引风机轴向振动高原因探讨

引风机轴向振动高原因探讨
北仑发电厂(浙江宁波　315800)　谢　澄
[摘　要]　通过分析引风机轴承轴向不同位置振动幅值的差异和轴承刚度计算式,认为轴向振动高的原因是由于风机基础沉降引起的轴承单头扬起所致,给出了处理方法。

[关键词]　引风机　轴承　轴向振动　轴承刚度
1　结构型式
北仑电厂2号炉引风机是由加拿大NOVE NC O公司制造的双吸、双速、离心式风机,高速590r/min,低速490r/min;对应轴功率2307kW～1357kW;风量由进口挡板调节;驱动电机由日本FU J I公司制造,电机极数为10/12极的感应异步电动机;联轴器为弹性蛇形弹簧连接,中间用橡胶块分隔定位。

风机的轴承固定在独立的轴承座上,形式为圆筒瓦,其中联轴器侧的轴承是支承、推力联合轴承。

润滑油是通过油环把轴承室内的油甩到轴承上,再用闭式循环冷却水冷却轴承室内的润滑油。

2　存在问题
1997年6月份,在一次试运转的过程中,发现引风机A联轴器侧轴承的轴向振动比以前大许多,当时检测用的是手持式振动检测仪(成都产),风机自由侧轴承的轴向振动也比以前大,但风机轴承其它几个方向的振动变化并不大。

在以后的正常运转中用同一测振仪又检测了几次,情况相差无几。

针对这种情况,用另外的振动数采仪对其进行了几次检测,得到的情况与成都产检测仪检测情况一样,也即轴向振动有变化,各道轴承的其它方向振动无多大改变,只是偶然有些升降,当属正常。

对引风机A 的二个轴承的各个结合面的三个方向进行了检测,联轴器处轴承检测点在锅炉侧,各点位置见上图(轴承两侧完全对称);自由端测点在烟囱侧,测点位置一样。

当时,机组负荷500多MW,基本接近满负荷,具体数据见表1。

表1　轴承振动数值表
位　置123456水
平
速度值/mm·s-10.480.530.960.97 1.60 1.90位移值/μm31.632.743.047.865.081.0加速度值/mm·s-20.0180.0400.051垂
直
速度值/mm·s-10.140.590.670.530.630.30位移值/μm8.6622.829.922.330.710.6加速度值/mm·s-20.0340.028轴
向
速度值/mm·s-1不能测0.48 1.56 1.76 2.7 3.65位移值/μm不能测25.970.385.6145.8157.0加速度值/mm·s-2不能测0.0700.0500.0500.0500.068表1为风机联轴器侧的轴承振动数值,自由侧的轴承振动比联轴器侧小得多,轴向振动也比水平振动小。

表1所列的数据均为通频值,工频是其主要的分量,另外各点尚有100H z、715H z、815H z和915H z的振动信号,高频成分虽有,但值很小。

“不能测”,是指振动探头放不进去。

运行中的轴承金属温度和回油温度正常,联轴器中的定位橡胶块已去掉。

从这时起的较长时间内,机组的负荷基本保持不变。

3　原因分析
由表1可见,垂直向各点振动的速度值、位移值均不大,且差别不大,可以确信各接触面之间连接牢固,各个连接螺栓强度足够;水平向的振动幅值变化也比较平缓,只是在测点5和6处,位移值增加了十几μm,因两点高度相差大,且是轴承座的中分面和顶部,当属正常;比较轴向位置各点振动,点2和点3高差近500
　经验交流
热力发电·2000(3)π∼　
mm ,但振动值相差达44μm ,几乎是2.8倍。

同样,点4
和点5高差约300mm ,振动值相差达60mm 。

而点3和点4,高差虽然只有60mm ,但振动值相差竟达15μm 。

点3和点4振动的加速度值并不大,在顶部也只有0.068mm/s 2,说明振动的冲击力不大。

由1997年10月的轴向振动的频谱图和时域图可见(图1),振动主频为工频,其余高频和低频与之相比极小。

时域图上振动冲击呈现周期性,夹杂一些不稳定的冲击。

图上还有一个较为明显的特征是正负幅值不对称,这显示冲击来自单向,而电机联轴器侧轴承比风机联轴器侧轴承的轴向振动加速度值小得多。

至此,完全可以相信振源不是来自于电机,
而来自于风机。

图1　上为频谱图、下为时域波形图
对于这么小的的冲击却引起如此大的振动。

根据轴颈承力中心沿轴向周期性变化引起轴承轴向振动的
单振幅α计算式
α=h b (2Q +2P 3K a -Q +P 3K b
)
式中　b ———轴承座轴向宽度;
h ———轴承座高度;P ———激振力;Q ———轴瓦载荷;
K a 、K b ———轴承座两侧的支承动刚度。

可以看出轴承座的轴向振幅与轴承座的高度成正比,与宽度成反比,也就是高度越高,稳定性越差。

轴承座越窄,稳定性也越差。

轴承座要是“瘦高个”,其支
承动刚度就更低,它的稳定性更差。

与北仑电厂1号炉引风机比较,其h/b 的值比2A 小一半。

轴承座宽度大了,支承动刚度K a 、K b 也就随之增大。

因此,尽管1号炉引风机轴承振动经常高,但其轴向振动一直不大,说明外形结构是很重要的一个因素。

在轴承座的轴向支承动刚度不够大的情况下,当外界有轻微的冲击扰动时,轴承座就会有剧烈的轴向振动。

此风机进口为双吸,风量两侧不对称的现象可不作考虑。

但是,如果轴承与轴的中心不正或前后两个轴承座的中心标高有差异的话,这样轴承座表面上看是刚性的,实际上仍有点弹性。

若转子有轻微的弯曲或轴向不对称位置不平衡,就会在轴承座上产生一个周期性的冲击。

此外,两个轴承座中心标高有差异的话,也会有一个单向的冲击力,图1的时域波形图即符合这种情况。

风机联轴器侧的轴承承担了风机的轴向冲击,所以其轴向振动远高于另一侧。

4　处理方法
1998年6月在2号机组中修时,对引风机的轴承
和风机内部进行了检查,确认轴承受力情况与叶轮均无问题。

但在检查轴承的扬度时却发现风机外侧的转子单头扬起,按理这么重的风机在正常情况下,依靠其自身的重量,中间垂下,两头上扬,等其正常运转时,转子可保持水平。

而目前单头1.7mm/m 的扬度足以产生较大的轴向力,使轴向发生大的振动,两台引风机均有类似的扬度问题。

经分析,产生如此大的扬度不是因为安装质量问题和检修质量问题,而是由于基础的沉降引起。

北仑电厂是建在海涂上,近几年沉降问题比较突出。

所以随着时间的推移和沉降量的积累,风机轴向振动逐渐表露出来。

对此处理可在联轴器侧的轴承座下面加垫片;也可通过刨削风机外侧轴承座台板来使其达到转子扬度要求。

参　考　文　献
1　施维新.汽轮发动机组振动.水利电力出版社,1991
2　袁宏义,牛明忠等.设备振动诊断技术基础.国防工业出版
社,1991
作者简介　谢　澄,1988年毕业于上海电力学院,现在浙江北仑发电
厂专职从事汽轮发电机组状态监测工作。

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