After determining the optimal performance requirements and the environmental factors that will impact performance, the next step is to select or design an appropriate pressure relief valve for the application. Unfortunately, it may be challenging to find a solution that meets every requirement. As with most decisions, there are trade-offs that must be considered. Some of these trade-offs are common sense. For example, decreasing the valve’s envelope size may result in a lower flow capacity. Changing a valve component’s material from plastic to metal will increase operating or burst pressure capability but will impact the valve’s weight. There are other performance aspects specific to pressure relief valves that may require more careful consideration during the selection process. These include:
Valve hysteresis is the difference in flow performance based on whether pressure is increasing or decreasing. As can be seen in the graph on page 12, the flow curve generated with increasing pressure from the closed position to the flow point pressure will be offset from the flow curve generated with decreasing pressure from the flow point pressure back to the closed position. As a valve opens, there may be an increase in the affected area the pressure acts upon. See the Affected Area figure to the right for an example of this. It is this change, along with other forces, that can cause a difference in increasing and decreasing pressure flow performance. Therefore, when a valve is open and pressure is decreasing, the pressure may need to decrease further for it to close as compared to the same pressure when it’s increasing from the closed position.
Valve gain is defined as the rate of increase in flow rate per increase in pressure. This is a measure of how efficiently a valve moves from a closed position to the flow point pressure. A high-gain valve is one that can fully open with minimal increase in pressure, which is desired in pop-off applications. A low-gain valve requires a more significant increase in pressure to achieve a fully open condition.
If a valve is designed such that the force balance results in a significant, instantaneous movement of the poppet from the mostly closed to the fully open position, the flow rate through the valve will experience a sudden, simultaneous increase in flow rate. This sudden increase in flow may be accompanied by a decrease in pressure differential across the valve. This instantaneous event is sometimes referred to as an “open break.” A valve with an open break will have very high gain, and may even achieve a “negative” gain, if the differential pressure decreases as the valve opens. A low-gain valve opens with a more significant increase in pressure but can potentially offer greater stability which is required for regulation applications. This greater stability generally comes from the lack of an open break.
Designing a pressure relief valve usually requires a trade-off between the valve’s gain and hysteresis. Typically, higher gain results in a larger hysteresis, and vice versa. These dynamic characteristics will affect the valve’s critical performance criteria, including its cracking pressure, flow point pressure, and reseat pressure. A design engineer may have to determine if it is more critical that a valve achieves its flow point at a pressure near the cracking pressure and reseats at a lower point, or that the valve requires more pressure to fully open, but can reseat closer to the cracking pressure. Some compromise between the two options is often required.
For pressure relief valves, response time is how long the valve takes to sense the increase in pressure and react by opening. This can be important in situations when the upstream pressure spike has an extremely high pressure rise rate. A valve with a slow response time with respect to a high pressure rise rate will allow the upstream pressure to momentarily exceed desired limits, even if the valve has sufficient flow capacity and is specified to the correct crack, flow point, and reseat pressures. This is likely to cause damage to the system or other components. Therefore, it is important to ensure the relief valve’s response time is designed to cope with the system’s expected pressure rise rates.
Even when a valve is designed with the previously discussed performance criteria and environmental factors considered, pressure relief valves may get damaged and fail to perform properly in service. It is important to be aware of certain failure modes to ensure a relief 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 potential failure modes for a specific application must be evaluated.
The most common failure mode for a pressure relief valve is damage due to ingestion of foreign material, or simply, contamination. Unfortunately, fluids can contain contaminants of various sizes and materials. This contamination can damage the valve’s internal components or sealing surfaces, or may become lodged inside the valve, which can negatively impact the valve’s performance. In a worst-case scenario, contamination can become trapped in a place that prohibits the valve from closing, such as between the sealing surfaces, causing the complete loss of system pressure and functionality as fluid drains through the valve. Adequate protection against contamination should be incorporated upstream of the relief valve.
As noted in the performance criteria, pressure requirements are important for valve selection. However, the minimum and maximum pressure differentials are not the only concern. The rates at which pressure increases and decreases generate additional forces on the valve due to the rate of acceleration of the moving parts. Unforeseen, extremely high rates of increasing or decreasing pressure may cause these moving parts to impact other components with forces high enough to cause damage. This may affect decisions regarding the valve’s damping characteristics, spring rate, and material selection.
A high-gain safety relief valve that is designed to have an open break may not be stable if the valve is forced to operate at an intermediate pressure between the closed and fully open positions. The instability is due to an imbalance of forces acting on the spring and moving mass. This instability results in unpredictable relief flow and system pressure as well as the potential for audible chattering while the valve repeatedly transitions between the open and closed positions. This instability may dramatically increase the number of cycles the valve is subjected to as well as generate very high pressure rise and decay rates within the valve, which may cause damage.
A pressure relief 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 fully open the valve, 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 safety relief valves that could result in excessive wear and damage. Thermal relief valves are intentionally designed for low flow rates. Pressure regulating valves are designed to operate properly when the valve is not fully open.
Improper installation of a relief valve can result in degradation of the valve’s performance. For example, if the valve is not installed correctly its envelope could become damaged during the installation process, which may result in an external leakage path. It’s also possible that damage to the valve could interfere with its internal components. To avoid these issues, it is critical that the installation instructions for a relief valve are followed closely