Overall structure design of centrifugal sanddredge pump
The centrifugal sand pump relies on the action of the rotating impeller on the liquid to transfer the mechanical energy of the prime mover to the liquid. Due to the action of the centrifugal sand pump, when the liquid flows from the inlet to the outlet of the impeller, its velocity energy and pressure energy are increased. The liquid discharged by the impeller passes through the pressure chamber, and most of the velocity energy can be converted into pressure energy, and then along the discharge pipe. At this time, a vacuum or low pressure is formed at the inlet of the impeller due to the discharge of the liquid, and the liquid in the suction pool is pressed into the inlet of the impeller under the action of the liquid surface pressure (atmospheric pressure). Inhale and expel fluid continuously.
When the flow is in the range of 5-20000m3/hour and the distance is in the range of 8-2800m, it is more appropriate to use a centrifugal pump. Because within this performance range, centrifugal sand dredge pumps have the advantages of high speed, small size, light weight, high efficiency, large flow, simple structure, stable performance, and easy operation and maintenance. Production practice at home and abroad shows that the output value of centrifugal sand pump is the highest among pump products.
The centrifugal sand dredge pump has its advantages, but also its disadvantages. The actual use efficiency of the centrifugal pump is low, and the flow rate changes with the pressure in actual use, the speed requirement is strict, the single-stage lift is low, the pump must be filled with liquid before starting, and the liquid viscosity also has a great impact on the pump performance. , can only be used for liquids whose accuracy is similar to water. For a certain flow centrifugal sand pump, there is a corresponding viscosity limit. If the liquid exceeds this viscosity limit, the efficiency of the pump will decrease rapidly or even fail to work.
Basic parameters of the pump
1.1 Traffic
Flow: The amount of medium (volume or mass) delivered per unit time. This value is generally determined by the design department.
Flow is divided into volume flow and mass flow
Volume flow is expressed by Q, unit: m3/h or m3/s,
Mass flow is expressed in Qm, unit: t/h or kg/s
The relationship between mass flow and volume flow is: Qm =ρQ
ρ—density of conveying slurry (t/m3; kg/m3), normal temperature clean water ρ= 1 (t/m3)
The flow rate is the most critical parameter, because the ultimate goal of the user is to deliver the required amount of slurry. This parameter is the actual delivery value of the system, and its determination method should be based on actual measurement or calculation, and should not be determined by empirical guesswork. The accuracy of this value plays an extremely important role in selecting the pump type. The provided value should not be too large or too small, and its accuracy should be within the range of plus or minus 10%.
1.2 Head
Pump head: refers to the increase in energy per unit mass of liquid transported from the inlet of the pump to the outlet of the pump (the outlet flange of the pump). That is, the effective energy obtained by the pump.
Pipeline system loss head: refers to the energy loss of the pipeline transportation system. This value is an important basis for pump selection.
The unit of head is meters (m). It is particularly pointed out that for the conveying of slurry medium, some people have used the slurry head to express that the slurry head is actually the outlet pressure P of the pump (unit MPa or kg/cm2), and the relationship between it and the head of the clean water is: P=0.01ρH ( The unit of P is MPa, the unit of ρ is t/m3), and the difference between the two is a medium density value.
The head of the centrifugal sand pump is theoretically independent of the type, nature and density (or specific gravity) of the fluid being transported, but only related to the state of motion. For the same pump to transport different media, such as water, mercury, and various slurries, the generated lift is the same, and the discharge height is the same. However, the loss head of the conveying pipeline for conveying different materials is different.
By the way: the lifts given by the slurry pump samples are all tested under the condition of clean water. Therefore, the system loss head must be converted into the clean water lift when calculating, so that the model can be selected according to the sample.
1.3 Speed
The speed is the number of revolutions per unit time of the pump, and the unit is: revolution/minute (r/min) 1.4 NPSH The flowing liquid creates bubbles and then ruptures to erode the part, known as cavitation. Cavitation allowance is the surplus amount that does not occur under certain conditions. It reflects the suction performance of the pump. The unit of NPSH is meters (m). 1.5 power N Power is the work done per unit of time in kilowatts per hour (kW/h). The power of the pump refers to the water power of the pump, and the calculation formula is: Nb = = (kW) (1) The formula for calculating the power output from the motor to the pump: ND=IvcosφηDηC (Kw) (2) In the formula: the unit of density ρ is t/h; the unit of flow Q is m3/h; the unit of lift is m. ηb—efficiency% of pump operating point; I—motor operating current A; V—motor operating voltage kV; cosφ—motor power factor; ηD—motor efficiency%; ηC—transmission efficiency%
2 Determination of the structural form
2.1 Selection of prime mover
The pump is driven directly by the engine through a coupling, or through a pulley on the engine.
2.2 Structural scheme selection
The smaller the volume of the pump, the better, as long as it meets the working requirements. The structure can be a double shell mechanism and a single shell machine structure.
2.3 Determine the suction caliber
The determination of the suction diameter of the pump mainly depends on the flow rate in the suction pipe. According to the economic flow rate analysis of domestic and foreign pipelines and relevant standards, the maximum flow rate in the suction pipe generally does not exceed 5m/s, and is usually about 3m/s. At low pressure, the suction diameter of the pump (D) = the discharge diameter (D). From the point of view of improving the cavitation performance, it is hoped that the flow velocity of the suction port of the pump is slower, but if the flow velocity is slower, the pipe diameter will be larger, which is not economical. Therefore, it is necessary to comprehensively analyze and compare on the basis of specific circumstances.
Table 4-1 of “Basics of Centrifugal Sand Pump Design” P110
An approximate value is chosen based on the calculated diameter, and the flow rate is:
Vs=2.3 Preliminary calculation of shaft diameter
Check the mechanical design manual for steel material allowable stress MPa;
Pump shaft calculation
N=
When calculating the shaft diameter, the calculated power Nc used is slightly larger than the shaft power. Because the shaft power is the power under the design condition, and the power corresponding to the maximum flow rate when the pump is running is greater than the shaft power N
The calculated power of the pump can be calculated according to the formula (4..5) of “Basic Design of Centrifugal Sand Pump”:
Nc =1.2N
3 Radial force, axial force and balance
3. 1 The radial force and balance acting on the impeller
The radial force acting on the impeller must be taken into account when calculating the shaft and bearings, since the pump does not always operate at the design flow, even at zero flow when starting and stopping. The radial force of the centrifugal sand pump can be calculated according to the following empirical formula
P = 0.36(1-)HB2D2ρg
where P is the radial force acting on the impeller (N);
Qd-design flow;
Q—actual work flow;pump head (m)
B2 The total width of the impeller outlet (including the front and rear cover plates) (m);
D2—the outer diameter of the impeller (m);
ρ——Liquid weight (kg/m’) (the weight of water is 1000 kg/m3).
3.2 Axial force and balance acting on the impeller
The single-suction impeller does not have the same symmetry as the double-suction impeller, and there is an axial force due to the unequal pressure acting on both sides of the impeller. The shape of the semi-open impeller is related to the specific speed. Usually the blade inclination is related to the specific revolution. The axial force acting on the semi-open impeller can be approximated by the following empirical formula:
F1=2πr1d1KH ρg
In the formula, K——the axial force coefficient takes K=0.8;
d1——the straight line of the circle whose center is on the inlet edge of the blade and is tangent to the outline of the impeller
The radius at the center of the circle is r1 (meters)
