Checking Out Check Valves – An Introduction
Last Updated June 11, 2021
Check valves are usually invisible in everyday applications but are critical for flow control
Introduction
Check valves, non-return or one-way valves, allow liquid or air to flow in only one direction. Manufacturers make them in a wide range of sizes and types. Tremendous varieties of applications use them to prevent costly or even dangerous backflow.
Even though many everyday applications use check valves, the basics of check valve operation are not always clearly understood. In this post, I will briefly review some basics of check valve function. I also include descriptions of some of the most common miniature check valve designs used for gas and liquid flow control.
This article is the first in a series of articles about check valves
- Plastic Diaphragm Check Valves – Check Valves Part II >>
- Spring Loaded Check Valves – Check Valves Part III >>
- How Do Diaphragm Check Valves Work? >>
- Inline Filters for Miniature Flow Control Component Protection >>
Some check valve basicsCheck valves are commonly two-port valves with two openings in the valve body. One port is for air or fluid (media) to enter the valve (referred to as the inlet port), and the other is for the media to exit (referred to as the exit port). Check valves require no external control to work, so they do not have a valve handle or stem.
Miniature, small-bore and compact check valves come in a wide variety of configurations and materials. Generalizations about their design are tricky because their designs are a little more complicated than larger industrial check valves. ISM is uniquely able to add check valve functionality to almost every fitting we offer. Still, all check valves usually share the same valuable characteristics.
Learn more about ISM's customized fittings services >>
Check valve uses and characteristics:
- Protect equipment from backflow damage
- Provide pressure relief for system safety
- Prevent contamination from backflow
- No human intervention is required to work
- Powered by flow and differential pressure
Normally open or normally closedMiniature, compact and small-bore check valves have two primary design differences.
There are check valve designs that use a free-floating valve sealing element to stop backflow through the valve. Backflow pressure moves the sealing element and pushes it back against the valve seat or sealing surface to stop the flow. These check valves are considered free-flowing, and at least some backflow pressure is required to close them. Because of this, they may sometimes be called normally open check valves.
The other primary check valve design uses a source of pressure, usually a spring or flexing elastomer, that pushes against the valve sealing element and holds it against the valve seat or sealing surface. Because these valves require at least some downstream pressure to open, they are often called "normally closed check valves."
The positive downstream pressure required to open a check valve and cause at least some detectible flow is called "check valve cracking pressure."
It is important to note that all check valves require at least some downstream flow to open fully and allow maximum flow through the valve. While the maximum flow rate is affected by the pressure difference across the valve, check valves generally require enough pressure from downstream flow to fully open and allow the highest flow possible to pass through the valve.
Cracking pressureCracking pressure is a crucial specification for normally-closed check valves. It is the minimum upstream pressure when a normally closed check valve begins to open and flow starts. It is possible to design precise spring pressures to build miniature check valves even with specific but very low cracking pressures.
Basic types of miniature or small-bore check valvesA miniature check valve is usually installed inline with the orientation necessary for an application's flow control requirements. There are three primary miniature check valve types:
- Duckbill
- Diaphragm
- Spring loaded or spring assist
Basic check valve designs
Spring loaded or spring assist check valvesBuilding a spring into a check valve's mechanism essentially eliminates the effect of gravity on check valve function. Without a spring, gravity and the valve's vertical orientation can become critical for a check valve to work correctly.
Spring loaded (spring assist) check valves reseal with the help of the force of the spring. They help close the check valve tightly enough to have a bubble-tight seal with no detectable flow.
Springs also provide additional reseal pressure. This spring-assisted sealing is often necessary for low-pressure applications.
Learn more about spring loaded check valves >>
Duckbill check valvesDuckbill check valves use a one-piece elastomer or rubber element molded into a shape resembling a duck's bill. Downstream pressure pushes open the beaked end of the valve to allow flow. When the downstream flow stops, material elasticity causes the slit tip of the duckbill to return to its flattened shape. The valve is then closed, which prevents backflow.
These valves are self-sealing, non-slamming, quiet and corrosion-resistant.
Other important duckbill check valve features include
- Work in both horizontal and vertical installations
- Low opening pressures with almost immediate free flow
- Straight, non-tortuous flow paths for reduced pressure drops across the valve
- No dead space volume, which provides compact designs and less turbulent flow
Check out ISM's selection of duckbill check valves >>
Diaphragm check valvesDiaphragm check valves use self-centering, free-floating, highly flexible elastomer or rubber diaphragms or discs to control backflow. The design of the check valve body keeps the diaphragm centered over the inlet and outlet valve seats.
The inlet sealing surface or seat of a diaphragm check valve is on the upstream side of the valve body. This inlet sealing surface has a smooth concave surface with an opening in the center that leads to the inlet port.
Backflow causes the flexible rubber disc to lie down across the inlet valve seat's smooth, curved sealing surface. Covering the sealing surface closes the opening to the inlet port and prevents upstream flow back through the inlet port.
There is a two-level valve seat on the outlet side of the valve body with built-in channels across it. Positive flow towards the outlet or exit port causes the disc to lie against the top of this seat. The liquid or gas then flows around the disc, through the channels, and out through the check valve's exit port.
Check out ISM's selection of diaphragm check valves >>
An essential point about check valve installationBecause check valves provide one-way flow, it is crucial to install them in the correct orientation. An arrow or the symbol (the fluidic/pneumatic logic symbol) on the outside surface of the body indicates flow direction. In these images, flow is from left to right. Some check valves have both the logic symbol and an arrow.
The image below is of two check valve schematic symbols. The top one is for a spring loaded or spring assist check valve. The bottom one represents a free-floating check valve.
Check valve schematic symbols
Technical specifications that matter
- End connection options
- Minimum cracking pressure
- Zero leakage or some leakage
- Normally closed or normally open
- How quickly a check valve closes
- Spring assist, gravity or flow dependent
- Materials, because of chemical compatibility
- Sealing pressure or backpressure or backflow
Other considerations
- Media viscosity
- Operating conditions
- Flow conditions (is there enough)
Learn more about viscosity and Viscometry at the Anton Paar GmbH website >>
ConclusionWhew! Maybe that was a bit much for a mere introduction, but I like to think that you may come away from this with a better understanding of check valve function. Additional articles in this check valve series focus on specific design types and technical issues associated with their use in pneumatic and fluidic flow control applications.
Part two of this series describes the design and function of plastic diaphragm check valves. These are among the most widely used miniature check valves because they are cost-effective, adaptable, and particularly suitable for low cracking pressure applications. Their widespread use and a broad range of applications require a detailed discussion of their own.
Articles in this series about check valves
- Checking Out Check Valves – An Introduction >>
- Plastic Diaphragm Check Valves – Check Valves Part II >>
- Spring Loaded Check Valves – Check Valves Part III >>
- How Do Diaphragm Check Valves Work? >>
- Inline Filters for Miniature Flow Control Component Protection >>