Complete Maintenance Process of Control Valves

In modern industrial production, the performance of control valves directly affects the stability and safety of the entire production process. Regular maintenance of control valves not only extends their service life but also ensures that they maintain optimal performance during operation. This article will provide a detailed introduction to the complete maintenance process of control valves, from disassembly, cleaning, inspection, and repair to reassembly and testing. Each step is crucial. Through a scientific and rational maintenance process, it is possible to effectively avoid production interruptions caused by control valve failures and ensure the smooth progress of industrial production.

Before installing a control valve, it is essential to ensure that its installation location allows for easy regular inspection and adjustment. This means that there must be sufficient space for maintenance personnel to access the valve, which usually requires the installation of ladders and platforms. Additionally, the installation location of the control valve should avoid becoming a collection point for pipeline contaminants. If the process fluid may contain rust, welding slag, or other foreign objects, a temporary screen or filter should be installed upstream of the control valve. These measures can effectively prevent damage to the control valve caused by contaminants.

The installation of the control valve must also ensure that the valve stem and the actuator above the valve body are in the same vertical plane. If the valve must be installed in a position where the valve stem moves horizontally, the actuator should be supported to prevent misalignment of the valve stem, which could lead to unacceptable variations and packing leakage.

When installing a control valve, it is crucial to ensure the tightness of the valve body and its connecting structures. However, control valves should not be used to align misaligned pipes. Any stress caused by improper pipe assembly must be eliminated elsewhere in the piping system. Otherwise, these stresses may cause the valve stem guide and seat system of the control valve to be misaligned, and may even lead to the detachment of the valve body flange, resulting in variations, seat or flange leakage, packing leakage, and other issues.

During the installation process, it is also essential to protect the precision-machined surfaces of the control valve, such as the valve stem, valve core, valve seat, and guide surfaces. Damage to these components can lead to a decline in the performance of the control valve, or even render it inoperable. Therefore, any unnecessary damage to these parts should be avoided during installation.

The maintenance of control valves is an ongoing process that requires regular inspection and adjustment. After a period of use, the packing needs to be adjusted, and the oiler also needs to be readjusted. In dusty or harsh conditions, a plastic or rubber sleeve should be installed around the valve stem to protect the packing gland and/or the bottom guide of the actuator. Additionally, in environments with corrosive gases or process fluid droplets, special protection should be provided for the control valve. Typically, a plastic bag can be used to cover the control valve without affecting its performance.

For the cleaning of control valves, the appropriate cleaning method must be selected based on the nature of the process fluid. In most cases, water washing or steam blowing is sufficient. However, some types of contamination may require acid washing or special heat treatment. The location for cleaning the equipment must avoid causing repeated contamination to other areas or equipment, and is usually chosen away from operating and repair equipment.

When it is necessary to maintain a control valve, it must first be removed from the pipeline. This is a process that requires special attention, as the control valve is a component of the process pipeline and may be contaminated by the process fluid. Before sending the valve to the repair workshop, all parts that have been soaked in the process medium must be cleaned. The cleaning method will vary depending on the nature of the process fluid. For example, for certain types of contamination, special chemical reagents may be required for cleaning.

When disassembling a control valve, it is essential to have a thorough understanding of its structure to avoid causing damage. For instance, if the valve seat is screwed into the valve body or held in place by other means, this will affect the actual disassembly process. During disassembly, appropriate tools such as packing ring removers, seat ring extractors, or flange spreaders must be used to prevent misuse and additional damage.

After all the parts of the control valve that have been soaked in the process medium have been cleaned, the valve must be reassembled. Before shipping to the workshop, the valve must be clearly marked to indicate that it has been cleaned. When the cleaned control valve is transported to the repair workshop, it should be accompanied by a work card that records detailed information about the valve and its use in the process.

The first step in the maintenance of the control valve is to mark the connection orientation of the actuator corresponding to the valve body flange, and then completely disassemble it. This includes separating the actuator from the upper valve cover, separating the upper valve cover from the valve body, and completely disassembling the actuator. After removing the upper valve cover and packing gland components, the valve core, valve stem, and lower flange cover should be removed from the valve body. All parts must be inspected to determine the scope of repairs and replacements needed.

When reassembling the control valve, it is essential to use gaskets of the appropriate size and material, ensure accurate alignment, and use bolts or studs of the correct size, length, and material. When tightening the upper valve cover and the lower flange cover of the valve body, it is important to evenly tighten the bolts on the diagonal to ensure even pressure on the gasket and prevent flange leakage. When tightening the bolts or studs on the valve body, the specific torque load recommended by the valve manufacturer must be followed.

After the control valve has been reassembled, a series of performance tests must be conducted to ensure that it can function properly. These tests include variation tests, air-tightness tests, and leakage tests. The variation test is used to determine whether all components are assembled correctly, the air-tightness test is used to check the seal of the diaphragm head, and the leakage test is used to check the seal of the valve body.

When conducting the variation test, special attention must be paid to special packing materials, such as V-shaped packing and special spring-loaded packing gland covers, which must be installed in different ways. To achieve a good packing gland seal, the instructions provided by the manufacturer for these special packing glands must be followed.

During the air-tightness test, air at a gauge pressure of 45 pounds per square inch (3 bar) can be introduced into the diaphragm chamber (do not exceed the shell pressure rating specified by the manufacturer), and soap solution can be applied to the flange edges for the air-tightness test. The actuator should be subjected to a variation test before being assembled with the valve body to ensure that the internal assembly of the actuator is in good condition.

During the leakage test, the valve body should be filled with test fluid, and the valve stem should be in the middle of its stroke. The pressure of the test fluid inside the valve body should be increased to the maximum process operating pressure, and the packing should be tightened just enough to stop the leakage. When tightening the packing, the valve stem should be continuously moved to properly rotate the packing and prevent binding of the valve stem.

Long-term maintenance of control valves is the key to ensuring their performance. This includes regular inspection and adjustment of the packing, oiler, and other components. In dusty or harsh conditions, the plastic or rubber sleeve around the valve stem should be regularly replaced to protect the packing gland and/or the bottom guide of the actuator. Additionally, the surface of the control valve should be regularly inspected to prevent corrosion and wear.

The actuator of the control valve should be inspected in detail piece by piece. Since all the impact forces of the valve are transmitted through the bracket, the bracket must be checked for any signs of cracks. Corrosion can cause poor fit between components and is one of the reasons for component replacement. The diaphragm must be inspected for any signs of cracks, hardening, or wear that may indicate potential future failures.

When reassembling the control valve, appropriate lubricants must be selected for all friction points. These friction points include the guides and seals through which the valve stem passes, spring thrust bearings, retainers, and spring adjustment nuts. After reassembly is completed, the diaphragm head needs to be subjected to an air-tightness test. This is the easiest to perform by introducing air at a gauge pressure of 45 pounds per square inch (3 bar) into the diaphragm chamber (do not exceed the shell pressure rating specified by the manufacturer) and applying soap solution to the flange edges for the air-tightness test.

The maintenance of control valves is a systematic project that requires careful operation by professionals and strict quality control. Through the detailed maintenance process described above, from disassembly, cleaning, inspection, and repair to reassembly and testing, every step is indispensable. Only by strictly following the standard procedures can we ensure that the control valve will operate stably after being put back into use and meet the demands of industrial production. Regular maintenance not only extends the service life of control valves but also effectively reduces production costs and improves production efficiency. It is hoped that the introduction in this article will provide valuable reference for relevant technical personnel and help them better complete the maintenance work of control valves.