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TKWL-1700系列电容式物位计

TKWL-1700系列电容式物位计

TKWL-1700seriescapacitancelevelmeter

工作原理

  电容式物位计由电容式物位传感器和检测电容的线路组成。其基本工作原理是电容式物位传感器把物位转换为电容量的变化,然后再用测量电容量的方法求知物位数值。电容式物位传感器是根据圆筒电容器原理进行工作的。其结构如2长度L 、半径分别Rr的圆筒型金属导体,中间隔以绝缘物质,当中间 所充介质是介电常数为ε1的气体时,两圆筒的电容量为:C1 =2πε1L/R/(lnR/r)(1)


如果被测介质为导电性液体时,电极要用绝缘物(如聚乙烯)覆盖作为中间介质,而液体和外圆筒一起作为外电极。假设中间介质的介电常数为ε3,电极被浸没长度为l,则此时电容器所具有的电容 量为: C=2πε3 L/R/(lnR/r) (3)

  其中:R 和r 分别为绝缘覆盖层外半径和内电极 外半径。由于ε3 为常数,所以C 与l 成正比。如果电极的一部分被介电常数为ε2的液体(非 导电性的)浸没时,则必须会有电容量的增量C 产Th(ε2>ε1),此时两极间的电容量C=C1+C假如电极被浸没长度为l则电容增量为: △C=2πε3 L/R/(lnR/r) (2)

当ε2、ε1、R、r不变时,电容量增量△C与电 极浸没的长度l 成正比,因此测出电容增量数值便可知道液位高度。

电容式物位计液位计在应用中应注意的几个问题:

1选型

由于被测介质的不同, 电容式物位传感器有不同的型式。

(1)测量非导电液体的电容物位传感器,当用于较稀的非导电液体(如轻油等)时,可采用一金属电极,外部同 轴套上一金属管,相互绝缘固定,以被测介质为中间绝缘 物质构成同轴套筒形电容器。

(2)测量导电液体的电容物位传感器,容器(规则)和 液体作为电容器的一个电极,插入的金属电极作为另一电极,绝缘套管作为中间介质,三者组成圆筒形电容器。当 容器为非导电体时,需另加一个接地极,其下端浸至被测容器底部,上端与安装法兰有可靠的导电连接,以使二电 极中有一个与大地及仪表地线相连,保证仪表正常测量。

(3)当测量粉状非导电固体料位和粘滞性非导电液 体液位时,可采用金属电极直接插入圆筒型容器的中央,将仪表地线与容器相连,以容器作为外电极,料或液体作为绝缘介质构成圆筒型电容器。所以应根据现场实际情况,即被测介质的性质导电特性、粘滞性)、容器类型(规则/非规则金 属罐、规则/非规则非金属罐),选择合适的电容物 位计。

2测量回路中接地点的处理

  仪表测量回路中接地点的正确、可靠与否直接影响被 测参数的测量。电容物位计对油品精制8碱液罐进行液面监控,由于碱液罐位于防爆区,所以最初设计测量回路中引入齐纳式输入安全栅(见图1)。

  安装调试发现,系统无法正常工作:安全栅2端电压高达23V,变送器(电容物位计)供电电压为0V, 也就是说已短路。仔细查找原因,发现是由于电容 物位计的电路结构所致。电容物位计的探头为等效电容的一极,对于外壁规则的金属容器,其罐壁为电容的另一极。因此变送器信号负接地,而齐纳栅 也接地则变送器被短路。于是将接地COM点悬空,观 察到开始时变送器输出在4mA,安全栅2端电压在 0.86V,不长时间变为1.6V,7V,变送器工作很不稳定,这是由于COM点悬空,系统回路受外部干扰所致。于是抛开安全栅与COM板,将24V电源直接送到变送器,串入标准电流表检测变送器工作情况,变 送器工作正常。得出结论:由于电容物位计信号负与大地(罐壁)相连,因此不能选用齐纳式安全栅。经与厂家协商,选用隔离式安全栅,因为电容物位计与隔离式安全栅已进行系统安全防爆联合取证。选用隔离式安全栅系统接线如图2所。由于隔 离式安全栅的电源、输入、输出信号三者隔离,避 免了系统间的相互干扰,以及系统多点接地问题。

Working  principle

  Capacitance level meter is composed ofcapacitive material level sensor and circuit checking capacitance, whose basicworking principle is the change of converting material level into capacitancewith capacitive material level sensor, and then calculate the value of materiallevel with the method of measuring capacitance.

  Capacitance level meter works based on theprinciple of cylinder capacitor, whose structure is similar to two cylindricalmetal conductors with length of L, radiuses of R and r respectively; the middleis separated with insulating material and when the filled medium in the middleis the gas with dielectric constant of ε1, the capacitance of two cylinders is: C1 =2πε1 L/R/(lnR/r)  (1)

  If the measured medium is conductive liquid,the electrode needs to be covered with insulator (such as polyethylene) asintermediate medium, while the liquid and external cylinder are as outerelectrodes. Presume the dielectric constant of intermediate medium is ε3 andthe immersed length of electrode is 1, and then the capacitance of capacitoris: C=2πε3 L/R/(lnR/r)  (3)

  In which R and r are outer radius ofinsulating coating and outer radius of inner electrode. As the ε3 is constant,so C and l are in direct ratio.

  If partof electrode is immersed by liquid (non-conductive) with dielectric constantatε2, and then there must be increase of capacitance Cproduced (becauseε2>ε1) , now the capacitance between these two electrodesis C=C1+C. Presume the immersion length of electrode is l, andthen the increase of capacitance is: C=2πε3 L/R/(lnR/r) (2)

  Whenε2, ε1, R, r keep unchanged, the increase of capacitance C and immersed length of electrode l are in direct ratio,therefore, the liquid level can be gotten after the value of capacitanceincrease is measured.

  A few problemsneed to be paid attentionto during the application of capacitance level meter

1.Selection

  Due to  the  differences  of  measured medium, capacitance  level meter has different models.

(1)When apply the capacitive level sensor of measuring non- conductive liquid to more dilute non-conductiveliquid (such as light oil), it can adopt one metal electrode and put a metal tube on externalcoaxial, mutually insulated and fixed, form coaxial sleevetype capacitor by taking measured medium as intermediateinsulatingmaterial.

(2)For capacitive level sensor of  measuring non-conductive liquid, take container (rule) and liquid as  one  electrode of capacitor, the inserting metal electrode as another electrode, the insulating tube as intermediate  medium, and  these  three  composecylindrical capacitor. When the  container  is non-conductive, it needs to add another earth electrode, whose bottom partwill be immersed to the bottom of the container and the top part will make reliableelectrical connection with installation flange to make one  of  the  electrodes connected with earth and meter ground line and ensure the normal measurement of  meter.

(3)When  measuring  the  material  level  of  powder non-conductive solid and the liquid level of viscousnon-conductive liquid, can adopt metal electrode to insert into the center  of cylindrical container directly,  connect  the  ground  line  of instrument with container and constructcylindrical capacitor by taking the container as outside electrode, material or liquid as insulatingmedium.

So it should choose suitable capacitance level meter based on real site situation, that is the property of measured medium (conductivity, viscosity) and type of container(regular/irregular metal tank, regular/irregular non-metal tank).

2. Treatment of ground point in measurementcircuit

  Whether the groundpoint in meter measurement circuit is correct and reliable or not affects themeasurement of measured parameters. Liquid monitoring for eight soda solutiontank of oil refining during overhaul. As the soda solution tank locates atexplosion proof area, so it introduces zenner diode safety barrier in theinitital design of measurement circuit (as shown in pic.1)

  It is found out during installation and debugging that the system  can’t work normally: voltage at two ends of safety barrier is as high as 23V, whle the powersupply voltage of transmitter(capacitance level meter) is 0V, whichmeans short circuit. After carefulchecking, it finds outthat it is caused by the circuit structure of capacitance level meter.The probe of capacitancelevel meter is one electrode   of equivalencecapacitance, for the metal containerwith regular  wall; thetank wall is anotherelectrode of capacitance. Therefore, the signalof transmitter is negative ground, while zenner barrier is grounded,which makes the transmitterbecome short circuit. Hangthe grounding COM point andit is observed that at the beginning stage,the output of transmitter is 4 mA, the voltage attwo ends of safety barrieris 0.86V, and then changes into1.6V, 7V shortly. The transmitter works unsteadily,which is caused byhang of COM point andthe system circuit is affectedby external disturbance. Therefore, throwoff safety barrier and COM plate,send 24V power totransmitter directly, string into standard current meter to examine theworking condition of transmitterand the transmitter works normally. It can draw a conclusionthat as the negative signalof capacitance level meter is connectedwith ground (wall of the tank), therefore,it can’t adopt zenner diode safety barrier.After negotiation with manufacturer,select isolating safety barrier, because capacitance level meter and isolating safety barrierhave made joint forensics forsystem safety and explosion  proof.Selecting isolating safety barriersystem wiring is as shownin picture 2. Due to the isolation amongpower, input and output signal of isolating safety barrier, it avoids the mutualinterference between systemsaswellasmultiplepointsgroundingproblemofsystem.




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