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关于轴封供汽的干货知识

2024-04-15

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一、轴封汽压力、流量的确定原则:

1、保证轴封汽不外泄。轴封汽发生外泄的主要原因是轴封供汽压力太高、轴封回汽不畅(如轴封加热器真空偏低、有轴封回汽阀的机组其回汽阀未调整好),轴封汽外泄的危害主要有:

1)、轴承箱进汽、水,从而使润滑油受到污染;

2)、污染空气(热污染和化学污染)。

2、保证空气不 内吸。发生空气内吸的主要原因是轴封供汽压力太低或轴封母管卸荷阀误动,空气内吸的危害有如下几方面:

1)、对负压(真空)系统来说,使真空不能正常建立或得以维持;

2)、在热态、极热态开机的低负荷阶段,低温的空气经高温轴封段而吸入汽缸,使高温轴封段急剧冷却、高温汽缸也以较高温降速率进行冷却。这种冷却的后果有两方面。一是在转子高温轴封段产生巨大的热应力,严重时将导致大轴弯曲;二是有可能造成汽缸内外壁负温差及上下缸温差超限,使汽缸发生不同程度的变形。总之,在热态、极热态开机的低负荷阶段空气内吸将使高温金属产生不同程度的热应力,使汽机寿命遭到额外消耗;

3)、在停机的低负荷阶段,空气内吸同样将使高温金属产生不同程度的热应力,使汽机寿命遭到额外消耗。

4)、无论在何种工况下,空气内吸均将导致高压、中压胀差的缩小。这在缸温较高的情况下尤应引起注意;

5)、空气内吸还有可能将外部的杂物(如保温材料)吸入轴封腔室,严重时吸入的杂物会嵌在动静轴封梳齿片间引起机组的振动;

因此,轴封汽压力的选择或确定应保证轴封汽不发生外泄和内吸。

二、轴封汽温度的确定原则:

对高、中压缸轴封而言,国内的一般要求是:根据汽机调节级内缸内上壁金属温度选择轴封汽温度,轴封汽温度与调节级内缸内上壁金属温度基本相一致为宜。

但是,由于系统设计的原因,这在热态、极热态启动时难以做到。尽管本公司各机组设计有轴封电加热器,东芝机组的系统还设计有主蒸汽供轴封汽子系统,可用主蒸汽与辅汽混温供轴封汽。事实上,要在热态、极热态启动时达到轴封汽温度与调节级内缸内上壁金属温度相一致是难以做到或者说根本无法做到。

轴封汽温过高,对高、中压轴封而言(尤其是机组冷态启动),不仅仅是产生额外的热应力,而且可能使机组胀差控制发生困难。

轴封汽温过低对高、中压轴封而言是一个冷却过程,既有热应力的产生,也有使高、中压胀差缩小的作用,这在热态、极热态启动时尤应注意。

对低压缸轴封而言,一般认为对应轴封压力的蒸汽具有20℃~40℃的过热度即可。低压轴封汽温度过高,易引起该区域金属过度膨胀、与低压缸为一体的轴承座中心线发生变化,情况严重时将导致机组发生异常振动。东芝机组的轴封蒸汽降温措施是:从轴封母管来的较高温度的蒸汽经低压缸内部饱和蒸汽冷却,再送至各低压轴封段。阿尔斯通的设计是使用减温水直接降温,这种设计在许多国家是不被许可的。其理由是一旦减温器控制故障,极易发生低压轴封带水、进而使低压末级叶片的安全受到威胁。

(来源:网络)

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