Blog 23: Engineering basics: Mechanical Valves?
Valves or mechanical valves are mechanical devices that are used to control the flow and pressure of liquids, gases, vapors, and slurries in a piping system. Valves are an important part of a piping system. Valves can be of different categories depending upon the way it works. There can be manual valves, self-actuated, actuated, pneumatic and hydraulic. But despite its many types, the basic operation of the valves remains the same that is:
- Starting or stopping the flow of liquid, gas, vapors, etc. based on the valve state
- Regulating flow and pressure within a piping system
- Controlling the direction of flow within a piping system
- Throttling flow rates within a piping system
- Improving safety through relieving pressure or vacuum in a piping system
Depending upon the vast industrial application, valves can be of different types depending upon their design, types, and material. But also ensuring that the function mentioned above comply too. Valve are an expensive product and have great value in the piping system so selecting the correct type of valve for the correct application is necessary
Categories of valves:
There are two basic categories of valves:
- Operation of valves
- The working capability of valves
Operation of Valves:
With respect to the operation of valves we can distribute them into 2 categories:
- Multi-Turn valves: Muti-Turn valves are the type of valve system that consists of a screw or a piston. It is used to control the flow rate of the liquid. When the valve handle is turned it rotates the screw or the piston that controls the plug, membrane, plate, or any other obstruction material for controlling the liquid. Each turns to allow a different rate of liquid to flow.
- Quarter turn valves: quarter turn valve also known as the 90- degree valve are the type of valve that is used where precision is not important but rapid action and quickly opening and closing operation is required.
The working capability of valves:
With respect to the working capability of valves we can distribute them into 3 categories:
- Manual Valves: Typically adjusted by hand, these valves use handwheels, hand levels, gear wheels, or chains to actuate.
- Actuated Valves: Actuated valves are remote-control valves that are often used for high-precision or large-scale applications. These valves are often connected to electric motors, air or pneumatic systems, hydraulic systems, or solenoids, that control their operation.
- Automatic Valves: Some valves activate when a specific flow condition is met. Examples include checking valves closing during backflow or pressure release valves activating when an over-pressure condition is detected.
Types of valves:
Some common types of valves are:
- Ball Valve: Predominantly equipped with quick-acting 90-degree turn handles, these valves have a ball to control flow to provide easy on-off control. Generally accepted by operators to be faster and easier to operate than gate valves.
- The flow resistance is the lowest.
- It can be used to control corrosive or low boiling point liquid reliably because it will not be stuck in operation.
- It has a strong seal and hence is used in high-pressure and temperature applications.
- It can realize quick opening and closing, and the opening and closing time of some structures is only 0.05-0.1s, so as to ensure that it can be used in the automation system of the test bench. When the valve is opened and closed quickly, the operation is free of impact
- The ball closing parts can be automatically located at the boundary position.
- The working medium is sealed reliably on both sides.
- When fully open and fully closed, the sealing surface of the ball and seat is isolated from the medium, so the medium passing through the valve at high speed will not cause the erosion of the sealing surface;
- It is considered the most reasonable valve structure for a low-temperature medium system because of its compact structure and lightweight;
- The valve body is symmetrical, especially the welded valve body structure, which can bear the stress from the pipe well;
- The closing parts can withstand the high-pressure difference when closing.
- A ball valve with a fully welded valve body can be buried directly underground, which makes the valve internals free from corrosion, and the maximum service life can reach 30 years, which is the most ideal valve for oil and natural gas pipelines.
- Because the main sealing ring material of the valve seat is polytetrafluoroethylene, it is inert to almost all chemical substances and has the characteristics of a small friction coefficient, stable performance, not easy aging, wide range of temperature application, and excellent sealing performance. But the physical properties of polytetrafluoroethylene, including high expansion coefficient, sensitivity to cold flow, and poor thermal conductivity, require the design of the seat seal to be around these characteristics. Therefore, when the sealing material becomes hard, the reliability of the seal is destroyed.
- The temperature resistance of polytetrafluoroethylene is low, and can only be used under the condition of less than 180 ℃. Above this temperature, the sealing material will age. But considering the long-term use, it will only be used at 120 ℃.
- Its regulating performance is worse than that of the stop valve, especially the pneumatic valve (or electric valve).
- Butterfly Valve: Using a compact design, the butterfly valve is a quick-acting rotary motion valve ideal for tight spaces thanks to its wafer-type design. Butterfly valve bodies are offered in many different configurations.
- It has a simple structure, small volume, lightweight and low consumption, and is not used in large caliber valves;
- The opening and closing are quick, and the flow resistance is small;
- It can be used in a medium with suspended solid particles and can be used in powder and granular media according to the strength of the sealing surface. It can be used for the two-way opening and closing of ventilation and dust removal pipelines and is widely used in gas pipelines and waterways of metallurgy, light industry, power, and petrochemical systems.
- The flow regulation range is not large, when the opening reaches 30%, the flow will enter more than 95%;
- Due to the limitation of the butterfly valve structure and sealing material, it is not suitable for high-temperature and high-pressure pipeline systems. The general working temperature is below 300 ℃ and PN40;
- The sealing performance is poor compared with the ball valve and stops valve, so it is not very high for sealing.
- Check Valve: Used to prevent backflow, these valves are typically self-activated allowing the valve automatically opens when media passes through the valve in the intended direction and close should flow reverse.
- Gate Valve: As one of the most common valve types, gate valves use linear motion to start and stop the flow. These are typically not used for flow regulation. Instead, they are used in fully open or closed positions.
- The fluid resistance is small;
- The torque required for opening and closing is small;
- It can be used in the ring network pipeline with the medium flowing in two directions, that is, the flow direction of the medium is not limited;
- When fully open, the erosion of the sealing surface by the working medium is smaller than that of the stop valve;
- The structure of the body is simple and the manufacturing technology is better;
- The structure length is shorter.
- The external dimension and opening height are large, and the space for installation is also large;
- In the process of opening and closing, the sealing surface is relatively friction, and the friction is large, even if it is easy to cause abrasion phenomenon in high temperatures;
- Generally, gate valves have two sealing surfaces, which add some difficulties to processing, grinding, and maintenance;
- The opening and closing time is long.
- Knife Gate Valve: Typically used for controlling the flow of media containing solids, the knife gate valve features a thin gate controlled through linear action which can cut through materials and create a seal. While not suited for high-pressure implementations, these valves are ideal for use with grease, oils, paper pulp, slurry, and linear action which can cut through materials and create a seal.
- Globe valve: Globe valves are typically applied in modulating control operations. Typically available in three body types, T-body (as shown above), Y-Pattern, and Angle body.
- Needle valve: Typically used in small diameter piping systems when fine, accurate flow control is needed, Needle valves get their name from the point on a conical disc used within.
- Pinch valve: Often used for handling solid materials, slurries and liquids with suspended solids, pinch valves use a linear motion. Typically Pinch Valves feature an internal sleeve to isolate the media.
- Plug valve: Using a quick-acting quarter-turn valve handle, these valves control flow using tapered or cylindrical plugs. They provide some of the best ratings when tight shutoff is essential and are reliable in high-pressure or high-temperature environments.
- Pressure relief valve: Used to help improve safety, these valves are spring-automated and will help to return a system to the desired pressure during over-pressure events.