The major type of damage that can be seen in a control valve is cavitation, flashing, erosion, chemical corrosion, fretting.
The cavitation is caused because of the vapor bubbles and this happens when the liquid pressure drops below the vapor pressure and the cavities will be created during the bubble collapsing, as the pressure recovers above the vapor pressure. The cavitation can be described as the vaporization and condensation of the liquid. The cavitation can be seen in some portion of the valve. The cavitation can be prevented by providing a tortuous flow path and also be expanding the flow area. We can also use harder trim material to prevent cavitation.
This could happen if the liquid pressure falls below the vapor pressure and thus the vapor bubble is created. Then the flow will be in the combination of the vapor and fluid. It is not possible to prevent flashing because of the process requirements and because of this, the flashing damage must be controlled by proper angle body design.
While selecting a control valve we must consider the velocity of the fluid because it can create erosion. If the velocity is high then the chances of erosion are high too.
Seat leakage is the passing of the fluid through the seat if the valve is in a closed position.
- Valve leak past the ball – it can be caused due to the damage of the seats or maybe the valve is not stopping at a proper closed position. The solution for this problem is the installation of a repair kit or to adjust the actuators limit switch
- Valve stem leaks – it can be caused by worn stem seals or by the damaged stem or bore. It can be solved by installing a repair kit or by replacing the valve stem or the valve.
- Valve body leaks – it can be caused by loose body bolts or excessive operating pressure or by defective body seals. This problem can be solved by examining the bolts and make sure if the application is in proper pressure rating. The defective body seals can be repaired if not possible then replace the valve.
- Excessive operating torque of the valve – it can be caused by the swollen seals or by the particulate buildup in the valve chamber, this problem can be solved by examining the compatibility of the valve with the product or by cleaning the valve, using a repair kit, etc. This problem can also be caused because of the overtightening of the valve bolts so in that case lose the bolts. This could also happen if the stem nut is too tight or because of the damaged stem seal in that case we can loosen the stem nut or install a repair kit if required
A choked flow is a flow rate of maximum or limiting flow rate, it is a condition in which the valve flow rate won’t change while the downstream pressure is lowered. So it is the condition of the maximum flow.
This is the other problem related to the control valve the stiction stands for static friction, it can be easily explained that the internals of the valve is sticky. The problems related to stiction can be avoided by doing certain things they are proper sizing of the positioner and actuator of the valve and it must be sized properly for the force needed to move the valve. We must also check the torque on the valve packing gland. We must check the internals of the valve for the scaling signs or for excessive wear and replace the valve trim if needed.
The major cause for hysteresis is because of the loose or worn mechanical linkages between the positioner, actuator, or the valve. Hysteresis can be defined as the difference between the position of the valve in upstroke and down-stroke in an input signal. If the positioner is present in the control valve it can reduce the hysteresis impact and we must check the positioner so that we can determine if it can work properly
A dead-band is the situation in which the output will be zero or there won’t be any signal or operation. So it is the signal to the control valve that won’t do any operation because the signal is dead. The dead-band occurs in a control valve during the reversal of the direction
Mechanical vibrations could be created in the valve because of the reaction of the internal components to the turbulent flow through the valve. The aerodynamic noise is formed because of the conversion of the mechanical energy of the flow into acoustic energy when the fluid passes through the flow restriction. The control valve noise can be handled by controlling the velocity of the fluid.
What are the factors that must be considered while installing a control valve, so that we can avoid future maintenance?
The control valve will function properly if the installation is done perfectly so we must consider certain factors while installing a control valve to operate the valve perfectly without and trouble they are
- We must avoid the valve installation where the vibrations or external stress may impair proper valve functions. So we must install it in a place which is suitable for the valve if it is inevitable then we must consider proper provisions against these.
- We must make sure that the control valve is installed in a place where it has sufficient space to operate and can be easily accessible to do the maintenance too.
- We can provide support to the valve so that it won’t be a burden to the process pipe due to the weight of the valve or we can provide supports to the pipe runs.
- We should not install a valve in places where it will submerge in water, snow, or in places of excessive freezing
- We must take precautions against the corrosive nature of the atmosphere
How to do the quick check and maintenance of a control valve
What are the major parts that must be inspected in a control valve?
- Actuator diaphragm
- Actuator shafts
- Trim components such as plug seat cage and stem
- Packing box must be inspected which is in the bonnet
- Gasket seating surface
- NDT test for body valve
- We must check the condition of the valve positioner
- Bolts and nuts of the body
- Actuator spring
- Handwheel if present
- The surface of the flange gasket
While doing the routine maintenance we need to check the gland nuts and tight it if there is any leakage through gland packing. We must also check the nuts on the top and bottom flanges if the gasket is leaking.