如何降低渣浆泵汽蚀的危害
    (1)降低泵的性能。汽蚀产生了大量的气泡,堵塞了流道，破坏了泵内液体的连续流动，泵的流量、扬程和效率明显下降,严重时出现图1- 16所示的断流工况,使泵不能工作。
    (2)产生振动和噪声。气泡溃灭时,液体质点互相撞击,同时也撞击金属表面,产生各种频率的噪声,严重时可听见泵内有“噼啪”的爆炸声,同时引起机组振动。
    (3)破坏过流部件。因机械剥蚀和电化学腐蚀的作用，使金属材料发生破坏，起初是金属表面出现麻点，继而表面呈现沟槽状、蜂窝状、鱼鳞状等痕迹;严重时可造成叶片或前后盖板穿孔甚至叶轮破裂。典型位置如图1-17所示。

汽蚀余量与汽蚀判别式

泵与吸入管路的分界面是泵的吸入口法兰s-s截面、泵内压力最低处位于k截面处，如图1-18所示。根据汽蚀过程可知，要避免发生汽蚀，应满足h.p.由于泵入口处液体压力比k截面压力高，因此，为保证，要求泵入口液体的静压头除了要高出液体的汽化压力外，还应当有定的富余能头，这 个富余能头称为汽蚀余量(又称为净正吸上水头)。

汽蚀余量又分为有效汽蚀余盘和泵必需的汽蚀余量。
1.有效汽蚀余量NPSH。
    有效汽蚀余量是指泵运行时，从基准面(卧式泵为过泵轴中心线的水平面，立式管道泵为吸入口中心线)算起,泵入口(s截面)的能头超过汽化压力p.的富余能头，用NPSH。(Net Positive Suction Head available)表示。 该值越大，说明泵入口的富余能头越多，越不容易汽蚀。根据定义,其计算公式为:

上式中cs≈0,代入式(1-23)得:

由上式可知，有效汽蚀余量NPSH,就是吸入罐液面上的压力能头ps/pg 在克服管路中的流动损失2 ha-，并把液体提高zp位差后，所剩余的超过汽化压力pv的能头。所以NPSH的大小与吸入装置的参数有关,而与泵本身的结构尺寸等无关。
    式(1- 24)中zg的正负以吸入罐液面为基准，当泵轴高于吸入液面时为正，泵轴低于吸入液面时为负,此时，一Z。称为倒灌高度或者灌注头,用符号Z: 来表示，则式(1-24)变为:
    在石油行业，离心泵的安装经常采用图1- 19(c)和图1-19(d)所示的灌注方式，这样既满足了灌泵的要求，也增大了有效汽蚀余量。
2.泵必需的汽蚀余量NPSHr
    泵必需的汽蚀余量是指液流从泵入口(s截面)到泵内压力最低点(k截面)的全部能头损失(含k截面处的速度能头),用NPSH,(Net Positive Suction Head required)表示。为了弄清NPSH,的物理意义,可以利用伯努利方程列出由泵入口s截面到泵内压力最低点k处的能量平衡关系:

上式中的h,加上k点的速度能头，即为从泵入口到泵内压力最低点(k处)的全部能头损失NSHI.该值起小，表示能头损失越小，泵的抗汽蚀性能越好，它可表示为流体从s截面流动到叶片进口截面位置1时的地对运动速度c1和相对运动速度w1的形式，其计算公式为：

NPSH仅与离心泵的结构参数及流量有关，详见第四节,通常由厂家根据实验测定.在美国石油学会(American Petroleum Institute)API 610标准中，规定是由于汽蚀导致扬程下降3%(对多级泵而言是首级扬程时的汽蚀余量。渣浆泵厂家
    从式(1- 24)可以看出,NPSH.是通过吸入装置参数来计算出来的，而这些参数均是由用户自己决定,所以用户自己需要计算或提供NPSH,值。而NPSH,是泵内的损失，与泵本身有关,需要由泵制造厂家提供。

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.