Cracking pressure is the minimum upstream pressure at the inlet of a check valve that is required in order to open the check valve and allow detectable flow through.

In this article I will present to you all the relevant information on check valve cracking pressure and why it is such an important specification.

Table of Contents

What is Check Valve Cracking Pressure?

Cracking pressure is essentially the check valve inlet pressure at which the first detectable sign of flow occurs. This detectable flow is when the check valve allows a small but steady flow of air to pass through the body of the valve and out its outlet on the other side.

A more detailed way to describe check valve cracking pressure is to label it a measure of the pressure differential between the valve’s inlet and outlet ports from when flow is first detected.

A check valve’s cracking pressure is a technical specififcation that is very important to the successful operation of an air compressor system, it is usually measured in psi (pounds per square inch) or the metric equivalent, bar.

Check valves are very common on not only air compressors but across other industrial processes that handle chemicals and other liquids.

How Does a Pressure Check Valve Work?

Check valves rely on pressure differential to work. What they require is a higher pressure on the input side of the valve than the output side in order for the valve to open and allow air through. If and when the pressure becomes higher on the outlet side than on the input side, the valve will close.

Of course, the closure mechanisms can vary across valve types, but unlike other valves on air compressors, they do not need any human interaction to correctly work.

For more information on Air Compressor Check Valve: How It Works, What It Does and Why It Exists please visit our guide!

Factors that Affect Cracking Pressure

There are a few factors that you must be aware of that can affect a check valves cracking pressure:

  • Check valve design
  • Condition of the check valve
  • Contamination in the pressurized air that is flowing through the valve
  • Orientation of the check valve

Cracking Pressure Considerations When Choosing a Check Valve

There are a number of things you must consider when choosing a check valve in relation to its cracking pressure. These are most importantly:

  • Springless vs. Spring Loaded Check Valves
  • Check Valve Orientation

Springless vs. Spring Loaded Check Valves

It’s important to consider whether you require a spring loaded or a springless check valves and what they can offer you in terms of cracking pressure.

Springless check valves provide almost zero pressure differential to allow free flow. They require sufficient back flow to close and cannot really provide a positive seal in most low pressure and low flow applications. They have very low cracking pressure.

Spring loaded check valves provide positive seals when they close as well as being far more resistant to leaking than springless check valves.

Check Valve Orientation

Installing the check valve horizontally or vertically can affect the resulting cracking pressure. Especially as gravity plays such a key role in check valve functions, so if you require vertical flow up or vertical flow down you must be aware of the effects.

Horizontal check valves are the most common and may not need so much consideration apart from orienting the check valve in the correct direction of flow.

Vertical check valves flowing upwards will result in an increase in the check valve’s cracking pressure, due to the weight of the air above the valve increasing the force to open it.

Vertical check valves flowing downwards will need a cracking pressure high enough to neutralize the weight of the air in the pipe above the check valve.

FAQs (Frequently Asked Questions)

What is reseal pressure and how is reseal pressure related to cracking pressure?

The reseal pressure is the required back flow pressure in order for the check valve to close tight enough so that there is no more detectable flow within the check valve. It can also be described as the measure of back flow pressure when the check valve is closed bubble or leak tight.

Spring loaded check valves for instance, reseal with the aid of the force of the spring. This results in a higher amount of reseal pressure required for lower cracking pressures to ensure a bubble tight seal.

What is cracking pressure and full flow pressure?

Cracking pressure is the check valve inlet pressure at which the first detectable sign of flow occurs. This detectable flow is when the check valve allows a small but steady flow of air to pass through the body of the valve and out its outlet on the other side.

When the valve is allowing through its full rated flow, it is termed as being in a state of full flow pressure.

How much pressure is needed to open a check valve?

The minimum upstream pressure that is required to open a check valve is called the cracking pressure, and it can generally be anywhere between 1 and 5 psi. Check valves tend to be designed with this number in mind, and it can vary depending on your requirements.

Flow rate determines the degree of opening on a check valve. Therefore, the higher the flow rate, the more open the check valve will become until it reaches it’s fully-open (maximum) position.

What is a positive seal?

A positive seal, or also known as leak tight or bubble tight seal (for liquids) is exactly what it’s known as, a seal that is leak tight. It’s on a check valve for when the valve closes and it reseals so that no back flow is allowed to leak past the sealing surfaces.