Common Causes and Remedies for Control Valve Malfunctions

Common Causes and Remedies for Control Valve Malfunctions

Control valves, essential components in fluid control systems, are susceptible to a range of malfunctions that can impede their performance. These issues demand tailored troubleshooting strategies to effectively address them and restore optimal functionality. From blockages caused by debris accumulation to filling leakage due to improper packing, and from control valve core and seat deformation to oscillations resulting from insufficient stiffness, each malfunction necessitates distinct approaches for resolution. 

Blockage:

Blockage occurs due to welding slag, rust, debris, etc., which cause obstructions at the throttle port, guiding parts, and balance holes of the lower valve cover in the pipeline, resulting in poor flow of the measured medium. Alternatively, overfilling of the stuffing box increases friction, causing the valve to fail to operate with small signals and to overreact with large signals.

Leakage:

Inappropriate length of the valve stem results in insufficient upward (or downward) movement distance, causing a gap between the valve core and the valve seat, preventing full contact and leading to internal leakage of the regulating valve.
Filling leakage occurs after the filling material is loaded into the stuffing box and axial pressure is applied through the gland. Due to the plasticity of the filling material, radial forces are generated, pressing it tightly against the valve stem, but the contact is not uniform. Some areas have loose contact, while others have no contact. During operation, relative movement exists between the valve stem and the filling material, known as axial movement. Due to the influence of high temperature, high pressure, and permeable fluid media, the stuffing box of the regulating valve is a common site of leakage. The main cause of filling leakage is interface leakage, which occurs as the contact pressure between the filling material and the valve stem gradually decreases and the filling material ages, allowing pressure media to leak outward along the contact gap between the filling material and the valve stem.
Leakage due to deformation of the valve core and valve seat is mainly caused by casting and forging defects during the production process of the regulating valve. Small defects such as pinholes and local abrasions can lead to intensified chemical corrosion of the medium. The passage of corrosive media and the flushing of fluid media can also cause leakage of the regulating valve. Corrosion mainly exists in the form of erosion or cavitation. Erosion or cavitation is caused by the flow of fluid media inside the valve body. When corrosive media such as strong acids and alkalis pass through the regulating valve, erosion and impact on the valve core and valve seat materials occur, causing them to become elliptical or other shapes, resulting in gaps between the valve core and valve seat and internal leakage.

Oscillation:

Insufficient stiffness of the spring in the control valve can lead to unstable output signals, causing rapid fluctuations and susceptibility to valve oscillation. Severe vibration of the pipeline and valve seat causes the control valve mounted on it to vibrate as well. If the sensitivity of the valve positioner is too high, even minor changes or drifts in the output signal of the controller are immediately amplified and transmitted to the control valve, resulting in unstable output signals from the controller. Selecting a flow capacity CV value that is too large may cause the control valve to operate at a small opening degree, resulting in a small shutoff gap and high flow velocity. This leads to large pressure changes before and after the valve, and when the stiffness of the valve is exceeded, the valve stability is compromised, leading to severe oscillations. Additionally, insufficient preload force in the control valve can also cause this phenomenon.

Membrane Leakage:

Continuous operation of the control valve over a long period causes the membrane to remain in a stretched state, leading to aging and deterioration. This weakens its elasticity, resulting in unstable operation and fluctuations in controlled parameters. When the control valve cannot fully open or close under an external pressure of 0.1MPa and gas leaks out from the external vent hole, it indicates membrane damage. Troubleshooting methods include clearing blockages by rapidly opening bypass valves or control valves to allow debris to be flushed away by the medium. Another method involves using pliers or locking pliers to clamp the valve stem and then applying force in both directions while rotating the stem to allow the valve core or stem to pass through the blockage. If this fails, increase the input signal pressure to 0.1MPa-0.14MPa to partially overcome the stuck valve stem or seat, thereby moving the stem or valve core. Repeat this motion several times to resolve the blockage issue. However, the additional pressure signal should not be too high, exceeding the gas source pressure of 0.14MPa, as excessive force may cause the valve stem to bend. For leaks caused by an inappropriate valve stem length, adjust the length (shorten or lengthen) of the control valve stem to ensure proper fit and prevent further leakage.

Filling Leakage:

To facilitate filling, the top of the stuffing box is chamfered, and a corrosion-resistant metal protective ring with a small gap is placed at the bottom of the stuffing box (the contact surface with the filler cannot be sloped) to prevent the filler from being pushed out by the medium pressure. The metal surfaces of the stuffing box and the parts in contact with the filler should be precision machined to improve surface smoothness, reduce filler loss. Flexible graphite is selected as the filler due to its good air tightness, low friction, easy maintenance, no change in friction after retightening the valve cover bolts, good pressure and heat resistance, resistance to erosion by internal media, and resistance to corrosion of the metal in contact with the valve stem and filler box internals. This effectively protects the sealing of the valve stem and stuffing box, ensuring the reliability and longevity of the filler seal. Leakage caused by deformation of the valve core and seat: First, strict quality control should be implemented, and products with defects such as spots and sand holes should be promptly removed. If the deformation of the valve core and seat is not severe, they can be polished with fine emery cloth to eliminate traces and improve the smoothness of the sealing surface to enhance sealing performance. If the damage is severe, new valves should be replaced.

Oscillation Treatment:

For vibration and slight oscillation, increasing stiffness can eliminate them. For example, using a spring with greater stiffness or switching to a piston actuator mechanism can help. Additionally, replacing the throttle component can eliminate resonance or increase friction against vibration. This approach is effective for minor oscillations.