[Repost] Practice of Using Cage-Type Plug Valves for Slag-Acid Regulation Valves in Rotary Drum Compound Fertilizer Production Units

Thread Content

yunrun.com.cn/tech/4232.html Since the rotary drum compound fertilizer production unit at Qinghai Yuntianhua International Fertilizer Co., Ltd. came online, there have been two issues with the slag acid control valve: First, operating at a low opening degree for an extended period, with the valve being open at around 5%, makes it difficult to control the system and prevents automatic operation； Second, the valve bushing is eroded and damaged, the valve core corrodes, which leads to internal leakage; in severe cases, the valve core breaks or gets stuck, causing the plant to stop operating. FLOWSERVE plug valves, YADI plug valves, SZZ ceramic valves, and FLYGER ball valves have all been used previously, but none of them were able to meet the requirements of long-term operation; the details are shown in Table 1. Table 1: Valve usage record. On March 27, 2019, an AZ cage-type plug valve was used; periodic inspections were carried out during operation to monitor its performance, and no abnormalities were detected ； During the major overhaul of the unit in 2020, a disassembly inspection revealed corrosion on the cage sleeve, but this did not affect the control of process parameters. New spare parts were replaced on May 9, 2020. The AZ cage-type plug valve has been in operation for 13 months, ensuring long-term operation of the device. Analysis of the reasons why the valve operates at a low opening degree: By reviewing the original design documents, it was found that the designed flow rate for the valve was 7.5~30 m3/h ； The process engineers have determined that the process flow rate, based on the current operating conditions of the unit, is 3.5–18 m3/h, as shown in Table 2 ； Therefore, the flow capacity of the originally designed valve is relatively high; it is necessary to calculate the Cv value of the valve based on the actual process parameters in order to select the appropriate control valve. Table 2: Process Parameter Table. Operating the control valve at a low opening degree for an extended period also leads to prolonged erosion of the valve inlet and outlet seat bushings; these bushings get damaged, resulting in internal leakage. This reduces the valve’s regulating capability, making it more difficult to control the flow rate of slag acid. 1. Calculate the flow capacity coefficient Cv of the control valve. The process parameters for the slag acid flow control valve FV-04601 are provided by the process; as shown in Table 2: Pressure before the control valve: P1 = 0.8 MPa; Pressure after the control valve: P2 = 0.45 MPa; Maximum flow rate of the control valve: Q = 18 m3/h; Temperature: T = 60°C; Specific gravity of slag acid: Gf = 1.45. ① First, determine whether it is a choked flow. Discriminant: FL2(P1-FF×Pv). The cage-type plug valve is similar to the V-ball valve; according to the table, FL=0.57. The sulfuric acid solution is estimated using water; per the table, the saturated vapor pressure at 60°C is Pv=0.02. From the critical pressure table, FF=0.95. Therefore, FL2(P1-FF×Pv)=0.572×(800-0.95×0.02)=260 kPa. Since ΔP=P1-P2=0.8-0.45=0.35 MPa=350 kPa, and ΔP＞FL2(P1-FF×Pv), flow obstruction occurs. ②Calculation of the flow coefficient: Based on the calculation of Cv, select from the standard product series the valve that has a value greater than the calculated Cv and is closest to that value. A globe valve with a Cv value of 31 was selected; simulations were conducted to verify the opening degree and flow rate, and these results met the requirements of the process operations, as shown in Table 3 (data sourced from AZ manufacturer’s report on Cv value verification prior to shipment). In accordance with the \"Regulations for the Selection of Automatic Instruments\" (HG20507-1992), for valves with equal percentage characteristics, the maximum opening degree is ≤90% and the minimum opening degree is ≤30%; the valve selection complies with these regulations. Table 3: Table showing the simulated opening degrees, Cv values, and flow rates. Table 4: Comparison table of the originally designed valves and those currently in use. This was also verified during actual operation; for example, on March 11, 2021, when producing ammonium sulfate, the flow rate was 11 m3/h and the valve opening degree was 65% ； For the production of products with other formulations, FV-04601 also operates stably. The original design featured a plug valve with a Cv value of 172, resulting in a flow capacity that was more than 5 times higher than what was actually required. The normal operating range for this valve was 5% to 10%, meaning the adjustable range was very limited; moreover, there was almost no means to adjust for internal leakage, so process parameters had to be controlled entirely through manual valves. Choosing a too high Cv value for the valve is also the fundamental reason behind the valve operating at a low opening degree for extended periods and being unable to be controlled accurately. 2. Analysis of internal leakage in plug valves: Internal leakage occurs mainly in FLOWSERVE plug valves and YADI plug valves. Upon disassembly and inspection, it was found that when the valves are opened to a small degree, the part of the PTFE bushing that comes into contact with the fluid is damaged, and white crystals appear on the surface of the valve core; these crystals are difficult to remove. There are two reasons for this analysis: First, at low opening degrees, the flow velocity of the medium is highest. According to the composition data of the medium, the solid content ranges from 5% to 20%. When the medium flows at high velocities or under high pressure losses, and when it contains solids, it causes erosion of the PTFE lining, resulting in the most severe wear ； Operating at a low opening for extended periods causes damage to the bushings, which in turn leads to increased internal leakage ； Secondly, the process water is mainly recycled water from the slag dump, and fluorosilicates are generated, such as potassium fluorosilicate and sodium fluorosilicate. Both are white, slightly insoluble crystals that are almost insoluble in cold water; in hot water, they hydrolyze to form potassium fluoride, hydrogen fluoride, and silicic acid. Their aqueous solutions are acidic, and trace amounts of these crystalline substances adhere to the valve core, increasing its surface roughness. Repeated opening and closing for adjustment can cause wear on the bushings. 3. Analysis of the reasons for ball valve jamming: During use, as the operating time of ceramic valves increases, these valves may experience jamming. White crystals appear on the surfaces of the valve core and the valve seat, and after about a month, the valve seat and valve core can break apart, as shown in Figure 1. Figure 1: Analysis of the reasons for the fragmentation of the internal components of a ceramic ball valve. First, as can be seen from the composition of the medium, it contains silicates, which over time cause slight adsorption on the internal components of the valve. This increases the frictional resistance between the valve core and the seat, thereby raising the torque required to operate the valve. When this torque exceeds the tolerance limit of the ceramic internal components, it leads to their fragmentation ； Secondly, during operation, depending on the operating conditions of the tubular reactor, medium-pressure steam is used to purge the pipes from time to time; the pressure is around 0.8 MPa and the temperature is around 170°C. As the hot and cold media alternate, the different coefficients of thermal expansion of the ceramic material and the valve body lead to increased fatigue in the ceramic ； Third, the medium is relatively dirty, containing foreign substances such as woven bags and tiny particles, which enter the valve chamber and get stuck between the valve core and the valve seat, increasing the switching torque ； Fourth, due to the presence of fluoride ions and hydrofluoric acid, the medium is corrosive to ceramics, which also accelerates the damage to the valve components. 4. Analysis of the causes of corrosion in valve internals: When producing compound fertilizers using phosphoric acid as the main raw material, the acid medium resulting from the process is complex. The wastewater from the slag storage area is used as the washing liquid in the final washing step; sulfuric acid is added to the pre-washing tank, while phosphoric acid is added to the reverse flow tank, with a ratio of 25%~30% ； The medium composition list shows the presence of ions such as F-, K+, Na+, CL-, and SO42-, which cause severe metal corrosion. The pipe material is 904L; the valve bodies and internal components of the FLOWSERVE ball valves, YADI ball valves, and AZ ball valves are all made of CD4MCu, a type of duplex stainless steel that offers better resistance to phosphoric acid corrosion than 904L. In April 2020, the production process was modified to a semi-slurry production method, and the main medium for control valve FV-04601 was changed to MAP solution. After 15 months of operation, severe general corrosion was found in the cage of the AZ plug valve. In the return water from the slag yard, the phosphorus content is less than 1%, but the sulfur content has increased (0.5–1.0 m3/h). Since 904L can address both overall corrosion caused by sulfur and phosphorus, as well as pitting corrosion due to chlorides, crevice corrosion, stress corrosion, and intergranular corrosion, using 904L for valve components in the semi-slurry production process provides better corrosion resistance; this needs to be verified through future use. 5. Considerations for the selection of slag acid control valves ① The Cv value of the control valve must be calculated accurately. The selection process should start with calculating the Cv value of the control valve based on the actual process parameters, in order to avoid operation at low opening degrees. The accuracy of Cv value calculation mainly depends on the accuracy of the process parameters provided by the product department, and this aspect must be given due attention. ②The selection of control valve type must be suitable for the actual production process. There are factors that cause crystallization in the phosphoric acid medium, and the medium also contains foreign substances such as fragments of woven bags and tiny particles; therefore, ball valves cannot be used. Cage-type plug valves are more suitable. First, the slight crystallization of the valve core prevents it from getting stuck to the PTFE seat; PTFE is softer than metal, and it possesses self-lubricating properties as well as resistance to deformation ； By adding a certain proportion of glass fibers to PTFE, the impact resistance and wear resistance of the bushing increase by 30%, allowing it to be used over long periods at temperatures between 220 and 250°C. When the second fluid passes through the cage plug valve, only the inner valve core (cage) is subjected to erosion, which effectively protects the liner from erosion and prevents internal leakage, allowing for long-term use. If it is damaged due to corrosion, it does not affect the normal operation of the control valve. Although the flow channel of the third cage-type plug valve has a necking, when fully open the valve core poses no obstruction to the flow of the medium; similar to O-ring ball valves, even if the medium contains debris, it will not cause blockage of the valve core. It also features a simple flow path, very low flow resistance, and high pressure recovery capability. Based on the operation conditions of the valves and the production process, when attempting to use new material semi-slurry processes, to address corrosion of the valve core, 904L material for valve internals can be considered ； It is also possible to try spraying special ceramics on the valve internals to improve their erosion resistance and corrosion resistance. 5. Summary: For the slag acid control valve FV-04601 in the rotary drum compound fertilizer production unit, a cage-type plug valve is selected, as it is suitable for the long-term operation of this unit ； Regarding the corrosion of valve internals, it is necessary to continue collecting process data, conduct analyses, and try new materials. Source: Changhui Instrument Network/Authors: Qi Weijun and Hu Yanwen (both authors are currently employed at Qinghai Yuntianhua International Fertilizer Co., Ltd.)