孔板流量计量表在电力行业里现场应用

当发电厂汽水管道不需要根据系统的要求调节不同压力，但管道的前后压差较大时，往往采用增加节流孔板的方式，其原理是：流体在管道中流动时，由于孔板的局部阻力，使得流体的压力降低，能量损耗，该现象在热力学上称为节流现象。该方式比采用调节阀要简单，但必须选择得当，否则，液体容易产生汽蚀现象，影响管道的安全运行。

一、汽蚀现象

节流孔板流量计的作用，就是在管道的适当地方将孔径变小，当液体经过缩口，流束会变细或收缩。流束的zui小横断面出现在实际缩口的下游，称为缩流断面。在缩流断面处，流速是zui大的，流速的增加伴随着缩流断面处压力的大大降低。当流束扩展进入更大的区域，速度下降，压力增加，但下游压力不会完全恢复到上游的压力，这是由于较大内部紊流和能量消耗的结果。如果缩流断面处的压力pvc降到液体对应温度下的饱和蒸汽压力pv以下，流束中就有蒸汽及溶解在水中的气体逸出，形成蒸汽与气体混合的小汽泡，压力越低，汽泡越多。如果孔板下游的压力p2仍低于液体的饱和蒸汽压力，汽泡将在下游的管道继续产生，液汽两相混合存在，这种现象就是闪蒸。如果下游压力恢复到高于液体的饱和蒸汽压力，汽泡在高压的作用下，迅速凝结而破裂，在汽泡破裂的瞬间，产生局部空穴，高压水以极高的速度流向这些原汽泡占有的空间，形成一个冲击力。由于汽泡中的气体和蒸汽来不及在瞬间全部溶解和凝结，在冲击力作用下又分成小汽泡，再被高压水压缩、凝结，如此形成多次反复，并产生一种类似于我们可以想象的砂石流过管道的噪音，此种现象称为空化。流道材料表面在水击压力作用下，形成疲劳而遭到严重破坏。我们把汽泡的形成、发展和破裂以致材料受到破坏的全部过程称为汽蚀现象。

闪蒸和空化的主要区别在于汽泡是否破裂。存在闪蒸现象的系统管道，由于介质为汽水两相流，介质比容和流速成倍增加，冲刷表面磨损相当厉害，其表现为冲刷面有平滑抛光的外形。闪蒸也产生噪音和振动，但其声级值一般为80 dB以下，不超出规范规定的许可范围。空化则不然，汽泡破裂和高速冲击会引起严重的噪音，管道振动大，在流道表面极微小的面积上，冲击力形成的压力可高达几百甚至上千兆帕，冲击频率可达每秒几万次，在短时间内就可能引起冲刷面的严重损坏，其表现为冲刷面会产生类似于煤渣的粗糟表面。而且，由液体中逸出的氧气等活性气体，借助汽泡凝结时放出热量，也会对金属起化学腐蚀作用。

不管是闪蒸还是空化，都会对管道造成不同程度的损害，对安全运行均是不利的，因此，选择节流孔板流量计时应避免这两种情况的发生。由于孔板下游的压力往往高于液体的饱和蒸汽压力，因此，选择节流孔板时，zui主要是防止空化的产生。

二、防止流体产生汽蚀的方法

对于汽蚀，冲刷面换用高级材料不是彻底解决问题的办法，控制缩流断面处的压力pvc，保持该压力不低于液体的饱和蒸汽压力pv，才是防止汽蚀产生的一项根本措施。对于压降较大的管道，可通过多级降压，确保介质经过每一个缩流断面时压力都大于液体的饱和蒸汽压力。

三、节流孔板流量计压差的计算

为了计算节流孔板的压差，需引入一个新的概念——阻塞流压差Δps。当孔板两端的压差Δp增加时，流量qm也增加，当压差Δp增大到一定值时，缩口处的压力pvc下降到流体饱和蒸气压力pv以下，一部分流体汽化，管道流量不再随压差增加而增加，即形成所谓阻塞流现象。此时，孔板两端的压差称为阻塞流压差Δps。当节流孔板的实际压差Δp小于其对应的Δps时，就可避免闪蒸或汽蚀的发生。当管道两端压差较大时，可采用多级减压，但每一级节流孔板流量计的实际压差Δp均应小于本级入口对应的Δps。

根据文献[1]，多级节流孔板的的压降按几何级数递减，当第1级节流孔板实际压降为Δp1时，第2级孔板减压至Δp1/2，第3级孔板减压至Δp1/22，第4级孔板减压至Δp1/23，……，第n+1级孔板减压至Δp1/2n，直减到末级孔板后压力接近所需压力为止。

以台山电厂凝补泵再循环管为例，在机组运行过程中，发现管道振动大。分析原因为：凝补泵在正常运行时，出口压力约1.5 MPa，补给水箱进口处的压力约0.12 MPa，当泵出口的除盐水经再循环管回流至补给水箱时，由于压差较大，且管道上只装了一个电动闸阀而非调节阀，因此引起振动。为了减少振动，在*次设计变更中，采用增加节流孔板的方式，实际运行后，泵出口的管道振动有所改善，但节流孔板后的管道出现汽蚀现象。说明靠增加节流孔板来进行降压的思路是对的，但孔板的选择应有所调整。

3.1孔板级数的确定

考虑管道受静压差44.012 kPa的影响，孔板两端zui大压差

式（1）至式（3）中：

p1——孔板入口压力；

pc——热力学临界压力，对于水，pc=22.5 MPa；

FL——液体压力恢复系数，暂定为0.9；

FF——临界压力比系数。

由于p1=1.5 MPa，p2=0.165 MPa，20 ℃时pv=2.338 5 kPa，根据式（1）至式（3），得Δp=1335 MPa，Δps=1213 MPa。由于Δp＞Δps，且p2＞pv，所以采用1级节流孔板将产生汽蚀现象。为了避免汽蚀的发生，至少应装2级节流孔板。

3.2孔板流量计压降的确定

根据前面的分析，当采用1级节流时，孔板压差大于阻塞流压差，采用多级节流降压后，第1级节流孔板的实际压差应小于阻塞流压差，其压差的大小取决于第2级孔板，多级节流孔板的压降按几何级数递减。因此，若采用2级节流孔板，则



其中Δp1=0.89 MPa，Δp2=Δp1/2=0.445 MPa。

为了防止节流孔板发生汽蚀，应以阻塞流压差Δps为准则，验算各级节流孔板压差：*级孔板的阻塞流压差Δps1=1.213 MPa＞Δp1；第二级孔板的阻塞流压差Δps2=0.92×[（1.5－0.89）MPa－0.957×0.002 338 5MPa=0.492 3 MPa＞Δp2。因此，每级节流孔板后都不会出现汽蚀现象，采用2级节流孔板是合理的。

四、节流孔板孔径的计算

根据DL/T 5054—1996《火力发电厂汽水管道设计技术规定》，水管道节流孔板孔径可按下式计算：

（4）

式中：dk——节流孔板的孔径；

ρ——水的密度。

仍以台山电厂凝补泵再循环管为例。根据现场的实际运行数据，正常运行时热井的补水量约20 t/h，泵出口压力约1.5 MPa，扣除泵进口压力，扬程约134 m，查性能曲线，对应的流量为136.8 t/h，即经再循环管回流至补给水箱的除盐水量约116 t/h。根据式（4）得：第1级节流孔板孔径dk1=40.68 mm，取40.7 mm；第2级节流孔板孔径dk2=48.37 mm，取48.5 mm。

在该管道的*次设计变更时，流量按常规泵的再循环量（zui大流量的30%）选取，取60 t/h，且压降没按几何级数递减考虑，两级孔板孔径均为33 mm。根据实际运行情况，经再循环管回流至补给水箱的除盐水量应约116 t/h，但由于节流孔板的限流作用，流经再循环管的水量zui大只能是第2级节流孔板阻塞流时的流量。因第2级节流孔板后的压力大于液体的饱和蒸汽压力，故第2级节流孔板后出现汽蚀现象，管道产生较大振动和噪音。

五、结束语

在实际工程应用中，将多级节流孔板用于减压系统是切实可行的，为了防止管道发生汽蚀，选择节流孔板流量计时，一定要根据管道的实际情况，计算出孔板数量和孔径.

英文版本：

When the steam pipeline of power plant without the need to adjust the pressure according to the requirement of the system, but the pipe before and after the pressure is large, often by increasing the throttle plate method, the principle is: the fluid flow in the pipeline, because of local resistance orifice plate, the fluid pressure is reduced, the energy loss, this phenomenon in thermodynamics known as the throttling phenomena. The way to simple than the regulating valve, but must be chosen properly, otherwise, easy liquid cavitation phenomenon, affecting the safety of pipeline operation. A, cavitation Orifice plate flowmeter role, is the appropriate place in the pipeline will be larger, when the liquid through the necking, beam will become thin or contraction. The minimum cross section beam in practical necking downstream, called the vena contracta. In the contraction flow section, velocity of flow is the biggest, the velocity increases with reduced greatly reduced flow cross section pressure at. When the beam expansion into larger areas, rate of decline, the increase of pressure, but the downstream pressure will not fully recover to the upstream pressure, this is because the turbulent large internal and energy consumption results. If the pressure PVC reduced flow cross section at the liquid temperature corresponding to the saturated steam pressure below PV, beam have steam and gases dissolved in water to form a mixed steam and gas escape, small bubble, the lower the pressure, the bubble number. If the pressure P2 downstream of the orifice plate is lower than the saturation vapor pressure of the liquid, the bubble will continue to produce in the lower reaches of the pipeline, liquid vapor two-phase mixture, this phenomenon is called flash. If the downstream pressure recovery than the saturation vapor pressure of the liquid, the bubble under high pressure, the rapid condensation and rupture, the bubble burst, cause local hole, high pressure water at high speed to the original bubble occupied space, forming a shock force. Because of the gas bubbles in steam and steam can not be compley dissolved in the moment and condensation, under the action of the impact force is divided into small bubbles, then condensation compression, high pressure water, thus formed repeatedly, and produce a sand flow through the pipeline is similar to we can imagine the noise, this kind of phenomenon called cavitation. The channel material surface under water hammer pressure, the formation of fatigue suffered serious damage. We put all the process of bubble formation, growth and rupture of material damage is called cavitation phenomenon. The main difference of flashing and cavitation is the bubble is broken. System pipeline has flash phenomenon, because the media is the steam water two-phase flow, medium volume and flow rate increase exponentially, scour surface wear quite powerful, its performance for the erosion surface smooth polished appearance. Flash also generate noise and vibration, but its level value is generally below 80 dB, do not exceed the specified scope. Cavitation is not, the bubble burst and high speed impact will cause severe noise, pipeline vibration, in the area of channel surface extremely small, the impact force formation pressure can be up to hundreds or even GPA, impact frequency can reach tens of thousands of times per second, in a short period of time may cause serious damage to the erosion surface, as the erosion surface will produce similar to a cinder rough surface. And, by the liquid oxygen and other reactive gases to escape, with the aid of bubble condensation heat release, also on the metal chemical corrosion. Whether the flash and cavitation, will cause varying degrees of damage to the pipeline, on the safe operation of all is negative, therefore, selection of throttle orifice plate flowmeter should avoid these two situations. Because the pressure downstream of the orifice plate is often higher than the saturation vapor pressure of the liquid, therefore, selection of throttle orifice, the most main is to prevent cavitation. Two, to prevent fluid cavitation For cavitation, erosion surface for advanced materials is not compley solve the problem, the control pressure PVC contraction section, keeping the pressure of not less than the saturated vapor pressure of liquid PV, is a fundamental measure to prevent cavitation. For the pressure drop of pipeline, through multistage depressed, to ensure that the media after each contraction section when the pressure is higher than the saturation vapor pressure of the liquid.  Three, to calculate the orifice flow meter pressure difference In order to calculate the orifice plate differential pressure, the need to introduce a new concept -- blocking flow pressure difference Δ PS. When the orifice plate at both ends of the pressure difference Δ P increases, the flow of QM also increases, when the pressure difference Δ P increases to a certain value, the pressure of PVC shrinkage at the mouth of the fluid saturated vapor pressure drops to below PV, a portion of the fluid is vaporized, pipe flow is no longer with the pressure difference increases, the formation of the so-called blocking flow phenomenon. At this time, the orifice plate at both ends of the pressure difference is called the pressure difference Δ PS blocking flow. When Δ PS actual pressure of throttle orifice difference Δ P is smaller than its corresponding, can avoid the occurrence of flash and cavitation. When the pipe at both ends of the pressure difference is larger, the multistage decompression, Δ PS but the actual pressure at every level of throttle orifice plate flowmeter difference Δ P should be less than the corresponding e According to [1], the pressure drop of multistage orifice of geometrically decreasing, when the first orifice actual pressure drop of Δ P1, class second hole plate decompression to delta p1/2, class third hole plate decompression to delta p1/22, class fourth hole plate decompression to delta p1/23,...... The n+1 class hole plate, pressure to p1/2n, straight down to the final orifice pressure close to the required pressure Taking Taishan power plant condensate make-up pump recirculation pipe as an example, in the operation process, found that the pipeline vibration. Analysis of the reasons for setting up: Pump in normal operation, the outlet pressure of about 1.5 MPa, feed water tank inlet pressure of about 0.12 MPa, when the pump outlet in addition to return after recirculation pipe brine to supply water tank, due to the pressure difference,