Check Valve Buyer’s Guide
- What is a check valve and how does it work
- The problems check valves solve
- Types of check valves
- Three things to look for in a check valve
- Materials and their effect on check valve performance
- When not to use a check valve
- The three most common mistakes made when buying a check valve
- Four ways to install a check valve
What is a check valve and how do they work:
Think of a check valve as a turn-style for fluid flow: It only allows fluid to flow in one direction. This is achieved by a poppet, flap, swinging door, or other mechanism that moves out of the way to allow fluid (this can be either a gas or liquid) to travel in the flow-direction. If the flow is reversed, the valve will seal shut and block fluid flow. This direction of flow is called in the check-direction.
The problems check valves solve:
Spring-loaded check valves can be used in three distinct ways.
- Back flow prevention
The first problem that check valves solve is undesired backflow. If a check valve is installed in a fluid path, the valve will keep backflow from occurring. This application is commonly used in pumps, anti-reversion systems, safety systems, etc. In the picture below you can see a typical installation of a B250 check valve that allows flow from right to left only.
- Pressure/Vacuum Relief
Check valves can be used is to vent or bleed pressure from a tube or system. This allows the valve to prevent over pressurization. When a check valve is being used in this manner, it becomes a relief valve. Relief valves vent when it’s open-pressure is reached. Imagine a 10 PSI relief valve installed on an air tank. When the tank reaches approximately 10 PSI, the valve will open to bleed-off pressure above 10 PSI. When the pressure in the water tank is below 10 PSI the valve will shut. Check valves can be used to maintain vacuum levels by venting in air if the valve direction is reversed. In the picture below the B250 check valve is venting out the top tube.
- Anti-leak device
A check valve can also be used as an anti-leak/anti-drip device. A valve can be installed at the end of a tube or nozzle that is being fed by a pump. It will seal the tube shut once pumping has ceased. This will prevent dripping. You can find check valves in dispensers of all types: printers, coffee makers, and other systems which benefits from a drip-free nozzle. Below a T250 check valve is installed in the end of silicone tube to prevent drips.
Types of check valves:
There are four basic types of small check valves that you will commonly encounter:
- Umbrella / Flat disk check valves have a small elastic flat disk or a disk with a stem resembling a miniature umbrella. Umbrella / flat disk check valves are typically made from plastic and rubber. These valves work in non-critical applications. They are inexpensive but will require back flow to seal the valve. There is no spring to assist closing the valve, so sealing is poor.
- “Duck Bill” check valves are typically molded from rubber or silicone in a wedged shape resembling a duck bill. Fluid pushes the duck bill open from one direction and shuts from the other direction. typically, these valves do not seal well. They are used on glue jars and other non-critical applications. Duck Bill check valves are typically the least expensive of all types of check valves.
- Ball Check Valves provide an above average seal depending on their design. They can be made very small. Ball check valves can be made spring-loaded or just use the fluid flow to shut the valve. With high pressure check valves, no elastomer is used. Valves with no elastomer will not seal at lower pressures.
- Poppet Check Valves utilize an O-ring and seat sealing. Poppet check valves are made with springs to enhance the sealing force resulting in an excellent seal. A side benefit of this valve is the opening pressure can be customized. Due to the advantages of the poppet check valve, it’s is all we sell at Precision Check Valve.
Three things to look for in a check valve:
Simply put, you should look for:
- Sealing performance
- Ability to withstand harsh environments
All of these factors are controlled by the manufacturing process, materials, and design. At Precision Check Valve we provide valves that excel in these areas.
Materials and their effect on check valve performance:
Body materials can have a dramatic effect on the overall performance. How you ask? In addition to providing physical strength and ability to withstand harsh environments, the material’s ease manufacture will dramatically affect surface finishes, that in turn effect the valves sealing performance.
Metal can be an ideal material provide great strength and quality. Commonly used metals are as follows.
303 Stainless steel is ideal overall as it produces an excellent finish, an excellent seal, has high strength, is corrosion resistant, is UV safe, and it is sterilizable in autoclaves. Due to these advantages Precision Check Valve’s standard material is 303 stainless steel.
Brass is also an industry favorite as it can also be machined to a high finish, but it is subject to corrosion over time in certain environments. Overall it is a great material for some applications.
Aluminium can produce good finishes akin to brass. It is subject to oxidizing unless it has an anodization coating to protect it.
Plastic can be molded or machined. Molding is the most typical process because it produces low-cost parts. There are some significant drawbacks to molding as it can produce significant imperfections such as sink, short shots, cavity misalignment, parting line flash, and other issues.
Acetal (Delrin), Acrylic, and ABS better than average seal
Polypropylene and Ultem average seal
Nylon tends to seal poorly due to its rougher surface finish
Acetal (Delrin), and ABS, and PEAK seals well
Polypropylene, Nylon, and Acrylic does not seal well due to their inability to machined smoothly.
When not to use a check valve?
Check valves work everywhere, right? Not so fast, there are some applications where a check valve just won’t work or hold up. First, a check valve cannot be used with any particulate or dry flows such as sand. These situations will keep the valves from sealing properly. Check valves may fail with high viscosities liquids (above the 20,000 cps range) such as cold honey. If the temperature or pressure exceeds the valve’s rating, failure is just a matter of time. If you’re not sure of check valve suitability just contact us for a recommendation of use.
The 3 most common mistakes made when buying a check valve:
Mistake 1:Believing open pressure and resealing pressure are the same
It would be nice if check valves worked in a binary fashion, but in reality, they function in an analog manner. Let’s bring back that example of the 10 PSI valve from the air tank. If we really dig down we can see it may start to open slightly at 9 PSI, then opens significantly at 10 PSI (0.4 litres/minute is how we rate all of our valves), and finally be fully open at 12 PSI. Due to historisis, a check valve will not seal until a lower pressure than it’s opening pressure is achieved. In our example the check valve resealed at 8 PSI. It is important to keep this in mind when picking an open pressure for your application.
Mistake 2: Using a failure-prone valve in a critical application
Plastic injection check valves may have imperfections from molding. If your application is critical you may experience an unfortunate failure. A plastic injection mold typically creates multiple parts at once. Usually in sets of 2, 4, 8, or 16 parts at the same time. Ideal settings can not be optimized for each part but are rather averaged for all parts molded at the same time. This can result in differences from part to part and other issues such as short shots, parting line flash, weld lines, warp, shrink, sink, misalignment or a host of other problems. By utilizing CNC machined valves Precision Check Valve avoids these issues and provides a superior product free from all plastic derived issues.
Mistake 3: Specifying the wrong O-ring compound
Choosing the right O-ring compound helps with chemical compatibility and sealing performance. If you choose an O-ring that is not compatibile with the fluid running through it, the O-ring will swell and dissolve over time. This will result in valve failure. Sealing performance is also affected because the durometer (hardness) of each O-ring differs from compound to compound. For sealing in low pressures applications silicone and buna (Nitrile) work well. For higher pressures Viton and EPDM O-rings work well. It is important to pick harder compounds to withstand high-pressure forces exerted on the O-rings. Precision Check Valve’s engineers can recommend an ideal O-ring compound for your specific application.
How can a check valve be installed
Here are four different ways to install small check valves:
Barb check valve:
Barb type check valves can be installed at a break in a hose inline. Due to the ease of installation, barbed check valves such as our B250 (shown above and below) are extremely popular. Note: double barbs allow for a sure seal.
Gland check valve:
Gland-type check valves are used when the valve needs to fit into a bore, manifold, or machined boss. The outer O-ring provides sure sealing in this compact space. It is important to allow enough space for the poppet to open outwards for full flow. The G250 is shown above and below.
Tubing inserted check valve:
The inserted-type check valves can be installed in the tubing itself. They are the ultimate in space-saving. They lend themselves to use as an anti-drip nozzle. The T250 is shown above and below.
NPT (National pipe thread):
NPT or National Pipe Thread style check valves can be threaded into a manifold or other fitting where NPT’s are used. Our P125 Check valve is shown above and below.
Need help with your application? We can help! Call or email us for a free engineering consult and a free sample valve.
Can’t find a check valve that meets your requirement? Precision Check Valve can design a custom valve in a little as four weeks! Just contact us for details.
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