摘要：孔板流量計量表在電力行業(yè)里現(xiàn)場應(yīng)用資訊由優(yōu)秀的流量計、流量儀生產(chǎn)報價廠家為您提供。當發(fā)電廠汽水管道不需要根據(jù)系統(tǒng)的要求調(diào)節(jié)不同壓力，但管道的前后壓差較大時，往往采用增加節(jié)流孔板的方式，其原理是：流體在管道中流動時，由于孔板的局部阻力，使得流體。更多的流量計廠家選型號價格報價歡迎您來電咨詢，下面是孔板流量計量表在電力行業(yè)里現(xiàn)場應(yīng)用文章詳情。

當發(fā)電廠汽水管道不需要根據(jù)系統(tǒng)的要求調(diào)節(jié)不同壓力，但管道的前后壓差較大時，往往采用增加節(jié)流孔板的方式，其原理是：流體在管道中流動時，由于孔板的局部阻力，使得流體的壓力降低，能量損耗，該現(xiàn)象在熱力學上稱為節(jié)流現(xiàn)象。該方式比采用調(diào)節(jié)閥要簡單，但必須選擇得當，否則，液體容易產(chǎn)生汽蝕現(xiàn)象，影響管道的安全運行。

一、汽蝕現(xiàn)象

節(jié)流孔板流量計的作用，就是在管道的適當?shù)胤綄⒖讖阶冃。斠后w經(jīng)過縮口，流束會變細或收縮。流束的zui小橫斷面出現(xiàn)在實際縮口的下游，稱為縮流斷面。在縮流斷面處，流速是zui大的，流速的增加伴隨著縮流斷面處壓力的大大降低。當流束擴展進入更大的區(qū)域，速度下降，壓力增加，但下游壓力不會完全恢復到上游的壓力，這是由于較大內(nèi)部紊流和能量消耗的結(jié)果。如果縮流斷面處的壓力pvc降到液體對應(yīng)溫度下的飽和蒸汽壓力pv以下，流束中就有蒸汽及溶解在水中的氣體逸出，形成蒸汽與氣體混合的小汽泡，壓力越低，汽泡越多。如果孔板下游的壓力p2仍低于液體的飽和蒸汽壓力，汽泡將在下游的管道繼續(xù)產(chǎn)生，液汽兩相混合存在，這種現(xiàn)象就是閃蒸。如果下游壓力恢復到高于液體的飽和蒸汽壓力，汽泡在高壓的作用下，迅速凝結(jié)而破裂，在汽泡破裂的瞬間，產(chǎn)生局部空穴，高壓水以極高的速度流向這些原汽泡占有的空間，形成一個沖擊力。由于汽泡中的氣體和蒸汽來不及在瞬間全部溶解和凝結(jié)，在沖擊力作用下又分成小汽泡，再被高壓水壓縮、凝結(jié)，如此形成多次反復，并產(chǎn)生一種類似于我們可以想象的砂石流過管道的噪音，此種現(xiàn)象稱為空化。流道材料表面在水擊壓力作用下，形成疲勞而遭到嚴重破壞。我們把汽泡的形成、發(fā)展和破裂以致材料受到破壞的全部過程稱為汽蝕現(xiàn)象。

閃蒸和空化的主要區(qū)別在于汽泡是否破裂。存在閃蒸現(xiàn)象的系統(tǒng)管道，由于介質(zhì)為汽水兩相流，介質(zhì)比容和流速成倍增加，沖刷表面磨損相當厲害，其表現(xiàn)為沖刷面有平滑拋光的外形。閃蒸也產(chǎn)生噪音和振動，但其聲級值一般為80 dB以下，不超出規(guī)范規(guī)定的許可范圍?？栈瘎t不然，汽泡破裂和高速沖擊會引起嚴重的噪音，管道振動大，在流道表面極微小的面積上，沖擊力形成的壓力可高達幾百甚至上千兆帕，沖擊頻率可達每秒幾萬次，在短時間內(nèi)就可能引起沖刷面的嚴重損壞，其表現(xiàn)為沖刷面會產(chǎn)生類似于煤渣的粗糟表面。而且，由液體中逸出的氧氣等活性氣體，借助汽泡凝結(jié)時放出熱量，也會對金屬起化學腐蝕作用。

不管是閃蒸還是空化，都會對管道造成不同程度的損害，對安全運行均是不利的，因此，選擇節(jié)流孔板流量計時應(yīng)避免這兩種情況的發(fā)生。由于孔板下游的壓力往往高于液體的飽和蒸汽壓力，因此，選擇節(jié)流孔板時，zui主要是防止空化的產(chǎn)生。

二、防止流體產(chǎn)生汽蝕的方法

對于汽蝕，沖刷面換用高級材料不是徹底解決問題的辦法，控制縮流斷面處的壓力pvc，保持該壓力不低于液體的飽和蒸汽壓力pv，才是防止汽蝕產(chǎn)生的一項根本措施。對于壓降較大的管道，可通過多級降壓，確保介質(zhì)經(jīng)過每一個縮流斷面時壓力都大于液體的飽和蒸汽壓力。

三、節(jié)流孔板流量計壓差的計算

為了計算節(jié)流孔板的壓差，需引入一個新的概念——阻塞流壓差Δps。當孔板兩端的壓差Δp增加時，流量qm也增加，當壓差Δp增大到一定值時，縮口處的壓力pvc下降到流體飽和蒸氣壓力pv以下，一部分流體汽化，管道流量不再隨壓差增加而增加，即形成所謂阻塞流現(xiàn)象。此時，孔板兩端的壓差稱為阻塞流壓差Δps。當節(jié)流孔板的實際壓差Δp小于其對應(yīng)的Δps時，就可避免閃蒸或汽蝕的發(fā)生。當管道兩端壓差較大時，可采用多級減壓，但每一級節(jié)流孔板流量計的實際壓差Δp均應(yīng)小于本級入口對應(yīng)的Δps。

根據(jù)文獻[1]，多級節(jié)流孔板的的壓降按幾何級數(shù)遞減，當?shù)?級節(jié)流孔板實際壓降為Δp1時，第2級孔板減壓至Δp1/2，第3級孔板減壓至Δp1/22，第4級孔板減壓至Δp1/23，……，第n+1級孔板減壓至Δp1/2n，直減到末級孔板后壓力接近所需壓力為止。

以臺山電廠凝補泵再循環(huán)管為例，在機組運行過程中，發(fā)現(xiàn)管道振動大。分析原因為：凝補泵在正常運行時，出口壓力約1.5 MPa，補給水箱進口處的壓力約0.12 MPa，當泵出口的除鹽水經(jīng)再循環(huán)管回流至補給水箱時，由于壓差較大，且管道上只裝了一個電動閘閥而非調(diào)節(jié)閥，因此引起振動。為了減少振動，在*次設(shè)計變更中，采用增加節(jié)流孔板的方式，實際運行后，泵出口的管道振動有所改善，但節(jié)流孔板后的管道出現(xiàn)汽蝕現(xiàn)象。說明靠增加節(jié)流孔板來進行降壓的思路是對的，但孔板的選擇應(yīng)有所調(diào)整。

3.1孔板級數(shù)的確定

考慮管道受靜壓差44.012 kPa的影響，孔板兩端zui大壓差

式（1）至式（3）中：

p1——孔板入口壓力；

pc——熱力學臨界壓力，對于水，pc=22.5 MPa；

FL——液體壓力恢復系數(shù)，暫定為0.9；

FF——臨界壓力比系數(shù)。

由于p1=1.5 MPa，p2=0.165 MPa，20 ℃時pv=2.338 5 kPa，根據(jù)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級節(jié)流孔板將產(chǎn)生汽蝕現(xiàn)象。為了避免汽蝕的發(fā)生，至少應(yīng)裝2級節(jié)流孔板。

3.2孔板流量計壓降的確定

根據(jù)前面的分析，當采用1級節(jié)流時，孔板壓差大于阻塞流壓差，采用多級節(jié)流降壓后，第1級節(jié)流孔板的實際壓差應(yīng)小于阻塞流壓差，其壓差的大小取決于第2級孔板，多級節(jié)流孔板的壓降按幾何級數(shù)遞減。因此，若采用2級節(jié)流孔板，則



其中Δp1=0.89 MPa，Δp2=Δp1/2=0.445 MPa。

為了防止節(jié)流孔板發(fā)生汽蝕，應(yīng)以阻塞流壓差Δps為準則，驗算各級節(jié)流孔板壓差：*級孔板的阻塞流壓差Δps1=1.213 MPa＞Δp1；第二級孔板的阻塞流壓差Δps2=0.92×[（1.5－0.89）MPa－0.957×0.002 338 5MPa=0.492 3 MPa＞Δp2。因此，每級節(jié)流孔板后都不會出現(xiàn)汽蝕現(xiàn)象，采用2級節(jié)流孔板是合理的。

四、節(jié)流孔板孔徑的計算

根據(jù)DL/T 5054—1996《火力發(fā)電廠汽水管道設(shè)計技術(shù)規(guī)定》，水管道節(jié)流孔板孔徑可按下式計算：

（4）

式中：dk——節(jié)流孔板的孔徑；

ρ——水的密度。

仍以臺山電廠凝補泵再循環(huán)管為例。根據(jù)現(xiàn)場的實際運行數(shù)據(jù)，正常運行時熱井的補水量約20 t/h，泵出口壓力約1.5 MPa，扣除泵進口壓力，揚程約134 m，查性能曲線，對應(yīng)的流量為136.8 t/h，即經(jīng)再循環(huán)管回流至補給水箱的除鹽水量約116 t/h。根據(jù)式（4）得：第1級節(jié)流孔板孔徑dk1=40.68 mm，取40.7 mm；第2級節(jié)流孔板孔徑dk2=48.37 mm，取48.5 mm。

在該管道的*次設(shè)計變更時，流量按常規(guī)泵的再循環(huán)量（zui大流量的30%）選取，取60 t/h，且壓降沒按幾何級數(shù)遞減考慮，兩級孔板孔徑均為33 mm。根據(jù)實際運行情況，經(jīng)再循環(huán)管回流至補給水箱的除鹽水量應(yīng)約116 t/h，但由于節(jié)流孔板的限流作用，流經(jīng)再循環(huán)管的水量zui大只能是第2級節(jié)流孔板阻塞流時的流量。因第2級節(jié)流孔板后的壓力大于液體的飽和蒸汽壓力，故第2級節(jié)流孔板后出現(xiàn)汽蝕現(xiàn)象，管道產(chǎn)生較大振動和噪音。

五、結(jié)束語

在實際工程應(yīng)用中，將多級節(jié)流孔板用于減壓系統(tǒng)是切實可行的，為了防止管道發(fā)生汽蝕，選擇節(jié)流孔板流量計時，一定要根據(jù)管道的實際情況，計算出孔板數(shù)量和孔徑.

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,

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