Mar. 01, 2022
Almost every large-scale application benefits from valve technology. Valves are devices that handle fluid/media flow through a pipeline and can insert control or regulatory functions. Their mechanical/electromechanical nature makes them easy to manufacture and operate, and these engineering tools are widely used and varied as a result. Our article on understanding valves provides a great introduction to these devices, but this article will take a deeper look at the rotary valve. We will examine what a rotary valve is, how it works, what it is used for, and how to select the right rotary valve if it seems useful for your project.
Rotary valves (also known as rotary airlocks, rotary feeders, etc.) are pneumatic valves that handle and meter the flow of granular bulk or powders. Material is fed into the valve via a hopper (see Figure 1) or other inlets, handled through the valve rotor, and then deposited onto a conveyor system in discrete packets, all with minimal pressure loss. They are most often used in air conveyor systems to minimize the loss of pressure across the valve, but rotary valves are also used to meter materials at a controlled feed rate. They work almost exclusively in air systems and function as airlocks, explosion/flame barriers, and process isolators, among other tasks. They can move dry to slightly sticky products, and find many applications in chemical and food, pharmaceutical, plastic, and other manufacturing markets.
Rotary valves are composed of 4 main components: an outer housing, a rotor, a rotor bearing and seal, and a drive. The housing is the rightmost component in Figure 2 and provides the inlet/outlet ports as well as the protective pathway for material through the valve. The rotor is held in place by the rotor bearing, and material is sealed within the valve inner housing via the bearing and shaft seal. The drive mechanism (typically an electric motor) attaches to the rotor shaft from the outside and controls the rotation speed and direction of the rotor. Product is fed into the valve, captured into equal pockets between rotor vanes, and is dispensed in even amounts at the outlets. These rotor vanes have a tight clearance with the housing and prevent pressure losses/material losses through the valve, ensuring pressure differentials remain intact.
There are many variations on the rotary valve, all suited for unique applications. Each component can be configured to meet different needs, and the below sections will briefly explain how certain parts differ to provide different kinds of rotary valves.
Housings can come as either inline or in offset arrangements, depending upon the need. Inline housings configure the inlet directly above the outlet, which is more efficient and is generally practical in most applications. Offset housings intentionally skew the inlet from the outlet to prevent product shearing and overfilling but are less efficient and more niche in their application. There are also plow like designs and one-off arrangements all meant to provide the best valve functionality to specific uses.
Rotor design is highly specific to the application at hand, and come in many, many types. There are closed-end rotors, open-end rotors, metering rotors, staggered pocket rotors, reduced volume rotors, helical rotors, adjustable rotors, flex tip rotors, radius pocket rotors, and many more one-off types that each fit unique material and needs. There are typically no less than 8 rotor pockets (and therefore 8 vanes) in a rotor, but increasing the rotor amount will increase sealing capacity at the cost of filling efficiency. Replaceable vanes are also popular, as rotors tend to wear with time, but are generally not used in large rotary valves.
Most rotary valves are powered via a synchronous electric motor connected to a reduction gearbox and chain setup, allowing for precise speed control and regulation. Additional speed monitoring equipment and variable speed controls can be added to these drives, making the rotary valve useful as rotary automation valves. There are also hydraulic motors that can power larger rotary valves, but they do not use a gearbox and are reserved for more specific uses.
Since these devices are dependent on the application, there is a lot to specify when choosing the right valve for the job. This section will detail some important specifications to understand when considering a rotary valve but know that this list is not fully comprehensive. Always consult your designers and suppliers about specifications, as they will have the most relevant information and resources.
Application + performance requirements
Understand what you are using the rotary valve for, and what it must accomplish; this means calculating the desired throughput rate, defining material characteristics (density, flow coefficients), estimating efficiency, and other important metrics. These values are dependent on all aspects of the valve, so be prepared to alter these specifications as needed.
Rotor speed will affect throughput rate, valve wear, valve size, and more. Understanding the right speed at which to run your rotary valve will help with efficiency and maintenance. It is generally said that a rotor should not exceed 40 meters/min, but again this is dependent on rotor type, material characteristics, and more.
Rated temperature and pressure
If the rotary valve will be used in a pressure or vacuum system, ensure it is rated for these conditions. Also be sure to define the working temperatures of the surroundings and of the material, so that no part of the valve is overtaxed. There are significant thermal effects between the rotor and the housing (especially at full speeds) as the rotor generally expands at a different rate, which can cause unwanted leaking or maintenance. Ensure the chosen rotary valve will not exhibit these effects in your application and can handle the heat.
How will the valve connect to your larger system, and how will it feed and discharge? These are all important specifications, as different feeding and discharge equipment can be used to differing successes. There are hopper feeders, blowers, simple piping, and other equipment, so figure out which one will be used in your project. Specifying these aspects will also allow you to find a rotary valve that will come with the correct connection characteristics for your greater application such as welding spots, through holes, etc.
Material of construction
Finally, determine what possible chemical and physical effects can occur between your product and valve, and choose a material that can stand up to these stresses. For example, stainless steel rotor blades will resist rust from wet media, while bronze rotor tips will not wear down as easily as other kinds. Choose a valve material that will keep your pressure/vacuum integrity if needed, and make sure that the material is rated for the estimated lifespan of your project.
Rotary valves are used in almost all pneumatic conveyor systems, but they also find uses in many other applications. This section will display just how indispensable these valves are, and how they can still find new uses.
Some notable applications of rotary valves are:
Food and chemical processing applications
Pharmaceutical manufacturing applications
Abrasive product conveyance/metering
Frequently cleaned/abiotic applications
Pneumatic conveyance and more.
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