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如何降低渣漿泵汽蝕的危害
(1)降低泵的性能。汽蝕產(chǎn)生了大量的氣泡,堵塞了流道，破壞了泵內(nèi)液體的連續(xù)流動(dòng)，泵的流量、揚(yáng)程和效率明顯下降,嚴(yán)重時(shí)出現(xiàn)圖1- 16所示的斷流工況,使泵不能工作。
(2)產(chǎn)生振動(dòng)和噪聲。氣泡潰滅時(shí),液體質(zhì)點(diǎn)互相撞擊,同時(shí)也撞擊金屬表面,產(chǎn)生各種頻率的噪聲,嚴(yán)重時(shí)可聽見泵內(nèi)有“噼啪”的爆炸聲,同時(shí)引起機(jī)組振動(dòng)。
(3)破壞過(guò)流部件。因機(jī)械剝蝕和電化學(xué)腐蝕的作用，使金屬材料發(fā)生破壞，起初是金屬表面出現(xiàn)麻點(diǎn)，繼而表面呈現(xiàn)溝槽狀、蜂窩狀、魚鱗狀等痕跡;嚴(yán)重時(shí)可造成葉片或前后蓋板穿孔甚至葉輪破裂。典型位置如圖1-17所示。
汽蝕余量與汽蝕判別式
泵與吸入管路的分界面是泵的吸入口法蘭s-s截面、泵內(nèi)壓力最低處位于k截面處，如圖1-18所示。根據(jù)汽蝕過(guò)程可知，要避免發(fā)生汽蝕，應(yīng)滿足h.p.由于泵入口處液體壓力比k截面壓力高，因此，為保證，要求泵入口液體的靜壓頭除了要高出液體的汽化壓力外，還應(yīng)當(dāng)有定的富余能頭，這 個(gè)富余能頭稱為汽蝕余量(又稱為凈正吸上水頭)。
汽蝕余量又分為有效汽蝕余盤和泵必需的汽蝕余量。
1.有效汽蝕余量NPSH。
有效汽蝕余量是指泵運(yùn)行時(shí)，從基準(zhǔn)面(臥式泵為過(guò)泵軸中心線的水平面，立式管道泵為吸入口中心線)算起,泵入口(s截面)的能頭超過(guò)汽化壓力p.的富余能頭，用NPSH。(Net Positive Suction Head available)表示。 該值越大，說(shuō)明泵入口的富余能頭越多，越不容易汽蝕。根據(jù)定義,其計(jì)算公式為:
上式中cs≈0,代入式(1-23)得:
由上式可知，有效汽蝕余量NPSH,就是吸入罐液面上的壓力能頭ps/pg 在克服管路中的流動(dòng)損失2 ha-，并把液體提高zp位差后，所剩余的超過(guò)汽化壓力pv的能頭。所以NPSH的大小與吸入裝置的參數(shù)有關(guān),而與泵本身的結(jié)構(gòu)尺寸等無(wú)關(guān)。
式(1- 24)中zg的正負(fù)以吸入罐液面為基準(zhǔn)，當(dāng)泵軸高于吸入液面時(shí)為正，泵軸低于吸入液面時(shí)為負(fù),此時(shí)，一Z。稱為倒灌高度或者灌注頭,用符號(hào)Z: 來(lái)表示，則式(1-24)變?yōu)?/span>:
在石油行業(yè)，離心泵的安裝經(jīng)常采用圖1- 19(c)和圖1-19(d)所示的灌注方式，這樣既滿足了灌泵的要求，也增大了有效汽蝕余量。
2.泵必需的汽蝕余量NPSHr
泵必需的汽蝕余量是指液流從泵入口(s截面)到泵內(nèi)壓力最低點(diǎn)(k截面)的全部能頭損失(含k截面處的速度能頭),用NPSH,(Net Positive Suction Head required)表示。為了弄清NPSH,的物理意義,可以利用伯努利方程列出由泵入口s截面到泵內(nèi)壓力最低點(diǎn)k處的能量平衡關(guān)系:
上式中的h,加上k點(diǎn)的速度能頭，即為從泵入口到泵內(nèi)壓力最低點(diǎn)(k處)的全部能頭損失NSHI.該值起小，表示能頭損失越小，泵的抗汽蝕性能越好，它可表示為流體從s截面流動(dòng)到葉片進(jìn)口截面位置1時(shí)的地對(duì)運(yùn)動(dòng)速度c1和相對(duì)運(yùn)動(dòng)速度w1的形式，其計(jì)算公式為：
NPSH僅與離心泵的結(jié)構(gòu)參數(shù)及流量有關(guān)，詳見第四節(jié),通常由廠家根據(jù)實(shí)驗(yàn)測(cè)定.在美國(guó)石油學(xué)會(huì)(American Petroleum Institute)API 610標(biāo)準(zhǔn)中，規(guī)定是由于汽蝕導(dǎo)致?lián)P程下降3%(對(duì)多級(jí)泵而言是首級(jí)揚(yáng)程時(shí)的汽蝕余量。渣漿泵廠家
從式(1- 24)可以看出,NPSH.是通過(guò)吸入裝置參數(shù)來(lái)計(jì)算出來(lái)的，而這些參數(shù)均是由用戶自己決定,所以用戶自己需要計(jì)算或提供NPSH,值。而NPSH,是泵內(nèi)的損失，與泵本身有關(guān),需要由泵制造廠家提供。
How to reduce the cavitation damage of slurry pump
(1) Reduce pump performance. Cavitation produces a large number of bubbles, blocks the flow passage, destroys the continuous flow of the liquid in the pump, and obviously reduces the flow, head and efficiency of the pump. In serious cases, the flow cutoff condition shown in Figure 1-16 occurs, which makes the pump unable to work.
(2) Produce vibration and noise. When the bubble collapses, the liquid particles collide with each other, at the same time, they also collide with the metal surface, producing noise of various frequencies. In serious cases, the explosion sound of "crackling" can be heard in the pump, and the vibration of the unit can be caused at the same time.
(3) Damage to overcurrent components. Due to the effect of mechanical erosion and electrochemical corrosion, the metal materials are damaged. At first, there are pockmarks on the metal surface, and then there are groove, honeycomb, fish scale and other traces on the surface. In serious cases, it may cause perforation of the blade or front and rear cover plates or even impeller rupture. Typical locations are shown in Figure 1-17.
NPSH and discriminant of NPSH
The interface between the pump and the suction pipeline is the S-S section of the suction flange of the pump, and the lowest pressure in the pump is located at the K section, as shown in Figure 1-18. According to the cavitation process, in order to avoid cavitation, H.P. because the liquid pressure at the pump inlet is higher than the section pressure of K, in order to ensure that the static head of the liquid at the pump inlet is required to have a certain surplus head besides the liquid vaporization pressure. This surplus head is called NPSH (also called net positive suction head).
NPSH can be divided into effective NPSH disk and necessary NPSH of pump.
1. NPSH.
The effective NPSH refers to the surplus energy head at the pump inlet (section s) which exceeds the vaporization pressure P. calculated from the reference plane (the horizontal pump is the horizontal plane passing the pump shaft centerline, and the vertical pipeline pump is the suction inlet centerline). NPSH is used. (net positive suction head available). The larger the value, the more surplus energy head at the pump inlet, the less cavitation. According to the definition, the calculation formula is:
In the above formula, CS ≈ 0 is substituted into formula (1-23) to obtain:
It can be seen from the above formula that the effective NPSH is the energy head PS / PG on the liquid surface of the suction tank, which overcomes the flow loss of 2 ha - in the pipeline and increases the liquid ZP potential difference. The remaining energy head exceeds the vaporization pressure PV. Therefore, the size of NPSH is related to the parameters of suction device, but not to the structural size of pump itself.
The positive and negative ZG in formula (1-24) are based on the liquid level of the suction tank. When the pump shaft is higher than the suction liquid level, it is positive. When the pump shaft is lower than the suction liquid level, it is negative. At this time, a Z. It is called pouring height or pouring head, which is represented by symbol Z:, then formula (1-24) becomes:
In the oil industry, the centrifugal pump is often installed in the way shown in Fig. 1-19 (c) and Fig. 1-19 (d), which not only meets the requirements of the pump, but also increases the effective NPSH.
2. NPSHr necessary for pump
The necessary NPSH of the pump refers to the total head loss (including the speed head at section K) of the liquid flow from the pump inlet (section s) to the lowest point of the pump internal pressure (section K), which is expressed by NPSH (net positive suction head required). In order to understand the physical meaning of NPSH, we can use Bernoulli equation to list the energy balance relationship from the section s of pump inlet to the lowest point K of pump pressure
H in the above formula, plus the velocity head at k point, is the total energy head loss nshi from the pump inlet to the lowest point (k) of the pump internal pressure. The smaller the value is, the smaller the energy head loss is, the better the anti cavitation performance of the pump is. It can be expressed in the form of ground to ground velocity C1 and relative velocity W1 when the fluid flows from s section to blade inlet section 1. The calculation formula is as follows:
NPSH is only related to the structural parameters and flow of centrifugal pump, see Section IV for details, which is usually measured by the manufacturer according to the experiment. In API 610 standard of American Petroleum Institute, it is specified that the head is reduced by 3% due to cavitation (for multi-stage pump, it is the cavitation allowance at the first head). Slurry pump manufacturer
It can be seen from equation (1-24) that NPSH. Is calculated by the parameters of suction device, and these parameters are determined by the user himself, so the user needs to calculate or provide NPSH, value. NPSH is the loss in the pump, which is related to the pump itself and needs to be provided by the pump manufacturer.
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