There are different common problems in control valves. Here we are going to discuss such problems:
1. Mechanical friction:
Moving parts of control valves can be subjected to friction, primarily between the valve stem and the stem packing. Some degree of friction is inevitable in valve packing and in some types of trim where components must move past each other throughout the full range of valve stem travel (e.g. cage-guided globe valves, rotary ball valves), and the goal is to minimize friction to a bare minimum while still maintaining a pressure-tight seal.
The presence of friction in a control valve increases the force necessary for the actuator to cause valve movement. If the actuator is electric or hydraulic, the only real problem with increased force is the additional energy required from the actuator to move the valve (recall that mechanical work is the product of force and parallel displacement). If the actuator is pneumatic, however, a more serious problem arises from the combined effects of static and dynamic friction.
When a fluid passes through the constrictive passageways of a control valve, its average velocity increases. As fluid velocity increases through the constrictive passages of a control valve, the fluid molecules kinetic energy increases. In accordance with the Law of Energy Conservation, potential energy in the form of fluid pressure must decrease correspondingly. Thus, fluid pressure decreases within the constriction of a control valve’s trim as it throttles the flow, then increases after leaving the constrictive passageways of the trim and entering the wider areas of the valve body.
If the fluid being throttled by the valve is a liquid, and its absolute pressure ever falls below the vapour pressure of that substance, the liquid will begin to boil. This phenomenon, when it happens inside a control valve, is called flashing.
Fluid passing through a control valve experiences changes in velocity as it enters the narrow constriction of the valve trim (increasing velocity) then enters the widening area of the valve body downstream of the trim (decreasing velocity). These changes in velocity result in the fluid molecules’ kinetic energies changing.
If fluid being throttled is a liquid, and the pressure at the constriction is less than the vapor pressure of that liquid at the flowing temperature, the liquid will spontaneously boil. This is the phenomenon of flashing previously described. If, however, the pressure recovers to a point greater than the vapor pressure of the liquid, the vapor will re-condense back into liquid again. This is called cavitation
4. Choked flow:
Both gas and liquid control valves may experience what is generally known as choked flow. Simply put, “choked flow” is a condition where the rate of flow through a valve does not change substantially as downstream pressure is reduced.
In a gas control valve, choking occurs when the velocity of the gas reaches the speed of sound for that gas. This is often referred to as critical or sonic flow.
In a liquid control valve, choking occurs with the onset of flashing52. The reason sonic velocity is relevant to flow capacity for a control valve has to do with the propagation of pressure changes in fluids.
5. Valve noise:
A troublesome phenomenon in severe services is the audible noise produced by turbulence as the fluid moves through a control valve. Noise output is worse for gas services experiencing sonic (critical) flow and for liquid services experiencing cavitation, although it is possible for a control valve to produce substantial noise even when avoiding these operating conditions.
Noise produced by a control valve also translates into vibration imposed on the piping, which may cause problems such as loosening of threaded fasteners over time. One way to reduce noise output is to use special valve trim resembling the trim used to mitigate cavitation.
A problem common to control valves used in slurry service (where the process fluid is a liquid containing a substantial quantity of hard, solid particles) is erosion, where the valve trim and body are worn by the passage of solid particles.Another cause of erosion in control valves is wet steam, where steam contains droplets of liquid water propelled at high velocity by the steam flow