[Q&A Question 122] 2017.05.02

Fully open the bypass line, and slowly close the control valve. After gradually closing the control valve, close the isolation valves before and after it.

Contact the control room to slowly open the double-line valve, close the valve upstream of the control valve, and have the operator monitor the flow changes

Use on-site secondary lines in conjunction with the central control system for adjustment and control

I. Methods to increase lifespan (8 methods) 1) Method of extending lifespan by operating at high opening degrees: Have the control valve operate at its maximum opening degree from the start, such as 90%. In this way, damages such as cavitation and erosion occur on the head of the valve core. As the valve core is damaged, the flow rate increases, causing the valve to close a little more; this process of continuous damage and gradual closure ensures that the entire valve core is utilized to the fullest extent, until the root of the valve core and the sealing surface are damaged and the valve can no longer be used. At the same time, operating at a large opening results in a larger throttle gap, which reduces erosion; this increases the valve’s lifespan by 1 to 5 times or more compared to operating it at intermediate or small openings from the start. If a chemical plant adopts this method, the service life of the valves is doubled. 2) Method of reducing S, increasing the operating opening, and extending lifespan: By reducing S, the losses in the system other than the control valve are increased, which leads to a lower pressure drop across the valve. To ensure that flow passes through the control valve, its opening must be increased; at the same time, the reduced pressure drop across the valve lessens cavitation and erosion. Specific methods include: installing a orifice plate behind the valve to create pressure drop through throttling ； Close the manual valve connected in series on the pipeline until the control valve reaches an optimal operating position. For when the valve is initially selected to operate at a small opening degree, this method is very simple, convenient, and effective. 3) Method of reducing the diameter to increase the operating opening and extend lifespan: This involves increasing the operating opening by reducing the valve’s diameter. Specific methods include: ① Replacing the valve with one of a smaller diameter, such as changing from DN32 to DN25 ； ②The valve body remains unchanged; only the valve core and seat with a smaller diameter are replaced. During a major overhaul of a chemical plant, replacing the throttle element dgl0 with dg8 doubled its service life. 4) Method of improving lifespan by shifting the location of damage: Moving the areas severely damaged to less critical positions in order to protect the sealing surfaces and throttling surfaces of the valve core and seat. 5) Method of increasing lifespan by enlarging the throttling channel: The simplest way to do this is to thicken the valve seat, thereby enlarging the valve seat hole and creating a longer throttling channel. On the one hand, it can delay the sudden expansion that occurs after throttling in a flow-blocking manner, thereby shifting the location of failure and keeping it away from the sealing surface ； On the other hand, it increases the throttling resistance, reduces the degree of pressure recovery, and thereby attenuates cavitation. Some designs feature stepped or wavy shapes in the valve seat hole, aimed at increasing resistance and reducing cavitation. This method is often used on the high-pressure valves in new installations and when upgrading old valves, and it is also very effective. 6) Method of improving lifespan by changing the flow direction: The fluid flows in the open direction; cavitation and erosion mainly affect the sealing surfaces, causing rapid damage to the root of the valve core as well as the sealing surfaces of the valve core and the valve seat ； In the flow-blocking type, flow moves in the direction of closure; cavitation and erosion occur after throttling, below the valve seat sealing surface, which protects the sealing surface and the root of the valve element, thereby extending its lifespan. For valves used in a deliberately flowing pattern, when extending their lifespan is a significant concern, simply changing the flow direction can double or triple their service life. 7) Method of improving durability by using special materials: To resist cavitation (damage in the form of honeycomb-like spots) and erosion (streamline-shaped grooves), special materials resistant to cavitation and erosion can be used to manufacture the throttling components. Such special materials include 6YC-1, A4 steel, Stellite, and cemented carbide. To resist corrosion, materials that are more resistant to corrosion and possess certain mechanical and physical properties can be used instead. This material is divided into two categories: non-metallic materials (such as rubber, PTFE, ceramics, etc.) and metallic materials (such as Monel, Hastelloy, etc.). 8) Method of improving lifespan by altering valve structure: The goal of increasing lifespan is achieved by modifying the valve structure or choosing valves with a longer service life, such as multi-stage valves, anti-cavitation valves, and corrosion-resistant valves. II. Methods to prevent clogging (sticking) of control valves (6 methods): 1) Cleaning method: Weld slag, rust, debris, etc. in the pipeline can cause blockages or sticking at the throttle openings, guide areas, and balance holes on the lower valve cover; this can lead to scratches and abrasions on the surface of the valve core and guide surfaces, as well as indentations on the sealing surfaces. This often occurs in newly commissioned systems and in the early stages after major repairs. This is the most common fault. In such cases, it is necessary to disassemble it for cleaning in order to remove debris; if the sealing surface is damaged, it should also be ground ； At the same time, open the bottom plug to flush out the debris that has fallen from the balance hole into the lower valve cover, and to clean the pipelines. Before putting it into operation, open the control valve fully; allow the medium to flow for a while before proceeding with normal operation. 2) External flushing method: When using ordinary valves to regulate media that tend to precipitate and contain solid particles, blockages often occur at the throttling ports and guide areas; in such cases, flushing gas and steam can be supplied at the bottom plug of the lower valve cover. When the valve becomes clogged or stuck, opening the external gas or steam valve allows for flushing to be carried out without moving the control valve, thereby restoring normal operation of the valve. 3) Pipe filter installation method: For small-diameter control valves, especially those with extremely low flow rates, the throttling gap is very small, and there must not be even the slightest amount of debris in the fluid. In the event of such blockage, it is best to install a filter on the pipeline ahead of the valve to ensure smooth flow of the medium. For control valves used with positioners, when the positioner does not function properly, the most common fault is a blocked throttle in its air supply line. Therefore, when a positioner is in use, it is necessary to take proper care of the air supply; a common approach is to install an air filter and pressure reducing valve on the air supply pipeline before the positioner. 4) Increasing the throttling gap: If solid particles in the fluid, or welding slag and rust that have been washed into the pipes, cause blockages or jams due to their inability to pass through the throttling opening, it is possible to use throttling elements with larger throttling gaps – valve cores or sleeves with window-like or open-shaped throttling areas. Since the throttling area in these cases is concentrated rather than distributed circumferentially, such problems can be easily resolved. For single or double-seat valves, the plunger-type valve core can be replaced with a valve core having a “V”-shaped port, or it can be changed to a cartridge valve or similar. For example, a chemical plant had a two-seat valve that would get stuck frequently; after it was recommended to replace it with a sleeve valve, the problem was resolved immediately. 5) Medium scouring method: Utilizes the scouring energy of the medium itself to scour away and remove substances that tend to precipitate or cause blockages, thereby enhancing the valve’s anti-blocking capability. Common methods include: ① modifying it for use in a flow-closed configuration ； ②Use a streamlined valve body ； ③Place the throttle orifice at the area most subject to erosion; when using this method, it is important to improve the erosion resistance of the material used for the throttle component. 6) Changing from straight-through to angular flow: The straight-through configuration involves an inverted S-shaped flow pattern, resulting in a complex flow path and numerous dead zones in the upper and lower chambers, which provide areas for the deposition of the medium. Angular connection: the medium flows as if through a 90° elbow, resulting in good scouring performance, a small dead zone, and ease of designing it into a streamlined shape. Therefore, when a slight blockage occurs with a straight-through control valve, it can be replaced with an angle valve. III. Solutions to leakage in control valves (6 methods) 1) Adding sealing grease: For valves that do not currently use sealing grease, it is possible to add such grease to improve the sealing performance of the valve stem. 2) Adding packing method: To improve the sealing performance of the packing around the valve stem, the method of adding more packing can be employed. Usually, a dual-layer or multi-layer mixed packing structure is used; simply increasing the number of elements, for example from 3 to 5, does not yield significant results.

It is important to know whether the control valve is of the FC or FO type. The general approach is to remove the control valve, activate the bypass valve, and then reinstall the control valve after repairs are completed.

First, slightly close the bypass valve, then gradually close the control valve until it is fully closed. Once the flow rate or pressure stabilizes, close the valve upstream of the control valve, followed by the valve downstream of it. Open the drain valve for venting, and hand over the system to the maintenance team for repairs

In coordination with the operator, slightly open the bypass valve, and gradually close the control valve (or the upstream valve), maintaining stable process parameters (pressure, flow rate) downstream of the valve until the control valve is fully closed. Verify that the process parameters (pressure, flow rate) downstream of the valve are the same as those when the control valve is operating normally. Close the upstream valve before closing the control valve, and then slightly open the backflow valve.

Slowly open the bypass line while slowly closing the upstream valve until there is a change in the readings; then slowly open the bypass line again, and close both the upstream and downstream valves

The two people work together: one opens the bypass valve of the control valve while the other closes the manual valve in front of the control valve, and once the pressure stabilizes, both manual valves are closed.

Use a walkie-talkie to communicate with the operator on-site, and while slowly opening the bypass valve, gradually close the upstream valve until it is completely shut off, with the bypass valve used for control. Close the downstream valve again and contact the instrumentation team for handling.