渣浆泵蜗壳、导叶及吸入室转能装置

蜗壳和导叶是离心泵的转能装置，它们的作用是把从叶轮甩出来的液体收集起来，使液体流速降低，把部分速度能头转变为压力能头后，再均勾地引入下一级成经过扩散管排出。

1. 蜗壳

蜗壳的形状通常是按照泵在设计流量下液体在叶轮中作稳定的相对运动，离开叶轮后不受外力作用,按其惯性作自由流动的轨迹而做成。当自叶轮流出的液体不受外力(摩擦力等)作用时，则液流对旋转轴中心的动量矩保持不变.故有:
    根据流体连续性方程，蜗壳任意半径R处的径向分速CRr (不计阻塞系数影响)为:
    若蜗壳宽度不变，即bR为定值时，则在定常流动时有:
                                CRuR=常数
    由此可见，液流在平行板式蜗壳内作自由流动时，其流动轨迹的方向角不变，为一条对数螺旋线，如图1-59所示。随着半径R的增加，对应的CR与ce.均减小，故液流速度CR及速度能头也将减小，而逐渐转换为静压能。如果想使速度能充分转换为压力能，液流必经流过较长的路程，径向尺寸必然过于庞大。因此，限制蜗壳螺旋线的包角不大于360。为了减小径向尺寸，根据cRr与蜗壳轴面宽度成反比的关系，用不断扩大的扩张形轴面宽度，如图1-60所示。这样，液流方向角a不再是常数,而是随半径R和6R增大而减小。但蜗壳截面的扩张角0不应大于60°，以免因蜗壳通流截面扩大太快使液流发生严重的边界层分离。蜗壳尺寸缩小后，液体在螺旋线部分只有小部分动能转变为静压能。为此，在螺旋线末端加一扩压管，其扩张角为8°~12°，长度为扩压管进口截面直径的2.5~3.0倍。在扩压管内可使80%~85%的动能转变为静压能。
    蜗壳的截面有圆形、矩形和倒梯形几种,其中，圆形截面用于高比转速泵，倒梯形截面用于中比转速泵,矩形截面用于低比转速泵。
    蜗壳结构多用于单级离心泵和水平中开式多级泵中。

2. 导叶

导叶的作用与蜗壳相同，多用于分段式多级泵中，按其结构形式,可分为径向式导叶和流

道式导叶。径向式导叶是由正向导叶、环形空间和反向导叶组成其结构如图1-61所示。 正向导叶内螺旋线AB部分是按照设计工况下液体的自由流动轨迹得出的，用于收集液体和保证液体在叶道中自由流动;而扩散段BC部分则用来把大部分动能转变为静压能;环形空间CD则用于改变液流方向。反向导叶DE的作用是消除旋转速度，并把液体在无预选条件下引入下一级叶轮进口。 导叶的叶片数与叶轮叶片数不应相等，一般为4~7片。
    流道式导叶如图1-62所示，其结构与径向式导叶基本相同，所不同的是径向式导叶从正向导叶出来的液体在环形空间内混合在一起，之后进入反向导叶。 而流道式导叶的正向导叶和反向导叶是铸在一起的 ,中间形成单独的小流道，各流道的液体不能混合,不易形成死角和突然扩散，速度变化比较均匀,水力性能较好,但结构复杂，制造工艺性差。
    与蜗壳相比，导叶具有外形尺寸较小通用性大和制造方便等特点.因为它可以用数量不同、尺寸相同的导叶组成叶轮尺寸相同的分段式多级泵。但是采用蜗壳作为转能装置的中开

式多级泵，具有安装、检修方便和泵的高效率区较宽的优点；而导叶作为转能装置的分段式多级泵，安装、检修不方便，高效率区较窄。因为在偏离设计工况时，液流对每个叶片都会产生冲击损失，而在蜗壳中只有一个隔舌，所以导叶式泵的H-Q及n-Q性能曲线均比蜗亮泵要陡,平均效率也较低。
3.吸入室
    吸入室位于叶轮前,其作用是将液体以最小的损失均匀地引入叶轮。吸入室有锥形管吸入室螺旋形吸入室和圆形吸入室三种形式。
    (1)锥形管吸入室用于小型单级单吸悬臂式离心泵中,其结构简单、 制造方便，如图1- 63(a)所示。在叶轮入口前使液流造成集流和加速度， 流速均匀，损失较小。
    (2)螺旋形吸入室如图1- 63(b)所示，流动情况较好，速度比较均匀,但液流进入叶轮前有预旋,在一定程度上会降低扬程 ,对低比转速泵,这种影响不明显。目前，我国悬臂式离心油泵和中开式多级蜗壳泵都采用这种吸入室。
    (3)圆形吸入室如图1- 63(o)所示,结构简单，轴向尺寸短,但液流进入渣浆泵厂家叶轮前有撞击和旋涡损失，液流也不太均匀，常用于多级分段式离心泵中。

Energy transfer device for volute, guide vane and suction chamber of slurry pump

The volute and guide vane are the energy conversion devices of centrifugal pumps. Their functions are to collect the liquid thrown out from the impeller, reduce the liquid flow rate, change part of the speed energy head into the pressure energy head, and then all lead into the next stage to discharge through the diffusion tube.

1. volute

The shape of the volute is usually made according to the relative movement of the liquid in the impeller under the design flow of the pump. After leaving the impeller, it is not affected by the external force, and it is made according to its inertia as the path of free flow. When the liquid from the impeller is not affected by external forces (friction, etc.), the momentum moment of the liquid flow to the center of the rotating shaft remains unchanged

According to the fluid continuity equation, the radial velocity CRR at any radius r of volute (excluding the influence of blocking coefficient) is as follows:

If the width of the volute is constant, that is to say, when BR is a constant value, there are:

CRuR= constant

It can be seen that when the liquid flows freely in the parallel plate volute, the direction angle of its flow path is unchanged, which is a logarithmic helix, as shown in Figure 1-59. With the increase of radius r, the corresponding Cr and CE decrease, so the liquid velocity Cr and velocity head will also decrease, and gradually convert to static pressure energy. If we want to convert the velocity into the pressure energy, the liquid flow must go through a long distance, and the radial dimension must be too large. Therefore, the wrap angle of spiral is limited to no more than 360. In order to reduce the size of the minor diameter, according to the inverse ratio between the CRR and the width of the volute axial surface, the expanding axial surface width is used, as shown in Figure 1-60. In this way, the flow direction angle a is no longer constant, but decreases with the increase of radii R and 6R. However, the expansion angle 0 of the volute section should not be greater than 60 ° to avoid serious boundary layer separation due to the rapid expansion of the volute flow passage section. When the volute size is reduced, only a small part of the kinetic energy of the liquid in the helix changes into the static energy. For this reason, a diffuser is added at the end of the helix, its expansion angle is 8 ° ~ 12 °, and its length is 2.5 ~ 3.0 times of the diameter of the inlet section of the diffuser. 80% ~ 85% of kinetic energy can be converted into static energy in the diffuser.

The section of volute includes circle, rectangle and inverted trapezoid. Among them, circle section is used for high specific speed pump, inverted trapezoid section is used for medium specific speed pump and rectangle section is used for low specific speed pump.

Volute structure is mostly used in single-stage centrifugal pump and horizontal split multistage pump.

2. guide vane

The function of the guide vane is the same as that of the volute. It is mostly used in the segmented multistage pump. According to its structure, it can be divided into radial guide vane and flow

Channel guide vane. Radial guide vane is composed of forward guide vane, annular space and reverse guide vane. Its structure is shown in figure 1-61. Part ab of the helix in the forward guide vane is obtained according to the free flow path of the liquid under the design condition, which is used to collect the liquid and ensure the free flow of the liquid in the blade passage; part BC of the diffusion section is used to convert most of the kinetic energy into the static energy; and part CD of the annular space is used to change the direction of the liquid flow. The function of the reverse guide vane De is to eliminate the rotation speed and introduce the liquid into the inlet of the next stage impeller without preselection. The number of guide vane and impeller vane should not be equal, generally 4-7.

The runner guide vane is as shown in figure 1-62. Its structure is basically the same as the radial guide vane. The difference is that the liquid from the radial guide vane is mixed together in the annular space, and then enters the reverse guide vane. The forward guide vane and the reverse guide vane of the runner type guide vane are cast together, and a separate small runner is formed in the middle. The liquid in each runner cannot be mixed, and it is not easy to form dead angle and sudden diffusion. The speed change is relatively uniform, and the hydraulic performance is good, but the structure is complex, and the manufacturing process is poor.

Compared with the volute, the guide vane has the characteristics of small size, large universality and convenient manufacture, because it can be used to form a segmented multistage pump with the same impeller size with different number of guide vanes of the same size. But the spiral case is used as the middle opening of the energy conversion device

The multi-stage pump of type B has the advantages of convenient installation and maintenance as well as wide high efficiency area of the pump, while the segmented multi-stage pump of the guide vane as the energy conversion device is inconvenient for installation and maintenance, and the high efficiency area is narrow. Because of the impact loss of the liquid flow to each blade when it deviates from the design condition, and there is only one tongue in the volute, the H-Q and n-q performance curves of the guide vane pump are steeper than those of the bright volute pump, and the average efficiency is lower.

3. inhalation room

The suction chamber is located in front of the impeller, and its function is to introduce the liquid evenly into the impeller with the minimum loss. There are three types of suction chamber: spiral suction chamber and circular suction chamber.

(1) The conical tube suction chamber is used in a small single-stage single suction cantilever centrifugal pump, with simple structure and convenient manufacture, as shown in Fig. 1-63 (a). In front of the impeller inlet, the liquid flow causes the collection and acceleration, the flow velocity is uniform, and the loss is small.

(2) As shown in Fig. 1-63 (b), the spiral suction chamber has good flow condition and uniform speed, but there is pre rotation before the liquid flows into the impeller, which will reduce the head to a certain extent, and the effect on the low specific speed pump is not obvious. At present, cantilever centrifugal pump and split type multi-stage volute pump all use this kind of suction chamber.

(3) As shown in Fig. 1-63 (o), the circular suction chamber is simple in structure and short in axial dimension, but there is impact and vortex loss before the liquid flows into the impeller of slurry pump manufacturer, and the liquid flow is not uniform, so it is commonly used in multistage segmented centrifugal pump.