Abstract 摘要 Inrecentyears，the control valve industry has seen an important debateaboutthe validity of limiting the valve trim exit velocity head toamaximum of 480 kPa in gas and steam applications。Thisvelocitylimitation is assumed to provide an acceptable noise levelandavoid problems that arise in control valve gas andsteamapplications。However，in a very large numberofapplications，adopting a velocity limiting approach may requiretheuse of expensive multi-stage or multi-turn trim designs。Thepurposeof this article is to demonstrate that low noise levels canbeachieved without following this overly conservative andexpensivetrim exit velocity head limitation。Also，this article willshow thathaving a trim exit velocity head lower than 480 kPa willstillgenerate a very high valve noise level if the valve outletMachnumber is high。
近年來，控制閥行業(yè)出現關于氣體或蒸汽應用中限制*高閥門內件節(jié)流出口速度頭小于480KPa有效性的爭論。它被假定為可以提供可接受的聲壓級，避免在氣體或蒸汽應用中出現控制閥難題。然而，在大量的應用中采用限制速度頭的做???需要采用保守昂貴的多級或多回路設計。這篇文章的目的是要說明低聲壓級可以在不用昂貴的速度頭限制方式下實現。另外，這篇文章顯示在流體速度頭低于480KPa，但閥門出口流體速度馬赫數較高的情況下仍會產生相當高的聲壓級。
Noise Prediciton inControlValves 控制閥噪音預測
TheIEC534-8-3 Aerodynamic Noise Standard is the most widely used inthevalve industry for noise prediction in control valves。Thisstandardcalculates the stream power and the acoustical efficiencyfactorsat various flow regimes。IEC 534-8-3 identifies five flowregimesthat are determined by the relationship between variouspressureparameters。This standard also provides methods to calculatetheinternal sound pressure，its corresponding peakfrequency，thetransmission loss，and the A-weighted noise levelat 1 mdownstream of the valve and 1 m away from the pipe wall。It isveryimportant to note that this IEC standard does notrecommendlimiting the trim exit velocity head to 480 kPa，in orderto achievea low noise level in the valve。
IEC534-8-3空氣動力學噪聲標準是控制閥領域使用的*廣的工業(yè)閥門噪聲預測標準。這一標準計算流體在各個方向的聲響效率因素，IEC534-8-3定義了由各種壓力參數確定的五個方向流體。這一標準還規(guī)定了計算內部聲壓的方法，其相應的峰頻率、傳輸損失和在閥門下游1米處及管道側面1m處的加權聲壓級。很重要的一點是IEC標準并沒有建議通過限制閥門縮流處速度頭為480KPa來實現閥門低聲壓級。
Valve trim exitvelocityhead 閥門內件節(jié)流出口速度頭
閥門內件節(jié)流出口速度頭計算式為：
KE=1/2＊ρ＊V2 （1）
公式中
KE閥門內件節(jié)流出口速度頭 Pa
ρ 節(jié)流處流體密度kg/m3
V 縮流處流體速度
聲速C計算公式為：
C=(γ＊R＊T)1/2 （2）
公式中
C 聲音速度 m/s
γ 氣體比熱
R 氣體常數R=8314/MW MW是氣體分子量
T 氣體溫度 K
從公式1和2可以解出：
KEregimellthruV= （1/2 ＊ρ＊C2）=1/2＊ρ＊γ＊R＊T （3）
Valve outlet machnumber閥門出口馬赫數
Averycritical parameter in gas and steam applications is thevalveoutlet Mach number，which is defined as the ratio of thefluidvelocity at the valve outlet，to the sonic velocity in thefluid atthe given temperature。This is a very important parameterindetermining not only the noise level in the valve，but alsothepotential for vibration in the valve/pipe system and potentialforerosion damage to the body if the outlet mach numberishigh。Typically，a control valve body has a close resonancefrequencyto the pipe system。Therefore，a high outlet mach number cangeneratea frequency that will match the valve/pipe systemresonancefrequency and this will lead tovibration。Unfortunately，the trimexit velocity head alone is not agood predictor of such animportant phenomena。For superheated steamand clean gas，a valveoutlet mach number below 0.4 is recommendedfor continuousoperation and below 0.5 for intermittentoperation。Keeping the machnumber low will result in a low noiselevel and will eliminate thepotential for vibration problems in thevalve/pipe system。Themaximum valve outlet velocity should belimited to mach 0.3 forsaturated steam，due to the thermodynamicproperties of the fluid。Avery small decrease in the temperature ofsaturated steam may leadto the formation of liquid droplets withinthe steam and this willbe extremely erosive at highvelocities。
氣體和蒸汽控制應用中閥門的出口馬赫數是一個非常關鍵的參數，其定義為在給定溫度下流體離開閥門出口的時候的速度是聲速的倍數。這是一個很重要的參數，如果閥門出口流速馬赫數過高，不僅造成閥門聲壓級高，而且容易造成閥門/管道系統(tǒng)振動及侵蝕破壞。通常，控制閥閥體的共振頻率和管道系統(tǒng)比較接近。因此，高的閥門出口馬赫數將能夠產生一個與管道系統(tǒng)頻率相配合的頻率，這將導致發(fā)生共振?？上?，單靠閥門內件節(jié)流出口速度頭并不能很好的預測這種現象。對于過熱蒸汽或干凈的氣體，閥門的出口馬赫數推薦在連續(xù)操作情況下低于0.4，間歇操作時候低于0.5。保持低馬赫數將能夠獲得低聲壓級，同時也將會避免發(fā)生閥門及管道系統(tǒng)振動等問題。由于流體的熱力學性質，對于飽和蒸汽*大的閥門出口馬赫數應限制在0.3。很小的飽和蒸汽溫度下降都有可能導致在蒸汽中形成液滴，這種高速液滴將會導致非常嚴重的破壞。
Example 舉例
Followingaretwo examples that will illustrate the validity of the argumentsmadeabove。The first example will demonstrate that forsomeapplications，when the time exit velocity head exceeds480kPa，thevalve can still generate a low noise level。The secondexample willshow that for some other applications having a trimexit velocityhead lower than 480kPa but a high mach number，thevalve willgenerate a very high noise level。
以下是兩個例子可以證明上述觀點是有效的。**個例子顯示在一些應用中，當閥門內件節(jié)流出口速度頭超過480KPa時，閥門仍能處于低聲壓級。**個例子顯示另外一些應用中，閥門內件節(jié)流出口速度頭低于480KPa，但有高的馬赫數時，閥門會產生很高聲壓級。
Example 1 例1
Consideranapplication with 30,000 kg/hr of air at 100℃。The valveinletpressure is 30 bar and is outlet pressure is 12 bar。
工況：介質空氣；流量30,000kg/h；溫度100℃；閥前壓力 30 bar；閥后壓力 12 bar
AccordingtoIEC 534-3-8，this application is represented by flow regimeIV。Nowconsider a 4" globe style valve with single-stage drilledholecage。For the service conditions given above，the noiselevelgenerated by the 4" valve is approximately 85 dBA with an 8"outletpipe schedule 40。The trim exit velocity is sonic and it isequal to337 m/s from Equation 2。The air density at the trim outletis11.17kg/m3。From Equation 3， the trim exit velocityheadis equal to 836 kPa。 The valve outlet mach number isapproximately0.24。
根據IEC534-3-8這屬于第四類?，F在考慮選擇一臺4"的單級籠式套筒閥。根據前面的工況條件計算，這個4"的閥門的聲壓級約為85分貝，閥門出口接8"的管道。由方程2可以計算得節(jié)流速度是音速，等于337米/秒。在閥門出口處的空氣密度是11.17kg/m3。根據方程3，閥門內件節(jié)流出口速度頭等于836KPa，閥門的出口馬赫數約為0.24。
Thus，theexample above demonstrates that a 4" valve with a singlestagedrilled hole cage can achieve a low noise level despite thefactthat the time exit velocity head is much great than the 480kPalimitations that some valve manufacturers advocate。Thevalvesolution described above is technically excellent andveryeconomical compared to a 4" valve with a torturousmultistagetrim。
因此，上例顯示了4"的單級籠式套筒閥可以實現低噪音，盡管閥門內件節(jié)流出口速度頭遠超過480KPa。這就是一些廠家主張的這個方法的局限性。這個閥門非常出色、經濟的解決了上述問題，省去了采用多級式閥。
Example 2 例2
Consideranapplication with 50,000 kg/hr of air at 100℃。The valveinletpressure is 50 bar and its outlet pressure is 3 bar。
工況：介質空氣；流量50,000kg/h；溫度100℃；閥前壓力50bar；閥后壓力3bar
From these service conditions therequiredCv=62。Thus，consider a 6" globe style valve with atorturouspath，with 24 stages，rated Cv = 70。The outlet pipe is 8"schedule40。For the given service conditions，the last turn in thetrim hasan exit velocity of 233 m/s and the fluid outlet density is2.8kg/m3。Thus，per equation 1， the trim exit velocityheadis 76 kPa， well below the maximum limitation of 480kPa。However，despite this very low trim exit velocity head， thevalve noiselevel is 98 dBA because the valve outlet mach numberis0.78。Now，take the same exact trim and install it in an 8"globestyle valve。In this case the valve outlet mach number is 0.4andthe valve noise level is approximately 85 dBA。
從條件計算得Cv=62。因此，考慮使用一個6"、具有24級流路的多級式球型閥，Cv=70。閥門出口管道為8"。根據以上條件，閥門的*后出口速度為233m/s，出口處的流體密度為2.8kg/m3。因此，根據方程1，閥門內件節(jié)流出口速度頭為76KPa，處于*大限制數480KPa下。然而，盡管閥門內件節(jié)流出口速度頭非常的低，但閥門的聲壓級卻是98分貝，顯然這是由于閥門的出口馬赫數為0.78造成的?，F在，采用同樣的閥門內件節(jié)流出口速度頭和8"球型閥。在這種情況下，閥門的出口馬赫數為0。4，閥門的聲壓級約85分貝。
Thisexampledemonstrates that a low trim exit velocity head does notresult ina low noise level in the valve，unless the valve outletmach numberis low。It is also noteworthy to mention that for thesameapplication，an 8" globe style valve with two drilled holedcages isan excellent solution for the application above since itwillgenerate a noise level of 80 dBA。These two drilled holed cagesaremuch less expensive than the 24-turn torturous path time。
這個例子顯示，低閥門內件節(jié)流出口速度頭不會保證閥門處于低聲壓級，除非閥門出口馬赫數很低。值得一提的是，在這個例子中一臺8"、具有雙層閥籠的球型套筒閥也是一個很好的解決方案，因為它產生的噪音水平為80分貝。雙層套筒相對于24級流路簡單了很多。
Conclusion 結論
Limitingthetrim exit velocity head to a maximum of 480 kPa is not practicalina large number of steam and gas applications。Asdemonstratedabove，this approach can be overly conservative in manygasapplications and it can be insufficient to ensure low valvenoiselevel in others。Low noise level can be achieved with trim thathasan exit velocity head higher than 480 kPa。On the other hand，avalvetrim exit velocity head lower than 480 kPa will have a veryhighnoise level if the valve outlet mach number is notlow。Whenproviding a control valve solution for gas applications，itis veryimportant not to be bound by the trim exit velocityheadlimitation，but rather profile the valve that gives：lownoiselevels， a low outlet mach number and is the mosteconomicalsolution available。
在大量的氣體和水蒸氣應用中。限制閥門內件節(jié)流出口速度頭*高為480KPa是不實用的，以上證明，這種方法過于保守，不足以確保閥門的聲壓級能處于較低水平。在閥門內件節(jié)流出口速度頭高于480KPa的情況下，低的閥門聲壓級仍能實現。另一方面，當閥門的閥門內件節(jié)流出口速度頭低于480KPa時，如果閥門的出口馬赫數不處于較低的水平，閥門仍會有較高的聲壓級。當提供一個控制閥解決氣體應用的時候，不受閥門內件節(jié)流出口速度頭限制是很重要的，概括起來講就是：使用低的閥門出口馬赫數來實現閥門低聲壓級是*經濟、可行的方式。
寫于2004年2月