Anti-corrosion regulations for urea equipment and test plan for stripping tower pressure drop

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As an experienced veteran in the urea industry, I am sharing here the equipment anti-corrosion regulations for a large-scale urea plant with a production capacity of 520,000 tons per year, as well as the testing plan for measuring resistance during the maintenance of stripping towers, for the reference of new technicians. To prevent new members from losing money, the attachment format will not be used; please forgive us, moderator. Regulations on Equipment Corrosion Prevention Management 1. Professional Management 1.1 Each enterprise shall assign a deputy plant manager or chief mechanic (deputy chief engineer) to be in charge of equipment corrosion prevention work, and include the implementation of these regulations as a basic requirement for the upgrading of the enterprise’s equipment. 1.2 The anti-corrosion work for equipment is uniformly managed by the machinery departments of each enterprise. 1.3 The production technology department shall assign full-time technical personnel to carry out the following tasks: (1) The collection and organization of data related to process parameters. (2) Analysis and statistics of urea intermediate and final nickel contents. (3) Analysis and statistics of chloride ions in circulating water, temperature-regulating water, steam condensate, and the shell-side condensate of high-pressure ammonium methanate condensers. 1.4 The mechanical department assigns dedicated technical personnel to carry out the following tasks: (1) Establishing and regularly updating equipment corrosion records. (2) Conduct regular inspections of stainless steel equipment and heat exchange equipment in conjunction with relevant professional institutions. (3) Fill in the statistical table on the corrosion status of urea plants in a uniform format every year ； If necessary, attach a more detailed technical report and submit it to the Mobile Operations Department, Technical Department 1 of the Production Department at the head office, as well as the Mobile Operations Department and the Large Nitrogen Fertilizers Department of the Fertilizer Division in the Chemicals Department. (4) A separate summary report on major corrosion damage and repair work shall be prepared in conjunction with the relevant specialized departments, and submitted in accordance with the provisions of the preceding paragraph. 2. Regulations during production 2.1 Temperature-raising passivation: Operate strictly in accordance with the specified temperature-raising rate. According to the specified starting conditions in the operating regulations, the passivation time must be no less than 8 hours (including passivation through steam and air heating). 2.2 Normal operation: Strict control is exercised over the oxygen addition level (the oxygen content in the CO2 fed to the stripping tower should be between 0.6% and 0.85%). Each plant determines the minimum oxygen addition level, as well as the ratios of NH3/CO2 and H2O/CO2, the temperatures for synthesis and stripping, and the steam pressure on the shell side of the stripping tower, based on its equipment and operational conditions. In the following situations, immediate system drainage should be carried out, and the equipment or system should be inspected for re-passivation. (1) Oxygen interruption for 1 minute, with oxygen concentration < 0.6% (Vt) for 5 minutes ； (2) Load below 65% for 2 hours; (3) Distillation column temperature above 175°C for 2 hours, or above 185°C for 10 minutes ； (4) The nickel content in the finished product exceeded 0.30 ppm on two consecutive occasions (the nickel content was analyzed using a standardized method). (5) Leakage is detected at the leak detection hole (the leak detection tube is checked once per shift to ensure it is unobstructed). (6) An increase in ammonia content was detected in the cooled water or steam condensate; analysis confirmed that there was a leak in a certain device within the high-pressure system. 2.3 Tower Sealing The process of sealing the tower while it is parked is essentially a corrosive process; therefore, tower sealing operations should be minimized as much as possible. Requirements: (1) The tower sealing time for each normal shutdown shall not exceed 12 hours ； The emergency stop shall not exceed 8 hours ； The second tower shutdown within 24 hours shall not exceed 4 hours, with an interval of at least 3 hours between shutdowns. (2) During the tower sealing period, the pressure in the synthesis tower should not drop too rapidly; it should be kept above 80×0.1 MPa as much as possible. 2.4 Water Quality (1) The chloride ion content in high and low pressure temperature-regulating water, steam condensate, and the shell-side condensate from high-pressure ammonium methoxide systems must not exceed 0.5 ppm; otherwise, replacement is required. (2) The conductivity of steam condensate should not be higher than 30 us/cm; if it is excessively high, an analysis of the NH3 content should be conducted immediately to identify the cause as soon as possible. (3) The chloride ion concentration in the circulating water of the urea feed unit shall not exceed 100 ppm. (4) The condensate at the bottom of the shell side of the high-pressure ammonium methoxide condenser should be drained regularly (once per shift, for 30 minutes each time). 2.5 Analysis Items To ensure adequate monitoring of the corrosion condition of the equipment, the following analysis items are specified: (1) The Ni content in the finished products shall be analyzed once daily, and the Ni content of all materials in the system shall be analyzed once a month. The main components are the synthesis tower, stripping tower, circulation heater, and the liquid phase in the urine tank. In special situations, such as starting up for the first time, starting up after tower sealing, when the stripping tower is overheated, or in the event of oxygen loss, sampling and analysis should be carried out promptly. (2) Analyze the Cl‑ content of the circulating water once a day. (3) Analyze the NH3 and Cl‑ contents of the warmed water and steam condensate once a week. 3. Equipment maintenance regulations (detailed contents relate to the maintenance of safety equipment and safety procedures). 3.1 It is not allowed to use pressure-containing plugging methods to address leaks in the four high-pressure urea equipment units. 3.2 Where the equipment permits, the pressure used for ammonia leak testing on the shell side of the high-pressure heat exchanger can be appropriately increased to reduce the pressure holding time and improve leak detection sensitivity. Suggestions: (1) Distillation column (1.0–1.6 MPa) ； (2) High-pressure condenser (0.4–0.5 MPa) ； (3) High-pressure washer (0.4–0.6 MPa). Maintain the pressure under the above conditions for no less than 4 hours. 3.3 The maintenance of various equipment shall strictly follow the \"Equipment Maintenance Procedures\". 3.4 Process equipment should be maintained in good insulation condition in accordance with standards ； Special attention should be paid to the end caps and lifting lugs; if necessary, additional pipes should be installed ； It should be avoided that rainwater flows into the insulation layer along the steam pipes. E2201 Lift Pipe Pressure Drop Test Procedure (Internal)
I. Preparation Work:
1. Items required for the lift pipe pressure drop test:
—— One electronic pressure gauge;
—— One pressure reducing valve;
—— 10 meters of rubber hose with a diameter of Æ mm;
—— One tee fitting with a diameter of Æ mm.
2. Connect the items required for the test in the manner shown in the attached diagram.
3. Establish a liquid seal at the lower part of E2201:
—— Connect and secure the lower end of the transparent rubber hose to the drainage main pipe; fix the upper end (which remains open) to the third-floor surface.
—— Close the second shut-off valve before LV029 and the second valve for CO2 inlet; open the main drainage valve, the first shut-off valve before LV029, the main flushing water valve, and the drainage main pipe’s drainage outlet.
—— Start G2702 to inject water into E2201; (if this test is conducted after calibrating the liquid level in C2201, the next steps can be carried out during the liquid drainage process.)
—— Stop injecting water (or draining liquid) when the liquid level indicated by the transparent rubber hose reaches the second-floor level (elevation of ~8 meters).
II. Determining the “Reference Pipe”:
1. Align the opening of the measuring pipe vertically with the opening of the lift pipe, press it down firmly, open the main instrument air supply valve, and slowly adjust the regulator knob until the pressure gauge reads a stable value of 2.5 kPa.
2. Select 40 pipes randomly from all the pipes to conduct pressure drop tests, and record the serial number of each pipe along with its test results.
3. Calculate the average pressure drop value for these 40 pipes, and select one pipe whose value matches this average as the “reference pipe”.
4. Repeat step “1” to ensure that the reading of the “reference pipe” remains stable at 2.5 kPa.
5. Mark the “reference pipe”. III. Testing: 1. Mark at 2.0 and 3.0 kPa on the electronic pressure gauge (i.e., the pressure drop difference across all tubes must not exceed ±0.5 kPa) ； 2. Measure the pressure drop for all pipes one by one in a specified order (for example, from south to north as a whole, and from east to west within each row); once the water column stabilizes, record the row number, pipe number, and pressure gauge reading for that pipe. 3. For pipes with readings greater than 3.0 kPa, clean them using a large volume of air and then measure again; if the value is still above the acceptable range, make a record and clearly mark it. 4. For pipes with readings less than 2.0 kPa, check whether there is any looseness in those pipes, take another measurement; if the value is still above the acceptable range, make a record and clearly mark it. 5. After measuring 100 pipes, calibrate the 2.5 kPa reading using a “reference pipe” once. 6. Before resuming measurements after an interruption, it is necessary to calibrate the 2.5 kPa reading again using a “reference pipe”. 7. Once all measurements have been completed, clean the fixing plate of the lift pipe and drain the liquid level in E2201. 8. Submit the measurement results to the relevant departments for verification and further processing. IV. Precautions: 1. When working inside the air lift tower, relevant safety regulations must be followed; 2. The lighting and measuring equipment used inside the tower must comply with relevant safety standards; 3. Persons entering the tower should wear canvas shoe covers and work uniforms, and must not carry small items that could easily fall off. E2201 Method for Determining the Pressure Drop in the Intake Pipe (External) I. Preparation: 1. Items required for determining the pressure drop in the intake pipe: —— One testing workbench ； ——One electronic pressure gauge; one —QGD—200 pneumatic setpoint regulator; 10 meters of Æ22 mm rubber hose; one glass tube flow meter. 2. Determine a stable gas source for measurement (service air or bottled nitrogen) ； 3. Connect the items required for measurement in the manner shown in the attached diagram; II. Determine the “reference tube”: 1. Randomly select 40 lift tubes from all the tubes ； 2. As shown in the diagram, vertically align the opening of the tube to be tested with the gas supply inlet, and press the handle to seal the air inlet hole ； 3. Open the main gas supply valve, and slowly adjust the setter knob; once the pressure gauge shows a stable reading of 2.5 kPa, observe and record the reading indicated by the glass tube flow meter ； 4. Measure the pressure drop for the 40 lift pipes, and record the serial number of each pipe along with the resulting pressure drop values. 5. Calculate the average pressure drop across all 40 pipes, and select one pipe whose value matches or is close to this average as the “reference pipe”. 6. Re-measure the pressure drop of the “reference pipe” by adjusting the knob on the regulator until the gauge reading for this pipe stabilizes at 2.5 kPa; record the reading indicated by the glass tube flow meter ； 7. Mark the “reference tube” and keep it safely for future use. III. Measurement: 1. Measure the pressure drop of all lift pipes one by one; pipes with electronic pressure gauge readings between 2.0 kPa and 3.0 kPa are considered qualified (i.e., the pressure drop difference for all pipes must not exceed ±0.5 kPa), and such pipes should be placed in the designated area. 2. During measurement, a visual inspection must also be carried out: check whether there is any obvious bending, deformation, or corrosion of the pipes; verify that there are no significant differences in the size of the three liquid dropping holes, and that there are no foreign objects blocking them or any burrs; check whether the mounting ports are deformed. 3. Pipes with electronic pressure gauge readings greater than 3.0 kPa or less than 2.0 kPa should be inspected and cleaned before being measured again; those that still show deviations should be placed separately or handled under the guidance of technical personnel from the maintenance department. 4. After measuring 400 pipes, calibrate the 2.5 kPa reading using a “reference pipe” once. 5. Before resuming measurements after an interruption, the 2.5 kPa reading must be calibrated again using a “reference pipe”. 6. Throughout the entire measurement process, it is necessary to constantly monitor the readings of the glass tube flow meter to prevent inaccurate results due to pressure fluctuations ； 7. After all the measurement tasks are completed, the results shall be submitted to the relevant departments for confirmation and processing. IV. Precautions: 1. During the measurement process, the lift pipe must be handled with care to avoid collisions; 2. The storage method for qualified and unqualified pipes must ensure that they do not get mixed up; 3. The ambient lighting and measurement tools must comply with relevant safety regulations. This post was last edited by Zizhu Youjing on 2007-12-14 at 17:36.]