【Q&A Question 107】April 17, 2018

This is caused by the resistance exerted by the sealing packing or dirt inside the valve body on the valve stem. Under normal circumstances, this leads to inflexible movement of the valve stem, or oscillations as the input pressure changes. In such cases, fluctuations in the parameter being regulated can be observed on the recording curve, resulting in severe delays in the operation of the control valve. In extreme situations, the valve stem may become unable to move, causing the control valve to lose all its regulatory function. It is advisable to use a visible-stem design along with coarse-traded trapezoidal threads in the design； During installation, a sturdy support should be provided at the valve to prevent the valve from shifting along with the pipeline and causing the valve stem to bend. The valve can be fixed to the insulated box using an elastic connection, thereby preventing the valve stem from deforming and becoming out of alignment with the valve body ； During the cold shutdown period, it is necessary to inspect and adjust the valve installation in a cooled state; if the valve becomes stuck after cooling, the fixing flange of the valve on the shell can be adjusted to enable smooth opening and closing. During operation, the pre-startup heating should be thorough, and when closing the valves, it is important to ensure no leaks by not using excessive force.

Phenomenon: The valve position signal changes, but the flow rate remains unchanged; it can be observed on-site that the valve stem does not move. Treatment: Switch to the bypass operation, close the manual valves before and after the control valve, open the drain valves for condensate and oil, and then have the instrumentation team repair the control valve.

The valve position signal changes, but the flow rate remains unchanged; it can be observed on site that the valve stem is not moving. Solution: Operation should be switched to a bypass line, the manual valves before and after the control valve should be closed, drain valves for condensate and oil should be opened, and then the control valve should be repaired by the instrumentation team.

Answer: Phenomenon: The valve position signal changes, but the flow rate remains unchanged; it can be observed on site that the valve stem does not move. Solution: Operation should be switched to a bypass line, the manual valves before and after the control valve should be closed, and after draining the condensate and oil, the control valve should be repaired by the instrumentation team.

The flow regulation is not timely, resulting in lagging. Solution: Divert the medium through a bypass line and contact the instrumentation team for handling

What are the symptoms of a stuck control valve? How to deal with it? Symptoms: poor flow of the medium, failure to respond to small signal levels, and excessive response to large signal levels. Solutions: cleaning, external flushing, installing pipeline filters, increasing the throttling gap, using the medium for flushing, and changing from a straight-through design to an angled design.

1) Cleaning method: Weld slag, rust, debris, etc. in the pipelines can cause blockages or jams at the throttle ports, guide sections, and balance holes on the lower valve cover, leading to scratches and abrasions on the surface of the valve core and its guide surfaces, as well as indentations on the sealing surfaces. This often occurs in newly commissioned systems and during the initial period after maintenance. This is the most common fault. In such cases, it is necessary to remove the component for cleaning in order to get rid of debris; if the sealing surface is damaged, it must 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 starting 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 points and guide areas; in such cases, flushing gas and steam can be introduced 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 before the valve to ensure smooth flow of the medium. Control valves used with positioners – when the positioner does not function properly, the most common fault is a blockage in the air circuit throttle. Therefore, when a regulator 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 regulator. 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 a throttling element with a larger throttling gap – such as a valve core or sleeve with window-like or open-shaped throttling areas. Since the throttling area in this case is concentrated rather than distributed around the perimeter, 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 switch to 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 most severely eroded area; 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 corner flow method: The straight-through configuration results in an inverted S-shaped flow pattern; the flow path is complex, with 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, offering good scouring performance, a small dead zone, and allowing for easy design in a streamlined shape. Therefore, when a slight blockage occurs with a straight-through control valve, it can be replaced with an angle valve. 7) For sleeve control valves used on steam pipelines, if the gap between the valve core and the sleeve is too small, or if different materials are used, it is very likely that the valve will get stuck due to differences in expansion coefficients when heated. It is possible to increase the clearance appropriately during processing (on the premise of meeting the flow control requirements) to prevent similar situations from occurring.

The outlet flow rate of the control valve is abnormal, and the liquid levels in the tanks before and after the control valve are abnormal. Adjusting the opening degree of the control valve yields no response or a slow response, with the valve not moving properly; this indicates that the control valve is stuck. Convert the control valve to a bypass line, close the manual valves before and after the control valve, open the drain before the control valve to drain all the medium, and then contact the instrumentation team to carry out maintenance on the control valve.

Phenomenon: The valve position signal changes, but the flow rate remains unchanged; it can be observed on site that the valve stem is not moving. Solution: Operation should be switched to a bypass line, the manual valves before and after the control valve should be closed, and then the control valve should be repaired by the instrumentation team after draining condensate and oil.

Phenomenon: The control valve cannot be fully opened or fully closed properly. Solution: Install pipeline filters, use external gas or steam for flushing, and remove valves for cleaning.