渣浆泵的汽蚀余量NPSH 怎么产生的

为了使泵在运行中不发生汽蚀，泵的进口处的液体所具有超过汽化压力的富裕能量(即相对基准面的入口绝对总水头与汽化压力水头之差)，称为汽蚀余量(NPSH)， 国外称净正吸入头，单位mH2O。根据不同情况汽蚀余量有:
    (1)临界汽蚀余量NPSH3 (老标准中为NPSHC) ;

(2)必需汽蚀余量NPSHR;

(3)可用汽蚀余量NPSHA (老标准中称有效汽蚀余量)。

一、 临界汽蚀余量NPSH3

在规定流量下，泵的第一级扬程下降3%时的汽蚀余量值，它是通过试验来得到的，即在汽蚀试验时，当泵发生汽蚀后，泵的性能(扬程、流量、功率、效率)下降的这一特性得到的。目前标准规定( 见第八章泵试验)，当泵的第一级扬程下降3%时的汽蚀余量值为临界汽蚀余量NPSH3。
二、 必需汽蚀余量NPSHR
    必需汽蚀余量NPSHR是对于给定的泵在规定的转速、流量和输送液体的条件下，泵达到规定性能的最小汽蚀余量值。其值由制造厂给出，即样本上给出的汽蚀余量值。它标志一台泵本身所具备的汽蚀性能的好坏，是由设计和制造来决定的，与泵装置无关。必需汽蚀余量NPSHR是由汽蚀试验得到的临界汽蚀余量值NPSH3加上容差系数或根据实际需要来确定的，即:

    NPSHR≥NPSH3 + A·NPSH3 

      或  NPSHR≥NPSH3 + C  

式中，A、C为容差系数，对2级试验精度，A=6%，C=0.3m;对1级试验精度，A=3%，C =0.15m。
三、可用汽蚀余量NPSHA
    可用汽蚀余量NPSHA,在以前的资料中称有效汽蚀余量、装置汽蚀余量。它是在规定流量下由装置条件确定的获得的汽蚀余量值。根据上述定义，可用汽蚀余量NPSHA应该是:

以式(3-5)看出，可用汽蚀余量NPSHA 是由几何安装高度H、吸入管路的阻力损失h、吸入液面的压力 Po的大小及液体的汽化压力值来决定的，即与泵的装置有关，故有人称它为装置汽蚀余量。
    为了在使用运行中不发生汽蚀，就必须满足:
NPSHA≥NPSHR + B 

式中，B为安全裕量，是为了确保泵运行中不发生汽蚀，根据液体介质的性质及使用场合的要求来确定的，由用户或设计院来选取。一般情况下取0.6m, 例多级泵、双吸泵、常温冷却泵等;锅炉给水泵、冷凝泵B取2. 1m;而石化厂真空塔底泵B取3.0m之多。
                             第三节吸上真空高度Hs
    在过上的一些泵的资料中常用吸上真空度H来表示泵的汽蚀性能。什么是吸上真空高
度呢?
    图3-3是一台泵的安装示意图， 列出吸水面和深进口处的柏努和方程:

从式(3-7)、式(3-8) 可以看出:当泵安装时，泵轴线离液面(即几何安装高度Hg)越大，泵进口的压力p1越小，吸上真空高度H.也就越大。当几何安装高度Hg大到一定值时，或进口压力降到一定时，也即吸上真空高度H.大到一定值时，泵将发生汽蚀。泵不发生汽蚀情况下的最大吸上真空高度H...称最大吸上真空度。一台泵的最大吸上真空度...目前还不能用计算来确定，而是通过试验来获得的。
    为了泵运行安全起见，在过去的标准中留有0.3m的容差量，(Hsmx-0.3)称允许吸上真空高度[H。]，也就是样本上给出的允许吸上真空度[H.]， 它是由设计制造确定的，与装置无关。
    为使泵在运行中不发生汽蚀，必须满足: H.≤[H]，即Hg +
    要注意的是:样本或说明书中给出的允许吸上真空高度[H,]是在大气压力为101325Pa ( 即760mmHg或10. 33mH,0)，液体温度为常温(T=20C) 下清水试验得到的，如果泵使用地点的大气压力、液体温度或液体的汽化压力不同时，允许吸上真空高度应进行修正 渣浆泵厂家

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