Aug. 13, 2024
The slanted seat check valve also offers enhanced resistance to water hammer. The valve has a double eccentric shaft position as well as an increased seating angle. This yields a shorter valve stroke, thus reducing the time taken for the door to close.
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Hydraulic dampers are recommended, especially when the valves are installed on a pumping station where high frequency opening and closing of the valve is required. The valve disc closes quickly over the first 85% of its angular travel before meeting the hydraulic damper. The damper then dissipates the kinetic energy of the disc and forces it to open slightly. The disc closes until it contacts the damper again and this cushions the disc until it returns to its fully closed position, sealing the valve. This function greatly reduces the onset of water hammer due to the damped and controlled method of valve closure.Hydraulic dampers are recommended, especially when the valves are installed on a pumping station where high frequency opening and closing of the valve is required. The valve disc closes quickly over the first 85% of its angular travel before meeting the hydraulic damper. The damper then dissipates the kinetic energy of the disc and forces it to open slightly. The disc closes until it contacts the damper again and this cushions the disc until it returns to its fully closed position, sealing the valve. This function greatly reduces the onset of water hammer due to the damped and controlled method of valve closure.
For more details, please see our slanted seat check valve product information.
Nozzle check valves are designed with the valve disc connected to the stem which is guided on the central horizontal axis. A spring is positioned between the disc and the diffuser sleeve. When flow enters the valve, the hydraulic force exerted onto the front face reacts against the spring, causing the spring to compress and allow the valve to open. When the flow stops, the spring forces the disc to return to the closed position.
Due to the spring-assisted closure and the short linear valve stroke, the nozzle check valve is one of the quickest acting check valves available and is commonly used in pumped systems where water hammer is a potential concern. Because the disc is constantly in the direct line of flow, the head loss characteristics of this valve are higher compared to that of conventional swing check valves.
For more details, please see our nozzle check valve product information.
In order to choose the right check valve for your application, several selection criteria should be considered. First, however, there is not one type of check valves being the best choice for all applications, and the selection criteria may not be equally important for all cases. Some of the things you may need to consider are fluid compatibility, flow characteristics, head loss, non-slam characteristics and total cost of ownership.
All check valves referred to in this article are designed for water and treated wastewater applications, but using the valves for raw wastewater/sewage applications may cause some issues. When selecting a valve for these fluids, you should consider how the presence of solids may potentially affect operation of the valve.
If a check valve closes very fast, it may prevent slamming against upstream equipment such as pumps. However, the rapid closure will not protect against the surges caused by pumps being started and shut down. If the valve opens (and closes) quickly, the flow will change rapidly and thus increase likeliness of surge occurrence.
Head loss is, among other things, a function of fluid velocity. The head loss through a valve is determined by the internal design of the valve and the opening degree. When a valve is designed with a restricted (narrowed) opening compared to the pipeline, the velocity will increase through the valve, increasing the head loss as a result. Vice versa, if a valve is designed with less restriction and bore is equal size to the pipeline, the head loss will be smaller, and the valve will in practice not affect the overall head loss of the system. There are a number of values for head loss, amongst others zeta values, Kv and Kvs values.
The costs for your check valve consist of more than just the purchase price. For some installations, the most important costs may be purchasing and installation, but in other cases, maintenance or energy costs may be equally or even more important. Also protecting more valuable equipment such as e.g. pumps has to be considered, and looking at the valve performance will be crucial. When considering costs as a selection criterion for your check valve, the total costs over the life of the valve should be considered. In general, the simpler the valve construction is, the lower are the maintenance requirements. The higher the Kv value, the lower the energy consumption. The higher the performance, the better the protection ability.
Check valve slam can affect pressure surges in a negative way. First step of the process is when the pump stops, starting the pressure surge. Second step is, when the flow is reversed, slamming against the fully closed check valve. If the check valve closes too fast, the kinetic energy is turned into high pressure, stressing the pipes, and causing high noise.
A slam sounds like if the disc or the ball from the check valve is hitting the seat and can make quite some noise. However, the sound is not caused by the physical closing but by a sound wave arising from a pressure spike stretching the pipe wall.
To prevent the occurrence of check valve slam, the valve should close in a controlled way and slower when near closed position. For a check valve to close slowly, it requires additional ancillary equipment, such as hydraulic dampers, which act as a cushion to the valve door, as it comes into its seated position. This slower closure allows the fluid to pass through the check valve until it closes, causing less kinetic energy turning into high pressure, and thus less energy to feed and maintain the surge. Consideration must be given to the upstream pump to ensure that it is suitable for reverse spin and flow.
As swing check valves have the disc in the flow stream, helping with rapid closure, they have better non-slam characteristics. However, today many pumps are frequency converted, enabling them to adjust the start-up and closure time to avoid water hammer.
Figure 1: Check valve
A check valve allows liquid and air to flow in only one direction. The primary purpose of a check valve is to prevent backflow in the system. Check valves are inexpensive, effective, and easy solutions to the potential issue backflow. Backflow can cause a problem if the flow is contaminated because it can contaminate upstream media. For example, a sewer line will have a non-return valve to ensure that waste can leave a system (e.g., residential plumbing system) but cannot re-enter. Check valves are also often referred to as one-way valves or non-return valves.
A check valve requires a minimum pressure differential between the inlet and outlet to open, allowing media to flow through it. This minimum upstream pressure at which the valve opens is called the check valve's cracking pressure. The specific cracking pressure changes based on the valve design and size, so ensure that the system's pressure can generate the cracking pressure of the chosen check valve(s).
If the upstream pressure falls below the cracking pressure or there is back pressure (flow attempting to move from the outlet to the inlet), the one way check valve will close. Typically, check valves have a gate, ball, diaphragm, or disc pressed against a seal to close the valve and block flow. Gravity or a spring can assist in the closing process.
As a one-way valve only allows flow in one direction, knowing the correct installation orientation is crucial. Typically, an arrow on the valve's housing (Figure 3) indicates the flow direction. If there isn't an arrow, examine the valve to ensure it is installed in the intended flow direction. If the valve is installed backward, the media cannot move through the system, and the resulting pressure build-up can cause damage.
A normally open check valve allows the medium to flow freely but shuts off the flow in case of backflow. A normally closed check valve prevents the flow of media through it until the cracking pressure builds up, at which point the valve opens.
Check valves operate differently depending on their design. The most common check valve is a spring-loaded in-line check valve.
In-line spring-loaded check valves are common, easy to understand, and have a simple design. Figure 2 shows a spring-loaded in-line check valve in the open and closed positions. The arrows indicate flow direction. When flow enters the input port of the valve, it should have enough pressure (force) to overcome the cracking pressure and the spring force. The pressure pushes the disc (Figure 2 labeled B), opening the orifice and allowing flow through the valve. When the input pressure is no longer high enough, or there is enough back pressure, the back pressure and spring (Figure 2 labeled C) push the disc against the orifice and seal the valve shut. The spring and the short travel distance for the disc allow for a quick reaction time for closing. The spring check valve design also prevents pressure surges in the line, preventing water hammer.
Common types of spring-loaded in-line check valves are also called:
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Figure 2: Spring-loaded in-line check valve open (left) and closed (right). The working components are the valve body (A), disc (B), spring (C), and guide (D).
Spring-loaded y-check valves operate similarly to in-line spring-loaded check valves. The difference is that the spring and movable disc are positioned at an angle. This creates a &#;y&#; shape, hence the name of the valve. It works the same way as an in-line valve, but since the moveable components are at an angle, they can be inspected and serviced while the valve is connected to the system. These valves are larger and take up more room within the system.
Figure 3: Y-check valve
A ball check valve uses a free-floating or spring-loaded ball that rests on the sealing seat to close the orifice. The sealing seat is normally conically tapered to guide the ball into the seat and create a positive seal, thereby stopping reverse flow. When the pressure of the fluid in the inlet side exceeds the cracking pressure, the ball is dislodged from its seat, which permits flow. When the inlet pressure doesn&#;t exceed the cracking pressure, or there is back pressure, the ball will close with the back pressure or via the spring, effectively closing the orifice. True union ball check valves allow for easy removal and replacement of balls, eliminating the need to buy a new valve. Read our article on ball check valves for more information.
Figure 4: Spring-loaded ball check valve in the open position allowing flow (A), and in the closed position preventing backflow (B)
Swing check valves are called &#;tilting-disc&#; or &#;flapper&#; check valves. They have a disc on a hinge (or trunnion) that swings open with an inlet pressure. The disc swings shut as the inlet pressure decreases or if there is backflow. If there is no spring to assist in closing, mounting orientation is important to consider to ensure that the disc swings shut with gravity. Figure 5 shows an example of a swing check valve. Read our article on swing check valves for more information.
Figure 5: Swing check valve. Bolted bonnet (A), hinge or trunnion (B), valve body (C), disc (D), seal (E)
A foot valve is a check valve combined with a strainer on the inlet side. The strainer prevents debris that could clog or damage something downstream from entering the check valve. This valve is installed at the end of a section of piping as its input doesn't have a connection point. Common check valve types included in a foot valve are in-line spring assisted or an in-line ball check valve. They are typically installed at the end of a pump suction line of a water well, fuel tank, or any other application where the suction line is situated below the pump. They can keep pumps primed, prevent liquid from siphoning back, and keep debris out of the line. Figure 6 shows an example of a foot valve.
Figure 6: Foot valve
Stainless steel check valves are renowned for their superior resistance to corrosion, heat, and low temperatures, making them ideal for demanding environments. They exhibit excellent mechanical properties, including high strength and durability, contributing to their long service life. These valves are well-suited for applications involving high temperatures and pressures, such as in industrial processing, chemical plants, and oil and gas industries.
Stainless steel check valves are available in various grades, with 304 and 316 being the most common. Grade 316 stainless steel offers enhanced resistance to chlorides and other corrosive substances, making it suitable for marine and coastal applications.
While stainless steel check valves provide exceptional performance, they may not be the most cost-effective solution for applications that do not require such high durability or resistance. In such cases, alternative materials like PVC or brass may be more economical.
PVC check valves are frequently used in irrigation and water management systems. They are corrosion-resistant to most corrosive media like seawater, acids, bases, chloride solutions, and organic solvents. However, they are not immune to aromatic and chlorinated hydrocarbons and typically have a max temperature resistance of around 60 °C.
Brass check valves are excellent for air, water, oil, or fuel applications. However, they are not resistant to seawater, purified water, or chlorinated water. Compared to stainless steel, brass is less resistant to heat and corrosion and is typically used for applications with low pressure.
Check valves have the following criteria to consider when selecting one for an application:
Due to how check valves function, they are typically used for one of four different reasons in various applications:
Due to their function, they are used in almost every industry. They are used on common household appliances, like dishwashers, washing machines, and wastewater lines. For industrial purposes, they are used on boilers, furnaces, gas systems, pumping applications, or vacuum systems. They are also frequently used as aquarium check valves on water and CO2 lines. Also, a miniature check valve is a popular choice where the space is limited, yet a reliable operation is essential. Two of the most common check valve applications are for water and air, which are discussed in more depth below.
A water check valve is used in numerous water applications, like drinking water and wastewater. These valves are simply called one-way water valves. Plumbing check valves for drinking water applications ensure that no environmental media (outlet side of the valve) can enter the system with the safe, clean drinking water and contaminate it. For wastewater applications, they ensure that the wastewater cannot re-enter the system and cause an overflow or additional contamination. For water pumping applications, a foot valve is often used to ensure no debris enters the line and keep internal pressure for priming purposes. Sump pump check valves ensure that the discharged water does not come back into the sump pump with gravity when the pump is turned off.
A pneumatic check valve, or air check valve, allows airflow and prevents it from going out. They are often simply called one-way air valves. The most common application is for an air compressor. A pneumatic check valve allows the compressor to keep certain parts pressurized and other parts de-pressurized. They can be located on an air receiver, discharge pipe, or as a piston check valve on the piston compressor's inlet and outlet sides.
The P& ID symbol for check valves is shown in Figure 7.
Figure 7: The check valve P&ID symbol points in the orientation that it allows the flow with a vertical line showing it doesn&#;t allow backflow.
A check valve is a unidirectional valve that passes fluid in one direction but prevents any flow in the opposite direction.
The main purpose of a check valve in a system is to prevent backflow, which could damage equipment or contaminate media upstream.
Common check valve problems are noise, water hammer, vibration, reverse flow, sticking, leakage, and component wear/damage.
A check valve can prevent water hammer if it is fast-acting like a spring-actuated check valve. This prevents pressure surges, which create shock waves throughout the media.
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