Pressure testing issues for four high-pressure units in the CO2 stripping urea plant

Moderators, have you ever experienced maintaining pressure for 8 hours? Can the pressure be sustained?

It is recommended to conduct the pressure testing using the test pressure of the high-pressure device with the lowest pressure among the four, with the testing duration being no less than 30 minutes. After the pressure testing is completed, pressure holding should be carried out based on the design pressure of the high-pressure equipment with the lowest design pressure among the four units, with a pressure holding time of 24 hours being appropriate. However, during actual pressure testing, due to the large number of equipment, pipes, connection points, etc., it is often impossible to maintain the pressure. However, through pressure testing and hold-pressure tests, the areas with obvious leaks need to be identified.

1 Since it is a pressure test of the equipment, it should be conducted separately to avoid masking any contradictions or defects； 2 These four devices are high-pressure equipment; the determination of their test pressure, as well as the test methods and requirements, must all be carried out in accordance with the provisions of GB150-98 to ensure production safety. The hydrostatic test pressure specified in GB150-98 is as follows: Pt = 1.25P/t. Where: Pt --------- Test pressure, in MPa; P ------------ Design pressure, in MPa; ------- Allowable stress of the container component material at the test temperature, in MPa; t ------ Allowable stress of the container component material at the design temperature, in MPa. Note: 1) If a maximum allowable operating pressure is specified on the nameplate, the design pressure in the formula should be replaced by this maximum allowable operating pressure ； 2) When the materials used for various components of the vessel (cylinders, heads, nozzles, flanges, fasteners, etc.) are different, the smallest value among the /t ratios of the materials of each component shall be adopted. 4 For specific regulations and requirements, refer to GB150-98 (Steel Pressure Vessels)

The pressure testing before starting up high-pressure equipment is primarily done to check whether there are any leaks at the various sealing points of the equipment, pipes, etc. Piping, equipment, instruments, etc. undergo pressure testing before leaving the factory, so users do not need to conduct further pressure testing. Unless there is suspicion of a defect in the device. As long as the operating pressure is tested and no leakage occurs after holding the pressure for 30 minutes, it is considered qualified and can be put into use with confidence. The poster used water for pressure testing – how much water is needed? It’s more economical to use a compressor to inject carbon dioxide gas for testing.

1. The leak detection for the four high-pressure devices is carried out simultaneously with the leak detection of the entire high-pressure system; there is no need to maintain a specific pressure – leak detection can be performed once the high-pressure system reaches its operating pressure, and a slightly higher pressure than the normal operating pressure is sufficient. 2. The four high-pressure units that have been installed cannot be pressure-tested individually (unless it is **mandated**).

First, add water using the methylamine pump and the liquid ammonia pump, and finally use the flushing water pump, with the pressure to be set at 20 MPA. If the flushing water pump doesn’t work, use a pump that applies pressure through an end door to add water. It is considered qualified if the pressure remains stable for 30 minutes.

External conditions: Contact the production department to prepare approximately 200 tons of qualified deionized water for the cold hydrostatic testing of the high-pressure system. Steps for cold hydrostatic testing of high-pressure systems: I. Preparatory work before testing: 1. Close the two isolation valves at the liquid outlet of the stripping tower, the main isolation valve for CO2 entering the stripping tower, and the 11/2” bypass valve ； Discharge valves, flushing valves for various pipelines, steam lines for cleaning and blocking, drain pipes, and sampling valves. 2. Open the dual valves for discharging from the high-pressure washer, the gas-phase shut-off valve of the high-pressure washer, HPV2206 (the vent valve of the synthesis tower) and its shut-off valve, the gas-phase sampling valve of the synthesis tower, HPV2203 (the control valve of the high-pressure injector), HPV2201 (the liquid outlet valve of the synthesis tower), XPV2201 (the rapid shut-off valve for high-pressure ammonia), XPV2202 (the rapid shut-off valve for high-pressure CO2), PPV2203 (the gas-phase outlet valve of the low-pressure absorption tower), LPV2205 (the liquid outlet valve of the low-pressure absorption tower) sowie the drain line behind these valves; then close their respective shut-off valves. 3. Start the steam condensate pump to supply fluid to the high-pressure flushing water pump; use the high-pressure flushing water and high-pressure discharge pipeline to fill the overflow pipe of the high-pressure washer first, then close the discharge valve on the fourth floor. The high-pressure flushing water pump fills the stripping tower from its bottom. 4. Open the pump inlet flushing water valve, start the ammonium methoxide pump and the ammonia pump, use a ejector to adjust the pressure difference between the front and rear to 1 Mpa, and fill the high-pressure scrubber, the high-pressure ammonium methoxide condenser, and the synthesis tower with liquid respectively. After the liquid is drained through the drainage lines between the gas phase of the urea synthesis tower, the sampling valve, the gas phase vent valve, the exhaust valve at the top of the high-pressure scrubber, and the liquid outlet valve of the low-pressure absorption tower, all these valves should be closed. The ammonia pump, the ammonium methoxide pump, and the high-pressure flushing water pump should also be stopped, along with all the flushing water valves, as well as the two shut-off valves at the outlets of the high-pressure ammonia pump and the ammonium methoxide pump. Check the system for any leaks, and fix them if present. 5. After confirming that there are no leaks in the system, start the high-pressure flushing pump. Pressure is increased for the high-pressure system via the CO2 pipeline at the bottom of the stripping tower, using the high-pressure flushing pump and HV2202 (with XPV2202 closed), at a rate of 3 MPa/h. As the system pressure continues to rise, it is necessary to check every 30 minutes for any leaks in the various components of the high-pressure system, at the connections of the pipelines, and at all leak detection points. If a leak is detected, the pressure must be reduced promptly to address it, after which the pressure can be increased again. 6. When the pressure of PI2201 (high-pressure system pressure) rises to 6 MPa, conduct a thorough inspection of the high-pressure system and eliminate any leaks. 7. When the pressure of PI2201 (high-pressure system pressure) rises to 10 MPa, conduct a thorough inspection of the high-pressure system and eliminate any leaks. 8. When the pressure of PI2201 (high-pressure system pressure) rises to 15 MPa, conduct a thorough inspection of the high-pressure system; any leaks should be repaired, after which the pressure is raised to 15.5 MPa. 8. The system is held under pressure for 15–20 minutes; if the pressure drops by no less than 15 MPa during this time, and there are no leaks, the pressure is maintained for another 30–60 minutes. If the pressure remains stable, the hydrostatic test is considered successful. II. Precautions: 1. Open the gas-phase valve of the high-pressure washer and the gas-phase vent valve of the urea synthesis tower to release gas, in order to eliminate inert gases from the system ； 2. Since the pipe from the high-pressure washer overflow tube to the high-pressure injector is too long, it is necessary to fill the system first from the high-pressure flushing water pipeline located in front of the high-pressure injector, thereby preventing the liquid from flowing down suddenly and causing damage to the high-pressure injector and the pipes ； 3. The liquid filling at the overflow pipe of the synthesis tower must be carried out in accordance with the method described above, to avoid accidents similar to those mentioned in point 2 ； 4. The dual valves for the discharge of the high-pressure washer shall remain open, with the pressure increase rate controlled at 3 MPa/h to protect the explosion-proof plate inside the high-pressure washer. 5. The deionized water used must be of qualified quality, with a CL- level of ≤5PPm; ideally, it should be less than 1PPm. A level exceeding 25PPm is strictly prohibited to avoid damage to the equipment materials. 6. For the new valve being installed, the flange in front of the valve must be removed or a drain line connected in front of the valve to check whether any liquid flows out. If any are present, make a record of them and address them after the voltage rise test is completed. Otherwise, after feeding the material, the solution in the high-pressure system leaks into the discharge pipeline and the high-pressure flushing pipeline, causing crystallization blockages that affect operations ； III. Pressure relief: First, open the gas-phase isolation valve of the high-pressure washer, and reduce the pressure in the high-pressure system at a rate of 5 MPa per hour, ensuring that the high-pressure system remains filled with cold water. IV. Liquid drainage: 1. When draining liquid, be sure to first drain the liquid from within the urea synthesis tower, and then the liquid from the overflow pipe of the urea synthesis tower, in order to avoid damaging the overflow pipe. 2. When draining liquid, the gas-phase vent valve of the urea synthesis tower and the exhaust valve at the top of the high-pressure scrubber should be opened to connect with the atmosphere, thereby preventing a vacuum from forming inside the equipment and causing damage to it. 3. After the pressure testing is complete, open the discharge valves on the fourth floor of the urea synthesis tower, the high-pressure ammonium methoxide condenser, and the high-pressure washer; finally, open the discharge valve of the stripping tower to drain all water accumulated within the equipment, and then close them. Safety precautions for starting up machinery: 1. The process of starting up machinery shall be under the unified command of the dispatcher and team leader; operations within the workshop should be carried out in accordance with the workshop’s specific procedures for starting up equipment. 2. The rate at which temperature and pressure are increased during the startup process must be strictly followed according to the operational guidelines, in order to prevent damage to the equipment.

6# ttlkk: Pressure testing and leak detection should be carried out using both water and air. First, pressure testing is done with air, followed by testing with water. The pressure held during the air testing phase is maintained for 24 hours, and records are kept to enable the calculation of the leakage rate. Although high-pressure equipment undergoes pressure testing upon leaving the factory, it still needs to be tested for pressure and leaks before being put into use

Thank you, upstairs user :victory: