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。
近年来，控制阀行业出现关于气体或蒸汽应用中限制*高阀门内件节流出口速度头小于480KPa有效性的争论。它被假定为可以提供可接受的声压级，避免在气体或蒸汽应用中出现控制阀难题。然而，在大量的应用中采用限制速度头的���法需要采用保守昂贵的多级或多回路设计。这篇文章的目的是要说明低声压级可以在不用昂贵的速度头限制方式下实现。另外，这篇文章显示在流体速度头低于480KPa，但阀门出口流体速度马赫数较高的情况下仍会产生相当高的声压级。
Noise Prediciton inControlValves 控制阀噪音预测
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空气动力学噪声标准是控制阀领域使用的*广的工业阀门噪声预测标准。这一标准计算流体在各个方向的声响效率因素，IEC534-8-3定义了由各种压力参数确定的五个方向流体。这一标准还规定了计算内部声压的方法，其相应的峰频率、传输损失和在阀门下游1米处及管道侧面1m处的加权声压级。很重要的一点是IEC标准并没有建议通过限制阀门缩流处速度头为480KPa来实现阀门低声压级。
Valve trim exitvelocityhead 阀门内件节流出口速度头
阀门内件节流出口速度头计算式为：
KE=1/2＊ρ＊V2 （1）
公式中
KE阀门内件节流出口速度头 Pa
ρ 节流处流体密度kg/m3
V 缩流处流体速度
声速C计算公式为：
C=(γ＊R＊T)1/2 （2）
公式中
C 声音速度 m/s
γ 气体比热
R 气体常数R=8314/MW MW是气体分子量
T 气体温度 K
从公式1和2可以解出：
KEregimellthruV= （1/2 ＊ρ＊C2）=1/2＊ρ＊γ＊R＊T （3）
Valve outlet machnumber阀门出口马赫数
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。
气体和蒸汽控制应用中阀门的出口马赫数是一个非常关键的参数，其定义为在给定温度下流体离开阀门出口的时候的速度是声速的倍数。这是一个很重要的参数，如果阀门出口流速马赫数过高，不仅造成阀门声压级高，而且容易造成阀门/管道系统振动及侵蚀破坏。通常，控制阀阀体的共振频率和管道系统比较接近。因此，高的阀门出口马赫数将能够产生一个与管道系统频率相配合的频率，这将导致发生共振。可惜，单靠阀门内件节流出口速度头并不能很好的预测这种现象。对于过热蒸汽或干净的气体，阀门的出口马赫数推荐在连续操作情况下低于0.4，间歇操作时候低于0.5。保持低马赫数将能够获得低声压级，同时也将会避免发生阀门及管道系统振动等问题。由于流体的热力学性质，对于饱和蒸汽*大的阀门出口马赫数应限制在0.3。很小的饱和蒸汽温度下降都有可能导致在蒸汽中形成液滴，这种高速液滴将会导致非常严重的破坏。
Example 举例
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。
以下是两个例子可以证明上述观点是有效的。**个例子显示在一些应用中，当阀门内件节流出口速度头超过480KPa时，阀门仍能处于低声压级。**个例子显示另外一些应用中，阀门内件节流出口速度头低于480KPa，但有高的马赫数时，阀门会产生很高声压级。
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。
工况：介质空气；流量30,000kg/h；温度100℃；阀前压力 30 bar；阀后压力 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。
根据IEC534-3-8这属于第四类。现在考虑选择一台4"的单级笼式套筒阀。根据前面的工况条件计算，这个4"的阀门的声压级约为85分贝，阀门出口接8"的管道。由方程2可以计算得节流速度是音速，等于337米/秒。在阀门出口处的空气密度是11.17kg/m3。根据方程3，阀门内件节流出口速度头等于836KPa，阀门的出口马赫数约为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，上例显示了4"的单级笼式套筒阀可以实现低噪音，尽管阀门内件节流出口速度头远超过480KPa。这就是一些厂家主张的这个方法的局限性。这个阀门非常出色、经济的解决了上述问题，省去了采用多级式阀。
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。
工况：介质空气；流量50,000kg/h；温度100℃；阀前压力50bar；阀后压力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。
从条件计算得Cv=62。因此，考虑使用一个6"、具有24级流路的多级式球型阀，Cv=70。阀门出口管道为8"。根据以上条件，阀门的*后出口速度为233m/s，出口处的流体密度为2.8kg/m3。因此，根据方程1，阀门内件节流出口速度头为76KPa，处于*大限制数480KPa下。然而，尽管阀门内件节流出口速度头非常的低，但阀门的声压级却是98分贝，显然这是由于阀门的出口马赫数为0.78造成的。现在，采用同样的阀门内件节流出口速度头和8"球型阀。在这种情况下，阀门的出口马赫数为0。4，阀门的声压级约85分贝。
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。
这个例子显示，低阀门内件节流出口速度头不会保证阀门处于低声压级，除非阀门出口马赫数很低。值得一提的是，在这个例子中一台8"、具有双层阀笼的球型套筒阀也是一个很好的解决方案，因为它产生的噪音水平为80分贝。双层套筒相对于24级流路简单了很多。
Conclusion 结论
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。
在大量的气体和水蒸气应用中。限制阀门内件节流出口速度头*高为480KPa是不实用的，以上证明，这种方法过于保守，不足以确保阀门的声压级能处于较低水平。在阀门内件节流出口速度头高于480KPa的情况下，低的阀门声压级仍能实现。另一方面，当阀门的阀门内件节流出口速度头低于480KPa时，如果阀门的出口马赫数不处于较低的水平，阀门仍会有较高的声压级。当提供一个控制阀解决气体应用的时候，不受阀门内件节流出口速度头限制是很重要的，概括起来讲就是：使用低的阀门出口马赫数来实现阀门低声压级是*经济、可行的方式。
写于2004年2月