Improved safety system in a nitric acid plant

Transcription

1 NITR IC ACI D ALAR ARM AND SAFE FETY SYSTEM Improved safety system in a nitric acid plant The existing alarm and safety system in a nitric acid plant in Croatia was replaced by a new microprocessor-based system in order to increase the safety requirements and to modernise the production process. The main task of such a system is to continuously monitor all important process parameters and quickly inform operators about potential dangers that could lead to disastrous consequences and associated hazards. Above: Petrokemija nitric acid plant in Kutina, Croatia. The nitric acid production process is a very demanding process from a safety Tstandpoint. Special attention must be Ttaken regarding power recovery by the turbo set and the reactor section. The turbo set is the mechanical equipment that drives the air and nitrous oxide compressors. In the reactor section, there are special preconditions relating to the exothermal reactions involving the oxidation of a gaseous mixture of ammonia and air. In order to prevent disastrous consequences and hazards, the nitric acid production process must be continuously monitored using control and safety systems. A control system is considered to be safety related if it provides functions that significantly reduce the risk of a hazard, and in combination with other risk reduction measures, reduces the overall risk to a tolerable level, or if it is required to function, maintains or achieves a safe state for the equipment under control. The project scope Petrokemija is a fertilizer producer located in Kutina, Croatia. Nitric acid is produced in a dual pressure process in two identical production lines with an overall production capacity of 810 t/d of 100 % HNO 3. The original alarm and safety system in the nitric acid plant at Petrokemija consisted of a combination of an electrical relay-based safety system and transistorised alarm modules. However, over the years, it had become very unreliable and it was necessary to revamp and upgrade it to a new microprocessor-based system. The original system provided the following functions, which were also required in the new system: logic inputs and outputs for the first failure sequence; switching off the motors; control using NO (normal open) contacts and alarm signal repetition with visual and acoustic control. The existing system was replaced in three phases: the analysis phase, the implementation phase, the operation and maintenance phase. All three phases were carried out in accordance with the international standard IEC and IEC for the functional safety of safety instrumented systems. Nitrogen+Syngas 322 March-April

2 Table 1: Main causes and effects for emergency shutdown procedure Possible causes Effects Emergency STOP pushbutton in the control room 1. Closing the two electrical solenoid valves at the pipe of the gaseous ammonia and opening the startup relief valve. Emergency STOP pushbutton at the local control panel of the turbo set 2. Closing the extraction valve of the nitric acid from the absorption tower. Steam turbine overspeed Tail gas turbine overspeed Axial displacement of air compressor rotor Axial displacement of nitrous gas compressor rotor Axial displacement of steam turbine rotor Axial displacement of tail gas turbine rotor Low pressure of the lubrication oil for the turbo set Low pressure of the vacuum in the steam turbine condenser Low temperature of the tail gas before the DeNO x reactor Low temperature of the tail gas after the tail gas turbine 3. Closing the inlet valve of the quenching water before the steam super heaters. 4. The recirculating pump for the steam super heaters is stopped. 5. Emergency shutdown procedure of the turbo set, quick trip, which comprises: 5.1. Closing the steam inlet valve for the steam turbine Opening the relief valve of the air compressor to the atmosphere Opening the relief valve of the nitrous gas compressor to the atmosphere Closing the inlet valve for the tail gas turbine Opening the bypass valve of the tail gas turbine to relieve it. 6. Closing the control valve of the liquid ammonia for the DeNO x system Fig 1: ESD and alarm system configuration ES/OS operator & engineering station AS41 7H /F PROFIBUS - DP (redundant) standard OS operator station industrial ethernet redundant failsafe signal Defects and safety effects in production Two different sets of defects were recognised at the Petrokemija plant. Defects I represents the most serious defects in production, after which the emergency shutdown procedure for the whole process (turbo set and process unit) must be conducted as soon as possible. In the case of the defects II, the process unit must first be shut down, while the power recovery with the turbo set may remain operational for three minutes to ensure the proper blow down procedure for all parts of the equipment and pipes in the nitric acid production process unit. Both defects I and II result in the corresponding safety effects in the production: the protection of process equipment and process staff in order to avoid possible hazardous situations. Table 1 shows the main causes and effects of the emergency shutdown sequence and Table 2 lists the same for the normal shutdown sequence. Each cause will automatically and simultaneously trigger all the effects that are listed in the right-hand column of the Tables 1 and 2. In addition to the already mentioned safety causes and effects, there are further alarm states, trips and interlocks for process parameters that trigger an alarm as a preliminary warning so the operator can 50 Nitrogen+Syngas 322 March - April 2013

3 Table 2: Main causes and effects for normal shutdown procedure Possible causes Very high level of liquid ammonia in the ammonia evaporator High pressure of the gaseous ammonia after ammonia evaporator Low pressure of the air for the oxidation with the gaseous ammonia Malfunction of the boiler feed water recirculation in the water jackets of the burners Malfunction of the recirculation of the boiler feed water in the boiler Very high level of the nitric acid in the separator before the inlet of nitrous gas compressor Low pressure of the cooling water Low pressure of the instrumental air High temperature of the catalytic gauzes Failure of the electrical power Normal STOP pushbutton in the control room Normal STOP pushbutton at the local control panel of turbo set Low temperature of the tail gas before the DeNO x reactor Low temperature of the tail gas after the tail gas turbine Effects 1. Closing the two electrical solenoid valve at the pipe of the gaseous ammonia and opening the startup relief valve 2. Closing the extraction valve of the nitric acid from the absorption tower. 3. Closing the inlet valve of the quenching water before the steam super heaters. 4. Stop of the recirculation pump for the steam super heaters. 5. Normal shutdown procedure of the turbo set after 3 minutes, slow trip, which comprises: 5.1. Closing the steam inlet valve for the steam turbine Opening the relief valve of the air compressor to the atmosphere Opening the relief valve of the nitrous gas compressor to the atmosphere Closing the inlet tail gas turbine Opening the bypass valve of the tail gas turbine to relieve it. 6. Closing the control valve of the liquid ammonia for the DeNO x system. take the necessary action to prevent an unexpected shutdown. The possible alarm states, trips and interlocks are listed in Table 3. They refer to both production lines. Determination of safety instrumented functions and safety integrity level One task when analysing all of the possible hazardous process states in the nitric acid production at Petrokemija, was generating the logic diagram that determines the recognised causes and consequential safety protection effects of the safety equipment and devices. The logic diagram represents every possible hazardous state listed in Tables 1 and 2. These possible causes are entered in an interactive digital logic simulator CEDAR LS in order to verify the correctness and functionality of the logic diagram. Other tasks included the identification of the safety instrumented functions and determining the safety integrity level by using the risk graph technique in a systematic team approach. With the help of the risk graph technique the safety instrumented functions as shown in Table 4 have been identified. It can be concluded that the safety integrity level is 1, which means that the probability of the failure on demand is between 10-2 and 10-1 per year with a risk reduction factor of between 10 to 100. Decision for a new process control and safety system The SIMATIC PCS 7 process control and safety system was chosen to replace the old Praxis electrical relay safety and transistorised alarm system. The new SIMATIC PCS 7 alarm and safety system from Siemens combines the functionality of a classic distributed control (DCS) and logical systems in a common hardware and software platform with integrated engineering tools and operator interface. Thanks to SIMATIC Safety Matrix, it meets safety standards up to Safety Integrity Level 3 (SIL3) according to IEC and IEC The system comprises (see Fig. 1) one redundant central controller SIMATIC AS 417-FH with integrated safety function, four SIMATIC I/O racks with redundant PROFIBUS DP interface, industrial Ethernet (system bus, terminal bus), and operator interface in the form of the combined operator/engineering station and operator station. It is supplied from a new UPS. All the process safety conditions have been implemented in the SIMATIC Safety Matrix, which is the basis for the new alarm and safety system. The key condition was the recognition of the first alarm responsible for the shutdown sequence, regardless of whether it is an emergency or normal shutdown procedure. The SIMATIC Safety Matrix was configured as engineering (ES) and operator station (OS). Siemens implemented the cause and effect method defined by the American Petroleum Institute in the API RP 14C guideline and safety standards in accordance with IEC and IEC providing functional safety up to SIL 3. The operator interface was defined in the form of the process diagrams, alarm and working groups. The standard and failsafe I/O modules for the digital/analog inputs/outputs of the process variables, including EX protection were installed in the four Simatic I/O racks. Finally the new system was connected to all process safety equipment in the field, including new solenoid valves, where the control voltage has been changed from 380 V to 24 V or 220 V. Nitrogen+Syngas 322 March - April

4 Table 3: This list shows the alarm states, interlocks and trips for common situations in both production lines Possible alarms, trips and interlocks Low and high level of the liquid ammonia in the ammonia evaporator Low temperature of the gaseous ammonia after ammonia evaporator High temperature of the high pressure steam after super heater Blocked oil filter in the oil system for the turbo set High temperature of the oil in the oil system for the turbo set High level of condensate in the steam turbine condenser Low pressure of the instrumentation air Low temperature of the catalytic gauzes in the burners Low and high level of the boiler feed water in the steam drum Low and high level of the nitric acid in the bleaching tower Low level of the nitric acid in the oxidation tower High level of the nitric acid in the separator at the inlet of the nitrous gas compressor Low and high level of the nitric acid in the absorption tower Low and high level of the nitric acid in the condenser of the weak nitric acid Low volume flow of the de-mineralized water for the absorption tower Low pressure of the high and low pressure steam Low level of the boiler feed water in the reactor Low and high level of the nitric acid in the storage reservoirs for the nitric acid Processing alarms, trips and interlocks Audible and visual alarms with the necessary information such as: 1. alarm condition 2. part of the plant affected 3. description of the required action 4. alarm priority 5. time of the alarm 6. status of the alarm 7. grouping and first-up alarms 8. has priority over lower grade alarms (e.g. the high alarm is suppressed when a high-high alarm is received) 9. suppression of the out of service plant alarms 10. suppression of the selected alarms during certain operating modes 11. automatic load alarm, load shedding and shelving Malfunction of the boiler feed steam pump Malfunction of the nitric acid pump for the end users Malfunction of the nitric acid circulating pump through the oxidation tower Malfunction of the extraction pump for the weak nitric acid from the condenser of the weak nitric acid Malfunction of the extraction pump for the condensate from the condenser of the steam turbine Malfunction of the boiler feed water circulating pump Malfunction of the nitric acid circulating pump through the absorption tower Malfunction of the de-mineralized water pump for the absorption tower Low and high temperature of all other process parameters involving all process streams (air, ammonia, nitric acid, steam, etc.) Easy engineering with Simatic PCS 7 Table 4: Values of the determined safety instrumented functions in the nitric acid production at Petrokemija Category of the safety instrumented function The consequential severity of the accident being prevented C2 Description Injury or occupational illness but no lost time The pre-safeguard likelihood of the accident W4 Expected to occur frequently (for example, once a month) The presence in the hazardous zone F1 Rare to more frequent exposure in the hazardous zone The probability of avoiding the hazardous event P1 Possible under certain conditions 52 Nitrogen+Syngas 322 March - April 2013

5 A final dedicated testing and training phase The implementation was followed by testing according to Tables 1 to 3, realised according to the guidelines for alarm systems such as EEMUA 191 and CHID Circular CC Tech safety 9 and finally the SIMATIC Safety Matrix. The upgrade was successfully concluded with a training and introduction course for the process staff (a team of 20 operators). From the very beginning, the system operated without any malfunctions. The complete project, from the analysis phase through commissioning and validation to training and implementation, took approximately one year, and was completed in January It was mainly carried out by the process and maintenance staff of Petrokemija. Siemens Croatia supported the migration and safe commissioning. A well-structured project, good engineering practice of the teams and required safety integrity level meant that the upgrade of the alarm and safety system in the nitric acid production at Petrokemija went smoothly. Further, this project formed the basis for the replacement of the existing pneumatic control system and for improvements in the DeNOx system as well as in trending and reporting. Above: Petrokemija nitric acid plant in Kutina, Croatia. Right: The absorption tower in Petrokemija s nitric acid plant in Kutina, Croatia The selected alarm and safety system is based on the SIMATIC Safety cause & effects matrix. This method has proven to be an extremely effective option for describing safety functions and for defining marginal and shut down conditions, says Mr. Nenad Zecevic, Head of DUKI 1, Petrokemija. With the upgraded alarm and safety system, improved safety measures have been implemented in the production of nitric acid at Petrokemija, and it has created the basis for further improving the production process in the form of better analysis of the safety issues, added Mr. Ivan Hoško, Lead Automation Engineer DUKI 1, Petrokemija. Acknowledgement This text is based on the article of Nenad Zečević, Ivan Hoško and Sven Pavlaković published in Kemija u industriji: N. Zečević, I. Hoško, S. Pavlaković, Nitric acid revamp and upgrading of the alarm and protection safety system, Kem. Ind. 61 (4) (2012) Nitrogen+Syngas 322 March - April 2013