What is a Pump?

- May 21, 2019-

What is a Pump?

 

Pump, is a kind of used to transport liquid, or make liquid supercharged machinery.
The first pumps were used for water supply, and we Chinese invented them:

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Of course, many people don't call it a pump. They call it a water mill.
Later, around 300 BC, Archimedes invented a pump called the Archimedes screw pump, which is still produced today. It looks like a modern pump except that the handle in the picture was replaced by an electric one.

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The purpose of the pump

(1) liquid from low pressure area to high pressure area

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(2) liquid from low level to high level

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(3) long-distance transport of liquid

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 centrifugal pump

There are many types of pumps, here we introduce the most basic "centrifugal pump", its structure is as follows:

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The working principle is: the liquid rotates with the impeller and moves to the outer edge along the passage between the blades under the action of centrifugal force, increasing the speed and improving the mechanical energy. The liquid leaves the impeller and enters the volute, the volute flow passage gradually expands, the fluid speed slows down, the liquid kinetic energy is converted into static pressure energy, and the pressure keeps rising, and finally flows out of the volute along the tangential direction and is discharged through the conduit:

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The animation effect of the working process is as follows:

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 vane wheel

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Impeller is the core component that transfers the mechanical energy of the prime mover to the liquid and increases the liquid energy. It has three types: open type, semi-open type and closed type impeller:

 

The open impeller has no front and rear plates but only blades. It is mainly used to transport the liquid containing impurities. For example, the impeller of sewage pump adopts the open impeller.
The semi-open impeller only has the rear disc.
The closed impeller has both front and rear disks, and the impeller of fresh water pump is closed impeller.

 

The impeller should have enough strength and steeliness, the shape of the flow channel should conform to the flow law of the liquid, and the surface roughness of the flow channel should be small. Materials should have good wear resistance; Simple structure, good manufacturing process. Centrifugal pump impeller is generally backward blade, by casting.

 

pump case

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The pump shell collects the liquid from the impeller, and converts the kinetic energy of some liquid into pressure energy, and finally directs the liquid evenly to the secondary impeller or guide outlet.
Single stage centrifugal pump housing is mostly spiral volute housing, it has the characteristics of convenient manufacturing, high efficiency. The output of the worm machine should be in the same direction as the rotation of the impeller:

 

sealing device

Stuffing box is mainly used to prevent liquid leakage when pressure is increased. There are many types of seals, the most commonly used are packing and mechanical seals.
Packing seal is some soft packing with a certain pressure on the shaft to achieve the purpose of sealing. Soft packing for oil or graphite coated asbestos rope, put it between the stuffing box and the pump shaft, the gland pressing pressure to produce deformation to seal. The packing will be damaged after using for a period of time and needs to be checked and replaced regularly.

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Mechanical seals have two hard and smooth surfaces, one static and one rotating. This sealing device can meet the requirements of good sealing, but can not be used in liquid delivery system containing impurities, because its smooth surface will be broken and lose the sealing effect. This type of sealing device is common in liquid circulation systems:

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Cavitation

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Cavitation corrosion is a phenomenon that cavitation corrosion occurs on the metal surface in contact with liquid under the condition of high-speed flow and pressure change.

Cavitation usually occurs in the high speed decompression area at the blade tip of a centrifugal pump, where holes are formed. The holes are crushed in the high pressure area and generate impact pressure.Damage the protective film on the metal surface, so that the corrosion speed is accelerated, which is characterized by the formation of many small pitting on the metal surface, and then gradually expand into caves.

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important parameter

(1) traffic
The volume of liquid discharged by the pump in unit time is called flow rate, expressed as Q, in cubic meters per hour (m3/h).
(2) lift
Unit weight of liquid through the pump after the energy obtained is called the head. Pump head including suction, approximately pump outlet and inlet pressure difference. The head is expressed as "H" in meters (m), that is, the height of the liquid column pumped by the pump.
(3) rotation speed
The revolution per unit time of the pump shaft is expressed as "n", and the unit is r/min.
(4) power
Pump power refers to the input power, that is, the power of the prime mover to the pump shaft, also known as shaft power, represented by P. The effective power of the pump is also known as output power, expressed by Pe. The unit is W.
The efficiency of a pump is the ratio of its effective power to its input power, expressed in vertical axes, in percentages.
(5) cavitation allowance
Cavitation margin refers to the excess energy over vaporization pressure per unit weight of liquid at the pump suction inlet, expressed by NPSH, in meters. According to the cavitation allowance, the "suction distance" of the pump can be calculated, that is, the vacuum degree of the pump allowed to absorb liquid and the geometric installation height allowed by the pump.

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characteristic curve

The relation curve describing the main performance parameters is usually referred to as the pump characteristic curve, including flow-head curve (q-h), flow-power curve (q-n), flow-efficiency curve (q-vertical), flow-cavitation margin curve (q-npsh).

 

The characteristic curve of the centrifugal pump is attached to the product sample by the manufacturer, which is the main basis for guiding the correct selection and operation of the centrifugal pump.

 

Centrifugal pump operation

(1) preparation before operation
Check valves, flange anchor bolts, couplings, thermometers and pressure gauges in pumping and inlet lines.
Check the operation of the pump, first turn, listen to whether there is noise, see whether flexible.
Open the inlet valve, discharge the gas in the pump body, fill the pump with the liquid to be delivered, and then close the outlet valve.
Fill the pump tank with oil or grease.
Give the cooling water, open the pressure gauge and see if it is sensitive.
Check safety equipment such as wheelset cover, grounding wire, etc.
See the preheating condition of the hot oil pump, so that the pump body temperature can not be lower than the boundary temperature 40 degrees.
Keep good contact with related posts and units. Get ready to go.

(2) Normal boot
After the preparation is checked, the pump can be started. After starting, pay attention to the ammeter, pump steering, pressure gauge, leakage and other conditions, and slowly open the outlet valve after everything is normal.
(the pump shall not operate for more than 3 minutes before opening the outlet valve, or the temperature of the liquid will rise after forced circulation in the pump, and the liquid vaporization will produce pumping and other phenomena.)
Check the bearing temperature of the pump shall not be more than 65 degrees, the motor temperature shall not be more than 70 degrees.
May use the pump outlet valve to adjust the flow.
Observe the fluctuation of outlet pressure gauge and ammeter.
Check pump operation, vibration and leakage.
Check the pump cooling water supply, lubrication oil level changes.
For the pump of sealing oil, the sealing oil pressure shall be at least 0.05-0.1mpa higher than the pump outlet pressure.
For the long cycle operation of the pump, to replace the lubricating oil or grease regularly, to ensure that the pump in a good state of lubrication.

(3) stop
Slowly close the exit valve.
Shut off the power after the entrance valve, pressure gauge valve.
Hot oil pump, stop cooling water and sealing oil after the pump body temperature decreases.
In winter, to stop the pump to release the liquid pump, and take the necessary anti-freezing measures.
Regular inspection and turning.