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hg8858.com Brief analysis of the structure and principle of water pumps and valves for water systems

Brief analysis of the structure and principle of water pumps and valves for water systems

Public number: Fluid Machinery Online Source: Time: 2020-01-09 01:12:31

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Water pumps for water systems mainly include: centrifugal pumps (single-stage vertical centrifugal pumps, single-stage horizontal centrifugal pumps, multi-stage vertical centrifugal pumps);

The valves of water system mainly include: gate valve, butterfly valve, ball valve, check valve.

Centrifugal pumps for water systems

Working principle and structure of centrifugal pump

Centrifugal pump device

In order for the centrifugal pump to work normally, the centrifugal pump must be equipped with certain pipelines and fittings. Such a centrifugal pump equipped with a certain pipeline system is called a centrifugal pump device. Figure 1-1 shows the general device of a centrifugal pump, which mainly has a bottom valve, a suction line, an outlet valve, and an outlet line.

There are many types of centrifugal pumps. Although the structures of various types of pumps are different, the main parts are basically the same.

The main parts are pump casing, pump cover, pump body, impeller, seal ring, pump shaft, mechanical seal or stuffing box, coupling, bearing, etc.

Schematic diagram of the working principle of centrifugal pump

When the centrifugal pump works, the impeller rotates at high speed, and the liquid gains energy under the action of inertia centrifugal force to increase the pressure. Before the centrifugal pump works, the pump body and the inlet pipeline must be filled with liquid medium to prevent cavitation. When the impeller rotates rapidly, the blades prompt the medium to rotate quickly. The rotating medium flies out of the impeller under the action of centrifugal force. After the water in the pump is thrown out, the central part of the impeller forms a vacuum area. While continuously inhaling the liquid, the inhaled liquid is constantly given a certain amount of energy to discharge the liquid. The centrifugal pump works so continuously.

Cavitation

Cavitation refers to: When the centrifugal pump starts, if there is air in the pump, the centrifugal force generated by the rotation is very small due to the low density of the air, so the low pressure formed in the central area of the impeller is not enough to lower the liquid level below the pump inlet The phenomenon that fluid cannot be pumped into the pump.

Hazard mechanism: Cavities are formed in the high-speed decompression area of the blade end of the centrifugal pump blade. The cavities are crushed in the high-pressure area and generate impact pressure, which damages the protective film on the metal surface and accelerates the corrosion rate. Cavitation is characterized by the formation of many small pits on the metal surface, and then gradually expands into caves.

  When the pressure of a liquid at its contact with a solid surface is lower than its vapor pressure, bubbles will form near the solid surface. In addition, the gas dissolved in the liquid may precipitate and form bubbles. Subsequently, when the bubble flows to a place where the pressure of the liquid exceeds the pressure of the bubble, the bubble collapses, and a great impact force and high temperature are generated at the moment of collapse.

Cavitation hazard:

、产生振动和噪声。 1. Generate vibration and noise.

、降低泵的性能。 2. Reduce the performance of the pump.

、破坏过流部件。 3. Destroy overcurrent components.

Classification of centrifugal pump

(1) There is only one impeller in a single-stage centrifugal pump. A single-stage centrifugal pump is a widely used pump. Because the liquid is energized only once in the pump, the head is low. Figure 1-2 shows a single-stage single-suction centrifugal pump.

(2) Multi-stage centrifugal pump A centrifugal pump with two or more impellers is called a multi-stage centrifugal pump. The more stages, the higher the pressure. Figure 1-4 shows a segmented centrifugal water pump. The impeller of this pump is generally single-suction.

Advantages of multi-stage centrifugal pumps

  People install several impellers on the same pump shaft, and each impeller forms a separate working room with the liquid guiding device around it. The system composed of this working room and impeller can be considered as a single-stage centrifugal pump. The chambers are connected in series back and forth to form a multi-stage pump. Compared with multiple single-stage centrifugal pumps in series, multi-stage pumps have the advantages of high efficiency, small footprint, low operating costs, and easy maintenance. The pump has a flow range of 5-720m 3 / h and a head of up to 2800m.

  In addition to the advantages of a single-stage centrifugal pump, the most important advantage of a multi-stage centrifugal pump is its high head. Multi-stage centrifugal pumps have a wide range of uses. For example, in the production of chemical fertilizers, multi-stage pumps are used to drive ammonia into a carbonization tower, and the ammonia absorbs carbon dioxide in a pressurized nitrogen-hydrogen mixture to produce ammonium bicarbonate; water for boilers; mountainous areas Deep well irrigation and so on.

Valves for water systems

Classified by purpose and function:

Truncation class: mainly used to truncate or connect the media stream. Such as gate valve, globe valve, ball valve, disc valve, plug valve, diaphragm valve

Non-return type: used to prevent the medium from flowing backwards. Including various structures of check valves.

Regulation: regulating the pressure and flow of the medium such as pressure reducing valve, pressure regulating valve, throttle valve

Safety: When the medium pressure exceeds the specified value, it is used to discharge excess medium to ensure the safety of the piping system and equipment.

Distribution category: change the medium flow direction, distribution medium, such as three-way plug, distribution valve, slide valve, etc.

Special applications: such as traps, vent valves, blowdown valves, etc.

See Nanba Encyclopedia in detail: "A Brief Overview of the Application of Valves in Air Conditioning Systems"

gate

Gate valve refers to a valve whose opening and closing body (valve plate) drives the valve seat sealing surface to move up and down by the stem, which can connect or cut off the passage of fluid. When the valve is partially opened, eddy currents are generated on the back of the gate, which easily causes erosion and vibration of the gate, and also easily damages the sealing surface of the valve seat, making repair difficult. The gate valve is usually suitable for working conditions that do not require frequent opening and closing, and keep the gate fully open or closed. Not suitable for regulation or throttling.

Structure of the gate valve

Features of gate valve

The gate valve is mainly used for cutting in the pipeline. Generally, it is mostly used for cutting devices with a diameter of DN≥50mm, and sometimes the cutting device with a small diameter also uses a gate valve.

The gate valve has the following advantages:

① The fluid resistance is small.

② The external force required for opening and closing is small.

③ The flow direction of the medium is not restricted.


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④ When fully open, the sealing surface is less eroded by the working medium than the stop valve.

⑤ The shape is relatively simple, and the casting processability is good.

Gate valves also have disadvantages:

① The external dimensions and opening height are both large. Large space required for installation.

② During the opening and closing process, there is relative friction between the sealing surfaces, which easily causes abrasion.

③The gate valve generally has two sealing surfaces, which adds some difficulties to processing, grinding and maintenance.

Type of ball valve

Structure of the ball valve

Ball valve features

Ball valve is mainly used in the pipeline to cut off, distribute and change the flow direction of the medium. It has the following advantages

1 . Simple structure, small size, light weight and easy maintenance.

流体阻力小,紧密可靠,密封性能好。 2. Small fluid resistance, tight and reliable, good sealing performance.

3 Convenient operation, quick opening and closing, and convenient remote control.

4 . The sealing surface of the ball and the valve seat is isolated from the medium, which is not easy to cause the erosion of the valve sealing surface.

5 . Wide range of applications, from small diameter to several millimeters, large to several meters, can be applied from high vacuum to high pressure.

Butterfly valve

The butterfly valve is composed of a valve body, a disc, a valve stem, and a handle. It adopts disc-type opening and closing parts. The disc valve disc is fixed on the valve stem. The valve stem can be turned 90 to complete the opening and closing function. At the same time, when the opening angle of the valve flap is 20 ° ~ 75 °, the flow rate and the opening angle have a linear relationship and have the characteristics of throttling.

Structure of butterfly valve

Operating characteristics of butterfly valves

Butterfly valves are widely used in various media with a pressure below 2.0 MPa and a temperature not higher than 200 ° C.

Features of butterfly valve:

(1) Simple structure, small size, short structure length, small size, light weight, suitable for large-caliber valves.

(2) When the valve seat channel is fully opened, the effective flow area is large, and the fluid resistance is small.

(3) The opening and closing is convenient and rapid, and the adjustment performance is good.

(4) The opening and closing torque is small. Since the discs on both sides of the rotating shaft are basically affected by the medium, the direction of torque generation is opposite, so the opening and closing is labor-saving.

(5) The sealing surface is generally made of rubber or plastic, so the low-pressure sealing performance is good.

Check valve

Check valve refers to the valve that automatically opens and closes the valve flap depending on the flow of the medium itself to prevent the medium from flowing backward.

Classification of check valves

Swing check valve

The flow resistance is small, and the sealing performance is not as good as the lifting type. It is suitable for occasions with low flow rate and infrequent flow, and is not suitable for pulsating flow.

(C) Multi-lobe type: reduce water hammer without damage to sealing surface, large diameter (DN> 600)

Lift check valve

直通式升降止回阀; (b) 立式升降止回阀 (a) straight lift check valve; (b) vertical lift check valve

Foot valve: pump suction port is set. Preventing reverse flow is conducive to starting the pump.

Features of check valve

The shape of the valve body of the lift check valve is the same as that of the shut-off valve (common with the shut-off valve), so its fluid resistance coefficient is large.

Swing check valve, the valve disc rotates around the pin outside the valve seat, the application is more common.

The disc check disc rotates around a pin in the valve seat. Its structure is simple, it can only be installed on horizontal pipelines, and its sealing is poor.

END

:暖通南社 Source : HVAC Nanshe

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