Flow (Fig.2. Surging Occurrence of Operating Point Trajectory Water Pump Technology 2001.3 Hump Shaped Performance Curve Li Lianggeng Liu Meiqing Liu Dexiang (Wuhan University of Hydraulic and Electric Studies, Class 9906) (Professor and Associate Professor, Department of Power Engineering, Wuhan University of Hydraulic and Electric Engineering; 43072) Proposed protection according to these three conditions Measures.
1 Surge phenomenon The phenomenon that the flow rate changes periodically and in a large range periodically during the unstable operation of the pump is called surge phenomenon or flying phenomenon in the project. Surge occurs in the hump-shaped pump on the performance curve. Accompanied by vibration and noise, it can lead to blade fracture in severe cases and affect the safe operation of the pump. Therefore, it should be prevented in the project. The surge phenomenon is related to the mechanical stability of the piping system of the pump device. Therefore, the fluid flow in the pipeline and the driving condition of the pump system should be considered comprehensively in the study of the surge phenomenon.
2 Surge analysis on the pump performance curve As shown, when surge occurs, the pump operating point always rotates counterclockwise around the closed curve. It can be seen that the hump-shaped working point curve of the rotation zone or of the hump-shaped performance curve rising region shows that the frequency range is 0.1 to 10 Hz, which is equal to the natural frequency of the pump system. In this area, the pump operation is unstable, as demonstrated below. For a pump hump-shaped performance curve, the figure /: is the highest point of the performance curve. If the operating point is in the falling section of the performance curve, such as point B, the pump operation is safe. However, if the operating point is in the rising section of the performance curve, such as 4 points, the pump operation is not stable. This is because when the /1 point moves to the right, the hydrostatic head produced by the pump is greater than the resistance overcome by the pipeline device, and thus the flow rate increases. Large, the flow increases, the work point continues to move right until K point; when / 1 point moves to the left, the hydrostatic head produced by the pump is less than the resistance overcome by the pipeline device, the flow rate decreases, the flow rate decreases, the work point continues Move to the left until the flow is zero. Therefore, in case of interference, the 4 points will move and can no longer return to the origin, which is why the rising section of the performance curve is unstable. The rule point is usually used as the critical point to distinguish between stability and instability. The left side of the ruler point is called the unstable work area, and the right side of the X point is called the stable work area. The // point in the area, such as /1 point, is called For the unstable point, point S is called the stable point, as indicated.
From the above analysis, if the pump performance curve does not rise, there will be no instability of the work and no surge will occur. Therefore, the pump should be designed so that its performance curve does not have a rising section. Otherwise, the work should be done. The range is always in the falling segment of the performance curve. However, whether or not there is an ascending section will certainly generate surging, which is a very easy-to-follow argument. The hydraulic and vibration related theories are used to qualitatively analyze the disturbances in the operation of simple pump systems and complex pump systems. And draw the corresponding conclusions.
3 simple pump system surge analysis As shown in the simple pump system, the water tank in the middle without tanks or gas tanks and other equipment, the pump outlet drain directly connected to the pipeline system. The mechanical motion system of the pump system consists of two parts, the fluid flow part and the fluid drive (including the pump impeller and the drager) parts.
For the first part, related to the characteristics of the pipeline, according to the Bernoulli equation, the following equation can be established: The actual head of Sapphire is the resistance loss along a constant value of 91, and the flow symbol is determined by the small p, deduction, and ―. Let's study the symbol determination method for these three coefficients, such as small I and the like.
If the slope of the tangent of the head curve is greater than zero, it means that the point is on the part of the head curve that is tilted to the upper right (that is, the unstable area); if it is less than zero, it means that the point is bowed to the lower right of the head curve. On the part (that is, the stable area).
For buckles, also, according to their geometrical meanings, can be expressed on the pipeline resistance curve. As shown, detonation + detonation represents the tangential slope of the resistance curve of the drainage valve at different opening degrees.
The three working points of the test pump are: 5, ¢: For 4 points, because of the | and | Thus, it can be concluded that in the simple pump system, whether the head curve is inclined to the lower right or to the upper right, or Say no hump shape, no surge phenomenon.
It is a positive value. Therefore, as long as it meets: 4 Complex Pump System Surge Analysis A complex pump system is a pump system with equipment such as a gas tank or water tank in the pipeline, as shown. It is assumed that the part of the front (the side with the pump) of the It gas tank or the water tank is the system 1, and the part after it is the system 2. The stability determination condition of the pump system is the same as that of the simple pump system, as shown in the figure above. For system 1, the following figure is for system 2. At this time, the head of system 1 /// is, for example, the resistance loss of system 2 along the way. Since 6 does not work, there is no influence on the system 1, and therefore, as in the case of the simple pump system, the following situation is studied to adjust the flow using the valve K. The head of the system 1 at this time is the head loss of the valve F2. A. It's quite large, so it's possible that the P point is more than a dead lift point. For 4 o'clock, the situation is the same as 4 o'clock in the middle, and it is an unstable point. Therefore, surge phenomenon occurs. The use of stability theory can also prove that this point is > 0 and is therefore an unstable point. The condition of system 2 is the same as that of a simple pump system, so surge phenomenon occurs.
Therefore, in a complex pump system, if the valve is used to adjust the flow rate, surge phenomenon occurs.
5 Analysis of the results from the above analysis shows that the hump-shaped performance curve is only a necessary condition for surging, rather than a sufficient condition, only with the following three conditions, the surge phenomenon can occur: (1) the performance curve of the pump was camel-shaped , And in the unstable (2) in the pipeline there is a gas tank, water tank or gas phase part;
Ordinary centrifugal pump surge has less destructive force, the reason for this, first, the centrifugal pump performance curve is more gentle, and second, the centrifugal pump start-up mode is different from the axial flow pump, usually the valve is closed, and then gradually open the valve to increase the flow Even if a hump-shaped centrifugal pump generates surge, the destructive force is small due to the short action time. However, for boiler feed pumps with a hump-shaped performance curve, due to the pumping of hot water, the gas phase is easily formed in the piping system. If the flow control valve is located at the downstream end far from the pump, the valve loss is very large and large Surging, we must take preventive measures to this end. Specific methods can be found in the following preventive measures (2).
6 Surge prevention measures According to the above three conditions, corresponding preventive measures can be formulated, mainly including: (1) In the above-mentioned complex pipelines, the use of a pump with a hump-shaped performance curve should be avoided as much as possible. Instead, the performance curve should be flat. Tilt the pump down.
(2) A bypass pipe is installed on the pump discharge side, and the pump presses the water pump technology water part back to the suction side, so that the working point moves from the A point to a certain point on the right side of the hump.
(3) To make the flow not less than under any conditions, otherwise recirculation pipes or automatic discharge valves should be installed.
(4) Using movable vanes to adjust the flow rate, when the outside world needs the flow rate to decrease, turning the rotor leaves the device angle to decrease, so that the performance curve moves downwards, the critical point moves to the lower left, and the output flow decreases, see.
The critical point of the line moves to the upper right; otherwise, the critical point of the performance curve will move to the lower left, thus reducing the unstable area on the performance curve.
(6) Suction valve is installed at the place where people are sucked. When the flow rate of the pipeline increases, the large suction valve is opened to move the critical point of the performance curve to the upper right; when the flow of the pipeline is reduced, the suction valve is closed to make the performance curve. The critical point moves to the lower left, thus reducing the instability zone on the performance curve, see.
So as not to press out the pipeline memory. The regulator or throttle valve is preferably located at the pump outlet.
(8) Ensure that the pump is operating at normal flow. If the load is reduced, the number of devices to be put into operation should be reduced in advance.
Mechanical vibration, water hammer vibration, etc. cause larger flow changes and aggravate surge strength.
(10) Surge is caused by the non-uniformity of fluid flow. Therefore, the homogeneity of flow should be ensured as much as possible.
Rotary stall vibration, cavitation vibration, and surge are vibrations caused by fluid flow, which can affect the safe operation of the pump. The three often exist at the same time and interact with one another, and it is difficult to tell.
To find out the difference between the three, we should focus on flow, vibration frequency and other factors to find out the source of vibration and its origin.
Rotational stall is the rotational de-aeration caused by the structural characteristics of the blade, which can cause the obstruction of the blade passage and the reduction of the energy of the pump head. The vibration caused by it is the result of the de-flow. Surge is the result of the coupling of the pump performance and the device oscillation and is related to the mechanical stability of the pump system.
Cavitation is a phenomenon that occurs when the pump inlet pressure is lower than the corresponding water temperature vaporization pressure. Both cavitation and surge are related to the performance of the pump and the performance of the device, but cavitation generally occurs in the larger flow area, and surge occurs in the smaller flow area, and the surge vibration frequency is relatively small at 0. 1~. 10Hz, and cavitation frequency range of 600 ~ 25000HZ is much larger than the surge.
The above three phenomena that can cause vibration, coupled with water hammer and mechanical vibration (such as vibration caused by imbalance of rotating parts, etc.), are vibration phenomena that are often encountered in the complex pump system and pose a threat to the safe operation of the system. In the meantime, we should pay attention to it and take corresponding measures to prevent and improve the pump system.
Surge phenomenon is a phenomenon caused by the abnormal fluid flow in the pump system. Compared with the industrial water hammer phenomenon, the economic loss caused by the phenomenon is small. In the project, it often fails to receive extensive attention, but in the operation of the high pressure pump The vibration and noise generated by surging is very strong, such as boiler feed pumps, so it must be guarded against. Once it occurs, it must be changed to the normal operating state by changing the opening of the regulating valve immediately after the pump.
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