The final control element is one that ultimately modify any feature of the process as directed by the controller. Depending on the type of process and objectives, there are a variety of these elements. From devices that receive signals from discrete type control to others that act to regulate the variable of interest within a certain range, such as fluid flow through a control valve, the speed of a motor through a variable speed or temperature of an oven using a heating element.
Let's start ...
A control valve is the final control element most commonly used in industry. This device changes the flow of material or energy to a process, setting an opening through which flows the material, is therefore a variable orifice in the line. According to Bernoulli's theorem the flow through an orifice is
Q = CA (Ap) 1 / 2
Where:
Q is the amount of flow
C is a constant flow conditions
A is the area of valve opening
Dp is the pressure drop across the valve
According to the above formula, the flow through the valve is proportional to the aperture area and the square root of the pressure drop through the valve. Both factors vary, the area with the percentage of stroke (position) of the valve and the pressure drop is referred to conditions outside of the valve and set the process (distribution pipes).
In actual conditions, a constant pressure drop is rarely encountered. Therefore, the user of the valve or system designer must consider the characteristics of the valve and so the process of combining the two to achieve the desired performance.
In a closed loop, the valve is the only resistive element can be controlled. The other resistances vary due to changes in flow due to the coating system or pipes. These variations are undesirable and should be compensated by the control valve.
What are the parts of a CONTROL VALVE?
Because the valves are most commonly used air, we will refer to these to detail the parts of a control valve. In general, a VCA consists of two main parts: Valve and actuator itself. The valve is the part through which it passes and found the fluid and the actuator is the element responsible for carrying out the control operation.
Figure 1 - Parts of a Control Valve
VALVE
It contains the following parts:
Valve Body
Internal elements, as the seat of the shutter, the shutter, the shutter rod, the guide itself, and so on. in contact with the medium control.
Stuffing box, through which the rod moves the shutter and accessories containing sealed to prevent leakage of fluid.
It usually contains the mounting means of the actuator.
VALVE BODY TYPES
There are a variety of valves for various applications. A classification of these is the following:
Valve Body Types Chart
Globe valves double seat are very popular due to its balanced design (the forces that tend to close the valve are only slightly different from those who have to open it). Also available with reversible seals.
Globe valves single seat usually not balanced (there are designs that they are). They are used mostly in small sizes which requires a more tight close as possible. Its use in small sizes is due to: its simple design and still be unbalanced not require large actuator sizes.
Figure 2 Globe Valves
Mention the most important features:
Butterfly valves are widely used in high flow rates and services of small losses. They have high rates of pressure recovery. They have many leaks unless soft seated design. Actuator require more power and do not have good control characteristics (especially near the end zone and the open area near total) except in special designs.
Butterfly Valve Figure 3
Ball valves are being widely used due to its high percentage of pressure recovery. Design are presented in full or segmented ball. In the latter case are very useful in viscous fluids with suspended solids. Capabilities have high CV. In many cases ideal for applications on-off control.
Diaphragm valves are used primarily in viscous and corrosive services. The seal prevents the membrane is fluid contact with the internal organs. They generally have poor control characteristics and the membranes are usually short-lived.
There are numerous other body types (three-way valve, split body, angular, etc..) Presenting; also advantages and disadvantages, which weighed properly can enable the use of them.
The valve cage is another type of valve balloon that has had a long boom in recent years. Its name comes from the shutter is guided by a "cage" immersed in the body of the valve. It is used in many applications replacing the double seat with some additional advantage such as greater capacity Cv, low noise, good stability, easy change of the internal organs (trim), use of trim in the same small size and reduced shutter problems with erosion.
4 Valve cage Ffigura
THE SHUTTER
Is that primarily defines the behavior or characteristic of the valve as previously mentioned. In general we have the following characteristics:
Quick opening, allows a quick change of the flow for a short tour of the rod. About 90% of valve capacity 30% is obtained by opening the valve and a linear relationship is achieved to that point. Service is mainly used for on-off valves or self-regulated. Are useful even in systems with constant pressure drops, where a linear characteristic is required.
Linear produces a flow proportional to the valve opening. A variation of 50% of the shaft causes an equal change in the flow, etc. This relationship produces a constant slope, so that each incremental change of the position of the cap produces a similar change in the flow valve if the pressure drop is constant. Are generally used in liquid level control, and in applications where a constant gain is required.
Equal Percentage: equal percentage characteristic is one in which equal increments of travel of the rod, there is a percentage equal to the existing flow. For example, when the flow is small, the change in the same (for one, incremental change) is small when the flow is large, change is always proportional to the amount that flows before the change. They are used in applications where control of pressure, a small percentage of the collapse of the system allows control of the valve.
Figure 5 Characteristic curves of control valves
Modified Dish: A curve of this type lies between linear and equal percentage. Aplicacionesien which is used in most of the pressure drop occurs in the system control valve.
Linear Modified: falls between the linear and aperture for low flows and high sensitivity of the valve is low, ie large piston stroke produces small changes in flow.
VALVE ACTION
The action of the valve body or valve itself is related to the effect of displacement of the valve stem on the opening of it, that is, pushing the valve stem, it creates a push-to-i effect close ( push-to CSOs) to cover or push (push-to-open). The closure is achieved physically with the cap or stopper.
Figure 6 Actions of valve body
MATERIALS
Construction materials, ie materials of the internal organs of the body as well as the same, are required to come for the type of process and conditions. The use of stainless steels is common in chemical applications, however due to the wide variety of them using some special materials. Usually, the best selection of materials for valves comes from the person who can know about the process fluid. In this regard, valve manufacturers often rely on the user's opinion for the selection of materials and catalogs have detailed help on this issue.
An interesting aspect in the selection of the internal organs of control valves is the subject of erosion. Erosion is caused by high pressure drops, causing fluid velocities through the through hole, which is considerably higher and, therefore, highly erosive effect. The effects of erosion are magnified in cases where there are solid particles in fluid suspension. The choice has to be done considering also the anti-erosive and resistance to corrosion by the fluid valve will be a driver.
