Changing the pump speed can change the pump's performance curve and change the operating point while the line curve remains the same. This adjustment is called variable speed adjustment.
Take urban water as an example. The amount of water required by the user is not uniform, and when the pump station is planned, the selection of the pump is selected according to the most unfavorable conditions, that is, according to the maximum design flow rate and design head. In fact, for most of the time, the water volume is less than the maximum design flow, and the pump is working at a small flow rate. From the characteristic analysis of the centrifugal pump, we know that the blades of the centrifugal pump are all back-bend, that is, β2<90°, the characteristic curve is downwardly inclined, and the lift is reduced with the increase of the flow rate, as shown in Fig. 1. . On the contrary, when the water output of the pump is reduced, the working head of the pump will increase accordingly. It is known from the pipeline characteristic curve that when the flow rate is reduced, the head loss is reduced, so the fixed speed pump is in a state of excess head for most of the time, this part The remaining heads cause a lot of energy waste.
If the speed control technology is adopted, the flow rate and head of the pump can be adapted to the required water quantity and head change. The following is an analysis of the operating condition of the speed control pump. In fact, the above analysis of the fixed speed pump is completely used for the speed control pump. It's just that the speed is different. In Fig. 1, A1-A2 is the characteristic curve of the water pump before the speed regulation (fixed speed pump), and the characteristic CB of the pipeline is a quadratic curve. As mentioned above, the centrifugal pump has a certain self-balancing ability, and it can always work stably at the point B1 of the characteristic curve of the pump and the characteristic curve of the pipeline. The flow rate is Qmax and the head is H. A2B2 is the characteristic curve of the water pump (n2) after speed regulation. Similarly, when the pump runs at the speed of n2, it also has the same self-balancing ability. After the speed adjustment (n2), the intersection of the pump characteristic A2B2 and the pipeline characteristic CB is the water pump. When the speed is n2, the flow rate is Qmin and the head is H2. When the water quantity changes between Qmax and Qmin, as long as the speed is changed accordingly, a series of pump characteristic curves can be obtained. The intersection of the characteristic curve and the pipeline characteristic curve is the operating point of the pump at different speeds. All of these working points fall on the pipeline characteristic curve CB, that is to say, the pump characteristics at different speeds can be obtained. Below we use the similar law to analyze. According to the similarity theorem, the flow rate, head and shaft power of the pump vary with the speed of the pump.
In the each formula: n1 and n2 are the speeds of the fixed speed pump and the speed governing pump respectively; Q1 and Q2 are the flow rates of the fixed speed pump and the speed regulating pump respectively; H1 and H2 are the heads of the fixed speed pump and the speed regulating pump respectively; P1 P2 is the shaft power of the fixed speed pump and the speed control pump respectively. These three formulas represent the same vane pump. When the speed n is changed, other performance parameters will change according to the above proportional relationship. The above three examples have a special form of similar law, called proportional law. The proportionality law is useful for users of pumps. It reflects the change in the main performance of the pump when the speed changes. The content of the shift regulation condition for the centrifugal pump device described later is converted by applying this proportional law. There are two cases of proportional law in the design and operation of the pumping station: (1) The curve of (QH)1 when the pump speed is n1 (see Figure 2), but the required operating point is not on the characteristic curve. At the coordinate point A2 (Q2, H2), I ask: If the pump needs to work at point A2, what is the speed n2?
The (Q-H)1 curve at the time of the water pump n1 is known, and the (Q-H)2 curve when the rotational speed is n2 is used for the trial proportional law. When the rotational speed n2 value is obtained by the graphic method, the A1 point similar to the A2 (Q2, H2) point condition must be found on the (Q-H)1 curve of the rotational speed n1, and its coordinate is (Q1, H1). The "similar parabola" method is used to find the A1 point. From the figure 1, 2, after eliminating the speed, you can get:
It can be seen that all the operating points that meet the proportional law relationship are distributed on a quadratic parabola with the coordinate origin as the apex. This parabola is called a similar case parabola (also called efficiency curve). Substituting the coordinate value (Q2, H2) of the A2 point into the equation (6-16), the k value can be obtained, and then according to the equation (6-17), the generalized H=kQ2 similar to the A2 point condition is written. The equation represents a parabola (k is a constant) similar to the A2 point case. It intersects the (Q-H)1 curve at speed n1 at point A1, which is the desired point similar to the A2 point condition. Substituting the coordinate values (Q1, H1) and (Q2, H2) of points A1 and A2 into equation (6-13):
After the speed n2 is obtained, the proportional law can be used to draw the (Q-H) 2 curve at n2. At this time, in the formulas (6-13), (6-14), n1 and n2 are all known values. Using the iterative method, take the (Qa, Ha)' point, the (Qb, Hb)' point, and the (QC, HC)' point on the (QH)1 curve of n1. (6-14), get the corresponding (Qa, Ha) 'point, (Qb, Hb) 'point and (QC, HC) 'point .... (generally 6 ~ 7 points is better), with smooth The curve connection gives a (QH)2 curve, as shown by the dashed line in Figure 3. This curve is the (Q-H)2 curve obtained by the graphical method at the speed of n2. For the same reason, you can also follow:
The P2 values corresponding to P1 are obtained. In this way, the (Q-H) 2 curve at the speed n2 can also be drawn. In addition, when we use the proportional law, we think that the corresponding efficiency under the operating conditions is equal. Therefore, as long as the efficiency of points a, b, c, d in Fig. 3 is known, the equivalent principle can be used to find out. The efficiency of the corresponding points a', b', c', d', etc. at the speed of n2, the connected (Q-η)2 curve is as follows:
The above discussion shows that for all ratios of equal efficiency points, the uniform constant k value can draw a parabola with similar efficiency and similar working conditions. That is to say, the efficiency of each point is equal on the parabola of similar working conditions, but in fact, according to the test, when the range of the water pump speed regulation exceeds a certain value, the efficiency of the corresponding point will change. The measured equal efficiency curve is different from the theoretical equal efficiency curve, and only coincides in the high efficiency range. However, the speed regulation method used in engineering practice greatly expands the high efficiency of the vane pump.
