What is a Check valve?
Most valves are operated by a human, air pressure, or even electricity. When these elements are unavailable or fail to perform, a check valve keeps on working. A check valve is intended to automatically shut off when pressure reduces or changes direction so that fluids or gasses are only able to travel in one direction. Another way to say this is that check valves prevent backflow. Inside a check valve, there is either a disc or ball that moves away from the inlet when pressure is applied. This pressure is created by the flow of a fluid or gas. The disc or ball moves back and blocks the inlet when pressure comes from the opposite direction. The components of a check valve usually included the body, seat, disc, and cover.
What to expect
You may recall from reading about gate valves and globe valves that normal valve components usually include a bonnet, body, disc, and stem. Check valves do not usually have a stem or variation in bonnets, and body style is not a defining feature. This is a very unique style of valve! Check valves are defined by the type of force that closes the flow. The forces that stop flow in a check valve are back-flow pressure, gravity, and springs.
The most common structure of check valve in the marine industry is the swing check. Typical swing check valves have a fluid that flows straight through it. Inside the valve, there is a disc that is seated over the inlet. When fluid goes in and applies pressure on the disc, the disc is pushed into the open position. When the pressure reduces or backward pressure occurs, the disc is pushed back into its seat in the closed position. This automatically prevents backflow from occurring.
A lift check uses flow pressure in a similar way to swing checks. Flow pressure moves the disc to the open position, and reducing pressure or changing direction moves the disc to a closed position. The main difference in a lift check is that flow does not go straight through the valve. Instead, it moves upward against the disc and then back down to the outlet. This means that when flow pressure decreases or changes direction, gravity helps push the disc closed and ensures a faster, tighter seal. The changing internal direction of fluid gives more opportunity for build-up of debris and residue, so this style is not recommended for viscous material.
A piston check is very similar in function to a lift check. The big difference is that the disc is spring loaded, meaning that a spring pushes the disc into a closed position. This closes the disc faster, causing less backflow in the line and reduces the possibility of hammer-stop. However, it can also result in a higher amount of pressure reduction since the flow pressure has to both lift the disc and press the spring-loaded piston upward.
Inline check valves are smaller-bodied check valves that are spring loaded. Most inline check valves are intended for vertical installation, though they can be used in any direction. It has a disc or globe inside of it that is forced by a spring to seat in the inlet, stopping backflow. Flow pressure against the disc or globe in the correct direction causes it to open, and when pressure begins reducing, the spring presses it back into a closed position again. This is great when a check is needed and flow is moving in a downward vertical direction because the check does not rely on gravity or solely on backflow pressure to close.
This style is also known as a silent check. The reason for this is that the spring pressure causes it to close gradually and directly as inflow pressure reduces which results in a quieter close. A silent close means less hammer-stop and less vibration, which means pipes and valves last longer.
Wafer check valves look a lot like butterfly valves at first glance. They are narrow, which makes them great when there isn’t enough room for a bulky valve. Usually with a wafer check valve, there is a minimum back-pressure requirement restriction, which means it may have some seepage in the closed position and allow some backflow. Because of its shape and thickness, it works very well for fluids that are viscous or contain debris.
Choosing your check valve
Check valves come in a myriad of variations. All of them have strengths and weaknesses that must be accounted for when choosing a check valve. Before you choose a check valve, make sure you know the application of the valve. Take into account fluid pressure, consistency of pressure, viscosity of the fluid, and allowable amount of backflow. For example, if you are checking mud, a piston gate valve will probably clog fast and not function well. If you are checking gasoline, a wafer check may allow too much backflow resulting in loss of material from your pumps dumping it. If you are sending sea-water downward through a vertical line, a standard check valve probably won’t stop much. Determine your needs and then check the ANSI ratings of valves before making your final decision.
Other Styles and terms
Earlier we stated that a check valve doesn’t usually have a stem. It should be noted that some check valves, like the Pima bronze scupper, have a gag. This is a stem that is used for throttling a check valve manually, though it’s usually left alone since the check valve should prevent backflow automatically.
You may hear about the cracking pressure of a check valve. This is the minimum amount of pressure required to move the disc off of the seat and allow positive flow through the valve. Determine the minimum backpressure of your line before choosing a check valve that has minimum pressure requirements.
There are plenty of other check valve styles that exist or are being developed. For example, scuppers are check valves that are designed to prevent the backflow of sewage, though mechanically they can be very similar to swing checks or, in some applications, wafer checks. A lot of the check valves we didn’t cover here aren’t used for marine purposes or are less common in the marine industry. This doesn’t mean that we don’t sell the other types though! We carry a huge variety of check valves, and if you have questions about these check valves or other types of check valves, feel free to connect with our experts at Tork Systems, Inc. We’re always on standby to share our knowledge and make your job easier.