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Dangerous chemical or gas leaks pose an inherent risk to many industrial processes, especially to the chemical or oil and gas industries. Each year, explosions and misfires cause millions of dollars in damage to the chemical or oil and gas industry. Because of this potential loss, SISs are commonly used in industrial production to safely insulate flammable or potentially toxic materials in the event of a fire or accidental fluid leakage. Specifically designed to protect people, equipment and the environment, SIS reduces the probability of occurrence and the severity of the impact of a particular emergency. This safety control system comes with final control components such as emergency stop valves, emergency drain valves, emergency isolation valves and critical dual position valves. These valves, unlike conventional control valves, are continuously adjustable and usually require a standstill in one location and require reliable operation in the event of an emergency. SIS system composition and evaluation SIS is usually composed of three basic components: The field sensor is used to collect the necessary information to determine whether there is an emergency situation. These sensors are for SIS use and have separate process signal lines separate from the normal process information. Logic system used to determine the information collected and make the appropriate response. The system uses a highly reliable logic calculator with fail-safe and fault-tolerant operation. The final control component is responsible for implementing the decisions made by the logic system. The typical final control unit is a pneumatic valve. It is impractical to achieve zero-risk operation or 100% reliability. Therefore, one of the primary tasks for SIS system designers is to conduct a risk tolerance analysis to determine which level of security is required. IEC Standard 61508 (Functional Safety of Electrical, Electronic and Programmable Electronic Systems) is a common standard that includes functional safety related to all process industries and manufacturing industrial installations. IEC Standard 61511 is specifically for the process industry. Each of these standards sets the level of safety necessary to achieve precision and requires that the production unit provide a measurable compliance check. Since these standards were first introduced, SIL (Safety Integrity Levels) has been used as a quantifiable way to establish safety performance goals for SIS systems. There are 4 discrete SIL grades (SIL1, SIL2, SIL3 and SIL4). The three grades listed in Table 1 are the most representative. Due to the minimal use of SIL4, SIL3 is the highest designated safety class. Among the three commonly used grades, SIL3 has the highest available average safety rate (RSA) and therefore the lowest current failure rate (PFD). RSA is the time fraction of safety functions that the safety system can implement its design during the operation of the equipment. As can be seen from Table 1, RSA is equal to 1 minus PFD. Some safety analysts often like to use risk reduction factor analysis, the factor is the reciprocal of the PFD. Judgment of the target SIL requires: 1. Identification of the relevant hazards; 2. Assessment of the risk of the identified hazards. In other words, what is the expected magnitude of the hazard and how often it will occur; 3. Assess other IPLs that may appear on site. Danger recognition can be achieved with many different technologies. HAZOP and other dangerous operational software will help to carry out this analysis. Subsequently, each of the defined risks must be identified for its risk factors. Risks are the probability (probability or frequency) of each dangerous event and the function (severity) of the result. Table 2 illustrates five identifiable levels of risk based on the probability of occurrence and Table 3 illustrates the five identifiable levels of risk based on the severity of the occurrence. The overall risk can be measured as a number between 1 and 25 obtained by multiplying the risk level factors in Table 2 and Table 3. If the result is between 15 and 25, then the risk is higher and SIL3 may be required. If the result is between 1 and 6, the risk is considered low and SIL1 is sufficient. For results in the 6 to 15 cases, the risk is moderate, you can use SIL2. By analyzing the under what circumstances the various parts of the SIS system fail, the current failure rate (PFD) can be calculated. SIS system failure has two basic ways. The first, often referred to as harassment or deceptive processes, often leads to unplanned but secure processes. In this failure, there will be no danger, but will bring the loss of operating costs. The second kind of trouble will not bring the imitation process or the process interruption. On the contrary, it will not be detected, but it will make the process operation in unsafe or dangerous conditions. In the event of an emergency, the SIS system will not respond appropriately. These faults are hidden and will put the system in a dangerous way. The PFD of a SIS system is the sum of the PFDs of all the components in the system. To find the PFD for each component, the analysis specialist needs historical fault rate data that each component can demonstrate (such as the mean time between failures-MTBF). This failure rate is used along with the test interval (TI) to calculate the PFD. TI refers to the length of time it takes for a hidden fault to be discovered through testing. The value of TI is linear with the PFD; that is, if the TI is doubled, the PFD will be twice that of the PFD and the difficulty of achieving the target SIL will be doubled. Control System for Safety Systems The SIS control standard stipulates that plant operators must measure and document the design, maintenance, inspection, testing and safe operation of equipment. It is estimated that 40% to 50% of the cycle operation problems are caused by the final control components, especially the discrete (on / off) safety valve. If these valves remain in one position for long periods of time, contamination and corrosion from the surrounding fluid will keep them stuck and inoperable when needed. Therefore, in order to reduce PFD to the target SIL, these valves must always be tested. The more frequent the test, the less likely the problem is. Parking Detection To thoroughly test the final control components, the only sure way is to check the valve from 0% to 100% (fully open / fully closed) along the line. In general, safety system testing needs to be done without a by-pass, with the entire process being severed, resulting in significant financial losses. So managers are reluctant to completely shut down the parking valve just to test safety equipment. By convention, they wait until the unit has completed a production cycle and shut down to detect these valves. In the past, a unit's production cycle was about two to three years. However, with the increased reliability of mechanical equipment and the introduction of preventative maintenance measures, the plant's production cycle has been increased to 5-6 years. Although this can increase productivity and gain greater economic benefits, it also means that the frequency of testing of the final control components of the safety system is reduced. This will greatly affect the PFD of the system so that the system can not reach the target SIL. Bypass Online Testing To solve the above problem, many companies have proposed their own on-line test SIS valve program to avoid testing the SIS valve to shut down the entire device. A typical solution is to install a bypass for each valve. By using bypass, the safety valve can be fully tested without interrupting the entire process. In addition, faulty valves can be replaced online to make the machine easier to maintain. However, there are some disadvantages to the method of online testing by setting bypass. The first is when bypassing work, the entire process is unprotected. In addition, there is the problem of inadvertent use of the bypass after testing, leaving the device unprotected. Although the bypass test is to reduce the PFD value, not all bypass tests can achieve this goal. In calculating the PFD, the system must be kept in bypass operation time points into account. When bypassing for a long time or testing too often, the negative impact on the PFD will outweigh the benefits of passing the test. Local Testing In order to eliminate the operational or economic problems associated with the use of bypass tests, a number of other approaches have been developed. First, they realized that the most common problem with a discrete stop valve was that the valve could easily get caught in its usual position. For this type of failure, it is not necessary to test by fully switching the valve. Only a small adjustment of the valve position is required to test whether the valve is stuck or not and most of the valve concealment faults can be checked as such. If this test is conducted online, it is possible to increase the PFD without sacrificing product. Mechanical Limit Method This method ensures less than 15% valve movement by using mechanical devices such as pins, stem rings, or valve manual lifter, etc. These mechanical limit devices compare to expensive and complicated pneumatic test panels The price is not expensive. The disadvantage of this test is that the test must be started manually in the field, which requires a lot of manpower and is prone to error. In addition, the method can not test the safety stop function of the device during the test and can not easily pass the random inspection Found that the valve may not be able to work properly for a considerable period of time, but the operator knows nothing about it. Intelligent Positioner The Intelligent Positioner is a digital valve controller that has a microprocessor-based controller with communication capabilities, pneumatic control, and internal logic capabilities. In addition to converting the current signal to a pneumatic signal to control the valve, the intelligent positioner also uses the HART communication protocol to facilitate access to critical information for process operation. Intelligent positioner can receive feedback on valve position and supplemental and motivational barometric pressure. This allows the intelligent positioner not only to diagnose itself, but also to diagnose the valves and actuators to which it is connected. Due to its logic capabilities, intelligent positioners replace specialized field test panels and automate the testing process. The final control with intelligent positioner allows for partial movement and on-line testing without the need for dedicated mechanical restraints or other specialized test instruments. Partial Stroke Tests Partial Stroke tests allow the valve to be tested without interfering with normal operation of the entire process. Testing can be done more often because there is no need to completely cut off the entire process. Because TI is inversely proportional to PFD, the more frequent the test, the less likely it is to fail. At the same time the entire test process can be programmed into the intelligent positioner, part of the stroke test can be carried out without the concern of the operator automatically. This makes it possible to determine smaller TI's (hourly, daily or weekly) as needed to meet the target SIL value. The entire test procedure is fully automatic and eliminates the possibility of harassing or deceptive processes. A typical partial stroke test moves the valve 10% in its original position, but can move 30% if the unit's safety guidelines permit. While partial stroke testing does not eliminate the full stroke test (the full stroke test is used to test the valve seat), the frequency at which the full stroke test needs to be done can be drastically reduced, allowing the test to be completed after completing one work cycle. Communication Function Because the intelligent positioner communicates via the HART protocol, part of the stroke test can be initiated with a portable HART communicator, such as a PC with positioning software or a push-button control panel connected to the positioner terminal. Eliminate the need for costly pneumatic test panels and skilled testers, reducing capital equipment investment, testing time and labor requirements. At the same time, but also to achieve remote testing, save on-site maintenance inspection time. Monitoring and alarm functions Since the intelligent positioner can provide both position and diagnostic information, the status and response time of the valve are monitored during the test. After each partial stroke test, the valve's tendency to change in performance is monitored and automatically analyzed, so that those valves that may fail can be identified in advance. A cycle counter and travel memory will show the degree of valve operation. After the locator part of the stroke test, it will keep checking the valve operating conditions, to determine whether there is a correct response. If not responding properly, the positioner will automatically exit the test and alarm. In this way, if the valve has been fully prolapsed, it can be avoided to suddenly shut down. If an emergency situation requires parking while testing, the intelligent positioner will automatically stop the test and put the valve in a safe place. Adaptation The intelligent positioner can be installed on a variety of structural and shape of the valve, including the linear slider type, rotary type and right angle rotary type, with the exciter also includes spring and diaphragm actuators, spring return piston exciter Or double acting piston exciter. There are two possible installation methods. Both of these methods use a solenoid valve with a backup pneumatic device, ie the pressure of the actuator will at all times ensure that the valve can be moved to a safe position by venting. If the solenoid valve fails, the pressure in the actuator will be vented through the pneumatic device in the intelligent positioner, and the actuator pressure will be discharged through the solenoid valve if the intelligent positioner fails. The intelligent positioner helps provide proactive maintenance by providing functional degradation analysis of the valve, which is important for critical safety-related valves, while reducing the scheduled maintenance. The intelligent positioner has time and date stamps on all tests and reports, fulfilling the requirements of the relevant regulations and facilitating the comparison and interpretation of diagnostic data.
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