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容積損失與渣漿泵容積效率比例
泵中的容積損失主要有下列幾種:
(1)葉輪入口處密封環(huán)泄露的容積損失，如圖2-34所示；
(2)軸向力平衡機(jī)構(gòu)的容積損失;
(3)在多級(jí)泵中，還有級(jí)間回流損失;
(4)軸封泄漏的容積損失。
1.葉輪入口處密封環(huán)的容積損失
在葉輪入口處，葉輪與泵體間有一個(gè)很小密封間隙。由于泵腔內(nèi)的壓力高于葉輪入口處的壓力，所以有一小股液體通過此密封間隙從葉輪出口回流到葉輪入口，通常把這部分能量損失稱為密封環(huán)泄漏損失q1。
密封環(huán)間隙寬度b一般可參考表2-1 選取，但對(duì)于耐腐泵、油泵、雜質(zhì)泵、液下泵及輸送黏性液體、帶顆粒液體(例泵油等)的泵間隙適當(dāng)放大。
從式(2-23)可以看出，密封環(huán)的泄漏量q1,與密封環(huán)處的直徑、 間隙寬度b、圓角系數(shù)、兩端壓差有關(guān)。為了減少容積損失，盡量減少間隙寬度b,當(dāng)密封環(huán)磨損增大時(shí)，要及時(shí)修復(fù)更換。為了減少兩端的壓差,應(yīng)增加密封環(huán)間隙的圓角系數(shù)、長(zhǎng)度L。尤其對(duì)高揚(yáng)程的泵，可將密封環(huán)做成迷宮形或鋸齒形，如圖2-35所示，以減小密封環(huán)的泄漏。
2.軸向力平衡機(jī)構(gòu)的容積損失
軸向力平衡方法很多，機(jī)構(gòu)也很多，將在第四章中介紹。常用的為平衡孔結(jié)構(gòu)和多級(jí)泵中的平衡盤、平衡鼓結(jié)構(gòu)。平衡孔的泄漏量仍可按式(2 -24)計(jì)算。
3.級(jí)間回流損失
如圖2-36所示級(jí)間間隙的泄漏液體，流經(jīng)葉輪與導(dǎo)葉間的側(cè)隙后，與葉輪流出的液體混合，經(jīng)導(dǎo)葉和反導(dǎo)葉，又經(jīng)級(jí)間間隙流向前級(jí)葉輪的側(cè)隙，如此循環(huán)。由于這部分液體不
經(jīng)過葉輪，不影響泵的流量，所以這部分能量損失不屬于容積損失，但卻損失了一部分功率。
4.軸封泄漏
軸封泄露較小，尤其是軸封采用機(jī)械密封時(shí)。但對(duì)于小泵也應(yīng)重視。尤其是當(dāng)軸封失效時(shí)，泄漏量較大。
泵的容積效率從 上面分析計(jì)算可看出與泵的結(jié)構(gòu)形式及比轉(zhuǎn)數(shù)n泵的流量大小有關(guān)。圖2-37為一般情況下離心系的容積效率。渣漿泵
Ratio of volume loss to volume efficiency of slurry pump
The volume loss in the pump is mainly as follows:
(1) volume loss due to leakage of sealing ring at the inlet of impeller, as shown in figure 2-34;
(2) volume loss of axial force balancing mechanism;
(3) in the multi-stage pump, there is also a loss of return flow between stages;
(4) volume loss of shaft seal leakage.
1. Volume loss of sealing ring at impeller inlet
At the inlet of impeller, there is a small sealing clearance between impeller and pump body. Because the pressure in the pump cavity is higher than the pressure at the impeller inlet, a small stream of liquid flows back from the impeller outlet to the impeller inlet through the sealing clearance. This part of energy loss is usually called the leakage loss of the sealing ring Q1.
Generally, the clearance width b of sealing ring can be selected with reference to table 2-1, but the clearance of anti-corrosion pump, oil pump, impurity pump, submerged pump and pump for delivering viscous liquid and liquid with particles (such as pump oil, etc.) can be appropriately enlarged.
It can be seen from equation (2-23) that the leakage of the sealing ring Q1 is related to the diameter, clearance width b, fillet coefficient and pressure difference at both ends of the sealing ring. In order to reduce the volume loss and reduce the gap width B as much as possible, when the wear of the sealing ring increases, it is necessary to repair and replace it in time. In order to reduce the pressure difference at both ends, the fillet coefficient and length L of the seal ring gap should be increased. Especially for pumps with high lift, the sealing ring can be made into labyrinth or serrated shape, as shown in Fig. 2-35, so as to reduce the leakage of the sealing ring.
2. Volume loss of axial force balancing mechanism
There are many methods of axial force balance and many mechanisms, which will be introduced in Chapter 4. Commonly used for the balance hole structure and multi-stage pump balance plate, balance drum structure. The leakage of balance hole can still be calculated according to formula (2-24).
3. Loss of return flow between stages
As shown in Fig. 2-36, the leakage liquid of interstage clearance flows through the side clearance between impeller and guide vane, then mixes with the liquid flowing out of impeller, flows through the guide vane and anti guide vane, and then flows to the side clearance of front stage impeller through interstage clearance, so it circulates. Because this part of the liquid doesn't
After the impeller, the flow of the pump is not affected, so this part of energy loss is not volume loss, but a part of power is lost.
4. Shaft seal leakage
The leakage of shaft seal is small, especially when mechanical seal is adopted. However, attention should also be paid to small pumps. Especially when the shaft seal fails, the leakage is large.
From the above analysis and calculation, it can be seen that the volume efficiency of the pump is related to the structure of the pump and the flow of the specific speed N pump. Figure 2-37 shows the volumetric efficiency of the centrifugal system in general. Slurry pump
