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渣漿泵的汽蝕余量NPSH 怎么產(chǎn)生的
為了使泵在運(yùn)行中不發(fā)生汽蝕，泵的進(jìn)口處的液體所具有超過(guò)汽化壓力的富裕能量(即相對(duì)基準(zhǔn)面的入口絕對(duì)總水頭與汽化壓力水頭之差)，稱為汽蝕余量(NPSH)， 國(guó)外稱凈正吸入頭，單位mH2O。根據(jù)不同情況汽蝕余量有:
(1)臨界汽蝕余量NPSH3 (老標(biāo)準(zhǔn)中為NPSHC) ;
(2)必需汽蝕余量NPSHR;
(3)可用汽蝕余量NPSHA (老標(biāo)準(zhǔn)中稱有效汽蝕余量)。
一、 臨界汽蝕余量NPSH3
在規(guī)定流量下，泵的第一級(jí)揚(yáng)程下降3%時(shí)的汽蝕余量值，它是通過(guò)試驗(yàn)來(lái)得到的，即在汽蝕試驗(yàn)時(shí)，當(dāng)泵發(fā)生汽蝕后，泵的性能(揚(yáng)程、流量、功率、效率)下降的這一特性得到的。目前標(biāo)準(zhǔn)規(guī)定( 見第八章泵試驗(yàn))，當(dāng)泵的第一級(jí)揚(yáng)程下降3%時(shí)的汽蝕余量值為臨界汽蝕余量NPSH3。
二、 必需汽蝕余量NPSHR
必需汽蝕余量NPSHR是對(duì)于給定的泵在規(guī)定的轉(zhuǎn)速、流量和輸送液體的條件下，泵達(dá)到規(guī)定性能的最小汽蝕余量值。其值由制造廠給出，即樣本上給出的汽蝕余量值。它標(biāo)志一臺(tái)泵本身所具備的汽蝕性能的好壞，是由設(shè)計(jì)和制造來(lái)決定的，與泵裝置無(wú)關(guān)。必需汽蝕余量NPSHR是由汽蝕試驗(yàn)得到的臨界汽蝕余量值NPSH3加上容差系數(shù)或根據(jù)實(shí)際需要來(lái)確定的，即:
NPSHR≥NPSH3 + A·NPSH3
或 NPSHR≥NPSH3 + C
式中，A、C為容差系數(shù)，對(duì)2級(jí)試驗(yàn)精度，A=6%，C=0.3m;對(duì)1級(jí)試驗(yàn)精度，A=3%，C =0.15m。
三、可用汽蝕余量NPSHA
可用汽蝕余量NPSHA,在以前的資料中稱有效汽蝕余量、裝置汽蝕余量。它是在規(guī)定流量下由裝置條件確定的獲得的汽蝕余量值。根據(jù)上述定義，可用汽蝕余量NPSHA應(yīng)該是:
以式(3-5)看出，可用汽蝕余量NPSHA 是由幾何安裝高度H、吸入管路的阻力損失h、吸入液面的壓力 Po的大小及液體的汽化壓力值來(lái)決定的，即與泵的裝置有關(guān)，故有人稱它為裝置汽蝕余量。
為了在使用運(yùn)行中不發(fā)生汽蝕，就必須滿足:
NPSHA≥NPSHR + B
式中，B為安全裕量，是為了確保泵運(yùn)行中不發(fā)生汽蝕，根據(jù)液體介質(zhì)的性質(zhì)及使用場(chǎng)合的要求來(lái)確定的，由用戶或設(shè)計(jì)院來(lái)選取。一般情況下取0.6m, 例多級(jí)泵、雙吸泵、常溫冷卻泵等;鍋爐給水泵、冷凝泵B取2. 1m;而石化廠真空塔底泵B取3.0m之多。
第三節(jié)吸上真空高度Hs
在過(guò)上的一些泵的資料中常用吸上真空度H來(lái)表示泵的汽蝕性能。什么是吸上真空高
度呢?
圖3-3是一臺(tái)泵的安裝示意圖， 列出吸水面和深進(jìn)口處的柏努和方程:
從式(3-7)、式(3-8) 可以看出:當(dāng)泵安裝時(shí)，泵軸線離液面(即幾何安裝高度Hg)越大，泵進(jìn)口的壓力p1越小，吸上真空高度H.也就越大。當(dāng)幾何安裝高度Hg大到一定值時(shí)，或進(jìn)口壓力降到一定時(shí)，也即吸上真空高度H.大到一定值時(shí)，泵將發(fā)生汽蝕。泵不發(fā)生汽蝕情況下的最大吸上真空高度H...稱最大吸上真空度。一臺(tái)泵的最大吸上真空度...目前還不能用計(jì)算來(lái)確定，而是通過(guò)試驗(yàn)來(lái)獲得的。
為了泵運(yùn)行安全起見，在過(guò)去的標(biāo)準(zhǔn)中留有0.3m的容差量，(Hsmx-0.3)稱允許吸上真空高度[H。]，也就是樣本上給出的允許吸上真空度[H.]， 它是由設(shè)計(jì)制造確定的，與裝置無(wú)關(guān)。
為使泵在運(yùn)行中不發(fā)生汽蝕，必須滿足: H.≤[H]，即Hg +
要注意的是:樣本或說(shuō)明書中給出的允許吸上真空高度[H,]是在大氣壓力為101325Pa ( 即760mmHg或10. 33mH,0)，液體溫度為常溫(T=20C) 下清水試驗(yàn)得到的，如果泵使用地點(diǎn)的大氣壓力、液體溫度或液體的汽化壓力不同時(shí)，允許吸上真空高度應(yīng)進(jìn)行修正 渣漿泵廠家
How to produce NPSH of slurry pump
In order to avoid cavitation during the operation of the pump, the liquid at the inlet of the pump has a surplus energy (i.e. the difference between the absolute total water head at the inlet relative to the reference plane and the vaporization pressure water head), which is called NPSH, and the net positive suction head in foreign countries, in mh2o. According to different conditions, NPSH is as follows:
(1) critical NPSH 3 (npshc in the old standard);
(2) necessary NPSHr;
(3) available npsha (known as effective npsha in the old standard).
I. critical NPSH 3
Under the specified flow rate, the NPSH value when the first stage head of the pump drops by 3%, which is obtained through the test, that is, during the NPSH test, when the pump has cavitation, the performance (head, flow, power, efficiency) of the pump drops. According to the current standard (see Chapter VIII pump test), when the first stage head of the pump drops by 3%, the NPSH value is the critical npsh3.
II. NPSHr required
NPSHr is the minimum NPSHr value for a given pump to achieve the specified performance under the conditions of specified speed, flow and liquid delivery. The value is given by the manufacturer, i.e. the NPSH value given on the sample. It marks the cavitation performance of a pump, which is determined by design and manufacture, and has nothing to do with the pump device. NPSHr is determined by the critical NPSHr value npsh3 obtained from the cavitation test plus the tolerance coefficient or according to the actual needs, namely:
NPSHR≥NPSH3 + A·NPSH3
Or NPSHr ≥ npsh3 + C
In the formula, a and C are tolerance coefficients, a = 6% and C = 0.3m for Level 2 test precision, a = 3% and C = 0.15m for level 1 test precision.
III. npsha available
Npsha can be used, which is called effective npsha and npsha in previous data. It is the NPSH value determined by the device conditions under the specified flow. According to the above definition, npsha available shall be:
It can be seen from equation (3-5) that npsha is determined by geometric installation height h, resistance loss h of suction pipeline, pressure Po of suction liquid level and vaporization pressure value of liquid, that is, it is related to pump device, so it is called npsha of device.
In order to avoid cavitation during operation, it is necessary to meet the following requirements:
NPSHA≥NPSHR + B
In the formula, B is the safety margin, which is determined according to the nature of liquid medium and the requirements of use occasions to ensure that no cavitation occurs during the operation of the pump. It is selected by the user or the design institute. Generally, 0.6m is taken, such as multistage pump, double suction pump, normal temperature cooling pump, etc.; 2.1m is taken for boiler feed pump and condensate pump B; 3.0m is taken for vacuum tower bottom pump B in petrochemical plant.
Section 3 vacuum height HS
In the data of some pumps, vacuum degree h is often used to represent the cavitation performance of the pump. What is high vacuum
How about it?
Figure 3-3 is the installation diagram of a pump, listing the bernould equation at the suction surface and deep inlet:
It can be seen from formula (3-7) and formula (3-8): when the pump is installed, the greater the distance between the pump axis and the liquid level (that is, the geometric installation height Hg), the smaller the pressure P1 at the pump inlet, and the higher the vacuum height H. When the geometric installation height Hg reaches a certain value, or when the inlet pressure drops to a certain value, that is, when the suction vacuum height h reaches a certain value, cavitation will occur. The maximum suction vacuum height h... When the pump does not have cavitation is called the maximum suction vacuum degree. The maximum suction vacuum of a pump... Can not be determined by calculation at present, but by test.
In order to ensure the safety of pump operation, a tolerance of 0.3m is reserved in the past standard, and the (hsmx-0.3) is called the allowable vacuum height [H]. ]That is to say, the allowable vacuum degree [h.] given on the sample is determined by the design and manufacture, independent of the device.
In order to prevent the pump from cavitation during operation, it must meet the following requirements: H. ≤ [H], i.e. Hg+
It should be noted that the allowable vacuum height [h,] given in the sample or the instruction manual is obtained from the clean water test under the atmospheric pressure of 101325pa (760mmhg or 10.33mh, 0) and the liquid temperature of room temperature (t = 20c). If the atmospheric pressure, liquid temperature or liquid vaporization pressure at the pump use site are different, the allowable vacuum height shall be corrected by the slurry pump manufacturer
