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Talking about Centrifugal Pump Combined with Practice

22 Aug 2020Email

Abstract: This article mainly describes the structure, working principle and its characteristic curve of the centrifugal pump, as well as the reasonable configuration of the pump in actual production, how to ensure the reliability, safety and economy of the safe operation of the pump equipment during use .

 

   The application of centrifugal pump is very extensive, and it is used in many sectors of the national economy. Because the centrifugal pump is an important equipment, and its operation consumes a lot of power! In order to select and use water pumps reasonably and economically to ensure water supply in water plants, it is necessary to understand the working principle and basic performance of centrifugal pumps.

 

 

The basic structure of a centrifugal pump is composed of six parts
The basic structure of a centrifugal pump is composed of six parts: impeller, pump body, pump shaft, bearing, stuffing box, etc.


   1. The impeller is the core part of the centrifugal pump. It has a high speed and large output. The blades on the impeller play a major role. The impeller must pass dynamic and static balance experiments before assembly. The inner and outer surfaces of the impeller are required to be smooth to reduce the friction loss of the water flow.


   2. The pump body is also called the pump housing, which is the main body of the pump. Play a supporting and fixing role, and connect with the bracket for mounting the bearing.


   3. The role of the pump shaft is to connect with the motor through a coupling, and transmit the torque of the motor to the impeller, so it is the main component that transmits mechanical energy.


   4. Bearings are components that are sleeved on the pump shaft to support the pump shaft. There are two types of rolling bearings and sliding bearings. Rolling bearings use grease as a lubricant, and the oil should be appropriate, generally 2/3 ~ 3/4 of the volume, too much will generate heat, too little will cause noise and heat! The sliding bearing uses engine oil as the lubricant, and the oil is added to the oil level line. Too much oil will seep along the pump shaft and drift low, and too little oil will overheat and damage the bearings and cause accidents! During the operation of the pump, the temperature of the bearing is up to 85 degrees and generally runs at about 60 degrees. If it is high, find the cause and deal with it in time!


   5. The stuffing box is mainly composed of packing, water seal ring, packing tube, packing gland and water seal pipe. The function of the stuffing box is mainly to close the gap between the pump casing and the pump shaft, prevent the water in the pump from flowing to the outside, and prevent the outside air from entering the pump. Always maintain the vacuum in the pump! When the pump shaft and the packing friction generate heat, the water seal pipe needs to be injected into the water seal ring to cool the packing! Maintain the normal operation of the pump. Therefore, special attention should be paid to the inspection of the stuffing box during the itinerant inspection of the pump operation! The packing should be replaced after 600 hours of operation.

 

 

Flowing parts of centrifugal pump
   The flow parts of the centrifugal pump include three parts: suction chamber, impeller, and press-out chamber. The impeller chamber is the core of the pump and the core of the flow component. The pump works on the liquid through the impeller to increase its energy. The impeller is divided into three categories according to the direction of liquid flow:
   (1) Radial flow impeller (centrifugal impeller) liquid flows out of the impeller along the direction perpendicular to the axis.
   (2) Diagonal flow impeller (mixed flow impeller) The liquid flows out of the impeller along the inclined direction of the axis.
   (3) The direction of liquid flow of the axial flow impeller is parallel to the axis.
   The impeller is divided into two categories according to the way of inhalation:
   (1) Single suction impeller (that is, the impeller sucks liquid from one side).
   (2) Double suction impeller (that is, the impeller sucks liquid from both sides).
   The impeller is divided into three categories according to the cover form:
   (1) Closed impeller.
   (2) Open impeller.
   (3) Semi-open impeller.
   Among them, the enclosed impeller is widely used. The aforementioned single-suction impeller and double-suction impeller all belong to this form.

 

 

The working principle of centrifugal pump
   The working principle of the centrifugal pump is: the centrifugal pump can send water out because of the centrifugal force. Before the pump works, the pump body and the water inlet pipe must be filled with water to form a vacuum state. When the impeller rotates quickly, the blades prompt the water to rotate quickly. The rotating water flies out of the impeller under the action of centrifugal force. After the water is thrown out, the central part of the impeller forms a vacuum area. The water of the water source is pressed into the inlet pipe through the pipe network under the action of atmospheric pressure (or water pressure). In this way, continuous water pumping can be realized. It is worth mentioning here that the centrifugal pump must be filled with water in the pump casing before starting, otherwise it will cause the pump body to heat up, vibrate, reduce the water output, and cause damage to the pump (referred to as "cavitation") Cause an equipment accident!
   There are many types of centrifugal pumps, and the common classification methods are as follows:
  1 According to the impeller suction mode: single-suction centrifugal pump, double-suction centrifugal pump.
  2 According to the number of impellers: single-stage centrifugal pump, multi-stage centrifugal pump.
  3 According to the impeller structure: open impeller centrifugal pump, semi-open impeller centrifugal pump, closed impeller centrifugal pump.
  4 According to working pressure: low pressure centrifugal pump, medium pressure centrifugal pump, high pressure centrifugal pump.
  5 According to the position of the pump shaft: horizontal centrifugal pump side, vertical centrifugal pump.

 

 

The following introduces several important performance curves of centrifugal pumps
   There is a certain relationship between the performance parameters of the water pump such as flow Q, head H, shaft power N, speed n and efficiency η. The relationship between the value changes between them is represented by a curve, which is called the performance curve of the pump.
   The mutual change relationship and mutual restriction between the performance parameters of the water pump: First, the rated speed of the water pump is a prerequisite.
   There are three main curves of water pump performance curve: flow-head curve, flow-power curve, flow-efficiency curve.
   1 Flow rate-head characteristic curve
   It is the basic performance curve of a centrifugal pump. The centrifugal pump with a specific speed of less than 80 has the characteristics of rising and falling (both in the middle and downward bending on both sides), which is called the hump performance curve. The centrifugal pump with a specific speed between 80 and 150 has a flat performance curve. Centrifugal pumps with specific speeds above 150 have a steep drop performance curve. Generally speaking, when the flow rate is small, the head is high, and the head gradually decreases as the flow rate increases.
   2 Flow-power curve
   The shaft power increases with the increase of the flow rate. When the flow rate Q=0, the corresponding shaft power is not equal to zero, but is a certain value (about 60% of normal operation). This power is mainly consumed by mechanical losses. At this time, the water pump is full of water. If it is operated for a long time, the temperature in the pump will continue to rise, and the pump casing and bearings will heat up. In severe cases, the pump body may be thermally deformed. We call this "stuffy head". When the head is at the maximum, when the outlet valve is gradually opened, the flow will gradually increase, and the shaft power will slowly increase.
   3 Flow rate-efficiency curve
Its curve is like a hill. When the flow rate is zero, the efficiency is also equal to zero. As the flow rate increases, the efficiency gradually increases, but after increasing to a certain value (high efficiency point), the efficiency drops, and the efficiency has a maximum value. , Near the highest efficiency point, the efficiency is relatively high, this area is called the high efficiency zone.

 

 

Reasonable configuration, safe operation, high-quality water supply
After understanding the centrifugal pump structure, working principle and characteristic curve in the above four aspects, how to reasonably configure the power of the motor pump is the key to ensure the safe operation of the pump, high-quality water supply, and reduce the production cost. The pump power is reasonably configured to maximize the performance of the pump. The safe operation of the work area and the actual conditions of our plant's water supply have fully explained the importance, reliability and economy of reasonable equipment configuration.
   1. The importance of reasonable configuration of pump equipment. The main task of the water plant is to ensure the city’s people’s production and domestic water, and the inflow and outflow can meet regional pressure. However, in recent years, with the relocation of houses and the migration of major water users, the quality of people’s lives has improved, and the demand for water quality has increased In order to ensure high-quality water supply, the company has adopted a series of measures: (1) Adjust the reasonable configuration of pump equipment and implement the best combination of man and machine. (2) Increase scientific and technological innovation and invest a lot of funds to transform the original backward water purification equipment. (3) Invest capital, transform old equipment, old pipe network, improve hydraulic conditions, install static mixer, etc. (4) Install meters and use modern monitoring systems to monitor and control the water quality throughout the entire process to ensure high-quality water.
   2. The reliability of the safe operation of pump equipment. In order to ensure the safe operation of pump equipment, the company has more standardized management of pump equipment. Annual overhaul, monthly secondary maintenance, and daily primary maintenance system. These ISO9002 quality management guarantees pump equipment Various measures for safe operation, in order to ensure the reliability of safe operation, the training and improvement of the technical quality of the operators, and the implementation of safe operation regulations must be strictly implemented. The implementation of these safe operation systems is to ensure the reliability of the operation of pump equipment Guarantee.
   3. The economy of safe operation of pump equipment. When it comes to economy, it is the cost of the company’s water production, including electricity, alum, chlorine, and ammonia. The safest operation method, the best dispatch mode, and the lowest water production cost should be used to control the economic activities of the company and improve the economy. Benefits, companies have accumulated a certain amount of experience in this regard.
   With the continuous development of science and technology, the modernization of water pumps has also been continuously improved, reducing a lot of human management operations. Now most of the automatic operation mode of computer monitoring is adopted, which puts forward higher requirements on the quality of the operators.

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