内容发布更新时间 : 2025/1/4 2:16:58星期一 下面是文章的全部内容请认真阅读。

This gives a more gradual change in direction to the fluid and more efficient conversion of kinetic energy into pressure energy than is obtained with the volute type．The angle of the leading edge of the fixed vanes should be such that the fluid is received without shock．The liquids flows along the surface of the impeller vane with a certain velocity whilst the tip of the vane is moving relative to the casing of the pump．The direction of motion of the liquid relative to the pump casing and the required angle of the fixed vanes—is found by compounding these two velocities．

uv is the velocity of the liquid relative to the vane and utis the tangential velocity of the tip of the vane; compounding these two velocities gives the resultant velocity u2 of the liquid. It is apparent, therefore, that the required vane angle in the diffuser is dependent on the throughput, the speed of rotation，and the angle of the impeller blades. The pump will therefore operate at maximum efficiency only over a narrow range of conditions． (1) Virtual head of a centrifugal pump

The maximum pressure is developed when the whole of the excess kinetic energy of the fluid is converted into pressure energy. As indicated below, the head is proportional to the square of the radius and to the speed, and is of the order of 60m for a single—stage centrifugal pump; for higher pressures, multistage pumps must be used. Consider the liquid which is rotating at a distance of between rand r?dr from the centre of the pump.

The mass of this element of fluid dm is given by 2?rdrd?，where ? is the density of the fluid and b is the width of the element of fluid。If the fluid is traveling with a velocity u and at an angle ? to the tangential direction . The angular momentum of this mass of fluid is dM(urcos?).

9

The torque acting on the fluid dτ is equal to the rate of change of angular momentum with time，as it goes through the pump

??d??dM(urcos?)?2?rb?dr(urcos?)

?t?tThe volumetric rate of flow of liquid through the pump：

?Q?2?rb

?tdr?Q?d(urcos?) The total torque acting on the liquid in the pump is therefore obtained integrating d? between the limits denoted by suffix 1 and suffix 2，where suffix 1 refers to the conditions at the inlet to the pump and suffix 2 refers to the condition at the discharge．

Thus，??Q?(u2r2cos?2?u1r1cos?1) (2) The advantages and disadvantages of the centrifugal pump

The main advantages are：

1. It is simple in construction and can，therefore， be made in a wide range of materials.

2. There is a complete absence of valves．

3. It operates at high speed(up to 100 Hz)and，therefore，can be coupled directly to an electric motor. In general，the higher the speed the smaller the pump and motor for a give n duty．

4. It gives a steady delivery．

5. Maintenance costs are lower than for any other type of pump．

6. No damage is done to the pump if the delivery line becomes blocked，provided it is not run in this condition for a prolonged period．

7. It is much smaller than other pumps of equal capacity．It can，therefore，be made into a sealed unit with the driving motor and immersed in the suction tank．

8. Liquids containing high proportions of suspended solids are readily handled． The main disadvantages are：

1. The single—stage pump will not develop a high pressure．Multistage pumps will develop greater heads bat they are very much more expensive and cannot readily be made in corrosion—resistant material because of their greater complexity．It is generally better to use very high speeds in order to reduce the number of stages required．

2. It operates at a high efficiency over only a limited range of conditions; this applies especially to turbine pumps．

3. It is not usually self-priming.

4. If a non-return valve is not incorporated in the delivery or suction line, the liquid will run back into the suction tank as soon as the pump stops．

5. Very viscous liquids cannot he handled efficiently．

10

3. Cavitation in centrifugal pump

(1)The term ‘cavitation’ comes from the Latin word cavus, which means a hollow space or a cavity. Webster’s Dictionary defines the word ‘cavitation’ as the rapid formation and collapse of cavities in a flowing liquid in regions of very low pressure.

In any discussion on centrifugal pumps various terms like vapor pockets, gas pockets, holes, bubbles, etc. are used in place of the term cavities. These are one and the same thing and need not be confused. The term bubble shall be used hereafter in the discussion.

In the context of centrifugal pumps, the term cavitation implies a dynamic process of formation of bubbles inside the liquid, their growth and subsequent collapse as the liquid flows through the pump.

Generally, the bubbles that form inside the liquid are of two types: Vapor bubbles or Gas bubbles.

1.Vapor bubbles are formed due to the vaporisation of a process liquid that is being pumped. The cavitation condition induced by formation and collapse of vapor bubbles is commonly referred to as Vaporous Cavitation.

2.Gas bubbles are formed due to the presence of dissolved gases in the liquid that is being pumped (generally air but may be any gas in the system). The cavitation condition induced by the formation and collapse of gas bubbles is commonly referred to as Gaseous Cavitation. (2)Important Definitions: To enable a clear understanding of mechanism of cavitation, definitions of following important terms are explored.

Static pressure; Dynamic pressure; Total pressure; Velocity head; Vapour pressure.

Static pressure ：The static pressure in a fluid stream is the normal force per unit area on a solid boundary moving with the fluid. It describes the difference between the pressure inside and outside a system, disregarding any motion in the system. For instance, when referring to an air duct, static pressure is the difference between the pressure inside the duct and outside the duct, disregarding any airflow inside the duct. In energy terms, the static pressure is a measure of the potential energy of the fluid.

Dynamic pressure：A moving fluid stream exerts a pressure higher than the static pressure

1due to the kinetic energy (mv2) of the fluid. This additional pressure is defined as the dynamic

2pressure. The dynamic pressure can be measured by converting the kinetic energy of the fluid stream into the potential energy. In other words, it is pressure that would exist in a fluid stream that has been decelerated from its velocity ‘v’ to ‘zero’ velocity.

Total pressure：The sum of static pressure and dynamic pressure is defined as the total pressure. It is a measure of total energy of the moving fluid stream. i.e. both potential and kinetic energy.

Velocity head：Vapor pressure is the pressure required to keep a liquid in a liquid state. If

11

the pressure applied to the surface of the liquid is not enough to keep the molecules pretty close together, the molecules will be free to separate and roam around as a gas or vapor. The vapor pressure is dependent upon the temperature of the liquid. Higher the temperature, higher will be the vapor pressure.

(3) Cavitation Damage：Cavitation can destroy pumps and valves, and cavitation causes a loss of efficiency in pumps immediately, and also a continuously increasing loss of efficiency as the equipment degrades due to erosion of the pump components by cavitation. Therefore It is important to understand the phenomena sufficiently to predict and therefore reduce cavitation and damage from cavitation, and also to diagnose and find practical solutions to cavitation problems。

1.Cavitation Enhanced Chemical Erosion

Pumps operating under cavitation conditions become more vulnerable to corrosion and chemical attack. Metals commonly develop an oxide layer or passivated layer which protects the metal from further corrosion. Cavitation can remove this oxide or passive layer on a continuous basis and expose unprotected metal to further oxidation. The two processes (cavitation & oxidation) then work together to rapidly remove metal from the pump casing and impeller. Stainless steels are not invulnerable to this process.

2.Materials Selection

There is no metal, plastic, or any other material known to man, that can withstand the high levels of energy released by cavitation in the forms of heat and pressure. In practice however, materials can be selected that result in longer life and customer value in their ability to withstand cavitation energies, so that attention to pump construction materials is valuable and productive.

Where cavitation is not a problem or not predicted to be a problem, common materials such as cast iron and bronze are suitable for pump construction. There are millions of cast iron and bronze pumps that work fine for 20 years or more without any problem even though many of those pumps experience some cavitation.

(4) Mechanism of Cavitation: The phenomenon of cavitation is a stepwise crocess as shown in Figure (below).

Step One, Formation of bubbles inside the liquid being pumped.

The bubbles form inside the liquid when it vaporises i.e. phase change from liquid to vapor. But how does vaporization of the liquid occur during a pumping operation?

Vaporization of any liquid inside a closed container can occur if either pressure on the liquid surface decreases such that it becomes

equal to or less than the liquid vapor pressure at the operating temperature, or the temperature of the liquid rises, raising the Phenomenon of Cavitation

12