1.What is a pump?
Answer: Generally, any machine that lifts liquids, transports liquids, or increases the pressure of liquids, that is, converts the mechanical energy of the prime mover into liquid energy, is collectively referred to as a pump.
2. Classification of pumps?
Answer: The uses of pumps vary. According to their working principles, they can be classified into three major categories:
① Volume pump ② Vane pump ③ Other types of pumps
3. How does a volumetric pump work? Can you give an example?
Answer: Utilize the periodic changes in the working volume to convey the liquid.
For example: Piston pumps, plunger pumps, diaphragm pumps, gear pumps, plunger pumps, screw pumps, etc.
4. How does a vane pump work? Give an example?
Answer: Utilizing the liquid interaction within the blades to transport the liquid.
For example: Centrifugal pumps, mixed-flow pumps, axial-flow pumps, vortex pumps, etc.
5. How does a centrifugal pump work?
Answer: The centrifugal pump transfers the mechanical energy from the prime mover to the liquid through the action of the rotating impeller. During the process when the liquid flows from the inlet to the outlet of the impeller, both its velocity energy and pressure energy increase. The liquid discharged by the impeller is converted into pressure energy in the outlet chamber and then sent out along the discharge pipeline. At this time, a vacuum or low pressure is formed at the side of the impeller inlet due to the liquid discharge. The liquid in the suction chamber is pressed into the impeller inlet under the action of the liquid surface pressure (atmospheric pressure). Thus, the rotating impeller continuously sucks in and discharges the liquid.
6. What are the characteristics of centrifugal pumps?
Answer: Its features are: high rotational speed, small size, light weight, high efficiency, large flow rate, simple structure, stable performance, easy operation and maintenance. The drawback is that before starting, the pump must be filled with liquid. High viscosity has a significant impact on the pump performance and it can only be used for liquids with viscosity similar to water. Flow range: 5 - 20,000 cubic meters per hour, head range: 8 - 2,800 meters.
7. How many types of structural forms does the centrifugal pump have? What are their respective characteristics and applications?
Answer: Centrifugal pumps are classified by their structural forms into: vertical pumps and horizontal pumps. The characteristics of vertical pumps are: small floor area, low construction cost, and easy installation. The disadvantages are: high center of gravity, not suitable for operation in situations without fixed foundations. The characteristics of horizontal pumps are: wide application range, low center of gravity, and good stability. The disadvantages are: large floor area, high construction cost, large volume, and heavy weight. For example: vertical pumps are pipeline pumps, DL multi-stage pumps, submersible electric pumps, etc. Horizontal pumps include IS pumps, D-type multi-stage pumps, SH type double-suction pumps, B-type, BA type, IH type, IR type. According to the requirements of head and flow rate and based on the impeller structure and the number of stages, they are classified as:
①, Single-stage single-suction pump: The pump consists of one impeller with one suction port. The general flow rate range is: 5.5 - 2000 cubic meters per hour, and the head range is: 8 - 150 meters. The characteristics are: small flow rate and low head.
②, Single-stage double-suction pump: The pump has one impeller with two inlet sections. The general flow rate range is: 120 - 20,000 cubic meters per hour, and the head range is: 10 - 110 meters. It has a large flow rate and a low head.
② Single suction multi-stage pump: The pump consists of multiple impellers. The first impeller has one suction port, the discharge chamber of the first impeller serves as the suction port for the second impeller, and so on. The general flow rate range is: 5 - 200 cubic meters per hour, and the head is between 20 and 240 meters. Its characteristics are low flow rate and high head.
8. What is a pipeline pump? What are its structural features?
Answer: The pipe pump is a type of single-suction single-stage centrifugal pump. It has a vertical structure. Because its inlet and outlet are on the same straight line and the inlet and outlet diameters are the same, it resembles a section of pipe and can be installed at any position on the pipeline, hence it is named "pipe pump".
Structural features: It is a single-suction single-stage centrifugal pump. The inlet and outlet are the same and located on the same straight line, perpendicular to the centerline of the shaft, and it is a vertical pump.
9. The structural features and advantages of the ISG type single-stage single-suction vertical centrifugal pump are as follows:
First, the pump is of a vertical structure. The motor cover and the pump cover are designed as a single unit. The appearance is compact and attractive, with a small floor area, low construction cost, and it can be placed outdoors when equipped with a protective cover.
Second, the inlet and outlet diameters of the pump are the same and they are located on the same central line. It can be directly installed on the platform like a valve, and the installation process is extremely simple.
Thirdly, the ingenious base design facilitates the stable installation of the pump.
Fourth, the pump shaft serves as the extended shaft of the motor. It solves the serious vibration problem that occurs when the conventional centrifugal pump shaft and the motor shaft use a coupling for transmission. The surface of the pump shaft is chrome-plated, which significantly extends the service life of the pump.
Fifth, the impeller is directly installed on the extended shaft of the motor. During operation, the pump produces no noise. The motor bearings use low-noise bearings, ensuring that the overall machine operates with very low noise, significantly improving the usage environment.
Sixth, the shaft seal adopts a mechanical seal, which solves the serious leakage problem caused by the conventional centrifugal pump's sealing mechanism. The static ring and the moving ring of the seal are made of silicon carbide, which enhances the service life of the seal and ensures a dry and tidy working environment.
Seventh, there are vent holes on the pump cover. On the lower side and both sides of the pump body, there are water discharge holes and pressure gauge holes, which can ensure the normal operation and maintenance of the pump.
Eighth, the unique structure enables the pipeline system to be maintained without having to be disassembled. All that is needed is to remove the pump cover nut, after which the maintenance can be carried out very conveniently.
10. How many types of pipeline pumps are there and what are the common features among them? And what are their respective applications?
Answer: ①, ISG type single-stage single-suction centrifugal water pump for clear water. It is used for industrial and domestic water supply and drainage, high-rise building pressure boosting, water supply, heating, refrigeration and air conditioning circulation, industrial pipeline pressure boosting transportation, cleaning, water supply equipment and boiler matching. The operating temperature is ≤ 80℃.
②, The IRG type single-stage single-suction hot water pipeline pump is used for increasing pressure and circulating the hot water from boilers in industries such as metallurgy, chemical engineering, textile, wood processing, papermaking, as well as in departments like hotels, bathrooms, and guesthouses. The maximum operating temperature is ≤ 120℃.
③, The IHG single-stage single-suction chemical pipeline pump is used for transporting chemically corrosive liquids in industries such as textiles, petroleum, chemical engineering, medicine, hygiene, food, and oil refining. The operating temperature is ≤ 100℃. It is an ideal product for replacing conventional chemical pumps.
④, YG type single-stage single-suction pipe oil pump. It is an ideal product for conventional oil pumps. It is suitable for oil depots, refineries, chemical industries, and power departments of enterprises and institutions for transporting oil and flammable, explosive liquids. The operating temperature should be below 120℃.
5. The GRG, GHG, and GYG single-stage single-suction high-temperature pipeline pumps are designed by adding a water-cooling cooling device to the ordinary type. The operating temperature is ≤ 185℃. Their application scope is similar to that of the ordinary type.
GRG is a high-temperature hot water pump, GHG is a high-temperature chemical pipeline pump, and GYG is a high-temperature pipeline oil pump.
11. Basic parameters of the pump?
Answer: Flow rate Q (m³/h), Head H (m), Speed n (r/min), Power (total power and applicable power) Pa (kW), Efficiency h (%), Suction and discharge head difference r (m), Inlet and outlet diameters φ (mm), Impeller diameter D (mm), Pump weight W (kg).
12. What is flow? Which letter is used to represent it? How many units of measurement are there? How is it converted? How can it be converted into weight and what is the formula?
Answer: The volume of liquid discharged per unit time is called flow rate. Flow rate is denoted by the letter Q.
Measurement units: cubic meters per hour (m3/h), liters per minute (L/min), liters per second (L/s)
1 liter per second = 3.6 cubic meters per hour = 0.06 cubic meters per minute = 60 liters per minute
G = Qr G represents weight r represents the specific gravity of the liquid
Example: The flow rate of a certain pump is 50 m³/h. What is the weight per hour when pumping water? The specific gravity of water r is 1000 kilograms/cubic meter (or 1 g/cm³).
Solution: G = Qr = 50 × 1000 (m³/h. kg/m³) = 50000 kg/h = 50 T/h
13. What is head? Which letter is used to represent it? What is the unit of measurement? How is it related to pressure conversion and the corresponding formula?
Answer: The energy gained by a unit weight of liquid after passing through the pump is called head.
The head of the pump, including the suction head, is approximately equal to the pressure difference between the pump outlet and the inlet. The head is denoted by "H" and is measured in meters (m). The pressure of the pump is represented by "P" and is measured in Mpa (megapascals), kilograms (Kg)/cm, H = P/r
For example, P = 1 kilogram/cmH = P/r = (1 kilogram/cm) / (1000 kilograms/m) = (10000 kilograms/m) / (1000 kilograms/m) = 10 MPa = 10 kilograms (Kg) / cm H = (P2 - P1) r (P2 - outlet pressure)
14. What is the efficiency of a pump? How is it calculated?
Answer: It refers to the ratio of the pump's effective power to its shaft power.
Effective power refers to the pump's head × flow rate × specific gravity (weight flow rate) Ne = rQH. The unit is kilowatts.
1 kilowatt = 102 kilograms meters per second 1 kilowatt = 75/102 horsepower
Shaft power and centrifugal pump power refer to the power transmitted from the prime mover to the pump, that is, the input power. The unit is kilowatts.
n = Ne/N = rQH / 102N where r is in tons per cubic meter, Q is in liters per second, and H is in meters.
n = Ne/N = rQH / (102 × 3.6N) r is in tons per cubic meter Q is in cubic meters per hour H is in meters
15. What do we mean by rated flow rate, rated rotational speed, and rated head?
Answer: The pump is designed based on the specified performance parameters for its operation. The optimal performance achieved is defined as the pump's rated performance parameters. These are usually the parameter values specified in the product catalog sample.
For example: A flow rate of 50 - 125 with 12.5 m3/h being the rated flow rate, a head of 20 m being the rated head, and a rotational speed of 2900 rpm being the rated rotational speed.
16. What is the term "suction head loss"? What is the term "suction lift"? What are their respective units and corresponding symbols?
Answer: When the pump is in operation, due to a certain vacuum pressure at the inlet of the impeller, liquid vaporization occurs. The vaporized bubbles, under the impact movement of liquid particles, cause peeling on the metal surfaces such as the impeller, thereby damaging the metal. At this time, the vacuum pressure is called the vaporization pressure. The cavitation margin refers to the excess energy that the unit weight of liquid at the pump suction inlet possesses over the vaporization pressure. The unit is the meter of liquid column, and it is represented by (NPSH) r.
The suction head is the necessary cavitation margin Δ/h: it is the vacuum degree at which the pump can suck liquid, and it is also the allowable geometric installation height of the pump. The unit is in meters. Suction head = standard atmospheric pressure (10.33 meters) - cavitation margin - safety margin (0.5). The standard atmospheric pressure can create a vacuum height of 10.33 meters on the pipeline.
For example: The necessary suction lift for a certain pump is 4.0 meters. Calculate the suction head Δh.
Solution: Δh = 10.33 - 4.0 - 0.5 = 5.67 meters
17. What is the characteristic curve of a pump? What aspects does it include? What is its function?
Answer: Generally, the curves or characteristic curves that represent the relationships between the main performance parameters are referred to as the performance curves or characteristic curves of the centrifugal pump. In fact, the performance curves of the centrifugal pump are the external manifestations of the movement laws of the liquid within the pump, and they are obtained through actual measurement.
The characteristic curves include: flow-head curve (Q-H), flow-power curve (Q-N), flow-efficiency curve (Q-η), and flow-allowable suction head rise curve (Q-(NPSH)r).
The function of the performance curve is that for any flow point of the pump, a set of corresponding values of head, power, efficiency and cavitation margin can be found on the curve. This set of parameters is called the working state, which is abbreviated as working condition or working point. The working condition with high efficiency is called the optimal working condition point. The optimal working condition point is generally the design working condition point. Generally, the rated parameters of a centrifugal pump, that is, the design working condition point and the optimal working condition point, coincide or are very close. In practice, operating within the high-efficiency range can achieve energy saving while ensuring the normal operation of the pump. Therefore, understanding the performance parameters of the pump is quite important.
18. What is a pump's full performance test bench?
Answer: The equipment that can accurately test all the performance parameters of the pump through precise instruments is the full-performance testing platform. The national standard accuracy for this equipment is level B.
The flow rate is measured using a precision rotameter.
The head is measured using a precise pressure gauge.
The suction height is measured using a precise vacuum gauge.
The power is measured by a precise shaft power meter.
The rotational speed is measured using a speedometer. The efficiency is calculated based on the measured value: η = Rqn / 102N.
The performance curve is plotted on the coordinate system based on the measured values.
19. Relationship between pump shaft power and motor equipped power
Answer: The pump shaft power is the power transmitted from the prime mover to the pump during design. During actual operation, the working conditions will change. Therefore, there should be a certain margin for the power transmitted from the prime mover to the pump. Additionally, the output power of the motor depends on the power factor and the shaft, so the common practice is to equip the motor with a power greater than the pump shaft power.
Axial power:
0.1 - 0.55KW 1.3 - 1.5 times
0.75 - 2.2 KW 1.2 - 1.4 times
3.0 - 7.5 KW 1.15 - 1.25 times
11KW and above 1.1 - 1.15 times
And it is customized according to the power specifications of the Y series motors as per national standards.
20. Model meaning: ISG50-160IA (B)?
Answer: ISG50-160 (I)A (B) Where:
I: A single-stage single-suction centrifugal pump that adopts the ISO2858 international standard and the performance parameters of the IS type single-stage single-suction centrifugal pump.
S: S Clear Type
G: Pipeline type
50: Nominal diameter (bore) for import and export (in millimeters) 50mm
160: Nominal size of the pump impeller (referring to the diameter of the impeller which is approximately 160mm)
I: I classifies the flow (without I flow at 12.5 m³/h, with I flow at 25 m³/h)
A (B): A condition where the pump efficiency is not high, while the flow rate, head and shaft power are all reduced.
A: The first cutting of the impeller
B: Second cutting of the impeller
What is cavitation phenomenon:
Answer 1. The lowest pressure in the unit pump occurs near the inlet of the impeller. When the pressure at this point drops to the saturation pressure corresponding to the current temperature, the liquid begins to vaporize, and a large number of bubbles escape from the liquid. When these bubbles flow with the liquid to the high-pressure area of the pump, under the action of external pressure, the bubbles suddenly condense into liquid. At this time, the liquid surrounding the bubbles, that is, it rushes towards the space where the bubbles were originally, and generates a very strong hydraulic impact. Due to the condensation of many bubbles per second, many large impact pressures are generated repeatedly. Under the continuous action of this local impact load, the surfaces of the flow components in the pump gradually become worn out, and many eroded spots appear, then they form a honeycomb-like pattern, and eventually lead to peeling off. In addition to the damage caused by the impact, when the liquid vaporizes, it also releases the oxygen dissolved in it, causing the flow components to oxidize and corrode.
This phenomenon where the flow components are damaged due to the combined effect of mechanical erosion and chemical corrosion is known as cavitation.
Answer 2. When a liquid is at a certain temperature and the pressure is reduced to the vaporization pressure at that temperature, bubbles are formed in the liquid. This phenomenon of bubble formation is called cavitation.
Answer 3. Cavitation refers to the situation where, when the pressure on the surface of the storage tank remains constant, if the pressure at the center of the impeller drops to be equal to the saturated vapor pressure of the current temperature of the liquid being transported, a large number of bubbles will form at the inlet of the impeller. These bubbles, along with the liquid, enter the high-pressure zone and are quickly crushed and condensed, resulting in a vacuum in the area where the bubbles are located. The surrounding liquid particles rush towards the center of the bubbles at an extremely high speed, causing an instantaneous impact pressure, thereby causing the impeller to be damaged rapidly. At the same time, there is pump vibration, noise, and a significant decrease in the pump's flow rate, head, and efficiency. This phenomenon is called cavitation.
Answer 4. If it is a water pump, the height between the pump and the water surface should be reduced. During the operation of the hydraulic cylinder, a certain amount of air is mixed into the liquid between the piston and the guide sleeve. As the pressure gradually increases, the air in the liquid will turn into bubbles. When the pressure reaches a certain limit value, these bubbles will burst under the high pressure, thereby rapidly applying high-temperature and high-pressure gas to the surface of the parts, causing the hydraulic cylinder to suffer from cavitation and resulting in corrosive damage to the parts. This phenomenon is called cavitation.
Jet Pump and Cavitation
The jet pump achieves the purpose of transportation by converting the energy of fluid flow. It can be used to transport liquids or gases. In chemical production, steam is often used as the working fluid of the jet pump, which is utilized to create a vacuum and generate negative pressure within the equipment. Therefore, it is commonly referred to as a steam jet pump.
Working principle: Under high pressure, the working steam is ejected from the nozzle at a very high velocity, bringing low-pressure gas or steam into the high-speed fluid. The inhaled gas mixes with the steam and enters the expansion tube. The velocity gradually decreases, and the static pressure increases accordingly. Finally, it is discharged through the outlet.
When conducting the two working conditions of changing the flow rate of the mixed liquid and altering the length of the throat and nozzle gap for the jet pump. When adjusting the flow rate of the mixed liquid, the flow rate of the power fluid also changes accordingly, and the speed of the power fluid passing through the nozzle also changes. This results in the weakening of the cavitation phenomenon as the flow rate of the mixed liquid decreases, until it is completely eliminated. Based on the experience of three different throat and nozzle gap lengths, it is found that increasing the throat and nozzle gap can increase the annular flow area between the nozzle and the throat. When the same amount of fluid passes through a larger area, the flow velocity will be lower and the pressure will be higher, making the cavitation phenomenon less likely to occur.
Analysis and Management of Pump Cavitation Phenomenon
I. Cavitation Phenomenon
When a liquid is at a certain temperature and the pressure is reduced to the vaporization pressure at that temperature, bubbles are formed in the liquid. This phenomenon of the formation of bubbles is called cavitation. The bubbles generated during cavitation flow to the high-pressure area and their volume decreases, causing them to burst. The phenomenon where bubbles disappear in the liquid due to the increase in pressure is called cavitation collapse.
During the operation of the pump, if, for some reason, a certain local area of the flow passage (usually somewhere slightly after the inlet of the impeller blade) experiences a decrease in the absolute pressure of the liquid being pumped to the vaporization pressure of the liquid at that temperature, the liquid begins to vaporize at that point, generating a large amount of steam and forming bubbles. When the liquid containing a large number of bubbles passes through the high-pressure area within the impeller, the high-pressure liquid surrounding the bubbles causes the bubbles to rapidly shrink and eventually burst. At the same time, the liquid particles fill the voids at a very high speed, generating a very strong water impact effect at this instant. This process of forming bubbles and their bursting causing damage to the flow components is the cavitation process in the pump. After the pump experiences cavitation, in addition to causing damage to the flow components, it will also generate noise and vibration, and lead to a decrease in the pump's performance. In severe cases, it may cause the interruption of the liquid in the pump and prevent it from working normally.
II. Basic Relationship Formula for Pump Cavitation
The conditions for pump cavitation are determined by both the pump itself and the suction device. Therefore, when studying the conditions for cavitation, one should consider both the pump itself and the suction device. The basic relationship equation for pump cavitation is
NPSHc≤NPSHr≤[NPSH]≤NPSHa
NPSHa = NPSHr (NPSHc) -- Indicates the start of cavitation for the pump
NPSHa > NPSHa > NPSHr (NPSHc) -- The pump has no cavitation.
In the formula, NPSHa - the net positive suction head available, also known as the effective suction head, the larger the value, the less prone to cavitation.
NPSHr - Pump Suction Suction Head Margin, also known as the necessary suction head margin or the pump inlet dynamic pressure drop. The smaller it is, the better the anti-suction cavitation performance.
NPSHc - Critical Suction Head Margin, refers to the suction head margin corresponding to a certain degree of decline in pump performance;
[NPSH] - Allowable Suction Lift, this is the suction lift margin used to determine the operating conditions of the pump. Usually, [NPSH] = (1.1 - 1.5) NPSHc.
III. Calculation of the Cavitation Margin of the Device
NPSHa = Ps/ρg + Vs/2g - Pc/ρg = Pc/ρg ± hg - hc - Ps/ρg
IV. Measures to Prevent Cavitation Occurrence
To prevent cavitation, it is necessary to increase the NPSHa. The measures to prevent cavitation by ensuring that NPSHa is greater than NPSHr are as follows:
1. Reduce the geometric suction height hg (or increase the geometric backflow height).
2. To reduce the suction loss hc, one can try to increase the pipe diameter, minimize the length of the pipeline, and reduce the number of bends and accessories.
3. Prevent prolonged operation under high flow conditions;
4. Under the same rotational speed and flow rate, using a double-suction pump can reduce the inlet flow velocity, thus making the pump less prone to cavitation.
5. When the pump experiences cavitation, the flow rate should be reduced or the speed should be decreased for operation.
6. The condition of the pump's suction tank has a significant impact on the pump's cavitation.
7. For pumps operating under harsh conditions, to prevent cavitation damage, materials resistant to cavitation can be used.
Types and Principles of Pumps | Cavitation Phenomenon | Basic Relationship Equations of Pump Cavitation
Answer: 1. Definition of pump types and principles: Generally, any machine that lifts liquids, transports liquids, or increases the pressure of liquids, that is, any machine that converts the mechanical energy of the prime mover into liquid energy to achieve the purpose of pumping liquids, is collectively referred to as a pump.
II. Working Principle of the Pump:
1. Volumetric pump - Suction of liquid through the periodic change in the volume of the working chamber.
2. Vane pump - This type of pump uses the interaction between the vanes and the liquid to convey the liquid.
3. Specific Uses of the Pump: The different uses of the pump, the different liquid media it transports, the different flow rates and head ranges, of course, also result in different structural types and materials. In summary, they can be broadly classified as: urban water supply, sewage systems, civil and construction systems, agricultural and water conservancy systems, power station systems, chemical systems, petroleum industry systems, mining and metallurgical systems, light industry systems, and ship systems.
4. Cavitation Phenomenon
When a liquid is at a certain temperature and the pressure is reduced to the vaporization pressure at that temperature, bubbles are formed in the liquid. This phenomenon of the formation of bubbles is called cavitation. The bubbles generated during cavitation flow to the high-pressure area and their volume decreases, causing them to burst. The phenomenon where bubbles disappear in the liquid due to the increase in pressure is called cavitation collapse.
During the operation of the pump, if a certain local area of the flow passage (usually a certain position slightly behind the inlet of the impeller blade) experiences a reduction in the absolute pressure of the liquid being pumped to the vaporization pressure of the liquid at that temperature, the liquid will start to vaporize at this point, generating a large amount of steam and forming bubbles. When the liquid containing a large number of bubbles passes through the high-pressure area within the impeller, the high-pressure liquid surrounding the bubbles causes the bubbles to rapidly shrink and eventually burst. At the same time, the liquid particles fill the voids at a very high speed, generating a very strong water impact effect at this instant. The impact force reaches several to several thousand atmospheres per second, and the impact frequency can reach tens of thousands of times per second. In severe cases, the wall thickness can be penetrated.
The process in which bubbles are generated and burst in the pump, causing damage to the flow components, is known as the cavitation process in the pump. After the pump experiences cavitation, in addition to causing damage to the flow components, it will also produce noise and vibration, leading to a decline in the pump's performance. In severe cases, it may cause the interruption of the liquid in the pump and prevent it from operating normally.
How to choose a pump:
Answer: Currently, when selecting micro pumps, such as micro vacuum pumps, micro air pumps, micro gas sampling pumps, micro gas circulation pumps, micro exhaust pumps, micro suction pumps, micro pumping pumps, micro gas filling pumps, and micro high-pressure gas pumps, these often involve these three concepts.
In simple terms, these three concepts respectively correspond to the dilute, normal and dense states of a gas.
Atmospheric pressure: It refers to one atmosphere of pressure, which is the pressure exerted by the gases in the atmosphere we are accustomed to living in. A standard atmospheric pressure is 101325 Pa (pascal - a common unit of pressure). 100,000 Pa = 100 KPa, so "a standard atmospheric pressure" is also commonly expressed as 100 KPa or 101 KPa. Due to differences in geographical location, altitude, temperature, etc. in each place, the actual atmospheric pressure there is not equal to the standard atmospheric pressure. However, for the sake of simplicity, sometimes it can be approximately considered that the normal pressure is a standard atmospheric pressure, that is, 100 KPa.
Negative pressure: This refers to a gas state with a lower pressure than normal atmospheric pressure, which is commonly known as "vacuum". For example, when drinking a beverage through a tube, the tube contains negative pressure; the inner part of an suction cup used to hang things is also under negative pressure.
Positive pressure: This refers to a gas state with a higher pressure than the normal atmospheric pressure. For example, when inflating the tires of a bicycle or car, the outlet end of the air pump or inflator generates positive pressure.
II. In numerous fields such as research, bioengineering, automatic control, environmental protection, water treatment, etc., gas sampling, gas circulation, object adsorption, etc. are often required. At such times, a vacuum pump is needed. Its main parameters include vacuum degree and flow rate, etc.
(1) "Vacuum degree" generally refers to the maximum pressure that a pump can achieve during operation. That is, it is the degree of thinness of the remaining gas after the pump has removed all the gas from a sealed container.
In industry, the term "limit pressure" can have two meanings. One is "absolute pressure", which is based on "absolute vacuum" (the theoretical absolute vacuum where no substance exists) as the zero point. The values marked are all positive numbers. The smaller the number, the closer it is to absolute vacuum, and the higher the vacuum degree. For example, we have a "high vacuum" micro vacuum pump VCH1028. Its limit pressure is 10 KPa (0.01 MPa). Among micro vacuum pumps, this is considered to have a very high vacuum degree.
The other type is "relative pressure", where the atmospheric pressure is taken as the zero point. Anything below the atmospheric pressure is represented by a negative value, hence it is called "negative pressure". The larger the absolute value of this negative value, the higher the vacuum degree. For example, we have a "high negative pressure micro vacuum pump" PH2506B with a negative pressure of -75KPa (-0.075MPa), while VCH1028 is high (VCH has -90KPa (-0.09Mpa)). Therefore, the suction force of PH2506B is not as strong as that of VCH.
The internationally accepted and most scientific way to denote pressure in the vacuum industry is to use "absolute pressure"; however, because the method of measuring relative pressure is simpler and the measuring instruments are more common (such as ordinary vacuum gauges are all relative pressure gauges), it is customary in China to denote pressure as "relative pressure".
The relationship between the two: Relative pressure = Absolute pressure - Local atmospheric pressure.
For example, the absolute pressure of VCH1028 is 10 Kpa. Its relative pressure = 10 - 100 = -90 Kpa (-0.09 MPa).
(2) In fields such as research, laboratories, and medicine, there are often applications of gas pressurization, such as inflating a container that already has a positive pressure, or when the resistance within the system is high and a pump is needed to overcome the resistance to deliver gas. At such times, a pump that can output a positive pressure higher than atmospheric pressure is required. This is usually expressed as "relative pressure". Our high-pressure miniature air pump and miniature vacuum pump can output a maximum positive pressure of >100Kpa (0.1MPa). They are dry-type vacuum pumps and do not require vacuum pump oil or lubricating oil, thus not polluting the working medium. They can operate continuously for 24 hours, and the exhaust port can be clogged, making them particularly suitable for these situations.
Comprehensive example: (Not particularly rigorous, just to illustrate the relationship among the three)
Assuming the pressure of the gas in the sealed container is at normal pressure, which means there are 100 gas molecules inside. Using the VCH1028 with a negative pressure of -90 Kpa, it can finally remove 90 of them, leaving 10. At this point, the negative pressure inside the container is -90 Kpa. If it is replaced with the PH2506B, it can only remove 75 of them, leaving 25. Correspondingly, the negative pressure inside the container is -75 Kpa.
If the PCF5015N is used to inflate this container, there will be 200 gas molecules inside the container at the end. Represented by absolute pressure, it is 200 Kpa; represented by relative pressure (positive pressure), it is 100 Kpa.
What are the criteria for selecting the pump?
Answer: To select the type of pump, it is necessary to determine its purpose and performance. This selection process begins by choosing the type and form of the pump. Then, on what principle should the pump be selected? And what are the basis for this selection?
I. Selection Principles
Ensure that the selected pump type and performance meet the requirements of the process parameters such as flow rate, head, pressure, temperature, cavitation flow, and suction height of the equipment.
2. It is necessary to meet the requirements of the medium characteristics. For pumps that transport flammable, explosive, toxic or valuable media, reliable shaft seals or leak-free pumps are required, such as magnetic drive pumps, diaphragm pumps, and shielded pumps. For pumps that transport corrosive media, the flow components must be made of corrosion-resistant materials, such as AFB stainless steel corrosion-resistant pumps and CQF engineering plastic magnetic drive pumps. For pumps that transport media containing solid particles, the flow components must be made of wear-resistant materials, and in some cases, the shaft seals should be flushed with clean liquids.
3. High mechanical reliability, low noise and small vibration.
4. Economically, it is necessary to comprehensively consider the total cost of equipment, operation, maintenance and management, ensuring that it is the lowest.
5. Centrifugal pumps have the characteristics of high rotational speed, small size, light weight, high efficiency, large flow rate, simple structure, no pulsation in liquid delivery, stable performance, easy operation and convenient maintenance. Therefore, except for the following situations, centrifugal pumps should be selected as much as possible:
When there are measurement requirements, the head requirement of the metering pump is very high, the flow rate is very small, and there is no suitable small-flow high-head centrifugal pump available. In such cases, a reciprocating pump can be selected. If the cavitation requirement is not high, a vortex pump can also be chosen. When the head is very low and the flow rate is very high, an axial flow pump and a mixed flow pump can be selected. When the medium viscosity is relatively high (greater than 650 - 1000 mm2/s), a rotor pump or a reciprocating pump (such as a gear pump or a screw pump) can be considered. When the medium contains 75% air and the flow rate is small with a viscosity less than 37.4 mm2/s, a vortex pump can be selected. For occasions where frequent starting is required or it is inconvenient to fill the pump, pumps with self-priming performance should be selected, such as self-priming centrifugal pumps, self-priming vortex pumps, and pneumatic (electric) diaphragm pumps.
II. General Procedure for Pump Selection
Based on various factors such as the layout of the device, terrain conditions, water level conditions, operating conditions, and economic scheme comparison, the selection of horizontal, vertical and other types (pipe type, right-angle type, variable-angle type, turning-angle type, parallel type, vertical type, upright type, submersible type, detachable type, submerged type, non-clogging type, self-priming type, gear type, oil-filled type, water-temperature filled type) should be considered. Horizontal pumps are convenient for disassembly and assembly, easy to manage, but have a large volume and a relatively high price, and require a large area; vertical pumps are often with the impeller submerged in water, can be started at any time, are convenient for automatic operation or remote control, and are compact, have a small installation area, and are relatively cheaper.
2. Based on the properties of the liquid medium, select the appropriate pump, such as a water pump, a hot water pump, an oil pump, a chemical pump, a corrosion-resistant pump, or an impurity pump, or use a non-clogging pump. For pumps installed in explosion zones, if the explosion zone level is known, an explosion-proof motor should be used.
3. The vibration quantities are classified as: pneumatic and electric (the electric type is further divided into 220v voltage and 380v voltage).
4. Choosing between single-suction pumps and double-suction pumps based on flow rate: Select single-suction pumps or multi-suction pumps based on the height of the head. For high-speed pumps or low-speed pumps (air conditioning pumps), multi-stage pumps have lower efficiency than single-stage pumps. If both single-stage pumps and multi-stage pumps can be used, it is advisable to choose the single-stage pumps.
5. Once the specific model of the pump is determined and a pump from a certain series is selected, the specific model can be determined on the type spectrum or series characteristic curve based on the two main performance parameters: the maximum flow rate and the head after adding 5% - 10% margin. By using the pump characteristic curve, find the required flow rate value on the horizontal axis and the required head value on the vertical axis. Draw vertical or horizontal lines from these two values in the respective directions, and the intersection point of the two lines exactly falls on the characteristic curve. Then this pump is the one to be selected. However, this ideal situation is rarely encountered. Usually, the following situations may occur:
A. The first case: The intersection point is above the characteristic curve. This indicates that the flow rate meets the requirements, but the head is insufficient. At this time, if the head differences are similar or within about 5%, they can still be selected. If the head differences are significant, then choose the pump with a larger head. Or try to reduce the pipeline resistance loss.
B. The second type: If the intersection point is below the characteristic curve and within the fan-shaped trapezoidal range of the pump characteristic curve, then this model can be preliminarily determined. Then, based on the difference in head, decide whether to cut the impeller diameter. If the head difference is very small, do not cut; if the head difference is large, calculate the impeller diameter according to the required Q, H, using its ns and cutting formula. If the intersection point does not fall within the fan-shaped trapezoidal range, select a pump with a lower head. When selecting a pump, sometimes it is necessary to consider the production process requirements and choose different shapes of Q-H characteristic curves.
The concept of cavitation in centrifugal pumps
Essentially, the cavitation phenomenon in centrifugal pumps is a kind of fluid dynamic cavitation effect, related to vortices. It refers to the situation where the pressure of the fluid drops below its critical pressure (generally the saturated vapor pressure) during its movement, causing local areas of the fluid to vaporize and generating tiny bubble clusters. These bubble clusters grow to a certain extent and then collapse and disappear under the influence of external factors (such as gas dissolution, steam condensation, etc.). In the local area, this causes water hammer action, with the stress reaching several thousand atmospheres. Clearly, this effect is destructive. From a macroscopic perspective, the cavitation phenomenon causes the surface of the flow channel to be eroded and damaged (a continuous high-frequency impact damage), triggering vibrations and generating noise; in severe cases, there is a break in the flow, resulting in flow channel blockage, and causing a decline in the performance of the pump.
From the above description, it can be seen that cavitation occurs due to the minimum absolute pressure present in the flow field. Where the absolute pressure is low, cavitation is more likely to occur. Therefore, controlling the minimum absolute pressure can control the cavitation effect and effectively reduce the occurrence of cavitation phenomena.
A pump is a machine that adds energy to a fluid. The fluid flows out through the impeller, and its pressure generally increases. Therefore, the place where the fluid has the lowest pressure in a pump is usually near the inlet of the impeller blades. Thus, ensuring that the fluid has sufficient absolute pressure at the inlet of the impeller blades becomes the key to avoiding cavitation in the pump.
The suction head required (NPSH) for the pump
Due to the complexity of fluid motion in turbomachinery, it is extremely difficult to theoretically calculate where cavitation might occur in the flow field. Moreover, the occurrence of cavitation is not only dependent on the fluid's flow characteristics but also on the thermodynamic properties of the fluid itself. Therefore, it is even more challenging to theoretically establish a criterion for the occurrence of cavitation. Thus, in practice, the method of combining experience with experiments is often used to propose the criterion for cavitation. The concept of cavitation margin of pumps is one of the important criteria among them. It not only has certain theoretical significance but is also one of the standards for product acceptance.
The cavitation margin of a pump has two concepts: The first one is related to the installation method and is called the effective cavitation margin NPSHA. It refers to the part of energy remaining above the critical pressure head after the water flows through the suction pipeline and reaches the pump suction inlet. This is the available cavitation margin and belongs to "user parameters". The second one is related to the pump itself and is called the necessary cavitation margin NPSHR. It is the pressure drop value from the pump suction inlet to the point of minimum pressure. This is the critical cavitation margin and belongs to "factory parameters". To ensure that the pump does not cavitate during operation, it is necessary to ensure that NPSHA ≥ K × NPSHR in the installation (K is the safety margin), and the latter is guaranteed by the manufacturer. From this perspective, reducing the cavitation margin of the pump means ensuring the absolute lifting height of the pump and meeting the usage requirements.
Analysis of 2NPSHR
Obviously, the size of NPSHR depends on the energy loss of the fluid flow at the pump suction inlet. Due to the short process, this loss mainly manifests as local flow losses. There are several factors as follows:
(1) The pump suction inlet converges to the impeller inlet flow channel, resulting in an increase in flow velocity and a pressure loss. The fluid motion changes from axial to radial at the turning point, and the uneven flow field at the turning point causes a pressure loss.
(2) The flow loss caused by changes in flow velocity is manifested as a decrease in pressure;
(3) The energy loss generated by the fluid flowing around the inlet edge of the blade;
(4) The squeezing effect of the blade thickness causes an increase in the inlet velocity, resulting in pressure loss.
(5) The impact loss of the flowing fluid at the leading edge of the blade under non-design operating conditions;
(6) The poor casting quality of the impeller and the uneven surface of the flow channel result in viscous losses during flow.
Among the above factors, the first two are difficult to completely avoid; while the latter ones can be reduced by improving the design and manufacturing quality. This requires designers to strive to make the flow passage from the pump inlet to the impeller inlet as close as possible to the streamline of fluid movement, in order to reduce the pressure loss of this section of flow; for an existing product pump, analyzing its cavitation performance should start from analyzing the flow loss of its inlet flow passage.
3 Analysis of Cavitation in a Centrifugal Pump
Now, let's conduct a qualitative analysis of the cavitation problem of the centrifugal pump mentioned earlier. The cavitation margin of this pump is relatively large, and the reason can be considered to be caused by the excessive pressure loss at the suction inlet of the pump. However, the large cavitation margin of this pump at low flow rates is different from the usual detection results, which may be related to the design and manufacturing. The increase in cavitation margin at low flow rates can be attributed to the increase in the inlet angle of the liquid flow, resulting in an excessive positive impact angle at the blade inlet and excessive leakage, thereby causing a large pressure loss; while at high flow rates, the increase in cavitation margin is mainly due to the increase in flow velocity, which leads to an increase in losses.
From both the design and manufacturing perspectives, apart from the cause of gap cavitation, the small angle of the blade inlet placement (either due to improper design or during casting), the large thickness of the blade inlet, and the poor casting quality of the blade surface may be the main reasons for the large cavitation margin of this type of pump.
4. Improvement Measures
For this pump, the following appropriate measures can be taken to reduce the possibility of cavitation occurrence:
If possible, the inlet edge of the blade can be moved forward, that is, a piece can be attached at the inlet edge, so that the fluid can come into contact with the blade earlier to obtain energy, and avoid the occurrence of situations below the critical pressure.
(2) Clean the inlet channel of the impeller, making it as smooth and flat as possible to enhance the surface finish of the inlet and reduce the flow resistance and pressure loss.
(3) Grind the blade head, sharpen it, in order to reduce the impact loss at the inlet and lower the sensitivity of the inlet angle.
(4) If the gap cavitation is severe, a solution can be to drill balance holes on the impeller to reduce the leakage flow rate, thereby alleviating the degree of cavitation.
Questions related to pumps
Question 1: What are the classifications of pumps?
Answer: Based on the different working principles, they can be classified into the following types:
(1) Vane pumps rely on the high-speed rotating vanes within the pump to convey liquids, such as centrifugal pumps and axial flow pumps, etc.
1. (2) Volume pumps: These pumps rely on the changes in the working volume within the pump to draw in or discharge liquids and increase the pressure energy of the liquids. Examples include piston pumps and rotary gear pumps.
(3) Jet pump: This type of pump utilizes the energy of the working fluid (liquid or gas) to convey liquids, such as water jet pumps and steam jet pumps, etc.
2. What are the components of a centrifugal pump?
Answer: The centrifugal pump unit consists of a centrifugal pump, an electric motor, an inlet pipe, an outlet pipe and valves, etc. Our company adopts a combined design of machinery and pump, which reduces the area by 30%.
3. What is the working principle of a centrifugal pump?
Answer: Before starting the pump, the suction pipe and the pump itself must be filled with liquid. After starting the pump, the impeller rotates at high speed. The liquid inside the impeller rotates along with the blades. Under the action of centrifugal force, the liquid is ejected away from the impeller and shoots out. The ejected liquid gradually slows down in the diffusion chamber of the pump casing and gradually increases in pressure. Then it flows out from the pump outlet and the discharge pipe. At this time, at the center of the blades, due to the liquid being ejected to the surrounding areas, a vacuum low-pressure area without air or liquid is formed. The liquid in the liquid pool is sucked into the pump through the suction pipe under the action of the atmospheric pressure of the pool surface. The liquid is continuously sucked up from the liquid pool and continuously flows out through the discharge pipe.
4. What is "traffic"? What is its unit?
Answer: The flow rate q refers to the volume of liquid that is discharged from the pump outlet and enters the pipeline within a unit of time. The unit of flow rate is m/h, m/s or L/s.
5. What is head? What is its unit?
Answer: The energy added per unit mass of liquid by the pump, which is the total head generated by the pump, is called head. The unit of head is meters.
6. What is cavitation?
Answer: Cavitation is a phenomenon where liquid vaporizes, causing damage to the flow components of the pump (the components that the liquid comes into contact with as it passes through the pump).
7. What is cavitation?
Answer: The lowest pressure in the pump is near the inlet of the impeller. When the pressure at this point drops to the saturation pressure corresponding to the current temperature, the liquid begins to vaporize, and a large number of bubbles escape from the liquid. When these bubbles flow with the liquid to the high-pressure area of the pump, under the action of external pressure, the bubbles suddenly condense into liquid. At this time, the liquid surrounding the bubbles rushes towards the space where the bubbles were originally, generating a very strong hydraulic impact. Due to the condensation of many bubbles per second, many strong impact pressures occur repeatedly. Under the continuous action of this local impact load, the surfaces of the flow components in the pump gradually become worn out, forming many eroded spots. Subsequently, they become connected in patches in a honeycomb-like pattern, and eventually, there is a phenomenon of peeling off. In addition to the damage caused by the impact, when the liquid vaporizes, it also releases the oxygen dissolved in it, causing the flow components to oxidize and corrode. This phenomenon where the flow components are damaged by the combined action of mechanical erosion and chemical corrosion is called cavitation.
8. What are the classifications of centrifugal pumps?
Answer: (i) According to the application of centrifugal pumps, they can be classified as: ⑴ Clear water pump; ⑵ Impurity pump; ⑶ Acid-resistant pump.
(II) According to the structure of the impeller, they can be classified as: ⑴ Closed impeller centrifugal pumps; ⑵ Open impeller centrifugal pumps; ⑶ Semi-open centrifugal pumps.
(3) According to the number of impellers, it can be classified as: ⑴ Single-stage centrifugal pump; ⑵ Multi-stage centrifugal pump.
(4) According to the way the pump sucks in the liquid, it can be classified as: ⑴ Single suction centrifugal pump; ⑵ Double suction centrifugal pump.
(5) According to the method of pump discharge, they are classified as: ⑴蜗壳式 centrifugal pump; ⑵ guide-flow type centrifugal pump
㈥ Classified by head: ⑴ Low-pressure pump; ⑵ Medium-pressure pump; ⑶ High-pressure pump.
㈦ According to the position of the pump shaft, they are classified as: ⑴ Vertical pumps; ⑵ Horizontal pumps.
9. What are the methods for balancing the axial force of a centrifugal pump?
Answer: ⑴ The balance of axial force for single-stage pumps is mainly achieved through three methods: opening balance holes, installing balance pipes, and using double-suction impellers.
(2) The balance of axial force for multi-stage pumps is mainly achieved through the symmetrical arrangement of impellers and by using methods such as balance discs and balance drums.
The key to the renovation of the condensate water recovery system lies in how to eliminate cavitation phenomenon while ensuring normal production. Cavitation refers to the phenomenon where hot saturated water will release steam under pressure reduction, and the generated steam will suddenly liquefy and condense into water when entering the high-pressure area, causing the bubbles to burst. If this process repeats, it will cause damage to the surface of the parts in this area, along with various related corrosion effects, ultimately resulting in sponge-like or honeycomb-like cavitation damage. The consequence of cavitation is to disrupt the continuity of the steam transmission process, increase resistance, block the flow path, and seriously affect the efficiency and normal production of the pump. In the past, manufacturers often reduced pressure to recover condensate water in order to release a large amount of flash steam to reduce the source of cavitation. However, this approach undoubtedly leads to energy waste. Therefore, the best way to solve the cavitation problem of the pump is to make the pressure entering the pump exceed the cavitation pressure, thereby fundamentally avoiding the occurrence of cavitation. The main working principle of the closed condensate water recovery technology is to utilize the pressurization principle of the jet pump, establish a cavitation prevention theory suitable for the transportation of hot saturated water, and finally design the jet pump reasonably to solve the cavitation problem of the pump.
In addition, the selection of the steam trap in this system is based on the most unfavorable operating conditions, thereby avoiding the energy waste caused by the contradiction between the selection of the steam trap and its actual operation in the original system. The water collection tank designed for the closed-type recovery pump is closed, which not only ensures that the recovery temperature of the condensate water is 120℃, but also makes full use of the flash steam.
As mentioned above, adopting the closed-loop condensate recovery technology to enhance the utilization efficiency of steam is very effective and feasible.
