Posted on : Friday 29th May 2020 04:51 PM
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.