Pressure Relief Valve Cracking Pressure

[Saurabh Cloudas]

Crackingpressure refers to the inlet pressure level at which the first sign of flow is present. It can also be described as a measure of the pressure differential between the inlet and outlet ports of the valve when flow is initially detected. Specifically, cracking pressure is the least differential pressure that the valve experiences during flow.

There are several factors that must be considered to ensure proper operation of a pressure relief valve within a system or pressure vessel. Neglecting to consider these factors can lead to reduced valve or system performance, damage to other components within the system, or a total system failure. The below chart highlights the critical performance points that are typically defined for a pressure relief valve. Further performance characteristics that should be considered when selecting a pressure relief valve are discussed below.

There are four pressure ratings that should be considered for any pressure relief valve: operating pressure, system pressure, proof pressure, and burst pressure. Operating pressures are the pressures the valve will be subject to during normal operation throughout its life, both in the relief flow direction and the checked, or opposite, direction. System pressure is the maximum nominal pressure that the system the valve is installed into will achieve. Proof pressure is the pressure the valve should be able to withstand without permanent deformation or degradation of performance when the system returns to operating pressure. Burst pressure is the pressure at which the valve should survive without rupturing or bursting. All four pressure ratings must be considered during design to ensure the valve and its components are durable enough for the application.

If the valve opens at a pressure that is too low, there is a risk of improper valve operation or a decrease in system efficiency. Alternately, if it does not crack open and pressure increases too much, there is a risk of experiencing the issues which the valve is supposed to prevent such as deformation to the system or its subcomponents. The flow curve on page 7 highlights the critical performance requirements, including cracking pressure, that are typically defined when selecting a pressure relief valve.

To ensure that the system pressure does not reach a critical point, the relief valve must allow a certain volume of fluid to exit within a limited period of time. This flow rate is based on the potential rate of a pressure increase (also called a pressure rise rate), the volume of fluid in the system, and the volume of fluid that needs to be displaced to alleviate the pressure increase.

For example, a pressure relief valve protecting a large pressure vessel from damage due to a powerful pump malfunction requires more flow capacity than a pressure relief valve protecting a small pressure vessel from the pressure increase created by thermal expansion on a hot day.

The flow rate the valve is designed to achieve is typically defined at a specific pressure known as the flow point pressure, overpressure point, or relief flow pressure. The flow point pressure is always higher than the cracking pressure and may signify when the valve is in its fully open state, at which point it will perform as a fixed restriction. This performance criteria ensures the pressure relief valve will relieve enough fluid at a low enough pressure to prevent further increases in pressure or to reduce system pressure back to safe levels.

The envelope is an important factor to consider when selecting a relief valve. The first consideration is the location of the valve within the system and the desired flow path for the relieved fluid. The system may require the relief valve to be located within a specific area, limiting external dimensions or overall size. The location may also dictate the flow path of the relieved fluid based on existing lines. The envelope must also account for installation, retention, and maintenance requirements. For example, some valves incorporate threaded fitting ends, while others are installed directly into manifold housings. Next, determine whether the installation must be permanent or removable. Finally, evaluate if the valve may be used in a system in which weight is a factor, such as a portable system or when fuel efficiency is paramount.

Relief valve performance is typically based on changes in pressure within the system or pressure vessel. However, the pressure relief valve envelope may be subject to other environmental pressures, such as those found deep underwater, underground, or even the absence of pressure found in outer space.

Pressure relief valves can be subject to forces external to the system in which it is installed, as well as the forces generated by operation of the system. These forces are typically vibration, shock, and G-forces. For example, some systems or vehicles which use pressure relief valves generate levels of vibration during normal operation. A shock may occur if the system or vehicle in which the valve is installed suddenly encounters another object. Lastly, the system or vehicle may generate G-forces during operation due to sudden or rotational movement.

The magnitude, frequency, and direction of the potential forces must be considered. The internal components of the valve move differently based on the direction of the force. The frequency of the vibration may be detrimental to the valve if it corresponds to the natural frequencies of the design. In most cases, a relief valve is required to withstand and even remain closed during these events.

A pressure relief valve may have trace amounts of fluids, debris, or dust on or within the valve. This contamination may be caused by the processes used to manufacture the valve or the environment it is subjected to during manufacturing, transportation, or storage. In most cases, the trace amount of contamination that is present on the valve at the completion of production is acceptable. If the end user cannot tolerate the presence of this level of contamination, the valve may need to go through special cleaning and packaging processes. Examples of these types of applications include a pressure relief valve used in a chemistry analyzer or in an oxygen system which provides breathable air to a person. Some industries, such as the space, automotive, and medical industries, have defined cleanliness levels that dicate these requirements for any component or system.

What is check valve cracking pressure?Cracking pressure is the minimum upstream pressure required to open a check valve enough to allow detectable flow. Detectable flow is when the check valve allows a small but steady flow of liquid or gas to pass through the valve body and out through its outlet port.

An inexact but informative way to test cracking pressureA simple air pressure test is an easy way to estimate the cracking pressure of a spring loaded check valve. It involves attaching a pressurized air line with a control valve and a pressure gauge to the inlet side of the check valve. The check valve is then placed in a container filled with water. The pressure of the air coming into the check valve can be gradually increased using the control valve.

The cracking pressure of the valve will be about the same as the pressure gauge measurement when there is detectable flow through the check valve. Detectable flow will be the first small but steady stream of bubbles to come out through the outlet port of the check valve.

What is a bubble tight seal or bubble tight shutoff?To describe a check valve seal as bubble tight is to describe the sealing ability of a valve. If a closed check valve is air pressure tested for backflow, any leaking around the valve seals will causes bubbling up through water similar to the case above. A bubble tight seal produces no bubbles.

Size the check valve for the applicationChoosing the right check valve size for an application helps prevent premature check valve wear and failure. It also helps ensure the check valve and the application perform as expected.

Sizing check valves is different from sizing many other types of flow control and shutoff valves. The best operating results are usually when a check valve has been sized for the application and not for the pipe or tubing size.

In a majority of check valve installations, normal operating conditions will produce a fairly steady flow. For this situation, a check valve will usually be considered properly sized when this flow keeps the valve between about 80% open and fully open.

Sizing check valves becomes more complex when an application has a range of normal operating flow rates. In this case, the best check valve size choice will probably be when, at the lowest operating flow rate, the check valve opens up between about 80% open and fully open.

Determining which is the right check valve and especially choosing its size might be a little tricky. It will probably involve getting and testing samples in real operating conditions. The good news is that spring loaded or spring assist check valves are designed with a wide range of very specific cracking pressures.

Need product samples?Fill out the form on this page and a member of our team will get in touch with you to find the right product samples for your unique application needs.

A review of check valve fundamentalsCheck valves allow liquid or gas to flow in one direction while preventing flow in the reverse direction. Flow in the reverse direction is called backflow or upstream flow.

What is reseal pressure and how is reseal pressure related to cracking pressure?Reseal, re-seal or resealing pressure is the backflow pressure required to close a check valve tightly enough so that there is no longer any detectable flow. It is also described as the measure of backflow pressure when the check valve closes bubble tight.

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