1. Why Does the Tightness of a Control Valve Decrease? How to Solve This Problem?
Decreased tightness of a control valve leads to medium leakage affecting process control accuracy and safe production. The core causes are divided into internal leakage and external leakage requiring targeted analysis and solutions:
01 Internal Leakage
The most common cause is failure of the sealing surface between the valve core and seat. On one hand it may result from long-term erosion of the valve core and seat by high differential pressure or particle-containing media causing scratches and pits on the sealing surface or erosion by cavitation. On the other hand it may be due to poor adaptability of the sealing structure such as deformation of soft sealing components used in high-pressure conditions or corrosion of the sealing surface from improper selection of non-corrosion-resistant materials for highly corrosive media.
02 External LeakageIt
Which mainly includes two types of causes: packing seal failure such as aging wear or improper installation of packing and valve body connection seal failure such as aging of flange gaskets or casting defects of the valve body.
For the above possibilities the following targeted maintenance measures can be taken for control valves with decreased tightness:
Replace aging or damaged packing. Select suitable packing type based on medium characteristics and working conditions such as graphite ring packing for high-temperature media and polytetrafluoroethylene packing for corrosive media.
Reinstall the packing correctly ensure sufficient compression force and make sure the packing fits the valve stem and packing chamber evenly.
Inspect the valve stem surface. Repair or replace the valve stem if there are scratches or corrosion.
Repair or replace relevant upper bonnet components if the sealing structure has defects such as damage to the stuffing box.
(Cavitation refers to the phenomenon where liquid vaporizes to form bubbles when the local pressure drops to the saturated vapor pressure at the current temperature while flowing through throttling components such as control valves. Subsequently as the fluid flows to the downstream area where pressure recovers the bubbles collapse rapidly generating strong shock waves and micro-jets which in turn cause equipment noise vibration and cavitation damage.)
2. Why Does Noise Occur During the Operation of a Control Valve? How to Maintain and Solve This Problem?
When noise occurs during the operation of a control valve we should first determine the type and cause of the noise. The noise generated by control valves mainly has two types: hydrodynamic noise and mechanical noise.
01 Hydrodynamic Noise
Hydrodynamic noise is the most common type which is further divided into cavitation noise flash noise and turbulence and vortex noise.
Cavitation noise refers to high-frequency noise caused by excessive differential pressure across the valve. When the fluid pressure at the throttling point drops below the saturated vapor pressure bubbles are generated and then collapse accompanied by cavitation damage to the valve core.Flash noise refers to the situation where the fluid pressure remains below the saturated vapor pressure after throttling forming a stable gas-liquid two-phase flow leading to turbulent noise. This type of noise mostly occurs in liquid medium conditions.Turbulence and vortex noise refers to uneven fluid velocity when flowing through the throttling port forming vortex shedding. Especially when the flow velocity approaches or exceeds the speed of sound the noise increases sharply. This type of noise is more likely to occur in gas medium.
02 Mechanical Noise
Mechanical noise comes from two sources: valve core or valve stem vibration or actuator noise. It refers to low-frequency vibration noise caused by valve core oscillation when operating at small openings or excessive clearance due to friction between the valve stem and packing or wear of the guide sleeve. Or it is generated when the vibration is transmitted to the valve body due to insufficient spring stiffness of the pneumatic diaphragm actuator wear of the gear rack in the piston actuator or resonance of the motor in the electric actuator.
For the two types of noise mentioned above we can avoid noise from the source by adjusting differential pressure and opening degree and controlling flow velocity. We can also reduce noise by using corrosion-resistant and wear-resistant materials or optimizing the valve core structure. For example the flow path design of V type ball valves and eccentric rotary valves can reduce vortices and soft-sealed valve cores can absorb part of the turbulent noise.
To reduce and prevent failures of control valves during fluid control we should select and maintain control valves correctly. For example confirm functional requirements and calculate parameters such as differential pressure and flow velocity in advance; regularly inspect the wear of valve cores and seats and replace aging packing and guide sleeves; regularly discharge sewage from pneumatic actuators and inspect motors and reducers of electric actuators.
Thank you for your reading and may you select the correct and high-quality valves !
