What are the important interlocks in nitric acid plants?
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What important interlocks have been installed to ensure the safe operation of the nitric acid plant?
We have several: gaseous ammonia pressure, temperature, drum liquid level, furnace temperature, ammonia-to-air ratio
A certain facility is designed to have an annual production capacity of 105,000 tons of dilute nitric acid with a concentration of 58–60% (equivalent to 100% purity), with an annual operating cycle of 8,000 hours, resulting in a production rate of 14.3 tons per hour. The main interlocks are as follows: 1. Process automatic shutdown – (1) Each process protection interlock triggers by closing the ammonia quick shut-off valve and the gaseous ammonia pressure control valve, thereby stopping the oxidation reaction. (2) The main process protection devices include: 1. Three process shutdown buttons. 2. Platinum mesh temperature in the oxidation furnace is excessively high: TISA126–128HH, three-out-of-two logic.
3. Outlet temperature of the ammonia superheater: TISA135LL.
4. Acid concentration in the NOX separator: TISA129HH.
5. NH3/Air ratio is excessively high: FFISA102HH.
6. Drum liquid level is excessively low: LICSA114LL.
7. Flow rate of boiler circulating water: FISA116LL.
8. Liquid level in ammonia evaporator A: LSA102HH.
9. Liquid level in ammonia evaporator B: LSA104HH.
10. Platinum mesh temperature in the oxidation furnace is excessively low: TISA102 1–3LL, three-out-of-two logic.
11. Ammonia-to-air ratio is excessively low: FFISA102LL.
12. Outlet pressure of the ammonia superheater: PISA103HH.
13. Liquid level in the nitrogen oxide separator: LSA106, low level.
14. Pressure in the instrument air main line is low: PSA120.
15. Fault in the 220VAC power supply.
16. Fault in the 240VDC power supply.
17. Shutdown of the rotary igniter triggers the operation of interlock valves, resulting in an immediate shutdown of the process. (3) If both of the following pumps fail and cannot start simultaneously, it will cause an automatic shutdown of the process: 1. Both boiler circulation pumps P102A/B stop operating; 2. When the dilute nitric acid pump stops, P104A/B also stop operating; 3. Both condensate pumps P113A/B stop operating. (4) By pressing the “Process Shutdown” button in either the control room or on-site, the ammonia quick-shut valve can be closed, thereby causing an automatic shutdown of the process. Delay by 3 minutes at the same time to stop the turbine compressor unit. (5) Any fault in the compressor or activation of the safety interlock system of the turbine unit will also immediately trigger a process interlock shutdown. (6) The methods for automatic process shutdown and planned shutdown are the same; the difference is that automatic process shutdown occurs at a higher load level. 2. Automatic shutdown of the unit: (1) In addition to the shutdown button on the local control panel of the unit, the turbine unit is equipped with two more shutdown buttons, one located in the control room and the other next to the turbine unit on site. By pressing these stop buttons, the turbine unit and the process can be interlocked to shut down simultaneously. However, this button can be used for emergency shutdown only in the event of a equipment failure or an injury. (2) The factors that can trigger the shutdown interlocks of the “four-in-one” unit are: 1. Low steam turbine inlet temperature; 2. High control oil pressure; 3. Low lubricating oil pressure; 4. Overspeed trip of the steam turbine; 5. Overspeed trip of the exhaust gas turbine; 6. Steam turbine overspeed; 7. Process shutdown signal; 8. Low control oil pressure; 9. Axial displacement out of limit; 10. Axial vibration out of limit; 11. Air compressor surge; 12. Nitrogen oxide compressor surge. (3) Other interlocks that can cause the shutdown of the “four-in-one” unit are: 1. Excessively high liquid level in the NOX separator; 2. Opening of the acid diversion valve leading to the startup acid tank; 3. Excessively low instrument air pressure; 4. Loss of power to the 380V AC supply (power supply); 5. Loss of power to the 100V DC supply (valve actuator power supply). (4) After the shutdown signal for the “four-in-one” unit is received, six interlock valves can be activated simultaneously to achieve automatic shutdown of the unit: 1. Closing of the main steam valve of the steam turbine; 2. Opening of the air compressor vent valve HICV1001A; 3. Opening of the air compressor vent valve HICV1001B; 4. Opening of the emergency circulation valve of the nitrogen oxide compressor EMV1004; 5. Closing of the exhaust gas turbine inlet valve HICV1002; 6. Opening of the exhaust gas turbine bypass valve HICV1003. (5) If it is confirmed that the shutdown was caused by a fault within the “four-in-one” unit itself, then restarting the unit is not allowed. If it is determined that the fault is not related to the unit itself, the unit should be restarted immediately, accelerated to 1000 rpm, and used to purge the process system at a speed below the critical speed for 2–3 hours. (6) Close the water addition valve for the absorption tower; handle the rest according to the normal shutdown procedure. Once the process system has been properly purged, shut down the ‘four-in-one’ unit.
3. Outlet temperature of the ammonia superheater: TISA135LL.
4. Acid concentration in the NOX separator: TISA129HH.
5. NH3/Air ratio is excessively high: FFISA102HH.
6. Drum liquid level is excessively low: LICSA114LL.
7. Flow rate of boiler circulating water: FISA116LL.
8. Liquid level in ammonia evaporator A: LSA102HH.
9. Liquid level in ammonia evaporator B: LSA104HH.
10. Platinum mesh temperature in the oxidation furnace is excessively low: TISA102 1–3LL, three-out-of-two logic.
11. Ammonia-to-air ratio is excessively low: FFISA102LL.
12. Outlet pressure of the ammonia superheater: PISA103HH.
13. Liquid level in the nitrogen oxide separator: LSA106, low level.
14. Pressure in the instrument air main line is low: PSA120.
15. Fault in the 220VAC power supply.
16. Fault in the 240VDC power supply.
17. Shutdown of the rotary igniter triggers the operation of interlock valves, resulting in an immediate shutdown of the process. (3) If both of the following pumps fail and cannot start simultaneously, it will cause an automatic shutdown of the process: 1. Both boiler circulation pumps P102A/B stop operating; 2. When the dilute nitric acid pump stops, P104A/B also stop operating; 3. Both condensate pumps P113A/B stop operating. (4) By pressing the “Process Shutdown” button in either the control room or on-site, the ammonia quick-shut valve can be closed, thereby causing an automatic shutdown of the process. Delay by 3 minutes at the same time to stop the turbine compressor unit. (5) Any fault in the compressor or activation of the safety interlock system of the turbine unit will also immediately trigger a process interlock shutdown. (6) The methods for automatic process shutdown and planned shutdown are the same; the difference is that automatic process shutdown occurs at a higher load level. 2. Automatic shutdown of the unit: (1) In addition to the shutdown button on the local control panel of the unit, the turbine unit is equipped with two more shutdown buttons, one located in the control room and the other next to the turbine unit on site. By pressing these stop buttons, the turbine unit and the process can be interlocked to shut down simultaneously. However, this button can be used for emergency shutdown only in the event of a equipment failure or an injury. (2) The factors that can trigger the shutdown interlocks of the “four-in-one” unit are: 1. Low steam turbine inlet temperature; 2. High control oil pressure; 3. Low lubricating oil pressure; 4. Overspeed trip of the steam turbine; 5. Overspeed trip of the exhaust gas turbine; 6. Steam turbine overspeed; 7. Process shutdown signal; 8. Low control oil pressure; 9. Axial displacement out of limit; 10. Axial vibration out of limit; 11. Air compressor surge; 12. Nitrogen oxide compressor surge. (3) Other interlocks that can cause the shutdown of the “four-in-one” unit are: 1. Excessively high liquid level in the NOX separator; 2. Opening of the acid diversion valve leading to the startup acid tank; 3. Excessively low instrument air pressure; 4. Loss of power to the 380V AC supply (power supply); 5. Loss of power to the 100V DC supply (valve actuator power supply). (4) After the shutdown signal for the “four-in-one” unit is received, six interlock valves can be activated simultaneously to achieve automatic shutdown of the unit: 1. Closing of the main steam valve of the steam turbine; 2. Opening of the air compressor vent valve HICV1001A; 3. Opening of the air compressor vent valve HICV1001B; 4. Opening of the emergency circulation valve of the nitrogen oxide compressor EMV1004; 5. Closing of the exhaust gas turbine inlet valve HICV1002; 6. Opening of the exhaust gas turbine bypass valve HICV1003. (5) If it is confirmed that the shutdown was caused by a fault within the “four-in-one” unit itself, then restarting the unit is not allowed. If it is determined that the fault is not related to the unit itself, the unit should be restarted immediately, accelerated to 1000 rpm, and used to purge the process system at a speed below the critical speed for 2–3 hours. (6) Close the water addition valve for the absorption tower; handle the rest according to the normal shutdown procedure. Once the process system has been properly purged, shut down the ‘four-in-one’ unit.