Handling of instrument air failure

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Which device experienced this air shutdown incident?

Typical units are equipped with a low-low shutdown interlock based on the three-out-of-two principle for instrument air pressure.

Some of them are too simple; it seems to be the accident procedures for a certain area

In fact, if the instrument air stops, it’s sufficient to handle it as an emergency shutdown; there are no other options

This post was last edited by Bingfeng Xueshang on 2015-10-27 09:22. Emergency plan for wind-stop incidents; reasons for purification air shutdown: 1. Power air compressors stopping operating unintentionally or failing. 2. Other departments made extensive use of air without contacting us in a timely manner. 3. There were severe leaks in the pipelines. Phenomenon: The pressure of the purge air and industrial air decreases. When the pressure of the purge air falls below 0.3 MPa, the pneumatic butterfly valves and gate valves may malfunction prematurely; when it drops below 0.15 MPa, all control valves cease to function, and instruments showing volume, density, pressure drop, etc., also stop working properly. When the industrial-style pressure is below 0.25 MPa, the loose points and back-blow points of the two devices may become blocked. Treatment: During normal operation of the device, the pressure of the purified air and industrial air should be maintained above 0.4 MPa; under normal operating conditions, this device has a pressure of around 0.6 MPa for both types of air. If a drop in the pressure of the cleaning air is detected during normal production, the plant dispatcher should be contacted promptly to identify the cause; the workshop must prepare for emergency situations, and personnel in all positions should return to their posts immediately. When the wind pressure is between 0.6 MPa and 0.4 MPa, the device maintains normal operation. If the wind pressure remains below 0.4 MPa and continues to drop, all personnel responsible for the relevant systems should prepare for self-protection measures. When the wind pressure falls below 0.3 MPa, the following actions must be taken: For the reaction unit: 1. Activate the self-protection system to cut off the main air supply; once this system is activated, the operator must immediately open the large and small vent valves at the compressor inlet in order to relieve pressure in the settler. The assistant operator immediately went to the site to check whether the one-way damping valve of the main air pipeline was closed, whether the safety cut-off valves for the feed of crude oil and reprocessed oil were closed, and whether the return valve for the crude oil feed was open. Are the regenerative slide valve and the non-regenerative plug valve fully closed? Close the primary and secondary air manual valves of furnace 101 at the same time. Close the manual isolation valves for the feed of crude oil and the feed of reprocessed oil. Close the manual valve of the pre-lift dry gas control valve. Close all feed wall valves of the riser. After all the above actions have been completed, return to the control room to inform the chief operator and prepare for feeding the crude oil through the feed loop line to the return line. After the feed loop for crude oil and slurry is opened, the feed self-protection system resets, and the feed returns to the distillation unit via the nozzle. Current status: feed cut off, both reactors cut off, main air cut off; the regenerator is bed-packed. Fractionation unit: Once the main air protection system in the reaction unit is activated, the reaction gas stream is interrupted, the heat supply to the fractionation tower is cut off, and the temperature of the entire tower drops. The feed is returned to the crude oil tank via the return line, causing the liquid level in the crude oil tank to rise rapidly. The fractionation unit shall be operated as follows: 1. Stop the top circulation pump and the intermediate pump based on the tower top temperature and diesel temperature, stop the diesel pump, and close the hand valve for using diesel as a reabsorbent. Close the crude gasoline destabilization manual valve. If the top circulation pump stops and the tower top temperature remains low, the crude gasoline pump can be stopped. 2. Control the circulation of the crude oil, reprocessed oil, and slurry streams to reduce the feed of cold and hot slag. Contact the dispatch and atmospheric pressure department; discharge the slurry externally and remove the cooled slag from the unit, while ensuring proper control of the liquid levels in T201, V201, and V202. 3. Stop the diesel stripping steam and the vapor from the bottom of tower T201. 4. The V203 acidic water is discharged directly from the unit; close the acidic water discharge control valve. Control the V203 level properly. Current status: Circulation within the tower in three streams – crude oil, reprocessed oil, and oil slurry. Stabilization unit: Upon the interruption of feedstock supply for crude gasoline and rich gas, the stabilization unit carries out the following operations: 1. Close the manual control valves of the regulators for dry gas, liquefied gas, and stabilized steam; direct the non-condensable gas from V303 to the flare, and close the manual control valve for the intermediate tank where this non-condensable gas is sent. 2. Heat removal from the bottom reboilers of T302 and T304 is stopped. 3. Stable system three-tower cycle. Maintains pressure and liquid level. Unit position: 1. Once the main air automatic protection system for the reaction unit is activated, the main fan enters a safe operating mode, and the operator immediately shuts down the electric butterfly valve at the outlet of the main fan. Set the anti-surge valve output signal to fully open, i.e., 100%. The assistant operator immediately went to the site to check whether the one-way damping valve at the outlet of the main fan was closed and whether the anti-surge valve was fully open. 2. Due to the decrease in the pressure of the purge air, the primary and secondary pressure control valves may fail to function. Since the primary pressure control valve is a pressure-open valve while the secondary one is a pressure-close valve, the primary pressure control valve will close and the secondary one will open. This causes an increase in the pressure in the lubricating oil main pipe. The assistant operator went to the site and switched the primary and secondary pressure control valves to be controlled by the bypass valves. During the switching process, be careful not to let the main pipeline oil pressure drop too low, which could cause the main fan to stop. 3. After the main air protection system for the reaction unit was activated, the rich gas supply was interrupted; the pressures and temperatures of medium-pressure and low-pressure steam became unstable, resulting in an emergency shutdown of the gas turbine unit. After the emergency stop, the DCS immediately closed the inlet butterfly valve of the rich gas compressor. The assistant operator went to the site to shut off the steam seal extraction steam, and checked whether the turbine emergency shutdown valve and throttle valve were closed, as well as whether the air-operated gate valve at the compressor outlet was closed. Manually close the medium-pressure steam inlet manual valve of the turbine. Open the flare valve at the secondary outlet of the air compressor, which is the bypass valve of the secondary outlet safety valve. Switch the primary and secondary pressure-controlled regulating valves to secondary line valve control; the method is the same as that for the main fan. 4. After the main air protection system for the reaction unit is activated, the booster pump’s protection system causes it to stop operating; preparations should be made to restart the booster pump. 5. Ensure the air pressure unit performs its barring rotation properly.

If the instrument air supply is interrupted, a small amount of unpurified air can be introduced into the system for a short period of time to address the issue promptly; thereafter, the air supply valve should be switched to the bypass mode, while the manual valve should be used for controlling it. This is generally how it’s handled: if there is a long period of downtime, it is treated as a shutdown.