An introduction to control valve
A control valve is used in the oil and gas industry to regulate the flow rate of the fluid in a pipeline or process (and the related process parameters as pressure, temperature, and level) according to signals managed by a controller. The role of a flow control valve in the complex petrochemical process is key, as the multiple loops involved in the process should be kept under strict and dynamic control to make sure that the process, as a whole, works as intended and produces the desired output in terms of quantity, quality and time.
The application of flow control valves is increasing in the last years, due to growing process automation in most industries.
These type of valves are used in irrigation systems, water treatment plants, oil and gas plants, power generation, fire prevention systems, food processing industries by streamlining the response to changes in processes and providing greater safety to personnel and equipment.
A flow control valve used in the oil and gas industry can have a globe, butterfly, or ball shape, and is available in multiple material grades and sizes. The most used type of actuator is the air-operated, as it involves less ancillary equipment (as cabling, switchgear) when compared to other types of actuators.
The opening and closing of the valve and its regulation are done by the combined effect of an electronic controller, a positioner and the actuator of the valve (which can be electric, pneumatic or hydraulic).
The actuator opens and closes the control valve in response to changes in key process parameters, such as changes in pressure, level, temperature, and flow.
By such action, the process parameters are maintained within the required target range to make sure the process, as a whole, works as intended and produces an end product in the desired quantity and quality.
The main components of a control valve are:
The selection of actuators is based on the required thrust of the rod and the movement of the valve.
Engineers must determine if the diagram or piston actuator is the most suitable type. Actuators are the primary energy source used to move and position the flow control element within the valve body.
A variety of types are available, including pneumatic, electric, hydraulic, electro-hydraulic and manual. Pneumatic actuators are the most widely used for throttling applications.
Considered the standard for automatic control for more than a century, three types of pneumatic actuators are available: piston, spring-and-diaphragm, and rotary vane. The first two can be adapted for use on either rotary or sliding-stem valves; the latter is used only on rotary valves.
Normally, the selection of accessories such as positioners transducers, boosters, solenoid valves, limit switches, handwheels and travel stop, snubbers, regulators, transmission lines, is based on engineering specifications.
Cost is a major factor in material selection.
Not just the cost of material in dollars per pound, but also the cost of fabrication and inspection contribute to the uninstalled cost of the valve. Installed cost includes not only the cost of installation but also the cost of any damage from improper installation and costs the inspection.
The latter consists of such things as analysis of material chemistry, radiographic and surface examination of castings and welds, and check to see that the installed valve is the correct one and that it is properly oriented.
The selection of the appropriate or optimal control valve type depends on the particular study of the pipe system and the conditions of its fluid, but the size of the control valve should be such that pressure drops through it and not the drop of pressure of the pipe is the one that controls the flow.
All valves, including steam control valves, are designed to meet an allowable internal leakage standard (FCI / ANSI). The higher the number of leaks, the lower the permissible internal leakage rate.
A Class I valve will have the highest internal leakage rate and usually the lowest cost; While a Class VI valve will have the lowest allowable internal leakage rate. Steam valves must be specified to have a leak rate of not less than Class IV. A class IV steam control valve will maintain a long service life.
Control valves can be classified into two main types, based on the stem movement:
Globe control valves can be either single or double seat.
Process engineers must consider many factors to select the correct control valve. The general criteria to be considered for selection are summarized below:
As the design criteria are known, process engineers can proceed to make a selection of the correct type of control valve and related equipment, as well as the material of the valve itself.
The next step is to create the detailed specification (generally drafted by a specialized piping engineer) and create a purchase specification describing the valve for suppliers.
The specification should contain, as minimum: a description of the type of control valve (rotary, sliding); bore size and pressure rating; end-connections; type of body joints; material specification for body components, trim, gaskets and bolting; positioner and controller requirements; a reference to an industry design standard for type of valve.
Generally, a computerized system help engineers determine the most suited control valve and its features based on process parameters.