How do instrument technicians perform calculations for instrument intrinsically safe circuits?

Thread Content

This post was last edited by yunrun on 2019-9-25 22:13. In industries such as petroleum and petrochemicals, potential explosive environments can arise; therefore, it is necessary to implement appropriate explosion-proof measures for the equipment located on-site within these systems. The explosion-proof technologies used in automatic control instrumentation mainly include various types such as intrinsically safe (E*), flameproof (Ex d), increased safety (Ex e), positive pressure (Ex p), and encapsulation (Ex m). Among the various explosion-proof technologies, intrinsic safety technology—a safety technique that suppresses the energy of ignition sources to prevent explosions—has been widely applied in engineering projects across various industries. It offers advantages such as a simple structure, small size, light weight, and the ability to perform maintenance, calibration, and part replacement while energized. In a certain chemical engineering project, the owner stated that, based on past project experience, if rigorous calculations are not carried out for the intrinsically safe circuits, there will still be around 30% of such circuits that pose safety risks during operation. Even though there are no specific requirements for intrinsically safe computing in the country at present, owners still demand that rigorous calculations for each intrinsically safe circuit be carried out during the project design phase. These calculations are based on the relevant provisions of International Electrotechnical Commission standard IEC 60079-14:2007. Calculation of intrinsically safe circuits: yunrun.com.cn/tech/2710.html. Basic requirements for the design of intrinsically safe circuits: Achieving the intrinsically safe and explosion-proof requirements by controlling the electrical parameters of the circuit (such as reducing the parameters of energy-storing components like inductors and capacitors), or by lowering the circuit’s current and voltage ； The components in the circuit must have sufficient power rating, and the connecting wires must have an adequate cross-sectional area, so that the high voltages and large currents that may occur under various fault conditions will not impair the performance of the components. The reliability of the circuit is thus ensured through the reliability of its components. This requires calculating the parameters of the relevant electrical components in the intrinsically safe circuit, that is, performing intrinsically safe circuit calculations, in order to meet the requirements of relevant safety standards and ensure safer production. 1. Introduction to Intrinsic Safety Explosion Protection Technology. The basic principle of intrinsic safety explosion protection technology is to achieve explosion prevention by limiting energy. This technology ensures that the energy in the circuit remains within an allowable range, both under normal and abnormal conditions, so as to prevent explosions of any hazardous gases present in the vicinity in the event of short circuits or damage to electrical components. The intrinsically safe explosion-proof system, also referred to as the intrinsically safe circuit system, consists of three components: field intrinsically safe devices, intrinsically safe cables, and associated equipment, as shown in Figure 1. The system circuits are divided into intrinsically safe circuits and non-intrinsically safe circuits, with the safety barrier serving as the boundary. The circuit formed by connecting field instruments to the safety barrier via intrinsic safety cables is an intrinsic safety circuit ； The circuits from the safety barrier to the DCS and to the power supply are non-intrinsic circuits. http://yunrun.com.cn/upload/201909/25/201909252207051288.png Figure 1 Schematic diagram of a typical intrinsically safe circuit. ① Field intrinsically safe devices ◆ Simple devices According to the standards IEC60079-14, passive components such as switches, junction boxes, resistors, and simple semiconductor components can be considered simple devices ； For active energy storage components such as capacitors and inductors, they can be considered simple devices when their design parameters ensure the overall safety of the system ； For active components that generate energy, such as thermocouples and photovoltaic cells, electrical devices whose generated energy voltage does not exceed 1.5V, current does not exceed 100mA, and power does not exceed 25mW can be considered simple devices. They do not require explosion-proof certification and can be freely configured in intrinsically safe circuits. ◆Intrinsic safety devices contain energy storage elements; they are intrinsic safety electrical devices that require explosion-proof certification, such as transmitters and proximity switches. ②Intrinsically safe cables: The connecting cables between instruments and control rooms have a certain amount of distributed capacitance and inductance. Particularly when the distance is relatively long or when the design parameters of the instruments are close to the maximum values permitted for intrinsic safety, it is necessary to consider these distributed parameters and limit the length of the wiring. This is because both distributed inductance and distributed capacitance store energy; when a fault occurs in the cable, this stored energy is released in the form of electric sparks or thermal effects, which to varying degrees increase the risk of ignition and affect the intrinsically safe performance of the system. Maximum allowable distributed capacitance of the cable: Cc = Ck × L. Maximum allowable distributed inductance of the cable: Lc = Lk × L. Here, Ck represents the distributed capacitance per unit length of the cable ； Lk is the distributed inductance per unit length of the cable ； L is the actual wiring length. ③Interconnecting devices (safety barriers): Safety barriers are the most commonly used interconnecting devices in intrinsically safe explosion-proof instrument systems. They are connected between intrinsically safe and non-intrinsically safe circuits. Their function is to limit current and voltage, preventing hazardous energy from entering the intrinsically safe circuits, thereby ensuring the safety of these circuits. There are mainly two types of safety barriers: Zener safety barriers and isolated safety barriers. Currently, Zener safety barriers are gradually being replaced by isolated safety barriers. 2. Certification methods for intrinsically safe explosion-proof instrument systems. Currently, international testing agencies use three methods for the inspection and certification of intrinsically safe explosion-proof instrument systems: system approval, parameter approval, and FISCO (Fieldbus Intrinsically Safe Concept) approval. ①System approval: System approval, also known as “combined certification,” refers to the approval of a system formed by combining the specified intrinsically safe instrument under inspection with the specified associated equipment also under inspection. Once approved, no device in the system may be replaced with any other model or specification of intrinsically safe equipment (instruments) or associated equipment that has not been tested and approved by a testing institution using this combination. Currently, the main method of accreditation for testing institutions in China is system accreditation. However, in situations where there are numerous types of intrinsically safe instruments and safety barriers, this certification method involves rather cumbersome procedures and is no longer well-suited to modern industrial design. ②Parameter approval: Parameter certification is more commonly used in foreign projects. Parameter certification refers to the inspection and certification of a single device (an intrinsically safe instrument or associated equipment), with a set of corresponding safety parameters being assigned. Typically, intrinsically safe devices approved using this method can be connected and used with associated devices that have compatible safety parameters. The U.S. FM (Factory Mutual) refers to this approval method as “overall approval,” and the corresponding safety parameters as “overall parameters.” These parameters include the following: ◆ The overall parameter Ui of the intrinsically safe equipment represents the maximum voltage that the intrinsically safe equipment can tolerate under fault conditions ； Ii is the maximum acceptable current for the intrinsically safe equipment under fault conditions ； Ci refers to the unprotected capacitors inside the intrinsically safe device ； Li is the unprotected inductance inside the intrinsically safe equipment. ◆Overall parameters of the associated equipment: Uo is the open-circuit voltage, that is, the maximum voltage that may reach a hazardous area under fault conditions; Io is the short-circuit current, that is, the maximum current that may reach a hazardous area under fault conditions ； Co-associated devices allow a maximum external capacitance ； Lo—allows the maximum external inductance for connected devices. With parametric approval, when designing an intrinsic safety system, users only need to compare the overall parameters of the associated equipment and the intrinsically safe equipment. When they satisfy the following relationship, they can constitute a intrinsically safe system without the need for approval from a testing institution. The safety parameter matching between safety barriers, intrinsically safe instruments, and cables must satisfy the five inequalities listed in Table 1:
Table 1: Five inequalities for calculating simple intrinsically safe circuits
Formula | Safety barrier parameters | Conditions for safety parameter matching | Intrinsically safe instrument parameters + Cable parameters
1 | Uo ≤ Ui | |
2 | Io ≤ Ii | |
3 | Po ≤ Pi | |
4 | Co ≥ Ci + ∑Cc | |
5 | Lo ≥ Li + ∑Lc | |
Note: ∑Lc and ∑Cc represent the distributed inductance and capacitance of the connecting cables, respectively. Parameter recognition is a widely used certification method in foreign projects, and it has also been recognized by the **-level Instrument and Meter Safety Supervision Station in China. The 5 inequalities in Table 1 are key to determining whether an intrinsically safe circuit meets the regulatory requirements. ③FISCO adopts the concept of intrinsical safety for field buses, which was developed by Germany’s PTB (Physikalisch Technische Bundesanstalt). FISCO makes it possible to install Profibus-PA in an intrinsically safe manner, allowing its use in Ex zones. FISCO is suitable for EE*a IIC and EE*b IIC/IIB intrinsically safe explosion-proof methods. Research shows that the number of field instruments connected to a Profibus-PA segment is limited only by the electrical characteristics of the segment coupler; the maximum number of field instruments that can be connected to each segment is reached. The segments can be expanded within the limits set by FISCO, without the need to perform new intrinsically safe calculations. Field instruments from different manufacturers can be interchanged without the need for additional intrinsically safe calculations, but the replacement instruments must be FISCO-certified. It is particularly important that no system certification is required; only verification of the matching of certain key parameters is needed. FISCO certification is subject to the following conditions. a) Only one power supply is allowed per section. b) All stations must comply with the FISCO model and pass verification. c) The cable length shall not exceed 1000 m (explosion-proof type IA)/1900 m (explosion-proof type IB). d) The cable parameters must meet the following values: R′=15～150Ω/km, L′=0.4～1mH/km, C′=80～200nF/km. e) All combinations of power supply units and field devices must ensure that, in the event of a fault, the allowable input variables (Ui, Ii, and Pi) for any field device must exceed the maximum possible and permissible output variables (Uo, Io, Po) of the corresponding power supply unit ； In the United States: Vmax, I max, Pmax). 3. Intrinsic safety circuit calculation: IEC 60079-14:2007, 12.2.5 states that “unless an intrinsic safety circuit has been certified, all intrinsic safety circuits shall be subjected to intrinsic safety calculations in accordance with this standard.” Based on past experience, the project owner knows that even if intrinsically safe instruments, intrinsically safe cables, and compatible safety barriers are selected, without performing intrinsic safety calculations, there will still be safety hazards in 30% of the circuits. To reduce risks, all intrinsically safe circuits used in the project must be accompanied by the intrinsically safety certification certificates for the relevant instruments and safety barriers, as well as calculation documents for the intrinsically safe circuits. ①Calculation of the intrinsically safe circuit for a connected device ◆ Simple intrinsically safe circuit The simple intrinsically safe circuit is shown in Figure 2. http://yunrun.com.cn/upload/201909/21/201909212207179401.png Figure 2: Simple intrinsically safe circuit. Calculations are performed using circuit LIA+S+100.01, as shown in Table 2. Table 2: Parameters of circuit LIA+S+100.01. http://yunrun.com.cn/upload/201909/22/201909220221569366.png After calculating these parameters, it is confirmed that they satisfy the requirements of the 5 inequalities listed in Table 1; therefore, this circuit passes the intrinsically safe calculation and can be used in explosion-proof areas. ◆In the design of the hybrid intrinsically safe circuit, the differential pressure level transmitter LT100.01 is installed on top of the tank. To facilitate data reading, a local indicating instrument LI100.01 is added on the ground; this instrument is connected in series with the aforementioned circuit, as shown in Figure 3. http://yunrun.com.cn/upload/201909/21/201909212213525326.png Figure 3 Mixed circuit LIA+S+100.01 According to 12.2.5.2 of IEC 60079-14:2007: “If both the total distributed inductance and the total distributed capacitance are greater than 1%Lo and 1%Co respectively, then Lo and Co should both be divided by 2 during calculations.” This type of hybrid loop has the following scenarios, as shown in Figure 4. Figure 4 Calculation logic for the mixed-loop intrinsically safe circuit (Figure caption: Li=Li1+Li2 ； Ci=Ci1+Ci2) a) If the circuit consists only of the cables Lc and Cc, then either the sum of the inductances Li of the field instruments or the sum of the capacitances Ci is 0, or one of them is 0; in such cases, the intrinsically safe calculation is carried out using the 5 inequalities listed in Table 1. b) If Ci≤1Co or Li≤1Lo, the intrinsically safe calculation is performed using the 5 inequalities in Table 1. c) If both Ci＞1Co and Li＞1Lo are satisfied, the calculations will no longer be based on the inductance and capacitance inequalities listed in Table 1; instead, the inductance and capacitance values of the safety barrier are divided by 2 for the calculations, as shown in Table 3. Table 3: Five inequalities for the calculation of mixed intrinsically safe circuits. Formulas, safety barrier parameters, conditions for matching safety parameters, and parameters of intrinsically safe instruments plus cable parameters: 1. Uo ≤ Ui 2. Io ≤ Ii 3. Po ≤ Pi 4. Co/2 ≥ Ci + ∑Cc 5. Lo/2 ≥ Li + ∑Lc. Using circuit LIA+S+100.01 as an example again, the calculations are shown in Table 4. An indicating instrument LI100.01 is connected in series in the circuit. Table 4 Parameters of the hybrid circuit LIA+S+100.01 http://yunrun.com.cn/upload/201909/22/201909220245022546.png Li=Li1+Li2=0.73+0.04=0.77mH ； Lo=4.2mH ； Li＞1%Lo ； Ci=Ci1+Ci2=22.5+0=22.5nF ； Co=83nF ； Ci＞1%Co ； When both inequalities are satisfied simultaneously: Li > 1Lo and Ci > 1Co, the calculations in the intrinsic safety analysis should be carried out according to the inequalities given in Table 3. However, the calculated results fail to meet the conditions Co/2 ≥ Ci + Cc and Lo/2 ≥ Li + Lc ； Therefore, this circuit has not passed the intrinsically safe calculation. Solution: Split one circuit into two separate circuits that feed into the system, and add a safety barrier for one of the AO outputs; this results in two simple intrinsically safe circuits (as shown in Figure 5). Calculations performed on each of these two circuits will both satisfy the intrinsically safe requirements. http://yunrun.com.cn/upload/201909/21/201909212358132679.png http://yunrun.com.cn/upload/201909/22/201909220002179350.png Figure 5: Splitting a mixed intrinsically safe circuit into simple intrinsically safe circuits. ② Calculation of complex intrinsically safe circuits: If an intrinsically safe circuit contains two or more interconnected devices, or if two or more intrinsically safe circuits are connected together, the entire intrinsically safe system must undergo rigorous theoretical calculations or spark tests as specified in IEC standards. However, since the calculation method for intrinsic safety circuits where one barrier is connected in series with two loops (as shown in Figure 6), or where one detection loop involves two barriers (as shown in Figure 7) is rather complex, and both the maximum allowable inductance and maximum allowable capacitance values must be determined by referring to the corresponding ignition curves, such circuits should be avoided in design whenever possible. http://yunrun.com.cn/upload/201909/22/201909220144278433.png Figure 6: Safety barriers connected in parallel to intrinsically safe circuits. http://yunrun.com.cn/upload/201909/22/201909220017326549.png Figure 7: An intrinsically safe circuit with one detection point connected to two safety barriers. ③ Temperature requirements for simple devices in intrinsically safe circuits: For simple devices, although some documents state that they do not require explosion-proof certification and can be freely used in intrinsically safe circuits. However, due to the impact of surface temperature on explosion protection, when the temperature of the medium being measured is high, heat can be conducted to the instrument, causing the surface temperature of the electrical components to rise above the temperature limits specified for the explosion-proof area. In such cases, it is necessary to use extended-type instruments or install them separately, so that the electrical equipment operates within the temperature range permitted by the certification document. ④The intrinsically safe calculation for the intrinsically safe circuits of FF buses: The explosion protection methods for FF field instruments mainly include flameproof type (EEx d), intrinsically safe type (EE*), and spark-free type (EEx nL). Considering error tolerance and safety during live maintenance, the Fieldbus Foundation generally recommends the EE* explosion protection method. a) Intrinsically safe main and branch circuits, used for Zone 1 explosion protection, as shown in Figure 8. This scheme involves placing the safety barrier between the H1 card, the power distribution unit, and the field instruments. Its advantage is that both the main line and the branches are intrinsically safe, allowing for maintenance while energized. However, this approach is often quite difficult to implement in practice. The reason is that the topology of each network segment is relatively complex, with a large variety of instruments, which makes it very difficult to calculate the distributed capacitance and inductance of those segments; in particular, it is impossible to calculate the distributed capacitance and inductance in the branched parts. Furthermore, due to the energy-limiting function of the safety barrier, the number of instruments that can be connected to each network segment as well as the cable length are greatly restricted. http://yunrun.com.cn/upload/201909/22/201909220043423777.png Figure 8: Intrinsically safe main and branch FF field buses also adopt the FISCO model. In simple terms, this involves adding additional requirements specific to field buses on top of the traditional certification standards for intrinsically safe instruments; by following these constraints when designing each network segment, it is not necessary to perform complex safety calculations, even when adding or modifying field instruments. This approach, in which both the main line and branches are intrinsically safe, has significant drawbacks. Firstly, since safety barriers cannot be redundant, this makes them a bottleneck for the security of the entire network segment ； Secondly, within the same network segment, it is not permitted to simply mix field instruments certified for Entity with those certified for FISCO. In other words, Entity-certified field instruments must be connected to an Entity power supply, while FISCO-certified field instruments must be connected to a FISCO power supply ； Third, although more instruments can be connected to the FISCO network segment than to the Entity network segment, the maximum number is still 6; the addition of more network segments results in high investment costs. b) Increased-safety main circuits and intrinsically safe branch circuits, for use in Zone 1 of explosive atmospheres. This solution was developed based on the advancement of Fieldbarrier (EEx meIICT4) technology. The idea is to move the safety barriers, which serve to limit energy supply, from the start of the main line to various branches, namely within the chicken-foot junction boxes, so as to restrict energy supply for each individual field instrument. This approach enables compatibility between FISCO and Entity instruments, allowing them to be used together in the same network segment. The intrinsically safe calculation for this type of bus only requires following the parameter certification method from the field safety barrier to the field instruments. This method is currently quite practical and is increasingly being accepted by more users. In recent years, with China’s full integration into the IECEx system, it has also largely aligned itself with international standards in terms of testing and certification technologies for explosion-proof electrical products. With the widespread use of intrinsically safe explosion-proof technology in hazardous industries such as petroleum and chemicals in China, intrinsically safe explosion-proof systems are receiving increasing attention. The design of intrinsically safe explosion-proof systems is also governed by more stringent standards; intrinsically safety calculations have become an essential part of engineering design, thereby providing a more reliable safety guarantee for the safe operation of industries in China that are prone to explosions, such as those in the oil, chemical, and coal sectors. Those who are interested can visit the “Changhui Instrument Network” to read more valuable articles that are helpful for improving the level of instrument automation technology. Author: Wang Zhen