Qt = 60 Ftnm3 / h

Where: F- effective cross-sectional area of ​​the pump cylinder, cm2; t-lead screw thread, m; n- active screw revolutions per minute.

Screw pump internal leakage Qs:

Qs = αp / σm

Where: p - working pressure of the pump; σ - viscosity of the liquid being discharged; α - coefficient relating to screw diameter and effective length; m = 0.3 - 0.5.

Pump pressure in the delivery of different viscosity of the liquid, the displacement will change. The relationship between displacement and viscosity can be expressed as:

Q2 = Qt- (Qt-Q1) (σ1 / σ2) m

Where: Q1-displacement when viscosity is σ1; Q2-displacement when viscosity is σ2.

3.2 power

Pump shaft power is generally the sum of the three parts of water power, frictional power and leakage loss power.

Water power Nc refers to the energy delivered to the liquid pump per unit time, also known as output power, can be calculated using the following formula:

Nc = PQ × 10-3 kW

Where: P-pump discharge pressure and suction pressure difference, Pa; Q-pump the actual displacement, m3 / s.

Frictional power refers to the frictional loss caused by liquid viscous resistance, which can be expressed as follows:

Nf = Kn1.5D2σm kilowatts

Where: n-speed; D-outer diameter of the active screw; - Viscosity; K-factor related to the length of the screw; m = 0.3-0.5.

Can be seen from the above, when the pump delivery of liquid viscosity is different, the pump shaft power will be different.

Leakage loss refers to the liquid from the high pressure leak back to the Department caused by the low power loss.

Therefore, when calculating the pump shaft power, if the theoretical displacement, the pump shaft power by the following formula:

N = NfPQt10-3 kilowatts

4 screw pump management

4.1 start

Screw pump suction stop valve should be fully open in case of starting to prevent overload or suction air.

Although the pump has dry suction capacity, but must prevent dry rotation, so as not to scratch the work surface.

If the pump needs to be started with very low oil temperature or high viscosity, it should be started with the suction valve and by-pass valve fully open to minimize the load when starting the pump. Until the prime mover reaches the rated speed, Bypass valve gradually closed.

When the by-pass valve is opened, the liquid is circulated and circulated in the pump under the condition of throttling, and the more the circulating oil is, the longer the circulating time is, the more severe the heat of the liquid is, and even the pump factor High temperature deformation and damage, we must pay attention.

4.2 operation

Screw pump must be in the direction of the established operation to produce a certain suction row.

Pump work, should pay attention to check the pressure, temperature and mechanical seal work. Shaft seal should allow a trace of leakage, such as leakage does not exceed 20-30 seconds / drop, then that normal. If the pump is noisy at work, it is often caused by too low an oil temperature, too high a viscosity of the fluid, air entering the fluid, loss of coupling, or excessive wear on the pump.

4.3 Parking

When the pump is stopped, the discharge stop valve should be closed first and the suction stop valve should be closed after the pump stops completely.

4.4 screw pump working screw length larger, less rigid, easily lead to bending, resulting in abnormal work. Shaft connection must be well centered; the best work is to be carried out after the installation of positioning, so as not to cause deformation of the pipeline implicated; connection pipe should be independently fixed, as far as possible to reduce the implication of the pump. In addition, the spare screw, the best preserved in the use of suspended fixed method to avoid distortion caused by uneven placement of the third century BC, Archimedes invented the spiral rod, the water can be smooth and continuous to a few meters high , The principle is still used by modern screw pump.

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