Stop check valves play a vital role in fluid systems across diverse industries, ensuring the efficient and reliable flow of liquids. In this article, we will delve into the functionality of stop check valves, offering a comprehensive overview of their purpose, design, and applications in various systems.
What Is a Stop Check Valve?
A stop check valve is a type of valve designed to control the flow of fluids in a piping system. It combines the functions of a stop valve and a check valve, offering a versatile solution for fluid control. The primary purpose of a stop check valve is to allow the flow of fluids in one direction while preventing backflow in the opposite direction.
Key Features and Functions of a Stop Check Valve
- Bidirectional Flow Control: Stop check valves are capable of regulating the forward flow of fluids in a manner similar to a stop valve. Additionally, they act as a check valve to prevent reverse flow.
- Disc Mechanism: The valve typically incorporates a disc or flap mechanism that allows free passage of fluid in the desired direction. When the flow attempts to reverse, the disc quickly closes, acting as a check valve to block backflow.
- Design Variations: Stop check valves come in various designs, including swing, lift, and tilting disc configurations. Each design is suited for specific applications based on factors such as system pressure, flow rate, and the nature of the fluid being handled.
- Swing Stop Check Valves: Utilize a hinged disc that swings freely to allow forward flow. The disc swings back into a closed position to prevent reverse flow.
- Lift Stop Check Valves: Feature a disc that moves vertically to control the flow. When forward flow is desired, the disc lifts, and when backflow is detected, it swiftly closes.
- Tilting Disc Stop Check Valves: Employ a disc that tilts to allow forward flow and returns to a horizontal position to block backflow.
Stop Check Valve Designs
Two basic pattern designs are available for these valves, T pattern and Y pattern. The stem in the T pattern is perpendicular to the pipe, which allows the internal disc to move vertically. Its weight helps it close quickly, so when the valve closes, it has minimal reverse velocity. Because of its weight, the valve requires a higher flow velocity in order to fully open, and it could also cause the valve to lose pressure.
Similarly to the T pattern, the latest design is the Y pattern, where the stem and disc are angled at 30 to 45 degrees. In this pattern design, the stem and disc are angled at 30 to 45 degrees to allow a lower full-open flow velocity, which is most commonly used on top of boilers. It is necessary, however, to pay more attention to disc guiding to ensure trouble-free operation with the Y pattern.
The straightway design creates a more direct flow path, reducing turbulence and fluid friction. This results in decreased potential for erosive damage to the valve, leading to lower pressure loss and improved overall flow characteristics within the piping system.
The angle valve design is suitable for upward-to-horizontal or horizontal-to-downward flow. In both cases, the flow is redirected 90 degrees within the valve, enabling the angle valve to function as both a flow control device and a 90-degree piping ell.
As these valves feature a cylindrical-shaped floating disc member as the sole pressure-actuated component, it is crucial to size them appropriately to ensure proper full disc lift throughout the valve’s service life. Oversizing may lead to disc flutter, increasing wear and limiting the valve’s longevity. Conversely, undersizing may result in higher pressure loss and velocity, potentially reducing service life. A light and well-guided disc can achieve maximum lift at minimum velocities, ensuring swift opening and minimal pressure loss. Additionally, the disc should be designed to prevent spinning, promoting low wear and an extended service life.
How Does a Stop Check Valve Work?
The functionality of stop check valves is rooted in their design, which includes a disc or flap mechanism. When fluid flows in the desired direction, the disc allows unrestricted passage, behaving similarly to a stop valve. However, when the flow attempts to reverse, the disc quickly closes, acting as a check valve to prevent backflow.
Advantages of Stop Check Valves
- Stop check valves can be used as shut-off valves.
- These valves operate automatically when used as check valves.
- Stop check valves are durable since they last for long periods. Also, these valves can be repaired when they break down thus enhancing longer service life span.
- These valves are versatile as they can be used in different industrial applications.
- Stop check valves do not allow reverse fluid flow.
- These valves are free from fluid leakage.
- Stop check valves can be used in high pressure and high-temperature applications.
- These valves are easy to install and repair.
- These valves have good shut-off capability.
Disadvantages of Stop Check Valves
- Stop check valves have a high-pressure drop.
- These valves allow fluid flow in one direction only.
- They cannot be inspected while in operation.
Stop Check Valves: Areas of Usage
These valves are primarily used in power plants, where they are used for boiler circulation, steam generation and boiler feed water, turbine cooling, starter water and safety systems. They are also used in other applications where piping designers want to combine the properties of a globe and check valve. A few examples of these applications include high-temperature service, heat recovery steam, high-pressure safety service, geothermal service, petroleum production and refining, safety systems, shutdown cooling, and hydrocarbon processing.
The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code covers how valves should be used in boiler applications. Boiler and pressure vessel valves have been connected to boilers for over 150 years. In most cases, the first valves on a boiler’s output line are stop-check valves, also called non-return valves or boiler stop valves. When more than one boiler is connected to the main steam header, they should be installed in the pipeline between each boiler and the header, so that the boiler pressure is beneath the disc.
The stop check valve can also be used as a blowdown valve. This valve allows excess water to be removed from a boiler, which is required occasionally.
Conclusion
In conclusion, stop check valves serve as indispensable components in fluid systems, providing a versatile solution for controlling the direction of flow. Their unique design, incorporating elements of both stop valves and check valves, makes them essential in preventing backflow and maintaining the efficiency of various systems.
