Check valves are integral components in a variety of fluid and gas control systems, allowing flow in only one direction while preventing reverse flow. One crucial specification when selecting a check valve for a given application is its cracking pressure. Understanding cracking pressure can help ensure that the valve performs optimally and prevents operational issues in your system. In this article, we’ll define cracking pressure, explain its significance, and explore why it’s important when choosing a check valve for different applications.

What is Cracking Pressure?

Cracking pressure refers to the minimum upstream pressure at which a check valve will begin to open and allow flow to pass through. It is essentially the pressure that “cracks” or “lifts” the valve’s closure element (like a disc, ball, or diaphragm) off its seat, allowing fluid or gas to flow in the desired direction.

This specification is crucial because it determines the pressure at which the check valve starts its operation. The cracking pressure must be sufficient to overcome the force exerted by the valve’s spring (or other mechanisms) that keeps it closed when no flow is present. However, if the cracking pressure is too high, the valve may not open properly under low-pressure conditions, which can impede the flow. If it is too low, the valve may open prematurely, leading to undesirable flow.

 

Why Cracking Pressure Matters

The significance of cracking pressure lies in its ability to influence how well the valve performs in different operating conditions. The ideal cracking pressure depends on the type of system and the specific requirements of the application. Below are some reasons why cracking pressure is so important:

• Flow Control: A check valve with the correct cracking pressure ensures that the flow is regulated accurately, without unwanted backflow or pressure loss. It provides consistent and controlled flow, optimizing the system’s efficiency.
• System Protection: In many systems, such as HVAC or water treatment, it is vital to prevent reverse flow to avoid damage to pumps, compressors, or downstream equipment. The right cracking pressure prevents early or late valve openings that could lead to system failures.
• Energy Efficiency: By controlling when the valve opens, cracking pressure ensures that the valve works effectively without wasting energy. It helps maintain the desired pressure and flow, contributing to more efficient operations.
• Preventing Overpressure: Incorrect cracking pressure can result in the valve not opening when needed, creating high-pressure buildup. This can lead to catastrophic failures, ruptures, or other hazards.

 

Factors Influencing Cracking Pressure

 Understanding the factors that influence cracking pressure is essential for ensuring that a check valve performs optimally in its specific application. Several design and operational variables play a role in determining the cracking pressure, and these should be considered when selecting the appropriate valve for a system.

Spring Tension (For Spring-Loaded Check Valves)

In spring-loaded check valves, the spring plays a vital role in determining the cracking pressure. The tension in the spring controls the force required to open the valve. A higher spring tension will result in a higher cracking pressure, while a lower spring tension will reduce the cracking pressure. This makes spring-loaded valves highly customizable, as manufacturers can adjust the spring tension to meet the specific pressure requirements of an application​

Valve Size

The size of the check valve impacts the amount of force needed to overcome the internal resistance and open the valve. Larger valves generally require a higher cracking pressure because they have larger components (such as a heavier flap or larger spring) that require more force to move. In contrast, smaller check valves tend to have a lower cracking pressure, making them suitable for applications with less demanding flow conditions​

Valve Design Type

swing check valve

Different check valve designs have varying mechanisms for opening. For example, in swing check valves, the cracking pressure is often influenced by the weight of the valve’s disc or flap. The weight of the flap in swing check valves affects how easily the valve opens. In contrast, piston check valves rely more on fluid pressure and may require higher cracking pressures to achieve a proper seal and prevent reverse flow​

Fluid Characteristics

The type of fluid being handled by the check valve also affects the cracking pressure. Denser fluids, such as oils or liquids with high viscosity, typically require more pressure to initiate flow compared to lighter fluids like water or gases. The specific gravity and viscosity of the fluid can change the amount of force needed to open the valve, thus influencing the cracking pressure required​

Installation Orientation

The orientation of the valve installation—whether horizontal or vertical—also plays a significant role in determining cracking pressure. When a valve is installed in a vertical position, gravity can either increase or decrease the pressure required to open the valve, depending on whether the flow is directed upwards or downwards. For example, in a vertical downward flow, gravity assists in closing the valve, reducing the net cracking pressure. Conversely, in an upward flow, gravity adds to the force required to open the valve​

System Pressure

The overall pressure conditions of the system in which the check valve is installed will affect the cracking pressure. The pressure differential between the upstream and downstream sides of the valve must be sufficient to open the valve. If the system operates at a relatively low pressure, a valve with a lower cracking pressure is required to open reliably under those conditions. In contrast, higher system pressures may require a valve with a higher cracking pressure to prevent premature opening or excessive wear​

Material Selection

The material of construction for the check valve, such as stainless steel, brass, PVC, or alloy steel, can influence the valve’s response to pressure. Materials like stainless steel offer durability and consistency in maintaining cracking pressure over time. On the other hand, materials like PVC may be better suited for lower pressure applications, as they can flex more easily under pressure changes. The valve material should be selected based on the fluid being controlled and the required performance under different pressure conditions​

 

How to Measure Cracking Pressure in Check Valves

Measuring the cracking pressure of a check valve is a critical step in ensuring the valve’s proper function within fluid or gas systems. The cracking pressure is defined as the minimum upstream pressure required to open the valve and allow flow in the desired direction. Properly measuring this pressure helps ensure the check valve will function as intended and prevent issues like backflow or inefficient system operation. Here’s how to measure it:

1.Set Up the Test Environment

To begin measuring cracking pressure, you’ll need a test rig that includes a check valve, a pressure source, and a pressure gauge. The check valve should be installed in the same orientation as it will be used in the actual system to ensure the test is accurate. Depending on the valve type (spring-loaded or gravity-assisted), ensure the system is primed for the specific type of check valve you are testing.

2.Apply Pressure Gradually

Once the valve is properly set up, slowly apply pressure from the inlet side. As pressure builds, monitor the pressure gauge closely. The cracking pressure is the point where the valve’s dynamic element (ball, disc, or flapper) begins to lift, allowing flow to pass through. This initial opening signifies the cracking pressure. The pressure at which this occurs will be displayed on the gauge.

3.Record the Measurement

Once the valve begins to open, record the pressure shown on the gauge. This is the valve’s cracking pressure. If the check valve has a high tolerance, you may need to repeat the test several times to ensure consistency.

4.Check for Leaks

After noting the cracking pressure, check the valve for leaks at low flow rates to ensure that it is properly sealing when closed. Any deviation from the expected behavior could indicate wear or damage to the valve, affecting its performance.

5.Adjusting the Cracking Pressure

If the valve is adjustable, you can modify the cracking pressure by changing the spring tension or by replacing the spring with one of a different rating. This adjustment is useful when tuning the valve for specific applications, such as low-flow systems, where lower cracking pressure is beneficial, or high-pressure systems, where higher cracking pressure might be required.

 

How Resealing Pressure is Related to Cracking Pressure 

Resealing pressure refers to the pressure required to reverse the flow and close the valve again. For a check valve to fully seal, the resealing pressure must be high enough to overcome the downstream pressure, forcing the valve to return to a closed state and prevent backflow.

The Relationship Between the Two:

Valve Spring Mechanics: The spring tension responsible for maintaining the valve in a closed position contributes to both the cracking and resealing pressures. For check valves with low cracking pressure, the resealing pressure must generally be higher to ensure that the valve closes tightly enough to prevent leakage.

Flow Conditions: After the valve opens at the cracking pressure, the resealing pressure ensures that there is no fluid leakage when flow reverses. If the resealing pressure is not reached, the valve may not close effectively, allowing backflow, which could be problematic for the system.

Higher Cracking Pressure Correlates with Lower Resealing Pressure: In cases where the cracking pressure is higher, the resealing pressure often required to close the valve is lower, as the force required to reverse the flow is not as significant. This is because the force to open the valve has already been overcome, and the closing spring simply needs to counteract any remaining back pressure.

 

In practical terms, both pressures must be balanced carefully. Too high a cracking pressure could lead to excessive wear on the valve mechanism, while too low resealing pressure might lead to inefficient sealing and potential leakage. Therefore, selecting the right valve with appropriate cracking and resealing pressures for the specific application is essential to achieving optimal performance and longevity of the check valve.

 

Industry Standards for Cracking Pressure

Industry standards and guidelines can help you determine the appropriate cracking pressure for different applications. The American Petroleum Institute (API) and International Organization for Standardization (ISO) provide guidelines for selecting check valves based on cracking pressure, taking into account fluid properties, system design, and safety considerations.

For instance, API standards often specify cracking pressure ranges for valves used in high-pressure pipelines, while ISO guidelines may provide standards for valves used in HVAC systems or water treatment plants.

 

Check Valve Cracking Pressure Chart

 

Conclusion

Understanding cracking pressure is essential for ensuring that a check valve performs efficiently and safely within a system. By selecting the appropriate cracking pressure, you can optimize the valve’s operation, protect your equipment from damage, and improve overall system efficiency. Whether you’re designing a new system or maintaining an existing one, always consider cracking pressure as a critical factor when choosing a check valve.

So, are you using the right cracking pressure for your application? If not,Tanggong Valve Group is here to help, contact us today to ensure optimal performance and avoid costly system failures.