Pump Basics

1. What is a pump?

A pump is a machine that transports liquids or increases the pressure of liquids. It transfers the mechanical energy of the prime mover or other external energy to the liquid, increasing the energy of the liquid. Pumps are mainly used to transport liquids such as water, oil, acid and alkali liquids, emulsions, suspensions and liquid metals. They can also transport liquid-gas mixtures and liquids containing suspended solids.

Pumps can usually be divided into three categories according to their working principles: positive displacement pumps, dynamic pumps, and other types of pumps. In addition to classification by working principle, pumps can also be classified and named by other methods. For example, according to the driving method, they can be divided into electric pumps and water turbine pumps; according to the structure, they can be divided into single-stage pumps and multi-stage pumps; according to the purpose, they can be divided into boiler feed pumps and metering pumps; according to the nature of the transported liquid, they can be divided into water pumps, oil pumps, and mud pumps.

There is a certain interdependent variation relationship between the various performance parameters of the pump, which can be represented by a curve, called the pump characteristic curve. Each pump has its own specific characteristic curve.

2. Definition and Historical Origin of Pump

A machine that transports liquids or increases the pressure of liquids. In a broad sense, a pump is a machine that transports fluids or increases their pressure, including some machines that transport gases. The pump transfers the mechanical energy of the prime mover or the energy of other energy sources to the liquid, increasing the energy of the liquid.

Water lifting is very important for human life and production. In ancient times, there were various water-lifting tools, such as the Egyptian chain pump (17th century BC), the Chinese tangerine (17th century BC), the windlass (11th century BC), the waterwheel (1st century AD), and the screw invented by Archimedes in ancient Greece in the 3rd century BC. Around 200 BC, the ancient Greek craftsman Ctesibius invented the most primitive piston pump-fire extinguishing pump. As early as 1588, there were records of 4-blade sliding vane pumps, and various other rotary pumps appeared one after another. In 1689, D. Papin of France invented the volute centrifugal pump with 4-blade impellers. In 1818, centrifugal pumps with radial straight blades, semi-open double-suction impellers and volutes appeared in the United States. From 1840 to 1850, HR Worthington of the United States invented the steam-direct-acting piston pump with the pump cylinder and steam cylinder facing each other, marking the formation of the modern piston pump. From 1851 to 1875, multi-stage centrifugal pumps with guide vanes were invented, making it possible to develop high-lift centrifugal pumps. Subsequently, various pumps were introduced. With the application of various advanced technologies, the efficiency of pumps has gradually improved, and the performance range and application have also expanded.

3. Classification basis of pumps
There are many types of pumps, which can be divided into the following according to their working principles: ① Dynamic pumps, also called impeller pumps or vane pumps, rely on the dynamic effect of the rotating impeller on the liquid to continuously transfer energy to the liquid, increase the kinetic energy (mainly) and pressure energy of the liquid, and then convert the kinetic energy into pressure energy through the extrusion chamber. They can also be divided into centrifugal pumps, axial flow pumps, partial flow pumps and vortex pumps. ② Positive displacement pumps rely on the periodic change of the volume of the sealed working space containing the liquid to periodically transfer energy to the liquid, increase the pressure of the liquid to force the liquid out, and can be divided into reciprocating pumps and rotary pumps according to the movement form of the working element. ③ Other types of pumps transfer energy in other forms. For example, the jet pump relies on the high-speed jet of working fluid to suck the fluid to be transported into the pump and mix it, and then exchange momentum to transfer energy; the water hammer pump uses part of the flowing water to be raised to a certain height during braking to transfer energy; the electromagnetic pump makes the energized liquid metal flow under the action of electromagnetic force to achieve transportation. In addition, pumps can also be classified according to the properties of the transported liquid, driving method, structure, purpose, etc.

4. Application of pumps in various fields
From the performance range of pumps, the flow rate of giant pumps can reach hundreds of thousands of cubic meters per hour, while the flow rate of micro pumps is less than tens of milliliters per hour; the pressure of the pump can range from normal pressure to as high as 19.61Mpa (200kgf/cm2) or more; the temperature of the transported liquid can be as low as -200 degrees Celsius or as high as 800 degrees Celsius. Pumps transport a wide variety of liquids, such as water (clean water, sewage, etc.), oil, acid and alkali liquids, suspensions, and liquid metals.
In the production of chemical and petroleum sectors, raw materials, semi-finished products and finished products are mostly liquids, and it takes a complex process to make raw materials into semi-finished products and finished products. Pumps play the role of transporting liquids and providing pressure flow for chemical reactions in these processes. In addition, pumps are used to adjust the temperature in many devices.

In agricultural production, pumps are the main irrigation and drainage machinery. my country's rural areas are vast, and a large number of pumps are needed every year. Generally speaking, agricultural pumps account for more than half of the total pump production.

In the mining and metallurgical industries, pumps are also the most used equipment. Mines need pumps to drain water, and pumps are needed to supply water during ore dressing, smelting and rolling processes.

In the power sector, nuclear power plants require nuclear main pumps, secondary pumps, and tertiary pumps, and thermal power plants require a large number of boiler feed pumps, condensate pumps, circulating water pumps, and ash pumps, etc.

In national defense construction, pumps are needed for the adjustment of aircraft flaps, tail rudders and landing gear, the rotation of warships and tank turrets, the sinking and floating of submarines, etc. High-pressure and radioactive liquids, and some also require pumps without any leakage.

In the shipbuilding industry, each ocean-going ship generally uses more than 100 pumps of various types. Other applications such as urban water supply and drainage, water for steam locomotives, lubrication and cooling in machine tools, transportation of bleach and dye in the textile industry, transportation of pulp in the paper industry, and transportation of milk and sugar in the food industry all require a large number of pumps.

In short, whether it is airplanes, rockets, tanks, submarines, drilling, mining, trains, ships, or daily life, pumps are needed everywhere and pumps are running everywhere. This is why pumps are classified as general machinery. They are an important product in the machinery industry.

5. Basic parameters of pumps

The basic parameters that characterize the main performance of the pump are as follows:

1. Traffic Q

Flow rate is the amount of liquid (volume or mass) delivered by the pump per unit time.

The volume flow rate is represented by Q, and the unit is: m3/s, m3/h, l/s, etc.

Mass flow is represented by Qm, and its unit is: t/h, kg/s, etc.

The relationship between mass flow and volume flow is: Qm=ρQ

Where ρ is the density of the liquid (kg/m3, t/m3), and for clean water at room temperature ρ=1000kg/m3.

2. Lift H

The head is the energy increment of the unit weight of liquid pumped by the pump from the pump inlet (pump inlet flange) to the pump outlet (pump outlet flange). It is the effective energy obtained by one Newton of liquid passing through the pump. Its unit is N·m/N=m, that is, the height of the liquid column pumped by the pump, usually referred to as meter.
3. Speed ​​n

The speed is the number of revolutions of the pump shaft per unit time, represented by the symbol n, and the unit is r/min.

4. NPSH

NPSH, also known as net positive suction head, is the main parameter to indicate cavitation performance. NPSH was once expressed as Δh in China.

5. Power and efficiency

The power of a pump usually refers to the input power, that is, the power transmitted to the pump shaft by the prime mover, so it is also called shaft power, represented by P;

The effective power of the pump is also called the output power, which is represented by Pe. It is the effective energy obtained by the liquid transported from the pump in the pump per unit time.

Because the head refers to the effective energy obtained from the pump per unit weight of liquid output by the pump, the product of the head, mass flow rate and gravitational acceleration is the effective energy obtained from the liquid output from the pump per unit time - that is, the effective power of the pump:

Pe = ρgQH(W) = γQH(W)

Where ρ is the density of the liquid pumped by the pump (kg/m3);

γ——density of the liquid delivered by the pump (N/m3);

Q——pump flow rate (m3/s);

H——pump head (m);

g——acceleration due to gravity (m/s2).

The difference between shaft power P and effective power Pe is the power loss in the pump, and its size is measured by the efficiency of the pump. The efficiency of the pump is the ratio of effective power to shaft power, expressed as η.

6. What is flow rate? What letter is used to represent it? How to convert it?

The volume of liquid discharged by the pump per unit time is called flow rate, which is expressed by Q. The unit of measurement is: cubic meter/hour (m3/h), liter/second (l/s), L/s=3.6 m3/h=0.06 m3/min=60L/min
G=Qρ G is weight ρ is liquid specific gravity
Example: A pump has a flow rate of 50 m3/h. What is the weight per hour when pumping water? The specific gravity of water ρ is 1000 kg/cubic meter.
Solution: G=Qρ=50×1000(m3/h·kg/ m3)=50000kg / h=50t/h

7. What is head? What letter is used to represent it? What unit of measurement is used? Conversion and formula with pressure?

The energy obtained by a unit weight of liquid passing through a pump is called head. The head of a pump includes the suction head, which is approximately the pressure difference between the pump outlet and the pump inlet. Head is represented by H, and the unit is meter (m). The pressure of a pump is represented by P, and the unit is MPa (megapascal), H=P/ρ. If P is 1kg/cm2, then H=(lkg/cm2)/(1000kg/m3) H=(1kg/cm2)/(1000kg/m3)=(10000kg/m2)/1000kg/m3=10m
1Mpa=10kg/c m2, H=(P2-P1)/ρ (P2=outlet pressure P1=inlet pressure)

8. What is NPSH? What is suction lift? What are the letters representing their respective units of measurement?

When the pump is working, the liquid will produce gas at the inlet of the impeller due to a certain vacuum pressure. The vaporized bubbles will erode the metal surface of the impeller and other metals under the impact movement of the liquid particles, thereby destroying the impeller and other metals. At this time, the vacuum pressure is called the vaporization pressure. The cavitation margin refers to the excess energy of the unit weight of liquid at the pump suction port that exceeds the vaporization pressure. The unit is marked in meters, using (NPSH) r. The suction head is the required cavitation margin Δh: that is, the vacuum degree allowed by the pump to absorb liquid, that is, the allowable installation height of the pump, in meters.
Suction head = standard atmospheric pressure (10.33 meters) - cavitation margin - safety amount (0.5 meters)
Standard atmospheric pressure can compress the pipeline vacuum height to 10.33 meters.
For example: The required cavitation margin of a pump is 4.0 meters. What is the suction head Δh?
Solution: Δh=10.33-4.0-0.5=5.83 meters
IX. What is the cavitation phenomenon of the water pump and its causes
1. Cavitation

When the liquid is at a certain temperature and the pressure is reduced to the vaporization pressure at that temperature, bubbles will be generated in the liquid. This phenomenon of bubble generation is called cavitation.

2. Cavitation collapse

When bubbles generated during cavitation flow to a high-pressure area, their volume decreases and they burst. This phenomenon of bubbles disappearing in the liquid due to rising pressure is called cavitation collapse.

3. Causes and hazards of cavitation

When the pump is running, if the absolute pressure of the pumped liquid in a local area of ​​the flow section (usually somewhere behind the impeller blade inlet) drops to the liquid vaporization pressure at the current temperature for some reason, the liquid will begin to vaporize at that location, generating a large amount of steam and forming bubbles. When the liquid containing a large number of bubbles passes through the high-pressure area in the impeller, the high-pressure liquid around the bubbles causes the bubbles to shrink sharply and even burst. While the bubbles are condensing and bursting, the liquid particles fill the cavities at a very high speed, generating a very strong water hammer effect at this moment, and hitting the metal surface at a very high impact frequency. The impact stress can reach hundreds to thousands of atmospheres, and the impact frequency can reach tens of thousands of times per second. In severe cases, the wall thickness will be penetrated.

4. Cavitation process

The process of bubbles being generated and bursting in a water pump, which damages the flow-through parts, is the cavitation process in the water pump. After cavitation occurs in a water pump, in addition to damaging the flow-through parts, it will also generate noise and vibration, and cause the performance of the pump to decline. In severe cases, the liquid in the pump will be interrupted and cannot work normally.

10. What is the characteristic curve of a pump?

The curve that represents the relationship between the main performance parameters is usually called the performance curve or characteristic curve of a centrifugal pump. In essence, the performance curve of a centrifugal pump is the external manifestation of the movement law of the liquid in the pump, which is obtained through actual measurement. The characteristic curve includes: flow-head curve (QH), flow-efficiency curve (Q-η), flow-power curve (QN), flow-cavitation margin curve (Q-(NPSH)r). The function of the performance curve is that any flow point of the pump can find a set of head, power, efficiency and cavitation margin values ​​on the curve. This set of parameters is called the working state, referred to as the working condition or working point. The working condition of the highest efficiency point of the centrifugal pump is called the optimal working point, which is generally the design working point. Generally, the rated parameters of centrifugal pumps, i.e. the design operating point and the optimal operating point, coincide or are very close. Operating in the practical efficiency range can save energy and ensure the normal operation of the pump, so it is very important to understand the performance parameters of the pump.

11. What is pump efficiency? What is the formula?

It refers to the ratio of the effective power of the pump to the shaft power. η=Pe/P
Pump power usually refers to input power, that is, the power transmitted to the pump shaft by the prime mover, so it is also called shaft power, represented by P.
Effective power is: the product of the pump head, mass flow rate and gravity acceleration.
Pe=ρg QH (W) or Pe=γQH/1000 (KW)
ρ: Density of the liquid transported by the pump (kg/m3)
γ: Specific gravity of the liquid transported by the pumpγ=ρg (N/ m3)
g: Gravity acceleration (m/s)
Mass flow rate Qm=ρQ (t/h or kg/s)

12. What is a full performance test bench for a pump? A

full performance test bench is a device that can accurately test all performance parameters of a pump through precision instruments. The national standard accuracy is Class B. The flow rate is measured with a precision worm flowmeter, and the head is measured with a precision pressure gauge. The suction distance is measured with a precision vacuum gauge. The power is measured with a precision shaft power machine. The speed is measured with a tachometer. The efficiency is calculated based on the measured value: n=rQ102.