centrifugal pump

First, the working principle of centrifugal pump

Figure 2- 1 shows the centrifugal pump installed in the pipeline. The main components are impeller 1 and pump housing 2. An impeller with several curved blades is installed in the pump housing and fixed on the pump shaft 3. The suction port 4 in the center of the pump housing is connected with the suction pipeline 5, and the side discharge port 8 is connected with the discharge pipeline 9.

Centrifugal pump is generally driven by motor, and the shell needs to be filled with conveyed liquid before starting. After starting the motor, the pump shaft drives the impeller to rotate together, and the liquid between the blades also rotates. Under the action of centrifugal force, the liquid gains energy in the process of being thrown from the center of the impeller to the outer edge, which increases the static pressure and flow rate of the liquid at the outer edge of the impeller, generally reaching 15 ~ 25m/s, that is, the kinetic energy of the liquid also increases. After the liquid leaves the impeller and enters the pump casing, the flow velocity of the liquid gradually decreases due to the widening of the flow channel in the pump casing, and part of kinetic energy is converted into static pressure energy, which further increases the pressure of the liquid at the pump outlet, so the liquid enters the discharge pipeline from the pump outlet at a higher pressure and is transported to the required place.

When the liquid in the pump is thrown from the center of the impeller to the outer edge, a low pressure area is formed in the center. Because the pressure above the liquid level of the storage tank is greater than the pressure at the suction inlet of the pump, under the action of pressure difference, the liquid is continuously sucked into the pump through the suction pipeline to supplement the position of the discharged liquid. As long as the impeller keeps rotating, the liquid is continuously sucked and discharged. It can be seen that centrifugal pump mainly relies on high-speed rotating impeller to transport liquid. Liquid gains energy and increases pressure under the action of centrifugal force.

When the centrifugal pump is started, if the pump casing and suction pipeline are not filled with liquid, there is air in the pump casing. Because the density of air is much less than that of liquid and the centrifugal force is small, the low pressure formed in the center of the impeller is not enough to suck the liquid in the storage tank into the pump. At this time, even if the centrifugal pump is started, liquid cannot be transported. This phenomenon is called gas viscosity, which means that the centrifugal pump has no self-priming ability, so the shell must be filled with liquid before starting. If the suction inlet of the centrifugal pump is above the liquid level of the suction tank, a one-way bottom valve 6 and a filter screen 7 should be installed at the inlet of the suction pipeline. The bottom valve is to prevent the injected liquid from leaking from the pump before starting, and the filter screen can prevent the solid substances in the liquid from being sucked and blocking the pipeline and the pump casing. A regulating valve 10 is installed on the discharge pipeline near the pump outlet for starting, stopping and regulating the flow.

Figure 2- 1 Schematic diagram of centrifugal pump device

1- impeller; 2- pump housing; 3- pump shaft; 4- suction port; 5- straw; 6- Bottom valve; 7- filter screen; 8- discharge port; 9- Discharge pipe; 10- control valve

Second, the main components of centrifugal pump

The main components of centrifugal pump are impeller, pump shell and shaft seal device. The following briefly introduces its structure and function.

(1) Impeller Impeller's function is to transfer the mechanical energy of the prime mover to the liquid, so that the hydrostatic and kinetic energy of the liquid can be improved.

The impeller of centrifugal pump is shown in Figure 2-2, and there are 6 ~ 12 curved blades 1 in the impeller. An impeller with a front cover plate 2 and a rear cover plate 3 on both sides of the blade shown in Figure (a) is called a closed impeller. After the liquid enters from the inlet of the impeller center, it flows to the outer edge of the impeller through the flow channel between the two cover plates and the blades. In this process, the liquid gets energy from the rotating impeller, and part of the kinetic energy is also converted into hydrostatic energy because of the gradual expansion of the flow channels between the blades. Some impellers without a front cover on the suction side are called semi-closed impellers, as shown in Figure (b). Impeller without front and rear cover plates is called open impeller. As shown in figure (c), semi-closed and open impellers can be used to transport slurry or liquid containing solid suspended matter. Because the impeller wheel is not easy to be blocked after the cover plate is removed, but because there is no cover plate, the liquid is easy to flow backwards when moving between blades, so the efficiency is also low.

Figure 2-2 Centrifugal Pump Impeller

(a) shut down; Semi-closed; (c) openness

When a closed or semi-closed impeller works, a part of high-pressure liquid leaving the impeller leaks into the cavities on both sides between the impeller and the pump casing, and the suction port on the front side of the impeller is low-pressure, so the pressure of the liquid acting on the front and rear sides of the impeller is not equal, resulting in an axial thrust in the direction of the suction port of the impeller, which causes the impeller to move to the suction port side, leading to wear at the contact between the impeller and the pump casing, and even leading to pump vibration in severe cases. Therefore, some small holes can be drilled on the back cover plate of the impeller (see 1 in Figure 2-3(a)). These small holes are called balance holes, and their function is to make a part of high-pressure liquid in the cavity between the back cover plate and the pump casing leak to the low-pressure area, thus reducing the pressure difference on both sides of the impeller, thus balancing a part of axial thrust, but at the same time reducing the efficiency of the pump. Balance hole is the simplest method to balance axial thrust of centrifugal pump.

According to the different suction methods, there are two kinds of impellers: single suction and double suction. The single suction impeller has a simple structure, as shown in Figure 2-3(a), and liquid can only be sucked from one side of the impeller. As shown in Figure 2-3(b), the double-suction impeller can suck liquid from both sides of the impeller at the same time. Obviously, the double-suction impeller has a large liquid absorption capacity and can basically eliminate the axial thrust.

Figure 2-3 Suction mode (1) Single suction type; (b) double suction type

(2) Pump shell The pump shell of centrifugal pump is also called volute, because there is a snail-shell-shaped flow channel with gradually expanding cross section in the shell, as shown in 1 in Figure 2-4. The impeller rotates in the shell along the gradually expanding direction of the volute channel, and the closer to the liquid outlet, the larger the cross-sectional area of the channel. Therefore, after the liquid is thrown out from the outer edge of the impeller at high speed, it flows along the volute channel of the pump casing to the discharge port, and the flow rate gradually decreases, thus reducing the energy loss and effectively converting part of kinetic energy into static energy. Therefore, the pump casing is not only a part to collect the liquid thrown by the impeller, but also an energy conversion device itself.

In order to reduce the collision when the liquid directly enters the volute, sometimes a fixed disk with blades is installed between the impeller and the pump casing. This disc is called the guide wheel, as shown in Figure 2-4 at 3. The guide wheel has many gradually turning flow channels, so that high-speed liquid can evenly and gently convert kinetic energy into hydrostatic energy and reduce energy loss.

Figure 2-4 Pump housing and guide wheel 1- Pump housing; 2- Impeller; 3- Guide wheel

(3) The seal between the pump shaft and the pump casing of the shaft seal device is called shaft seal. The function of shaft seal is to prevent high-pressure liquid from leaking from the pump casing around the shaft, or to prevent external air from leaking into the pump casing in the opposite direction. Commonly used shaft seal devices include packing seal and mechanical seal.

The shaft seal device used in ordinary centrifugal pump is stuffing box, commonly known as packing box, as shown in Figure 2-5. 1 in the figure is a stuffing box connected with the pump casing; 2. Soft fillers, generally asbestos ropes impregnated with oil or coated with graphite; 4. Packing gland can be tightened with screws, so that the packing is pressed between the stuffing box and the rotating shaft to achieve the purpose of sealing; 5 is an inner bushing, which is used to prevent stuffing from squeezing into the pump. Because the contact between the pump casing and the rotating shaft may be the low pressure area in the pump, in order to better prevent air from leaking into the pump from the loose part of the packing box, a liquid sealing ring 3 is installed in the packing box. As shown in Figure 2-6, the liquid sealing ring is a metal ring with some radial holes, which can communicate with the discharge port of the pump through the small tube on the stuffing box shell, so that the high-pressure liquid in the pump flows into the liquid sealing ring along the small tube to prevent air from leaking into the pump, and the flowing liquid also plays the role of lubricating and cooling the stuffing and the shaft.

Figure 2-5 stuffing box

1- stuffing box; 2- Soft filler; 3- liquid sealing ring; 4- packing gland; 5- Inner Bushing

Figure 2-6 Liquid Seal Ring

When transporting acid, alkali and flammable, explosive and toxic liquids, the requirements for sealing are relatively high, and air is not allowed to leak into the air and liquid is not allowed to seep out. In recent years, shaft sealing devices called mechanical seals have been widely used. It consists of a moving ring installed on the rotating shaft and a static ring fixed on the pump casing. The end faces of the two rings cling together by spring force, making relative movement and playing a sealing role, so it is also called end face sealing. Figure 2-7 shows the structure of domestic AX mechanical sealing device, and the left side of the device is connected with the pump housing. Screw 1 to fix the transmission base 2 on the rotating shaft. A spring 3, a push ring 4, a moving ring sealing ring 5 and a moving ring 6 are installed in the transmission seat, and all these components rotate together with the shaft. Static ring 7 and static ring sealing ring 8 are installed on the sealing end cover and fixed by anti-rotation pin 9. All these parts are stationary. In this way, when an important official rotates, the moving ring 6 rotates, while the fixed ring 7 does not move, and the two rings are closely attached by the elastic force of the spring. Because the machining of the end faces of the two rings is very smooth, the liquid leakage of the end faces of the two rings is very small. In addition, the gap between the moving ring 6 and the pump shaft is blocked by the moving ring sealing ring 5, and the gap between the fixed ring 7 and the sealing end cover is blocked by the fixed ring sealing ring 8. There is no relative movement between these two gaps, so it is difficult to leak. Moving rings are usually made of hard materials, such as high silicon cast iron or hard-faced carbide. Non-metallic materials used for fixing rings are usually made of impregnated graphite and phenolic plastics. In this way, in the mutual friction between the moving ring and the static ring, the static ring is easy to wear, but from the structure of the mechanical sealing device, the static ring is easy to replace. The sealing ring of moving ring and static ring is usually made of synthetic rubber or plastic.

Figure 2-7 Mechanical sealing device

1- screw; 2- Transmission seat; 3- spring; 4- push ring; 5- Moving ring sealing ring; 6- moving ring; 7- static ring; 8- Static ring sealing ring; 9- Anti-rotation pin

When installing mechanical sealing device, it is required that the moving ring and the static ring are strictly perpendicular to the axis center line, and the friction surface is smooth. By adjusting the spring pressure, the end sealing mechanism can form a thin liquid film between the two friction surfaces during normal operation, thus achieving better sealing and lubrication.

Compared with packing seal, mechanical seal has the following advantages: good sealing performance, long service life, hard wear of shaft and low power consumption. Its disadvantages are high machining accuracy, complex machining, strict installation technical requirements, troublesome handling and replacement of parts, and much higher price than stuffing boxes.

Three. Main performance parameters and characteristic curves of centrifugal pump

1. Main performance parameters of centrifugal pump

In order to choose and use centrifugal pump correctly, it is necessary to know the performance of the pump. The main performance parameters of centrifugal pump are displacement, working pressure (head) efficiency and input power. These parameters are marked on the nameplate of the pump and their meanings are as follows.

The displacement of (1) displacement centrifugal pump refers to the liquid conveying capacity of the pump, and refers to the volume of liquid conveyed by the centrifugal pump in unit time, which is expressed by qv, and the unit is often 1/s or m3/h. The displacement of centrifugal pump depends on the structure, size (mainly the diameter of impeller and the width of blade) and rotating speed of the pump.

(2) Working pressure The working pressure of a centrifugal pump can be expressed by the pressure head or the lift of the pump, which refers to the effective energy that the pump can provide for the liquid under the unit weight. The working pressure is expressed by kPa or MPa, and the head is expressed by the height m of water column. The working pressure of centrifugal pump depends on the pump structure (such as impeller diameter, blade deflection, etc.). ), speed and flow. For a certain pump, there is a certain relationship between working pressure and displacement at a specified speed.

When the pump is working, the pressure can be measured experimentally, as shown in Figure 2-8. A vacuum pump and a pressure gauge are installed at the inlet and outlet of the pump respectively, and Bernoulli equation is established between the vacuum gauge and the pressure gauge, namely

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or

Where pM—— refers to the reading of pressure gauge (gauge pressure), and the unit is Newton/square meter (n/m2);

PV-vacuum degree read by vacuum gauge (n/m2);

V 1, V2—— velocity of liquid in suction pipe and extrusion pipe (m/s);

∑hf—— Head loss of two sections (m).

Figure 2-8 Installation Diagram of Pump Pressure Measurement

1- flowmeter; 2- pressure gauge; 3- vacuum gauge; 4- centrifugal pump; 5- storage tank

Because the pipeline between the two sections is very short, the head loss ∑hf can be ignored. If hM and hv are used to represent the readings of pressure gauge and vacuum gauge respectively, and the liquid column height m is used to calculate, (2- 1) can be rewritten as

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(3) Efficiency In the process of conveying liquid, when the external energy is transferred to the liquid through the impeller, there is bound to be energy loss, so the work done by the rotation of the pump shaft cannot be completely obtained by the liquid, and the energy loss is usually reflected by the efficiency η. These energy losses include volume loss, hydraulic loss and mechanical loss, and the reasons are as follows:

Volume loss Volume loss is caused by the leakage of the pump. When the centrifugal pump is running, a part of the high-pressure liquid with energy leaks back to the suction port through the gap between the impeller and the pump casing, or leaks out of the pump casing from the stuffing box. Therefore, the actual discharge of the pump is lower than the theoretical discharge, and its ratio is called volumetric efficiency η 1.

Hydraulic loss Hydraulic loss refers to the fact that when the fluid flows through the impeller and pump casing, the fluid will have an impact in the pump body, and energy will be lost due to the change of speed and direction, so the actual pressure of the pump is lower than the pressure that the pump can theoretically provide, and the ratio is called hydraulic efficiency η2.

Mechanical loss Mechanical loss is the energy loss caused by friction between pump shaft and bearing, between pump shaft and stuffing box, and between the outer surface of impeller cover plate and liquid when the pump is running. It can be expressed by mechanical efficiency η3.

The total efficiency η (also called efficiency) of the pump is equal to the product of the above three efficiencies, namely

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For centrifugal pumps, the efficiency of small pumps is generally 50% ~ 70%, and that of large pumps can reach 90%.

(4) Shaft Power The power of a centrifugal pump is the power required by the pump shaft. When the pump is directly driven by the motor, that is, the output power transmitted by the motor to the shaft is represented by n, and the unit is w or kW. Effective power is the power obtained by discharging the liquid from the impeller into the pipeline, which is expressed by ne. Due to volume loss, hydraulic loss and mechanical loss, the shaft power of the pump is greater than the effective power, that is,

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Effective power can be written as

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Where qv—— is the displacement of the pump (m3/s);

H—— the lift of the pump (m);

ρ-density of liquid to be transported (kg/m3);

G—— acceleration of gravity (m/s2).

If Ne in formula (2-5) takes kW as the unit, then

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Pump power is

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P is the working pressure of the pump.

2. Characteristic curve of centrifugal pump

As mentioned above, the main performance parameters of centrifugal pump are displacement, working pressure (head), pump power and efficiency, and the relationship between them is measured through experiments. A set of measured relationship curves is called the characteristic curve or working performance curve of centrifugal pump, which is provided by the pump manufacturer and attached to the pump sample or manual for reference by the user department when selecting the pump and operating it.

Figure 2-9 shows the characteristic curve of domestic 4B20 centrifugal pump at n = 2900r/min, which consists of three curves: h-qv, N-qv and η-qv. The characteristic curve is measured at a fixed speed and only applies to this speed, so the value of speed n is expressed on the characteristic curve.

(1)h-qv curve shows the relationship between pump head and displacement. The working pressure of centrifugal pump generally decreases with the increase of displacement (there may be exceptions when the displacement is extremely small).

(2)N-QV curve shows the relationship between pump shaft power and displacement. The power of centrifugal pump increases with the increase of displacement, and the shaft power is the minimum when the displacement is zero. Therefore, when the centrifugal pump starts, the outlet valve of the pump should be closed to reduce the starting current and protect the motor.

(3)η-qv curve shows the relationship between pump efficiency and displacement. As can be seen from the characteristic curve shown in Figure 2-9, when QV = 0 and η = 0, with the increase of displacement, the efficiency of the pump increases and reaches the maximum; If the displacement is increased in the future, the efficiency will be reduced. It shows that the centrifugal pump has a maximum efficiency point at a certain speed, which is called the design point. It is the most economical for the pump to work under the displacement and lift corresponding to the highest efficiency, so the values of qv, H and N corresponding to the highest efficiency point are called the best working condition parameters. The performance parameters indicated on the nameplate of centrifugal pump refer to the working condition parameters with the highest efficiency when the pump is running. But in fact, it is often impossible for centrifugal pumps to operate under such conditions, so generally only one working range can be specified, which is called the high efficiency area of the pump, usually about 92% of the highest efficiency. When choosing a centrifugal pump, the pump should work within this range as much as possible.

Figure 2-9 4b 20 Characteristic Curve of Centrifugal Water Pump

3. Influence of centrifugal pump speed on characteristic curve.

The characteristic curves of centrifugal pumps are all measured at a certain speed, but in actual use, it is often necessary to change the speed. At this time, the speed triangle will change, and so will the pump pressure, displacement, efficiency and pump power. When the viscosity of the liquid is not large and the efficiency of the pump remains unchanged, the approximate relationship between the displacement, head, shaft power and rotation speed of the pump is as follows:

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Where qv 1, h 1 and n 1 are the performance parameters of the pump when the rotating speed is N 1;

Qv2, h2, n2- Performance parameters of the pump when the rotating speed is N2.

When the speed change is less than 20%, it can be considered that the efficiency is unchanged, and the calculation error with the above formula is not big.

4. Influence of impeller diameter on characteristic curve

If only cutting the impeller makes the diameter smaller and the change is not big, it can be considered that the efficiency is basically unchanged, then qv is proportional to D, at a fixed speed, H is proportional to D2, and then N is proportional to D3. The approximate relationship between impeller diameter and pump displacement, pump head and shaft power is as follows:

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Where qv 1, h 1 and N 1 are the performance parameters of the pump when the impeller diameter is D 1;

Qv2, h2, N2-performance parameters of the pump when the impeller diameter is D2.

The above relationship is only available when the diameter change does not exceed 20%.

The pumps belonging to the same series are completely similar in geometry, and the ratio of impeller diameter to thickness is fixed. For this kind of pump with similar geometry, qv is proportional to D3, H is proportional to D2 and N is proportional to D5. The approximate relationship between impeller diameter and displacement, pressure head and power is:

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Where qv 1, h 1 and N 1 refer to the performance of the pump when the impeller diameter is D 1;

Qv2, h2, N2—— The performance of the pump when the impeller diameter is D2.

5. Influence of physical properties of liquid

The characteristic curve of centrifugal pump provided by the pump production department is generally measured by experiments with clean water at a certain speed and pressure at room temperature. When the performance of the transported liquid is quite different from that of water, the influence of viscosity and density on the characteristic curve should be considered.

(1) Influence of viscosity The greater the viscosity of the liquid delivered by a centrifugal pump, the more energy is lost in the pump. Results The working pressure and displacement of the pump will decrease, the efficiency will decrease and the power will increase, so the characteristic curve will change.

(2) The influence of density can be seen from the basic equation of centrifugal pump. The lift and displacement of centrifugal pump have nothing to do with the density of liquid, so the efficiency of the pump does not change with the density of liquid, so the h-qv and η-qv curves remain unchanged. But the shaft power of the pump varies with the density of the liquid. Therefore, when the density of transportation is different from that of water, the N-qv curve provided by the original product catalogue is no longer applicable, and the shaft power of the pump can be recalculated according to Formula (2-9).

(3) Influence of Solute If the transported liquid is an aqueous solution, the change of concentration will inevitably affect the viscosity and density of the liquid. The higher the concentration, the greater the difference with clear water. The influence of concentration on the characteristic curve of centrifugal pump is also reflected in viscosity and density. If the conveying liquid contains solid substances such as suspended solids, the characteristic curve of the pump is not only affected by the concentration, but also by the type and particle size distribution of the solid substances.

Four, the installation height of centrifugal pump and cavitation phenomenon

(A) cavitation phenomenon

Centrifugal pump works on liquid by rotating impeller, which increases liquid energy (including kinetic energy and static pressure energy). In the process of impeller movement, the speed and pressure of liquid also change. Usually, the inlet of centrifugal pump impeller is the place with the lowest pressure. If the pressure of the liquid in this place is equal to or lower than the saturated vapor pressure pv of the liquid at this temperature, a large amount of vapor will escape from the liquid and form many small bubbles mixed with vapor and gas. When these small bubbles flow with the liquid to the high pressure area, because the vapor pressure inside the bubbles is saturated and the surrounding of the bubbles is greater than the saturated vapor pressure, a pressure difference is generated. Under the action of this pressure difference, the bubbles burst under pressure and re-condense. In the process of condensation, liquid particles accelerate from the periphery to the center of the bubble. At the instant of rapid condensation, particles collide with each other, resulting in local high pressure. If these bubbles burst and condense near the metal surface, the liquid will keep hitting the metal surface like countless small warheads. Under the continuous impact of high pressure (hundreds of atmospheres) and high frequency (tens of thousands of times per second), the metal surface is gradually destroyed by fatigue, which is called cavitation. When the centrifugal pump runs in a state of serious cavitation, the cavitation parts are quickly destroyed into honeycomb or sponge, which greatly shortens the life of the pump. At the same time, the pump body vibrates due to cavitation, and the liquid absorption capacity and efficiency of the pump are greatly reduced. In order to ensure the normal operation of centrifugal pump and avoid cavitation, the water absorption height of the pump should not exceed the specified value, so as to ensure that the pressure at the pump inlet is greater than the saturated vapor pressure at the liquid conveying temperature.

(2) Installation height of centrifugal pump

In the specification of centrifugal pump in China, two indexes are adopted to limit the installation height of the pump to avoid cavitation. Now these two indicators are introduced as follows.

1. allowable vacuum height

The allowable suction vacuum height hs refers to the maximum vacuum that can be achieved by the pump inlet pressure p 1, and its expression is

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Where, hs-allowable suction height of centrifugal pump, m liquid column;

Pa-atmospheric pressure (n/m2);

ρ —— density of liquid to be transported (kg/m3).

In order to determine the relationship between the allowable suction vacuum and the allowable installation height hg, a centrifugal pump suction device can be provided as shown in Figure 2- 10. According to the liquid level of the storage tank, the Bernoulli equation of the section between the storage tank surface 0-0 and the pump inlet 1- 1 is listed, and then

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Where ∑hf is the head loss (m) when the liquid flows through the suction pipeline. Since the storage tank is open, p0 is the atmospheric pressure pa.

The above formula can be written as

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Substitute formula (2- 10) into the above formula, and then

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This formula can be used to calculate the installation height of the pump.

Figure 2- 10 Schematic diagram of suction port of centrifugal pump

From the above formula, sum ∑hf should be reduced as much as possible in order to improve the allowable installation height of the pump. In order to reduce, under the same flow rate, the suction pipe with a slightly larger diameter should be selected, the suction pipe should be as short as possible, the elbows should be reduced as much as possible, and the stop valve should not be installed.

Pump manufacturers can only give hs value, not hg value directly. Because the use conditions of each pump are different, the arrangement of suction pipeline is also different, and there are different sum ∑hf values, so hg can only be determined by users according to the specific arrangement of suction pipeline.

Hs given in the pump sample or instruction manual refers to the value at atmospheric pressure of 10mH2O and water temperature of 20℃. If the operating conditions of the pump are different from this state, the hs value given in the sample should be converted into the H value under the operating conditions, and the conversion formula is as follows

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Where h refers to the allowable upper vacuum height (MH2O) when conveying liquid under operating conditions;

Hs—— allowable suction vacuum degree (MH2O) given in the pump sample;

Ha—— atmospheric pressure (MH2O) at the pump working place;

Hr—— refers to the saturated vapor pressure (mH2O) of the liquid at the operating temperature.

The higher the altitude of the pump installation site, the lower the atmospheric pressure and the smaller the allowable suction vacuum. If the temperature of the liquid is higher or the liquid is more volatile, the higher the saturated vapor pressure, the smaller the allowable suction vacuum of the pump. See table 2- 1 for the atmospheric pressure at different heights.

Table 2- 1 Atmospheric pressure at different altitudes

2. Cavitation allowance

Cavitation allowance △h refers to that the sum of static head and dynamic head of liquid at the inlet of centrifugal pump exceeds a certain minimum specified value of saturated vapor pressure head of liquid at working temperature, namely

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Where △ h-cavitation allowance (m);

PR-saturated vapor pressure of liquid at working temperature (N/m2).

Combining equations (2- 1 1) and (2- 14), the relationship between cavitation allowance △h and allowable installation height hg can be deduced as follows.

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Where p0 is the pressure above the liquid level, and if it is an open liquid level, then

p0=pa

It should be noted that the △h value on the pump performance table is also specified according to the delivery of 20℃ water. When conveying other liquids, it needs to be corrected.

It can be seen from the above that the installation height of the pump can be determined as long as any one of the allowable vacuum height hs and cavitation allowance △h is known.

Verb (abbreviation of verb) Types and Selection of Centrifugal Pump

1. Type of centrifugal pump

In industrial production, the nature, pressure and flow rate of the transported liquid are very different. In order to meet various requirements, there are various types of centrifugal pumps. According to the properties of liquid, it can be divided into water pump, corrosion-resistant pump, oil pump and impurity pump. According to the impeller suction mode, it can be divided into single suction pump and double suction pump; According to the number of impellers, it can be divided into single-stage pump and multi-stage pump. Various types of centrifugal pumps become a series according to their structural characteristics, and one or several Chinese phonetic letters are used as series codes. In each series, because of different specifications, different letters and numbers will be attached to distinguish them. The types of centrifugal pumps commonly used in factories are briefly described.

(1) Water pumps (type B, type D, type Sh) can be used to transport clean water and clean liquids with physical and chemical properties similar to water.

The most widely used is the single-stage single-suction cantilever centrifugal pump, the series code is B, and its structure is shown in Figure 2- 1 1. The pump body and pump cover are made of cast iron. The total lift ranges from 8 to 8~98m, and the displacement ranges from 4.5 to 360m3/h. ..

If the required pressure head is very high and the flow rate is not too large, a multi-stage pump can be used, as shown in Figure 2- 12. A plurality of impellers are connected in series on a shaft, and the liquid flowing from one impeller passes through the guide wheel in the pump casing, which guides the liquid to change its flow direction, and at the same time converts part of kinetic energy into hydrostatic energy, and then enters the inlet of the next impeller, so that the liquid can receive energy from several impellers for many times, thus achieving a higher pressure head. The series of multistage pumps produced in China is code-named D, which is called D-type centrifugal pump. Generally from Grade 2 to Grade 9, up to Grade 12. Full range of lift range 14 ~ 35 1m and displacement range10.8 ~ 850m3/h. ..

If the liquid flow is large and the required pressure head is not high, double suction pump can be used. The impeller of the double-suction pump has two inlets, as shown in Figure 2- 13. Because the ratio of the thickness to the diameter of the impeller of the double suction pump is increased and there are two suction ports, the infusion volume is large. The series of double-suction centrifugal pumps produced in China is code-named Sh, and the range of lift of the whole series is 9 ~ 140m, and the range of displacement is120 ~12500m3/h. ..

(2) When transporting corrosive liquids such as acid and alkali, a corrosion-resistant pump (type F) should be used. Its main feature is that the parts in contact with liquid are made of corrosion-resistant materials. The corrosion-resistant pump is made of various materials, which requires simple structure, easy replacement of parts and convenient maintenance. F used as the serial code of corrosion-resistant pump. Add another letter after F to indicate the material code to show the difference. The F-type pump produced in China is made of many materials, such as:

Fig. 2- 1 1B pump structure diagram

1- pump body; 2- Impeller; 3- sealing ring; 4- protective sleeve; 5- Rear cover; 6- pump shaft; 7- brackets; 8-axis ink assembly

Figure 2- 12 Schematic Diagram of Multistage Pump

Fig. 2- 13 schematic diagram of double suction pump

Gray cast iron-material code H, used for conveying concentrated sulfuric acid;

High-silicon cast iron-material code G, used for conveying low-pressure sulfuric acid or mixed acid with sulfuric acid as the main component;

Chromium-nickel alloy steel-material code B, used for conveying low-concentration nitric acid, oxidizing acid, alkali liquor and other weakly corrosive liquids at room temperature;

Cr-Ni-Mo-Ti alloy steel-the material code is M, which is most suitable for nitric acid at room temperature and high concentration nitric acid;

Polytrifluorochloroethylene plastic-material code is S, which is suitable for sulfuric acid, nitric acid, hydrochloric acid and lye below 90℃.

Another characteristic of corrosion-resistant pump is high sealing requirements. Because it is difficult to completely solve the problem that the packing itself is corroded, F-type pump adopts mechanical sealing device as needed.

The lift range of all series F pumps is15 ~105m, and the displacement range is 2 ~ 400 m3/h. ..

Figure 2- 14b pump series characteristic curve

Table 2-2 Performance Table of Type B Water Pump (Part)

Note: Figures in brackets refer to the power of JO motor.

(3) The impurity pump (P-type) is a commonly used impurity pump, which is used to transport suspension and viscous slurry. It is widely used in non-metallic mineral processing. The series code is P, subdivided into sewage pump PW, sand pump PS, mud pump PN, etc. The requirements for this kind of pump are: not easy to be blocked by impurities, wear-resistant, easy to disassemble and easy to clean. Therefore, it is characterized by wide flow channel and few blades, and often adopts semi-closed or open impeller. Some pump casings are lined with wear-resistant cast steel guards or rubber linings.

In the product catalogue or sample of the pump, the model of the pump is a combination of letters and numbers, representing the type and specification of the pump. Now, for example.

8B29A:

Where 8 is the diameter of the suction port of the pump, in inches, that is, 8× 25 = 200mm;

B- single stage single suction cantilever centrifugal pump;

29—— Head of pump, m;

A—— The impeller diameter of this pump is one level smaller than that of the basic 8B29.

In order to facilitate the selection, the pump production department often provides the series characteristic curves of the same type of pumps, and Figure 2- 14 shows the series characteristic curves of type B pumps. Draw a section of h-qv curve corresponding to the higher efficiency range of the same type of pump on the general drawing. In the figure, the upper arc on the sector represents the basic type, and the lower arc represents the A type whose impeller diameter is one step smaller than the basic type. If the sector has three arcs, the middle arc represents type A, and the lower arc represents type B whose impeller diameter is one step smaller than the basic one. The symbols and numbers in the figure are explained in the figure.

2. Selection of centrifugal pump

The selection of centrifugal pump can generally be carried out according to the following methods and steps:

(1) Determine the flow and working pressure (pressure head) of the conveying system. The transportation capacity of liquid is generally stipulated by the production task. If the flow rate changes within a certain range, the pump should be selected according to the maximum flow rate. According to the layout of the pipeline in the transportation system, the required pressure head of the pipeline at the maximum flow rate is calculated by Bernoulli equation.

(2) Select the type and model of the pump. Determine the type of pump according to the nature of the liquid to be transported and the operating conditions. According to the determined flow Qe and head he or working pressure P, select the appropriate model from the pump sample or product catalog ... The displacement Q and head H that the selected pump can provide may not be completely consistent with the Qe and head he or working pressure P required by the pipeline. Moreover, considering the change and potential of working conditions, the selected pump can be slightly larger, but under this condition, the efficiency of the pump is relatively high, that is, the coordinate position of the point (Qe, he) is close to the h-qv curve corresponding to the high efficiency range of the pump.

After the pump model is selected, the performance parameters of the pump should be listed (Table 2-2 is the performance table of type B pump (part)).

(3) Calculating the shaft power of the pump If the density of the conveyed liquid is greater than that of water, the shaft power of the pump can be calculated according to Formula (2-7).