The Use and Selection of Control Valves

An analysis of the causes of damage to the slag acid slurry control valve FV-04601 for the inlet tube reactor of the rotary drum compound fertilizer unit, including internal leakage and valve trim damage, along with operational data considerations, supports the selection of a cage-type plug valve, ensuring stable and long-term unit operation. Since the start-up of the rotary drum compound fertilizer unit, two problems have occurred with the slag acid control valve: first, long-term operation at a small opening (approximately 5%) makes control difficult and prevents automatic operation; second, the valve bushing is eroded and damaged, causing the valve core to corrode, leading to internal leakage and even breakage and jamming, resulting in shutdown. Various valves, including the FLOWSERVE plug valve, YADI plug valve, SZZ ceramic valve, and FLYGER ball valve, have been tested, but none have met the process requirements for long-term operation.
 
The AZ cage plug valve was installed on March 27, 2019. Periodic inspections were conducted during operation to monitor the valve's status, and no abnormalities were detected. In 2020, the valve was overhauled and disassembled for inspection. Corrosion was found on the cage, but it did not affect the control of process indicators. The corroded parts were replaced with new ones on May 9, 2020. The AZ cage plug valve has been operational for 13 months, ensuring the long-term stability of the system.
 

Internal leakage primarily occurs in FLOWSERVE and YADI plug valves. Disassembly inspection reveals that when the valve is slightly opened, the PTFE bushing's contact area is damaged, and white crystals form on the valve core's surface, which are difficult to remove. The analysis identifies two causes: First, at a small opening, the medium's flow rate is the highest. According to the medium composition Table 5, the solid content ranges from 5% to 20%. When the medium with a high flow rate or pressure loss and containing solids flows through the PTFE bushing, it causes severe scouring and wear; prolonged operation at a small opening damages the bushing, as shown in Figures 1 and 2, leading to increased internal leakage. Second, the process water is primarily slag field return water, containing fluorosilicates such as potassium fluorosilicate and sodium fluorosilicate, which form white, slightly insoluble crystals. These are almost insoluble in cold water but hydrolyze into potassium fluoride, hydrogen fluoride, and silicic acid in hot water. The aqueous solution is acidic, and trace crystals adhere to the valve core, increasing its surface roughness. Prolonged opening and closing adjustments cause wear on the bushing.


Figure 1 Ordinary plug valve with small opening, and flushing bushing
 

Figure 2 Plug valve bushing erosion damage
 

As the operational time of ceramic valves increases, the valve may become stuck, and white crystals can form on the surfaces of the valve core and seat. The valve seat and core may break within about a month, as shown in Figure 3. First, the medium contains silicate, which gradually adsorbs onto the valve trim, increasing friction resistance between the valve core and seat, thereby raising the switching torque. When this torque exceeds the ceramic valve trim's tolerance, the trim may break. Second, during start-up, based on the tubular reactor's operating status, medium-pressure steam is used to purge the pipeline irregularly. The pressure is about 0.8MPa, and the temperature is about 170°C. The alternating cold and hot media, along with the differing thermal expansion coefficients of the ceramic and valve body, increase ceramic fatigue. Third, the medium is relatively dirty, containing woven bags, tiny particles, and other foreign matter that enter the valve body chamber, becoming lodged between the valve core and seat, increasing the switching torque. Fourth, the medium's corrosiveness, due to gas ions and hydrofluoric acid, further accelerates valve trim damage.
 

Figure 3 Broken ceramic ball valve internal parts
 

When phosphoric acid is the primary raw material for producing compound fertilizer, the slag acid medium becomes complex, and slag field return water is used as the washing liquid in the tail-washing process. Sulfuric acid is added to the pre-wash tank, and phosphoric acid is added to the pipe return tank, at a ratio of 25% to 30%. Table 5 shows the presence of F-, K+, Na+, Cl-, and SO₄^2- ions, which are highly corrosive to metals. The pipeline is made from 904L, while the valve body and internal parts of the FLOWSERVE, YADI, and AZ plug valves are made from CD4MCu duplex stainless steel, which has superior corrosion resistance to phosphoric acid compared to 904L. In April 2020, the production process was adjusted to a semi-slurry method, changing the primary medium of the control valve FV-04601 to MAP solution. After 15 months of operation, the cage exhibited significant overall corrosion. The phosphoric acid content in the slag field return water is less than 1%, but the sulfuric acid amount has increased to 0.5 to 1.0 m³/h. Since 904L can address overall corrosion issues with sulfuric and phosphoric acids, as well as gasification pitting, crevice, stress, and intergranular corrosion, selecting 904L for the valve trim in the semi-slurry production process increases corrosion resistance. This requires verification in future applications.
 

The selection of the control valve's Cv value should be accurate. The selection process should first involve calculating the Cv value of the control valve based on actual process parameters, and then choosing a type that avoids small opening operations. The accuracy of the Cv value calculation primarily depends on the accuracy of the process parameters provided by the product department, which must be taken seriously.
 
The type of control valve should be selected to match the actual production process. The slag acid medium has factors that cause crystallization, and it contains foreign objects such as woven bag debris and tiny particles; therefore, ball valves should not be selected. Cage plug valves are more suitable because a small amount of crystallization on the valve core will not cause it to stick to the PTFE valve seat. PTFE is softer than metal, with self-lubricating properties and anti-deformation capabilities. Adding a certain proportion of glass fiber to PTFE increases the impact strength and wear resistance of the bushing by 30%, allowing it to be used for extended periods at 220 to 250°C. Second, when fluid passes through the cage plug valve, only the inner valve core (cage sleeve) is subjected to washing, effectively protecting the bushing from erosion, as shown in Figure 4, thereby avoiding internal leakage and allowing long-term use. Even if damaged by corrosion, it will not affect the normal operation of the control valve. Third, although the flow channel of the cage plug valve narrows, the valve core does not obstruct the medium flow when fully opened. Similar to the O-type ball valve, the medium will not obstruct the valve core, even if debris is present. Additionally, the flow path is simple, with minimal flow resistance and high pressure recovery capability.

 
Figure 4 Cage plug valve with small opening, and bushing protection

In the semi-slurry production process, to address the corrosion of the valve core, 904L  valve trim can be tried; alternatively, special ceramics can be sprayed on the valve trim to improve erosion and corrosion resistance.
 

In the semi-slurry production process, to address the corrosion of the valve core, 904L valve trim can be tried; alternatively, special ceramics can be sprayed on the valve trim to improve erosion and corrosion resistance.