Types of Control Valves used in Industrial Sectors
In the petrochemical industry, getting oil and/or natural gas from one place to another requires highly skilled people, like technicians and instrumentation engineers, and superior products, such as pipes, thermocouples, and flow control valves. Different valves not only have different functions but are also made from different materials.
The Gate Valve
This type of valve regulates the flow by inserting a circular or wedge-shaped plane into the liquid. Most of the time, gate valves are used to control straight-line flows so that the pressure exerted by the liquid is equal on on sides of the gate. One kind of gate valve, known as through-conduits, has a circular aperture in the wedge that allows liquid passage through it instead of underneath it. A second subset of these valves is called knife gate valves, which are installed on lines that carry high-viscosity fluids that need a valve with an extra-sharp edge to cut through the surface tension. Also known as sluice valves, gate valves are generally installed at junctions that aren't used a lot because they require manual operation.
The Globe Valve
In the 21st century, the term globe valve is a bit of a misnomer. Most globe valves no longer have the distinctive spherical chamber that gave them their name. Rather, they have a rectangular or other-shaped chamber. Instead of creating a new name, the instrumentation community just continued to call them globe valves. A globe valve closes off the liquid flow by using a round disc that screws into place in the center of a plate. In most cases, the stems of globe valves are not threaded. Instead, they use an actuator assembly to raise and lower the disc to open or close off the flow. Unlike gate valves, globe valves are specifically designed for flow lines that see constant use.
The Pinch Valve
Unlike other valves, which are constructed exclusively of rigid materials, pinch valves, by their very nature, require flexible materials. As the name indicates, pinch valves shut off the flow of liquid by squeezing the opposite sides of the fluid path together. Pinch valves' particular advantage is in handling fluids that have a high concentration of sediments or suspended particulates. They guarantee a 100-percent seal even on these kind of fluids whereas other valve types can't keep the particulates from squeezing through the cracks due to sediment buildup, which jams the sealing plates. There are two general kinds of pinch valves: pneumatic and electric. Electric valves use an actuator assembly to achieve the pinch while pneumatic valves use air pressure alone.
The Diaphragm Valve
Diaphragm valves are similar to globe valves in that they use what is usually a horizontal piece of solid material to achieve the seal. Instead of a plug, however, they use a thin membrane as the diaphragm. Also, diaphragm valves have zero wasted space within the assembly, which is crucial when dealing with natural gas or low-viscosity fluids that flow under low pressure. There are two chief kinds of diaphragm valves: weir and straight-flow. Both require regular maintenance because of the thinness of the diaphragms. The weir configuration uses less force to achieve the seal and is the more useful when it comes to gases or low-viscosity liquids. The straight-flow configuration is better at handling fluids with suspended particles but is not as good as a pinch valve. It uses more force than the weir configuration and needs diaphragm replacement more often. Less common that the weir or straight-flow versions, the diaphragm check valve outperforms them. It is much more expensive to produce, however, because of the special materials from which the diaphragms must be fashioned to be able to withstand the extra force applied.
The Needle Valve
Needle valves are used when the controlled flow must be both slow-moving and precisely measured as part of the process. Unlike other, straight-flowing valves, needle valves have the fluid pass through a 90-degree turn before passing through a small orifice. Needle valves are also useful when the flow runs through delicate gauges that might be damaged by a larger, faster flow. Because they don't just "slam shut," needle valves are perfect for tasks where the liquid flow must be tapered off slowly or started slowly.
There are several valves in this category, and their primary function is to regulate flow in a quick, on-off fashion. A ball valve, for example, rotates 90 degrees on command and can swivel nearly instantly. Plug valves achieve the same thing by moving up and down vertically like a bathtub stopper. Butterfly valves have a circular vane similar to a diaphragm. They rotate in place, however, to allow flow. Check valves operate in concert with these and other valves and prevent backflow. Pressure relief valves don't regulate flow themselves. Instead, they exist on the flow circuit as a safety precaution that releases excess pressure that could either damage or destroy the system. They can be preset to "go off" at a certain level of pressure and then shut again once the pressure drops below an acceptable level.
Although these valves cover many of the instrumentation processes involved in the petrochemical industry, there are many jobs for which they are not suited. In that case, it's generally possible for a company to get custom-made valves for any process it might be running. Valve makers will take the following criteria into consideration when constructing customized valves:
- Required flow rate and the characteristics of the handled substance
- Pressure and temperature parameters of the substance and process
- Corrosion and erosion resistance
- Possible actuator requirements
- Maintenance schedules, upkeep, and repair considerations
In all cases, specialty or general, the valve used must meet the specifications of both the process itself and safety. It must also conform to applicable laws and regulations.