First, the principle of selection
The pump is a general-purpose mechanical equipment with a wide range of applications. It is widely used in petroleum, chemical, electric metallurgy, mining, ship selection, light industry, agriculture, civil and defense sectors, and plays an important role in the national economy. According to statistics in 2009 years, China's pump output reached 1256 million units. Pump power consumption accounts for more than 21% of national electricity consumption. Therefore, it is of great significance to reduce the energy consumption of the pump and to save energy.
In recent years, our pump industry has designed and developed many energy-efficient products, such as IHF, CQB, FSB, UHB and other pump products, which have played a positive role in reducing pump energy consumption. However, in various fields of the national economy, many pumps are in an unreasonable operation condition due to unreasonable selection, low operating efficiency, and a large amount of energy is wasted. Other pumps are not suitable for use because of the unreasonable selection, or the maintenance cost is increased, and the economic benefit is low. It can be seen that the rational selection of pumps is also of great significance for energy conservation.
The so-called rational selection of pumps is to comprehensively consider the comprehensive technical and economic indicators such as the investment and operating costs of pump units and pumping stations, so that they are in line with the principles of economy, safety and application. Specifically, there are the following aspects:
The flow rate and head requirements must be met, that is, the operating point of the pump (the intersection of the characteristic curve of the device and the performance curve of the pump) is required to be kept in an efficient range, which saves power and is not easy to damage the machine.
The selected pump is small in size, light in weight, inexpensive in cost, and has good characteristics and high efficiency.
It has good anti-cavitation performance, which can reduce the excavation depth of the pump house without causing cavitation of the pump, stable operation and long service life.
The pump station is built according to the selected pump, which has less project investment and low operating cost.
Second, the selection steps
(1) List basic data:
1. Characteristics of the medium: medium name, specific gravity, viscosity, corrosivity, toxicity, etc.
2. The particle diameter and content of the body contained in the medium.
3. Medium temperature: (°C)
4, the required traffic
General industrial pumps can ignore the amount of leakage in the piping system during the process, but must consider the impact of the process on the flow. Agricultural pumps that use open channels should also consider leakage and evaporation.
5. Pressure: suction tank pressure, drain pressure, pressure drop in the piping system (head loss).
6. Pipeline system data (pipe diameter, length, type and number of pipe attachments, geometric elevation of sink to pressure tank, etc.).
The device characteristic curve should also be made if necessary.
When designing the layout of the pipeline, you should pay attention to the following items:
A. Reasonably choose the diameter of the pipeline, the diameter of the pipeline is large. Under the same flow rate, the flow velocity is small, the resistance loss is small, but the price is high, and the pipe diameter is small, which will lead to a sharp increase in the resistance loss, so that the lift of the selected pump is increased. With increased power, both cost and operating costs increase. Therefore, it should be considered from a technical and economic perspective.
B. The discharge pipe and its pipe joints should take into account the maximum pressure that can be withstood.
C. The pipe arrangement should be arranged as straight pipe as much as possible. Minimize the accessories in the pipe and minimize the length of the pipe. When turning, the bending radius of the elbow should be 3 to 5 times the diameter of the pipe, and the angle should be as large as 90. °C.
D. The discharge side of the pump must be equipped with a valve (ball valve or shut-off valve, etc.) and a check valve. The valve is used to adjust the operating point of the pump. The check valve prevents the pump from reversing when the liquid is reversed, and prevents the pump from hitting the water hammer. (When the liquid flows backwards, it will generate a huge reverse pressure, which will damage the pump)
(2) Determining the flow head
Determination of traffic
a. If the minimum, normal, and maximum flow rates have been given in the production process, the maximum flow rate should be considered.
b. If only normal flow is given in the production process, consideration should be given to leaving a certain margin. For the large flow rate of ns>;100, it is not intended to lift the pump, the flow margin is 5%, the pump with a small flow rate of ns<;50 and the pump, the flow balance is 10%, 50≤ns≤100, the flow is the remaining The amount is also 5%. For pumps with poor quality and poor operating conditions, the flow balance should be 10%.
c. If the basic data is only for weight flow, it should be converted into volume flow.
What is flow capacity? What letter is used to indicate? How many units of measurement are used? How to convert? How to convert to weight and formula?
Answer: The volume of liquid discharged by the pump per unit time is called flow rate. The flow rate 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 the weight ρ is the liquid specific gravity
Example: The flow rate of a pump is 50 m3/h. What is the hourly weight when pumping? The specific gravity ρ of water is 1000 kg/m3.
Solution: G=Qρ=50×1000(m3/h·kg/ m3)=50000kg / h=50t/h
What is the head? What letter is used to indicate? What unit of measurement is used? And pressure conversion and formula?
A: The energy obtained by the unit weight liquid through the pump is called the lift. The lift of the pump, including the suction stroke, is approximately the difference between the pump outlet and the inlet pressure. The lift is indicated by H and the unit is meters (m). The pressure of the pump is expressed in P, the unit is Mpa (MPa), H = P / ρ. If P is 1kg / cm2, then H = (lkg / cm2) / (1000kg / m3) H = (1kg / cm2) / (1000 kg/m3) = (10000 kg/m2) / 1000 kg / m3 = 10 m
1Mpa=10kg/c m2, H=(P2-P1)/ρ (P2=exit pressure P1=inlet pressure)
What is the efficiency of the pump? What is the formula?
Answer: Refers to the ratio of the effective power of the pump to the shaft power. η=Pe/P
The power of the pump usually refers to the input power, that is, the power transmitted by the prime mover to the pump shaft, so it is also called the shaft power, which is denoted by P.
The effective power is the product of the pump head and mass flow and gravity acceleration.
Pe=ρg QH (W) or Pe=γQH/1000 (KW)
ρ: density of pumped liquid (kg/m3)
γ: the weight of the pump to deliver liquid γ = ρg (N / m3)
g: gravitational acceleration (m/s)
Mass flow Qm=ρQ (t/h or kg/s)
What is rated flow, rated speed, rated head?
A: The pump design is based on the set performance parameters of the pump, and the best performance achieved is the rated performance parameter of the pump, usually the parameter value specified in the catalog or sample.
Such as: 50-125 flow rate 12.5 m3 / h for the rated flow, head 20m for the rated head, speed 2900 rev / min for the rated speed.
What is the NPSH? What is a suction? The respective units of measurement represent letters?
Answer: When the pump is working, the liquid will generate vapor at the inlet of the impeller due to a certain vacuum pressure. The vaporized bubbles will ablate the metal surface of the impeller under the impact movement of the liquid particle, thereby destroying the metal such as the impeller. The pressure is called vaporization pressure, and the NPSH is the excess energy of the liquid per unit weight at the pump suction port that exceeds the vaporization pressure. The unit is marked with meters and (NPSH)r. The suction stroke is the required NPSH Δh: the vacuum that the pump is allowed to absorb, that is, the allowable installation height of the pump, in meters.
Suction stroke = standard atmospheric pressure (10.33 m) - NPSH - safety (0.5 m)
The standard atmospheric pressure energy pipeline has a vacuum height of 10.33 meters.
For example: a pump must have a NPSH of 4.0 meters, and ask for a suction Δh?
Solution: Δh=10.33-4.0-0.5=5.83 m
What is the characteristic curve of the pump? Including several aspects? What is the role?
Answer: The curve indicating the relationship between the main performance parameters is usually called the performance curve or characteristic curve of the centrifugal pump. In essence, the centrifugal pump performance curve is the external expression of the movement law of the liquid in the pump, which is obtained through actual measurement. The characteristic curves include: flow-head curve (QH), flow-efficiency curve (Q-η), flow-power curve (QN), flow-cavitation curve (Q-(NPSH)r), and the performance curve is Any flow point of the pump can find a set of heads, power, efficiency and NPSH values on the curve. This set of parameters is called working state, referred to as working condition or working point, centrifugal pump. The working condition at the highest efficiency point is called the best operating point, and the best operating point is generally the design operating point. Generally, the rated parameters of the centrifugal pump, that is, the design operating point and the optimal operating point coincide or are close. In the practice selection efficiency range, that is, energy saving, and can ensure the normal operation of the pump, it is very important to understand the performance parameters of the pump.
What is the pump's full performance test table?
A: The equipment that can accurately test all the performance parameters of the pump through precision instruments is a full performance test bench. The national standard accuracy is Class B. The flow rate was measured with a precision worm gear flowmeter, and the lift was measured with a precision pressure gauge. The suction stroke was 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 according to the measured value: n=rQ102.