進口調(diào)節(jié)閥的選型知識
選擇進口調(diào)節(jié)閥時，首先要收集完整的工藝流體的物理特性參數(shù)與調(diào)節(jié)閥的工作條件，主要流體的成份、溫度、密度、粘度、正常流量、最大流量、最小流量、最大流量與最小流量下的進出口壓力、最大壓差等。而在技術(shù)方面主要掌握和確定調(diào)節(jié)閥本身的結(jié)構(gòu)、流量特性、額定流量系數(shù)Kv值、口徑大小、工藝允許壓差計算及執(zhí)行機構(gòu)的選擇、材料和安裝等方面的內(nèi)容。
選擇進口調(diào)節(jié)閥時一般應遵循的原則有如下幾點。一、首先從動力源來選擇，美國威盾VTON根據(jù)方便和反應速度的要求來選擇用什么驅(qū)動方式的調(diào)節(jié)閥，主要有進口電動調(diào)節(jié)閥，進口氣動調(diào)節(jié)閥，進口自力式調(diào)節(jié)閥。二 調(diào)節(jié)閥的結(jié)構(gòu)型式：應能滿足介質(zhì)溫度、壓力、流動性、流向、調(diào)節(jié)范圍以及嚴密性的要求。按結(jié)構(gòu)分類主要有進口單座調(diào)節(jié)閥，進口雙座調(diào)節(jié)閥，進口套筒調(diào)節(jié)閥。三、 調(diào)節(jié)閥的流量特性：應能滿足系統(tǒng)特性進行合理的補償。調(diào)節(jié)閥的流量特性是指介質(zhì)流過閥的相對流量與閥桿相對位移間的關(guān)系，數(shù)學表達式如下：Q/Qmax=f(l/L)，式中Q/Qmax為相對流量，為調(diào)節(jié)閥在某一開度時流量Q與全開流量Qmax之比；l/L為相對位移，調(diào)節(jié)閥在某一開度時閥芯位移l與全開位移L之比。選擇的總體原則是調(diào)節(jié)閥的流量特性應與調(diào)節(jié)對象特性及調(diào)節(jié)器特性相反，這樣可使調(diào)節(jié)系統(tǒng)的綜合特性接近于線性。選擇流量特性通常在工藝系統(tǒng)要求下進行，但是還要考慮下述實際情況。1、直線性流量特性適用范圍：①差壓變化小，幾乎恒定；②工藝流程的主要參數(shù)的變化呈線性；③系統(tǒng)壓力損失大部分分配在調(diào)節(jié)閥上(改變開度，閥上差壓變化相對較小)；④外部干擾小，給定值變化小，可調(diào)范圍要求小。2、等百分比特性適用范圍：①實際可調(diào)范圍大；②開度變化，閥上差壓變化相對較大；③管道系統(tǒng)壓力損失大；④工藝系統(tǒng)負荷大幅度波動；⑤調(diào)節(jié)閥經(jīng)常在小開度下運行。3、除了以上兩種常用的流量特性之外，還有拋物線特性和快開特性等其他流量特性的調(diào)節(jié)閥。四 調(diào)節(jié)閥的口徑：應能滿足工藝上對流量的要求。有進口大口徑調(diào)節(jié)閥，進口小口徑調(diào)節(jié)閥。根據(jù)已知的流體條件，計算出必要的Kv值，選取合適的調(diào)節(jié)閥口徑。
Control valve selection and common sense Select the control valve, the first complete collection of the physical characteristics of the process fluid control valve parameters and operating conditions, the main fluid composition, temperature, density, viscosity, normal flow, maximum flow, minimum flow, maximum flow and minimum flow rate import and export pressure, maximum pressure and so on. In the technical aspects of the main control valve itself and determine the structure, flow characteristics, the rated flow coefficient Kv value, aperture size, technology allows the calculation of pressure and actuator selection, materials and installation aspects. Select the control valve when the general principles to be followed are the following. A valve structure type: should be able to meet the medium temperature, pressure, flow, flow, adjust the scope and rigor of the requirements. Two valve flow characteristics: the system should be able to meet the characteristics of reasonable compensation. Control valve flow characteristics is the medium flow through the valve stem relative to flow and the relationship between the relative displacement, mathematical expressions are as follows: Q / Qmax = f (l / L), where Q / Qmax for the relative flow rate, as adjusted When opening the valve at a flow rate Q and the ratio of full-flow Qmax; l / L for the relative displacement of the valve when the valve opening in a displacement l and the ratio of full-shift L. The overall principle is the selection control valve regulating the flow characteristics should be the object characteristics and regulator features the contrary, it will give a comprehensive control system features close to linear. Select the flow characteristics in the process is usually carried out under the system requirements, but also consider the following facts. 1, the linear flow characteristics of the scope of application: ① pressure change is small, almost constant; ② The main process parameter changes linearly; ③ loss of most of the distribution system pressure control valve on the (change in opening the valve on the pressure change is relatively small); ④ external interference, given the value of small changes, small adjustable range of requirements. 2, the percentage of properties such as the scope of application: ① actual adjustable range; ② opening change, relatively large changes in differential pressure valve; ③ large piping system pressure loss; ④ process system load fluctuations; ⑤ valve often in small opening run. 3, in addition to the above two common flow characteristics, there are parabolic features and other features quick opening flow characteristics of control valve. Three valve diameter: should be able to meet the technical requirements on the traffic. According to the known fluid conditions, necessary to calculate the Kv value, select the appropriate valve diameter.
