Cracking Valve

[Armonia Bunda]

Cracking pressure 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.

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.

Spring pressure alone provides a bubble tight sealSpring 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.

Spring pressure plus back pressure provide a bubble tight sealSpring 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.

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.

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.

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.

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.

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.

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:

Horizontal, vertical flow up or vertical flow downGravity can play an important role in check valve function and affect its cracking pressure. This is true for both springless and spring loaded designs.

Horizontal installationCheck 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.

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 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.

Why the weight and density of fluids passing through a check valve matterLiquids 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 valveA 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 valvesUse a material chemical compatibility guide to be sure the check valve component materials are compatible with the media passing through it.

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.

Are you developing a design for or maintaining a low-pressure (less than 125 psi, 8.6 bar) small-bore air, gas or fluid system? Then ISM has the valve for it. Get a catalog and find out more about all the different types of stainless steel valves we offer.

Check out our complete lineup of miniature flow control valves. Keep in mind we carry thousands of miniature flow control valves. Plus, thousands of related products like connectors, adapters, gauges and so on.

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