Weekly Topic: Discussing measures for dealing with dual-pump failures – Issue 21, July 4–10, 2011

When both diesel pumps used in coking fail at the same time, it is necessary to carry out repairs promptly or obtain other pumps of the same model from the company； The unit operates by reducing the feed volume to maintain production; fractional distillation adjustments are used to increase the flow rate of wax oil in order to control the temperature in the distillation tower. The intermediate circulation oil is sent out of the unit via a diesel pipeline using an intermediate circulation pump (i.e., as diesel). The maximum amount of reflux is used at the top of the distillation tower, while there is no control over the dry point of the coker gasoline ； Once the diesel pump is repaired, operations can be resumed

For different processes, there are various handling procedures. 1. For processes that cannot stop operating for even a moment, such as catalytic slurry pumps, if both pumps come to a halt, it is necessary to initiate an emergency shutdown procedure. 2. For processes where strict requirements do not apply, such as tower top reflux, it is possible to reduce the feed rate and maintain a low processing volume. By slightly opening the safety valve at the top, the pressure in the tower can be maintained to prevent overpressure, which in turn helps to gain time for the maintenance of the pumps.

Device name: Catalytic Cracking; Plant capacity: 90 WT/A. Location of the failure: Slurry pump (one in service and two in standby). Description of the failure: Under normal operating conditions, the slurry pump is designed to operate with one unit in service and two in standby; at that time, one of the standby pumps was under repair due to end-face leakage caused by improper preheating. Due to the high solid content, the impeller of the running pump wore out severely, so it was switched to another standby pump. However, the motor was overloaded, which caused a fuse to blow in the electrical system, preventing the standby pump from operating properly as well. Fault handling: 1. The operator on duty should promptly reduce the output to maintain stable operation. 2. Apply the re-refining oil pumping slurry process at the separation unit to ensure that slurry circulation is kept to a minimum. 3. Contact maintenance promptly to carry out emergency repairs on the pump with leakage at the original end face. Fault summary: 1. The routine maintenance of the backup pump needs to be strengthened. 2. Be aware of the risks associated with an increase in solid content. 3. Ensure the quality of repairs to the pump.

Due to a complete failure of the 1 million-ton catalytic slurry pump or leaks in the system pipelines, the plant’s production capacity was reduced by 80 tons; circulation between Unit 2 and Unit 1 was initiated to maintain production. A temporary vent was created at the bottom of the tower to connect to the pump, thereby maintaining an external flow rate, and this arrangement was in place for about 10 hours. The crude gasoline pump is completely faulty, which results in a reduction in the plant’s output. Gasoline is forced together with wastewater into the wastewater tank; from there, it is sent to the flare line through the top of the tank. After separation in the liquid separation tank, it is pumped to the waste oil tanks in the storage area. Hold on until one of the pumps is fixed. Isn’t that creative!

The last edit to this post was made by lijianhuai on 2011-7-6 at 15:40. When both liquid hydrocarbon pumps stop operating, it has a significant impact on the absorption and stabilization system: the reflux at the top of the stabilizer is interrupted, the load on the top of the stabilizer increases, the temperature there rises, the proportion of heavier components in the liquefied gas increases, the C5 content in the liquefied gas rises as well, the volume of liquefied gas increases, and the pressure in the stabilizer becomes extremely high. In such cases, the following actions should be taken: (1) Contact mechanics, electrical engineering, and other relevant teams to carry out emergency repairs on the liquid hydrocarbon pumps; (2) Take the compressor out of service and operate it at low speed, and contact the dispatch team to divert the liquefied gas to tanks designated for unqualified products； (3) Coordinate with the reaction unit to reduce the processing volume and treatment depth ； (4) Appropriately reduce the temperature at the bottom of the stabilizer tower, maintain the liquid level at the bottom of the tower at an appropriate level, and minimize the amount of gasoline components that reach the top of the tower ； (5) When the pressure in the stabilizer tower is excessively high, activate the pressure relief line from the stabilizer tower to the fuel pipeline network to bring it back to normal pressure ； (6) Close the valves for dry gas and liquefied gas outlet to maintain system pressure ； (7) Stop rich gas washing ； (8) The crude gasoline continues to be fed into the absorption stabilizer; the bottom temperature of the stabilizer must be well controlled to ensure that the 10% cut point and vapor pressure of the stabilized gasoline are within acceptable limits ； (9) Maintain the liquid level and pressure in the stabilizer top reflux tank; once the pump is repaired, activate the top reflux ；

Our facility has three centrifugal pumps of the same model, with a flow rate of 630 liters per hour per pump. Under normal circumstances, only one pump is operated at a time. However, due to the quality of our water, the pumps suffer from severe scaling; even the backup pumps are affected by this problem. It is common for the operating pump to fail, with the backup pump failing to start as well. Finally, the following two measures were taken: 1. A connecting pipe was installed between this pipeline and the fire water pipeline for emergency use. 2. The standby pump is started once a week and run for a while.

Such an accident has not occurred in our facility, but we recently prepared an emergency response plan. We would appreciate your advice: In the delayed coking unit with a capacity of 4.2 million tons per year, both diesel return pumps have failed. The basic process for diesel return is as follows: Diesel collection tank > Diesel return pump > Reboiler at the bottom of the desorption tower (in addition, about 50 tons of diesel is directly returned to the lower trays of the collection tank as heat reflux) > Pump for low-absorption diesel > Air coolers and water coolers for the low-absorption diesel > Reabsorption tower > Return to the distillation tower. Actions to take: 1. Reduce the flow rate and contact maintenance personnel to repair the faulty pumps urgently. 2. Slightly increase the amount of wax oil being returned. 3. Allow the diesel collection tank to reach its full level so that excess diesel overflows (this overflowed diesel can be used as heat reflux); adjust the amount of diesel leaving the facility based on the temperature in the lower part of the collection tank, in order to control the amount of overflowed diesel. 4. Adjust the quality standards for diesel products. 5. Fully open the auxiliary lines of the diesel return pumps and start both pumps for low-absorption diesel, in order to maintain a sufficient flow rate. At the same time, the amount of gasoline returning to the system can be increased appropriately in order to maintain the temperature at the top of the distillation tower (this can improve the cooling effect on the diesel product, and reducing the temperature of the diesel returning to the system helps to control the temperature at the top of the distillation tower). 6. Conduct frequent analyses of the quality of gasoline products, dry gas, and liquefied gas; if any defects are detected, switch the absorption and stabilization system to a three-tank circulation mode

Valve replacement time: June 25, 2011. Valve replacement location: Inlet valve of Diesel A pump. Handling procedure: At 8:18 a.m., the workshop ordered that the processing volume be reduced to 63 T/H. At 8:20, the diesel and rich diesel in the reabsorption tower were isolated, and the manual valve in front of the control valve was closed. At 8:20, the amount of diesel supplied externally was increased to lower the liquid level in the diesel tank; at 8:40, the temperature at which diesel was drawn out was reduced by increasing the flow rates of the intermediate and top circulation streams. (The opening degree of the mid-section temperature control valve is 90% at 8:30, 92.5% at 8:40, 95.5% at 8:56, and 99% at 9:06); it drops to 3% at 9:05. At this point, the cross-line valves for the diesel-sludge oil heat exchanger, the diesel-enriched diesel heat exchanger, as well as those for the diesel-water-cooled sections are opened to a position of 4–6 turns – this is done to reduce the resistance in the flow of fluid and to prevent pressure buildup due to vaporization of light oils, which could damage the heat exchangers. Afterwards, attention should be paid to the temperature of the diesel being sent out; during the processing, this temperature does not exceed 56°C. 9:06 Close the 2 diesel extraction tower wall valves and 1 diesel gas return valve on the fractionation tower. At 9:10, open the two diesel purge lines on the fractionation tower; simultaneously open the outlet valves of the two diesel pumps, slightly open the pump bypass valves, and also slightly open the inlet drain valve of diesel pump B (the purpose is to observe when vapor appears in the purge streams). At 9:20, Pump B for diesel was emptied and shut down. At this point, the top temperature cannot be controlled; turn on the cold reflux to keep the top temperature in check. Increase the amount of wax oil sent out based on the liquid level in the wax oil collection tank. At 9:24, vapor was detected in the drain at the inlet of diesel pump B; at 9:26, vapor was detected in the drain at the inlet of diesel pump A (a large amount of coke powder was present in the inlet valve of pump A, causing obstruction, which is why vapor was detected only after dealing with the inlet drain). At 9:27, the temperature of the diesel being drawn out dropped to 200°C. Close the tank wall valve of the diesel tank, simultaneously stop the steam supply, close the outlet valves and crossover valves of the two diesel pumps, and fully open the inlet drain valves of the two pumps (to release pressure in the pipelines). At 9:28, the construction team began to disassemble the inlet valve of diesel pump A. At 10:00, they tightened half of the bolts on the diagonal of the flange attached to the inlet valve of pump A (the lower flange could not be tightened because it was stuck due to coke particles; cleaning of the coke was carried out first). At this point, the two tower wall valves for diesel extraction and the gas return valve were opened, and the liquid level in the diesel tank began to rise. By 10:13, the liquid level had reached 75%. At 10:20, the inlet valve of diesel pump A was closed, while the extraction valve of the diesel tank was opened. By 10:30, the flanges on the valves were connected, the inlet valve of diesel pump B was opened to fill the pump, and the cross-flow valves for the diesel-sludge heat exchanger, the diesel-enriched diesel heat exchanger, as well as those for the diesel water-cooling circuit and the sludge circuit were closed. At 10:34, the diesel B pump was started; the flow rate was not good and there were fluctuations, so the pump was adjusted. Adjustments were made to restore the various reflux levels and temperatures; normal operations were resumed at 11:50. 12:45 Diesel and rich diesel were put into use in the reabsorption tower.

In atmospheric and vacuum distillation, liquid hydrocarbon reflux pumps are used. To ensure zero leakage of liquefied gas during the initial phase of plant construction, magnetic pumps were chosen; however, these pumps are very delicate and prone to damage. When both pumps fail at the same time, the heat source supply to the stabilizer and desorber towers is cut off, gasoline is routed through an alternative path out of the plant, while dry gas continues to flow out along its normal route.