What is Cracking Pressure? - Draft
Why check valve cracking pressure is such a critical specification
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.
A check valve’s cracking pressure is a technical specification and is usually provided as psi or psig (pounds per square inch or pounds per square inch gauge) or bar (the metric equivalent of psi and psig) or both.
A more precise way to describe check valve cracking pressure is to say that it is a measure of the pressure differential between the valve’s inlet and outlet ports when flow is first detected.
An inexact but informative way to test cracking pressure
A 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.
This is obviously a very rough-and-ready approach and cracking pressure quality control testing rigs are much more rigorous and carefully designed.
One thing a simple air pressure test clearly demonstrates is what it means to say a check valve’s cracking pressure has been reached because there is detectable flow.
On a related point, this is also useful for understanding where the phrases “bubble tight seal” and “its shutoff is bubble tight” come from.
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.
The key takeaway from this is to realize that there is a significant difference between a check valve’s flow rate at detectable flow and its flow rate when it is fully open. This is an important difference to be aware of when “sizing” a check valve for a specific application.
Size the check valve for the application
Choosing 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.
Learn more about estimating valve flow and valve sizing at Valve Flow and Sizing from IEEE GlobalSpec Engineering360.
A review of check valve fundamental
Check 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.
Because check valves provide automatic flow control while using relatively simple and durable designs, they are widely used in all sorts of applications.
Cracking pressure is a key specification
Cracking pressure comes up whenever you talk about check valves because it is such an important check valve specification.
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.
Spring loaded (spring assist) check valves reseal with the help of the force of the spring. This means the lower the cracking pressure, the higher the reseal pressure needed for a bubble tight seal.
What does this mean practically (two cases)?
Spring pressure alone provides a bubble tight seal
Spring loaded check valves that have a cracking pressure higher than about 3 psig (0.21 bar) to 5 psig (0.34 bar) will usually close or reseal bubble tight because of the force of the spring alone.
In these cases, if a check valve’s rated reseal pressure specification is provided, it will usually be lower than its cracking pressure.
Spring pressure plus back pressure provide a bubble tight seal
Spring loaded check valves that have a cracking pressure lower than about 3 psig (0.21 bar) to 5 psig (0.34 bar) will usually not return to a bubble tight seal with just the force provided by the spring. They usually need a little additional back pressure from the system in order to form a bubble tight seal.
These low cracking pressure check valves will have reseal pressures that are higher than their cracking pressure, sometimes even significantly higher.
Other factors that may affect a check valve’s reseal pressure
- The design of the check valve
- The speed of back pressure buildup
- The pressure and temperature of the environment surrounding the check valve
- The pressure and temperature of the air, gas or liquid moving through the valve
What is back pressure?
Back pressure is when there is upstream flow pressure in the system. In other words, the pressure at the check valve outlet port is higher than the inlet port. Check valves without springs require backflow and the resulting backflow pressure in order to close. This includes both free floating elastomer diaphragm check valve designs and springless (no spring) ball, poppet, cartridge and piston check valve designs.
Our Vacuum Pressure Unit Conversions Chart is a handy reference to compare and convert between the most commonly used vacuum and pressure units of measure.
Factors that affect cracking pressure:
- Check valve design
- Condition of the valve
- Orientation of the valve
- Contamination in the pressurized air, gas or fluid passing through the valve
Check valve designs matter
A more detailed look at diaphragm check valves and check valves with rigid sealing elements (ball, poppet, cartridge, piston and so on).
Diaphragm check valves
- Floating elastomer disc or diaphragm
- Reseals with minimal backflow pressure
- Generally, they do not provide bubble tight seals
- Probably not a good choice for thick or viscous fluids
- A low-pressure check valve for low-pressure applications
- Opens with minimal downstream flow or pressure differential
Free floating elastomer discs or diaphragms do not provide a positive seal even though they require very low backflow pressure to close. This can lead to some leakage around the seal, especially in low flow applications.
Learn more about diaphragm check valves in our blog post How Do Diaphragm Check Valves Work?
What is a positive seal?
A positive seal, also called a leak tight or bubble tight seal, is exactly that. For a check valve this means that when the check valve closes, it also reseals so that no backflow leaks past the sealing surfaces. As I mentioned earlier, some very low cracking pressure spring loaded check valves may require back pressure in addition to the spring pressure to create a positive, bubble tight seal.
Check valves that use springless or spring loaded rigid sealing elements
Many types of springless (no spring or floating) and spring loaded check valves use a rigid internal part that moves to either allow downstream flow or prevent backflow. This rigid internal part is usually called the sealing element.
The shape of the check valve sealing element is generally used to describe its type. Ball, poppet, cartridge or piston for example:
- Poppet check valves tend to have higher flow rates
- Ball check valves are relatively simple and cheap to make
- Piston and cartridge check valves provide especially good positive seals
In general, the interior design of a valve’s parts and sealing mechanism determines its cracking pressure, how well it reseals, its flow efficiency and its maximum flow rate.
Important cracking pressure considerations
Spring loaded vs springless (no spring) check valves
Springless check valves
- Very low cracking pressure
- Very little downstream flow pressure opens springless check valves
- Backflow and backflow pressure alone close springless check valves
Springless (no spring) check valves provide a near zero pressure differential to allow free flow. Springless check valves also require sufficient backflow to close and cannot provide a positive seal in most low pressure and low flow applications.
Spring loaded check valves
- Positive seal
- More resistant to leaking
Spring loaded check valves provide positive seals when they close and are more resistant to leaking than springless (no spring) check valves.
Check valve orientation matters
Proper installation is important
Because check valves are one-way valves, they need to be installed in the correct flow direction or orientation in order to work properly.
The inlet port or inlet valve connection is the upstream side of the valve. The outlet port is on the downstream side of the valve. Because check valves must be installed in the direction of flow, they usually have the direction of flow marked on the valve bodies. This marking is either a check valve symbol or an arrow that indicates flow direction.
What if there are no markings on a check valve to indicate flow direction?
For some customized check valve components, flow direction may not be marked on the check valve body. In this situation there are two ways to know the correct flow direction when installing the check valve:
- The manufacturer or part design and customization service provides documentation indicating flow direction. This is pretty straight forward when there is a clearly observable difference between the connection types at the inlet and the outlet ports.
- A simple air pressure test can determine which connection is the check valve inlet port.
Horizontal, vertical flow up or vertical flow down
Gravity can play an important role in check valve function and affect its cracking pressure. This is true for both springless and spring loaded designs.
The specific gravity or density of the liquid in an application determines how much a pipe or tube full the liquid weighs.
Check valves in horizontal lines are the most common type of check valve installation. In this case the only important thing to remember is to orient the check valve correctly for the direction of flow.
Vertical installation - flow direction up
When a spring loaded check valve is installed vertically, with the flow direction up, the weight of the liquid above the valve increases the amount of force required to open the valve. This type of installation increases the check valve’s cracking pressure.
A springless check valve mounted in the same way will have a cracking pressure determined by the weight of the liquid above it.
Vertical installation - flow direction down
If the valve flow direction is down, the weight of media pushes against the valve spring. This means the valve’s cracking pressure needs to be selected so that it is high enough to neutralize the weight of the liquid in the pipe or tube above the valve. Generally, springless (no spring) check valves will not work properly when they are installed vertically with downward flow.
Learn more about check valve flow orientation at A 360° Look at Check Valve Flow Orientation from Triangle Fluid Controls.
Why it matters what chemicals are passing through a check valve
Check valve designs frequently include more than one material option. This includes material options for all the check valve’s internal components too. There will probably be material choices to be made for the check valve body, the sealing element (diaphragm, ball, piston, poppet, cartridge and so on), the elastomer seal or seals and the spring metal.
One example of how chemical exposure can cause problems is the case of plastics and elastomers. Chemicals can potentially cause these materials to swell or the sealing surfaces to become sticky. To avoid this, evaluate what chemicals that will be passing through the check valve. Use chemical compatibility guides to make the best material choices.
What are wetted surfaces?
The wetted surfaces or wetted parts of a valve are the internal surfaces and components that will be exposed to gases or liquids passing through the valve. When selecting a check valve for an application, all the component materials used in the valve should be evaluated for their compatibility with chemicals they will be in contact with.
Learn more about the meaning of wetted parts at Wetted Parts of pressure sensors – definition and overview from WIKA.
Why the weight and density of fluids passing through a check valve matter
Liquids passing through an application have mass or weight. The mass of the liquid in a system is a function of its volume and its density. A heavy liquid will have a relatively higher mass per volume of fluid than a lighter liquid.
The mass of the liquid in an application affects how rapidly a check valve will be able to respond to pressure changes to either open or to reseal. In general, a dense liquid with a low flow volume will tend to take longer both to fully open and to reseal.
Viscosity of fluids passing through the valve
A viscous or thick liquid provides resistance to the movement of check valve sealing elements like the free-floating elastomer disc, ball, poppet, cartridge or piston. In low-flow systems, viscosity can affect both the cracking pressure of a check valve and its ability to reseal when there is backflow. A simple way to think about this to the difference between honey and alcohol on a cold day.
Some final tips about selecting check valves
Use a material chemical compatibility guide to be sure the check valve component materials are compatible with the media passing through it.
A large number of miniature and compact small-bore check valves use elastomer seals. In general, the temperature rating of a particular elastomer seal material is usually what limits a check valve’s operating temperature range.
Check valves are frequently used in series. Using two check valves in series can reduce the amount of reseal pressure required for bubble tight sealing.
Installing filtration upstream of a check valve to improve its performance. Filters capture particles and debris that can interfere with check valve sealing surfaces and resealing mechanisms. Keep in mind that switching out components as well as system repair and maintenance may introduce dirt, chips, scale or other contaminants into the system.
Learn more about the importance of protecting valves with filters in our blog post In-line Filters for Miniature Flow Control Component Protection.
Some related posts
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About the author
Steven C. Williams is the technical writer and an inbound marketing specialist at Industrial Specialties Manufacturing (ISM), an ISO 9001-2015 supplier of miniature pneumatic, vacuum and fluid circuitry components to OEM's and distributors all over the world. He writes on technical topics related to miniature pneumatic and fluidic components as well as topics of general interest at ISM.
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