After determining the optimal performance requirements and the environmental factors that will impact performance, the next step is to select or design an appropriate check valve for the application. Finding a solution that meets every requirement may take time and effort. As with most decisions, some trade-offs must be considered. For example, decreasing the valve’s envelope size may result in a lower flow capacity. Changing the valve’s material from plastic to metal will increase operating or burst pressure capability but will impact the valve’s weight. Other performance aspects specific to check valves may require more careful consideration during the selection process. These include:
Valve gain is defined as the rate of increase in flow rate per pressure increase; it measures how efficiently a valve moves from a closed position to the flow point pressure. A high-gain valve can fully open with minimal increase in pressure. A low-gain valve requires a more significant increase in pressure to achieve the fully open condition. However, a lower gain valve may offer greater stability when the system’s flow rate is too low to open the check valve fully.
The response time indicates how quickly a check valve goes from free flow to fully closed; this occurs when pressure reverses and generates a pressure differential in the checked direction. Response time may be important when the direction of pressure changes quickly and increases at an extremely high rate. A valve with a slow response time to a rapid pressure reversal and high rise rate may briefly allow flow and pressure in the checked direction until the valve closes. This may negatively affect system performance or may even cause damage to upstream components the check valve is intended to protect.
Valve materials were previously discussed about their ability to withstand the required environment. Significant performance trade-offs are also related to the choice of materials for sealing components. Seals made of more rigid material, such as metal, can often withstand higher pressure and wider temperature ranges than softer materials like plastic or rubber. However, the softer materials are usually capable of providing tighter leakage control. Leakage will also vary based on the force applied to the seals. Selecting materials that balance leakage rate, pressure range, temperature range, and durability is critical to system performance.
Even when a valve is designed with the previously discussed performance criteria and environmental factors considered, check valves may get damaged and fail to perform adequately in service. It is important to be aware of certain failure modes to ensure a check valve is designed appropriately and the proper measures are in place within the system to mitigate the risk of failure. Below are some examples of potential failure modes. This is not a complete list, so all possible failure modes for a specific application must be evaluated.
A check valve’s most common failure mode is damage due to ingesting foreign material or contamination. Unfortunately, fluids can contain contaminants of various sizes and materials, or the system can introduce them. This contamination can damage the valve’s internal components or become lodged inside the valve. This may cause the valve to leak or become stuck in either the open or closed position. In a worst-case scenario, contamination can become trapped in a place that prohibits the valve from closing, such as between the sealing surfaces, allowing fluid to flow in the checked direction unexpectedly. Adequate protection against contamination should be incorporated upstream of the check valve.
As noted in the performance criteria, pressure requirements are essential for valve selection. However, the minimum and maximum pressure differentials are only some concerns. The rates at which pressure increases and decreases generate additional forces on the valve due to the speed of acceleration of the moving parts. Unforeseen, extremely high increasing or decreasing pressure rates can cause these moving parts to impact other components with forces high enough to cause damage unexpectedly and potentially. This may affect decisions regarding the valve’s material selection and internal design.
A check valve opens due to the overall differential pressure between its inlet and outlet ports, and this overall pressure differential is created by a series of pressure differentials across the valve’s internal components. The valve is designed to achieve a flow rate at or before a certain differential pressure across the moving components to move them to a fully open position. If the pressure at the inlet port exceeds the valve’s cracking pressure, but the valve is not supplied with sufficient flow to generate the pressure differential across the moving components to open the valve fully, the valve will be compelled to close after it has cracked open. Once closed, the pressure will again build until the cracking pressure is exceeded, and the process repeats. This repeated opening and closing results in an unstable condition for the valve. This may be a problem for check valves, resulting in excessive wear and damage. Insufficient flow through a check valve can also result in a problem commonly called silting: the collection of contamination and debris between the valve’s seat and seal. It may occur when the capacity of a valve is significantly higher than the system’s flow rate, causing the valve to only partially open. This renders the valve more susceptible to trapping small dust or debris within fluid normally flowing through a fully open valve without adverse effects.
Improper installation of a check valve can result in the valve’s performance degradation. For example, if the valve is not installed correctly, its envelope could become damaged during installation, which may result in an external leakage path. It’s also possible that damage to the valve could interfere with its internal components. The installation instructions for a check must be followed closely to avoid these issues.
Selecting and designing an ideal check valve requires detailed knowledge and striking balances between various performance requirements, environmental factors, and potential trade-offs. We understand that this can be a complex process. Consider downloading our Engineer’s Guide to Selecting a Check Valve for an in-depth walkthrough for more insights and guidance.
Moreover, if you have specific concerns or need tailored advice, do not hesitate to contact one of our Technical Sales Engineers. They are skilled and ready to provide the required guidance for your unique situation and assure that you make the most out of your check valve design and selection process. Remember, each check valve directly influences your system’s performance, making its careful selection a matter of great importance.