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渣漿泵運行工況的調(diào)節(jié)
如果工作點流量大于或小于所需要的輸送量，應(yīng)設(shè)法改變工作點的位置。改變運轉(zhuǎn)泵的工作點稱為工況調(diào)節(jié)。既然泵的工作點為管路特性曲線與泵特性曲線的交點,因此，進(jìn)行工況，調(diào)節(jié)有兩種途徑，即改變管路特性和改變泵的特性，下面分別加以論述。
1.改變管路特性
1) 管路節(jié)流調(diào)節(jié)
這是使管路特性變化的最簡單、最常用的方法，即在排出管路上安裝調(diào)節(jié)閥,開大或關(guān)小調(diào)節(jié)閥的開度，從而改變管路中局部阻力，管路特性系數(shù)k改變，使管路特性的斜率發(fā)生變化。在泵性能曲線不變的情況下,工作點發(fā)生變化，達(dá)到調(diào)節(jié)流量的目的。
如圖1- 52所示，當(dāng)泵排出管路上調(diào)節(jié)閥全開時，設(shè)管路特性曲線為1.與泵H-Q性能曲線交點為M，對應(yīng)的流量為Q1.隨著調(diào)節(jié)閥逐漸關(guān)小，管路特性系數(shù)k逐漸變大，管路特性2和3相應(yīng)地變陡,工作點變?yōu)?/span>M2和M3，流量逐漸減少為Q2和Q3，而泵的揚程從H1逐漸增大為H2和H3。
由圖1-52可以看出，當(dāng)用關(guān)小調(diào)節(jié)閥使流量由Q1減小到Q2或Q3時，泵的效率往往會有所下降，因為一般情況下總是按閥全開時所確定的工作點M來選泵的。至于功率，一般低、中比轉(zhuǎn)速的離心泵，其功率性能曲線都是隨流量減小而下有所下降的。但是從泵提供的能頭利用程度來看，調(diào)節(jié)閥開度減小卻增力加了附加阻力損失。設(shè)調(diào)節(jié)閥全開時，流量為Q1,管路特性系數(shù)為k1，則管路中流動損失為h=k1Q2;節(jié)流后，流大量變?yōu)?/span>Q2，總的流動損失為h=k2Q2,其中用于使液體在管路中進(jìn)行輸送時需要克服的流動損失僅為h=k,Qz 2，其余能頭(k2 – k1)Qz 2則為節(jié)流調(diào)節(jié)損失。
由此可見，用關(guān)小排出調(diào)節(jié)閥的方法改變管路特性來調(diào)節(jié)流量時,管路中局部阻力損失增加，需要泵提供更多的能頭來克服附加的阻力損失，使整個裝置效率不高,長期這樣調(diào)節(jié)是不經(jīng)濟(jì)的。特別是對具有陡降揚程性能曲線的離心泵，采用這種方法調(diào)節(jié)就更不經(jīng)濟(jì)。但由于
該方法調(diào)節(jié)方法簡單，且調(diào)節(jié)很方便，故仍被廣泛的用于離心泵工況調(diào)節(jié)中。
2)旁路調(diào)書
如圖1-53所示，在泵出口設(shè)有旁路與吸液罐相連通。此管路上裝一調(diào)節(jié)閥。離心泵在旁路調(diào)節(jié)裝置上工作就如同在分支管路中一樣，設(shè)h-Q1是主管路的管路特性，h-Q2是旁路
的管路特性，則并聯(lián)后的管路特性為h-Q.當(dāng)旁路調(diào)節(jié)閥完全關(guān)閉時,泵的性能曲線H-Q與主管管路特性h-Q1的交點為b1;旁路閥打開時,H-Q曲線與h -Q的交點為a。按分支管路中求各管中的流量方法，過a點作水平線交h-Q于a1點，交h-Q2于a2點，則通過主管的流量為Q.,旁路中流量為Qao由圖可知,泵的流量變大，但主管中的流量比關(guān)閉旁路閥時主管中流量為小，所以流量得到了調(diào)節(jié)。
這種調(diào)節(jié)方法也不經(jīng)濟(jì)，因為旁路中的流體浪費了功耗。若泵的軸功率隨流量增加而減小，則用此方法調(diào)節(jié)較適宜。
此外，當(dāng)排液罐中液位變化時，也將使管路特性上下移動，工作點和流量均變化。
2.改變泵的特性
除上述利用泵串聯(lián)、并聯(lián)工作以改變性能曲線達(dá)到工況調(diào)節(jié)外,常見的改變泵性能曲線的方法還有改變工作轉(zhuǎn)速、切割葉輪外徑以及換葉輪調(diào)節(jié)等。
1)改變工作轉(zhuǎn)速
由Hr.-42C2.m和Q:=nD:b-E2C20可知，離心泵的場程和流量都和轉(zhuǎn)速有關(guān)。當(dāng)n增大時，由比例定律可知，流量和揚程相應(yīng)地與轉(zhuǎn)速近似地按次二次方的正比關(guān)系變化，即系的H-Q性能曲線向右上方移動:當(dāng)n減小時，H - Q性能曲線向左下方移動，如圖1- 54所示、當(dāng)管路特性h-Q不變時，就可得到不同的工作點，使流量改變。另外式(1-71)給出了調(diào)速的泵特性方程。
用變轉(zhuǎn)速調(diào)節(jié)流量是比較經(jīng)濟(jì)的,因為它沒有節(jié)流引起的附加能量損失。但是渣漿泵這種調(diào)節(jié)要求使用能改變轉(zhuǎn)速的原動機(jī)來驅(qū)動，如直流電動機(jī)變頻電動機(jī)和汽輪機(jī)等。目前廣泛使用變頻器與可變速交流電動機(jī)，或加液力聯(lián)軸器驅(qū)動等方式。
Adjustment of operation condition of slurry pump
If the flow rate of the working point is greater than or less than the required conveying capacity, try to change the position of the working point. Changing the working point of the running pump is called condition regulation. Since the working point of the pump is the intersection of the pipeline characteristic curve and the pump characteristic curve, there are two ways to adjust the working condition, i.e. changing the pipeline characteristic and changing the pump characteristic, which are discussed respectively below.
1. Change pipeline characteristics
1) Pipeline throttling regulation
This is the simplest and most commonly used method to change the characteristics of the pipeline, that is, install a regulating valve on the discharge pipeline, open or close the opening of the regulating valve, so as to change the local resistance in the pipeline, change the coefficient K of the pipeline characteristics, and change the slope of the pipeline characteristics. When the performance curve of the pump is constant, the working point changes to adjust the flow.
As shown in Figure 1-52, when the regulating valve on the discharge pipeline of the pump is fully open, the pipeline characteristic curve is set as 1. The intersection point with the H-Q performance curve of the pump is m, and the corresponding flow is Q1. As the regulating valve is gradually closed, the pipeline characteristic coefficient K is gradually increased, the pipeline characteristics 2 and 3 are correspondingly steeper, the working points are m2 and M3, the flow is gradually reduced to Q2 and Q3, and the pump lift is gradually increased from H1 to H2 and H3.
It can be seen from figure 1-52 that when the flow is reduced from Q1 to Q2 or Q3 by turning down the regulating valve, the efficiency of the pump will often be reduced, because in general, the pump is always selected according to the working point m determined when the valve is fully open. As for the power, the power performance curve of low and medium specific speed centrifugal pump decreases with the decrease of flow rate. However, from the utilization degree of the energy head provided by the pump, the reduction of the opening of the regulating valve increases the force and the additional resistance loss. When the regulating valve is set to be fully open, the flow rate is Q1, and the characteristic coefficient of the pipeline is K1, then the flow loss in the pipeline is h = K1q2; after throttling, the flow becomes Q2, and the total flow loss is h = k2q2, among which the flow loss to be overcome when the liquid is transported in the pipeline is only h = k, QZ2, and the other energy head (K2 – K1) QZ2 is throttling regulation loss.
It can be seen that when the pipeline characteristics are changed to regulate the flow by turning down the discharge regulating valve, the local resistance loss in the pipeline increases, and the pump is required to provide more energy head to overcome the additional resistance loss, so that the efficiency of the whole device is not high, and it is uneconomical to adjust in this way for a long time. Especially for the centrifugal pump with steep drop head performance curve, it is more uneconomical to adjust by this method. But because of
This method is simple and convenient to adjust, so it is still widely used in the condition adjustment of centrifugal pump.
2) Bypass Book Adjustment
As shown in Figure 1-53, a bypass is set at the pump outlet to connect with the liquid suction tank. A regulating valve is installed on the pipeline. The centrifugal pump works on the bypass regulating device as in the branch pipeline. H-q1 is the pipeline characteristic of the main pipeline, and h-q2 is the bypass
When the bypass control valve is completely closed, the intersection point of the pump performance curve H-Q and the main pipeline characteristic h-q1 is B1; when the bypass valve is opened, the intersection point of the H-Q curve and H-Q is a. According to the method of calculating the flow in each pipe in the branch pipeline, the flow through the main pipe is Q and the flow through the main pipe is qao when crossing point a as the horizontal line and point A1 and point A2 as the h-q2. It can be seen from the figure that the flow of the pump increases, but the flow in the main pipe is smaller than that in the main pipe when the bypass valve is closed, so the flow is regulated.
This method of regulation is also uneconomical because the fluid in the bypass wastes power. If the axial power of the pump decreases with the increase of the flow rate, this method is suitable for regulating.
In addition, when the liquid level in the drain tank changes, the pipeline characteristics will move up and down, and the working point and flow will change.
2. Change pump characteristics
In addition to the above-mentioned use of pumps in series and parallel operation to change the performance curve to achieve the condition regulation, the common methods to change the pump performance curve include changing the working speed, cutting the outer diameter of impeller and changing the impeller regulation.
1) Change the working speed
It can be seen from HR. - 42c2. M and Q: = Nd: b-e2c20 that the field path and flow rate of centrifugal pump are related to the rotating speed. When n increases, it can be seen from the proportional law that the flow rate and head change in proportion to the rotational speed approximately according to the quadratic power, i.e. the H-Q performance curve of the system moves up and to the right: when n decreases, the H-Q performance curve moves down and to the left, as shown in figure 1-54, when the pipeline characteristic H-Q remains unchanged, different working points can be obtained to change the flow rate. In addition, equation (1-71) gives the pump characteristic equation of speed regulation.
It is more economical to adjust the flow with variable speed because it has no additional energy loss caused by throttling. However, the adjustment of slurry pump requires the use of prime mover which can change the speed to drive, such as DC motor frequency conversion motor and steam turbine. At present, frequency converter and variable speed AC motor are widely used, or with hydraulic coupling drive.
