渣浆泵汽蚀判别式分几种
由式(1- 24)和式(1- 26)可以看出:
    (1)NPSHa = NPSHr时，是泵发生汽蚀的临界值，通过汽蚀实验测定的就是这个临界值。   

(2)NPSH,>NPSH,时.泵内不发生汽蚀。但为了安全起见，在设计和使用过程中，一般在NPSH的基础上加上一个安全余量.称为允许汽蚀余量用NPSHJ表示[NPSH =NPSH. +0.3。在设计和使用泵的过程中，为保证不发生汽的NSH,要大FINPS:图1-20给出了泵的有效汽蚀余量和必需汽蚀余量随流量的变化曲线。由图可则，随着流量的增加，泵装置提供的有效汽蚀余量在减少，而泵本身所必需的内油余质在均加。

吸上真空度和泵的安装高度

1.吸上真空度

由于装置的有效汽蚀余量所表示的泵入口的液体能量不易直接测量出来，而泵入口法兰处的真空表读数(以液柱高度表示)可直接测量出来，可用于监测泵是否正常运行。吸上真空度就是泵入口处的真空表读数，用Hs来表示，单位为m.其计算公式为:
                    Hs=Pa/pg- Ps/pg
式中 Pa----泵使用地点的当地大气压.Pa;

Ps----泵入口处的绝对压力.Pa。
吸上真空度可用于表示有效汽蚀余量，将式(1-27)与式(1-23)联立可得二者的关系式为:
             NPSHa=Pa-Pv/pg+cs/2g+Hs
Hs越大，即泵入口处压力Ps越小，则NPSHa就越小，泵越容易发生汽蚀。
    [例1-1]如图1- 19所示,用一台离心泵输送水，泵吸入口直径为200mm,安装高度Zg= 5m,流量为280m3 /h,此时泵的允许汽蚀余量[NPSH:]=4.4m。已知液面下降速度CA≈0,液面压力为大气压pa=1.013X 105 Pa,该液体在罐内温度条件下的饱和蒸气压p.=2. 35X10Pa,吸入管路阻力损失hf=0.5m。问该泵是否汽蚀?泵的吸上真空度Hs是多少?
解:根据已知条件可知，泵的有效汽蚀余量NPSH.为:

用汽蚀试验可得到刚发生汽蚀时的吸上真空度，称为最大吸上真空度,用Hx表示。运行时若真空表读数H,小于Hmnx ,说明不发生汽蚀。但为安全起见,在设计和使用过程中，一般用允许吸上真空度[H.]作为泵的汽蚀特性参数,[H,]= Hsmax-0.3,该值需要由泵制造厂家提供。例如,IS65- 50- 125型单级单吸悬臂式离心泵的[H,]为7m,则泵工作时泵入口处的真空表读数就应该小于7m,这样泵才是在安全状态下工作，不会发生汽蚀。
    泵样本上的[H;]值，是在标准大气压下抽送20°C 清水时测得的。如果泵的运行条件与上述条件不同，不能直接采用样本上提供的[H,]值，应将其换算成泵使用现场条件下的[H]。换算关系如下:

泵的安装高度
    泵的安装高度是由用户自己确定的，安装高度确定的正确与否，决定了泵工作时是否发生汽蚀。因此，用户需要根据泵制造厂家给定的汽蚀参数[NPSH,]和[H]来确定允许安装高度[zg]。实际安装高度定要小于[zg]，这样 才能保证泵工作时不发生汽蚀。

1)根据[NPSH,]确定[z]由式(1 - 24)得:

刚汽蚀时,NPSH. = NPSH,，则此时的安装高度是最大安装高度qgmx，有:
为安全起见，一般采用允许安装高度，用x]表示,1ξsJ=zm -0.3.则:

根据Hs确定  的安装高度

将式(1-23)与式(1 - 27)联立，可得：

刚汽蚀时:
故得到允许安装高度为:
若吸入罐液面上的压力pA为当地大气压，即pa=p.上式简化为:
    采用式(1- 30)进行计算时需要注意，一般来说[H,]是随着流量的增加而下降的(流量增
加，流动损失和速度水头均增加，使得ps降低)，如图1-21所示。因此，必须以泵在运行中可能出现的最大流量所对应的[Hs]为准，而泵铭牌上的[Hs]是指最高效率点的值。

[例1-2]用一台水泵从湖中将水泵送到水池，流量Q为0.2m3/s，湖面高程(海拔)500m，水池水面高程530m,吸水管和排水管直径d为500mm,长度分别为l吸=10m和l排= 100m，沿程摩阻系数λ=0.22,水泵允许吸上真空度[Hs]=4.5m,已知底阀的局部阻力系数ξ =2. 5,90°弯头的局部阻力系数ξ b.=0.3，水泵入口前变径的局部阻力系数ζ。=0. 1，设水温为10C，试确定水泵允许安装高度。

解:泵的允许安装高度为:

由于该泵运行条件不是在标准大气压下抽送20C清水，因此需对水泵的允许吸上真空度进行修正。查表1-1, 海拔500m时,大气压为9.7m;查表1-2,水温为10C时，饱和蒸气压

为0.12x 10-Pa(0.12m)，则:

吸入管内的流速为:渣浆泵厂家
所以泵的允许安装高度为:
[例1一3]如图1一19(d)所示，用一台离心泵输送罐内的液体,已知罐内液面下降速度CA≈0,罐内液面压力pA= 10.1m,该液体在罐内温度条件下的饱和蒸气压pv=6.3m,吸入管路阻力损失hf=2m,泵的允许必需汽蚀余量[NPSH,]=7m。若保证不汽蚀，则罐的最低液位是多少?
解:根据已知条件可知，泵的允许安装高度为:
说明当罐最低液位为5.2m时，可以保证泵不发生汽蚀。

Several types of cavitation discriminant for slurry pump

It can be seen from formula (1-24) and formula (1-26):

(1) When npsha = NPSHr, it is the critical value of pump cavitation, which is determined by cavitation experiment.

(2) When NPSH > NPSH, no cavitation occurs in the pump. However, for the sake of safety, in the process of design and use, a safety margin is generally added on the basis of NPSH, which is called allowable NPSH expressed by npshj [NPSH = NPSH. + 0.3]. In the process of designing and using the pump, in order to ensure the NSH without steam, finps: figure 1-20 shows the change curve of effective NPSH and necessary NPSH with flow. It can be seen from the figure that with the increase of flow, the effective NPSH provided by the pump device is decreasing, while the necessary internal oil surplus of the pump itself is being added.

Suction vacuum and pump installation height

1. Vacuum degree

Because the liquid energy at the pump inlet indicated by the effective NPSH of the device is not easy to be measured directly, and the reading of the vacuum gauge at the pump inlet flange (indicated by the liquid column height) can be measured directly, which can be used to monitor whether the pump operates normally. Suction vacuum is the reading of the vacuum gauge at the pump inlet, expressed in HS, in M. its calculation formula is:

Hs=Pa/pg- Ps/pg

Where PA is the local atmospheric pressure at the place where the pump is used. Pa;

PS ---- absolute pressure at pump inlet. PA.

The suction vacuum degree can be used to express the effective NPSH, and the relationship between equation (1-27) and equation (1-23) can be obtained as follows:

NPSHa=Pa-Pv/pg+cs/2g+Hs

The larger HS is, the smaller PS is at the pump inlet, the smaller npsha is, and the more likely the pump is to cavitation.

[example 1-1] as shown in Figure 1-19, a centrifugal pump is used to transport water. The diameter of the pump suction port is 200mm, the installation height is ZG = 5m, and the flow is 280M3 / h. at this time, the allowable NPSH of the pump [NPSH:] = 4.4m. It is known that the falling speed of liquid level Ca ≈ 0, the pressure of liquid level PA = 1.013 x 105 PA, the saturated vapor pressure P. = 2.35x10pa and the resistance loss HF = 0.5m of suction pipeline under the temperature condition in the tank. Is the pump cavitating? What is the suction vacuum HS of the pump?

Solution: according to the known conditions, the effective NPSH of the pump is:

When cavitation happens, the suction vacuum degree can be obtained by cavitation test, which is called the maximum suction vacuum degree, expressed by HX. If the reading h of the vacuum gauge is less than hmnx during operation, it means no cavitation. However, for the sake of safety, in the design and use process, the allowable suction vacuum degree [h.] is generally used as the cavitation characteristic parameter of the pump, [h,] = hsmax-0.3, which needs to be provided by the pump manufacturer. For example, if the [h,] of is65-50-125 single-stage single suction cantilever centrifugal pump is 7m, the reading of vacuum gauge at the pump inlet should be less than 7m when the pump is working, so that the pump is working in a safe state without cavitation.

The value of [h;] on the pump sample is measured when 20 ° C clean water is pumped under the standard atmospheric pressure. If the operating conditions of the pump are different from the above conditions, the value of [h,] provided on the sample cannot be used directly, it shall be converted to the value of [H] under the field conditions of pump use. The conversion relationship is as follows:

Installation height of pump

The installation height of the pump is determined by the user. Whether the installation height is correct or not determines whether cavitation occurs during the operation of the pump. Therefore, the user needs to determine the allowable installation height [ZG] according to the cavitation parameters [NPSH,] and [H] given by the pump manufacturer. The actual installation height must be less than [ZG], so as to ensure that no cavitation occurs during the operation of the pump.

1) According to [NPSH,] it is determined that [Z] is obtained from equation (1-24):

When NPSH. = NPSH, the installation height is the maximum installation height qgmx, including:

For the sake of safety, the allowable installation height is generally adopted, expressed by x], 1 ξ SJ = ZM - 0.3

Installation height according to HS

If equation (1-23) and equation (1-27) are combined, we can get:

In case of rigid cavitation:

Therefore, the allowable installation height is:

If the pressure PA on the liquid level of the suction tank is the local atmospheric pressure, that is, PA = P. the above formula is simplified as:

When using formula (1-30) for calculation, it should be noted that [h,] generally decreases with the increase of flow (flow increases

In addition, both flow loss and velocity head increase, which makes PS decrease), as shown in Figure 1-21. Therefore, it must be based on the [HS] corresponding to the maximum flow that may occur during the operation of the pump, and the [HS] on the pump nameplate refers to the value of the highest efficiency point.

[example 1-2] a water pump is used to pump water from the lake to the pool, with the flow Q of 0.2m3/s, the lake surface elevation (altitude) of 500m, the pool surface elevation of 530m, the diameter D of the suction pipe and the drainage pipe of 500mm, the length of L suction = 10m and l discharge = 100m, the friction coefficient along the way λ = 0.22, the allowable suction vacuum degree of the pump [HS] = 4.5m, and the local resistance coefficient ξ = 2.5, the local resistance coefficient ξ B. = 0.3 of the 90 ° elbow , the local resistance coefficient ζ of the variable diameter in front of the water pump inlet. =0.1, set the water temperature to 10C, and try to determine the allowable installation height of the water pump.

Solution: the allowable installation height of the pump is:

Since the operation condition of the pump is not to pump 20c clean water under standard atmospheric pressure, it is necessary to correct the allowable suction vacuum of the pump. Look up Table 1-1, when the altitude is 500m, the atmospheric pressure is 9.7M; look up table 1-2, when the water temperature is 10C, the saturated vapor pressure

0.12 x 10-pa (0.12M), then:

The flow rate in the suction pipe is: slurry pump manufacturer

Therefore, the allowable installation height of the pump is:

[example 1-3] as shown in Figure 1-19 (d), a centrifugal pump is used to transport the liquid in the tank. It is known that the falling speed of the liquid level in the tank Ca ≈ 0, the pressure of the liquid level in the tank PA = 10.1m, the saturated vapor pressure of the liquid at the temperature in the tank PV = 6.3m, the resistance loss of the suction pipeline HF = 2m, and the allowable necessary NPSH of the pump [NPSH,] = 7m. If no cavitation is guaranteed, what is the minimum liquid level of the tank?

Solution: according to the known conditions, the allowable installation height of the pump is:

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