Checking Out Check Valves – An Introduction

Learn more about ISM's selection of miniature check valves.Check valves are mostly invisible in everyday applications but are critical for flow control

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 they are widely used for everyday applications, the how and why of check valve operation is not always clearly understood. In this post, I will briefly review some basics of check valve function as well as some generic descriptions of some of the most common miniature check valve designs used for miniature pneumatic and fluidic flow control.

This is the first in a series of articles about check valves

An exploded view of an ISM modular check valve. Click here to go to a landing page where you can get more information about these valves.

Modular Check Valves

We've taken spring-loaded check valves to a whole new level. Mix and match imperial and metric connections. Watch the video.

A play button for a video about ISM's modular check valves. Click here to go to a landing page where you can watch the video.

Some check valve basics
Check valves are commonly two-port valves with two openings in the valve body. One opening 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 and so most do not have any sort of valve handle or stem.


Miniature check valves actually come in a wide variety of configurations and materials. Generalizations about their design are a little more difficult than for large industrial check valves. ISM is even in the unique position of being able to add check valve functionality to almost every fitting we offer. Nonetheless, all check valves usually share the same useful characteristics.

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 closed
Check valve basics include whether the design begins with the valve normally open or normally closed. If it is a normally open design, flow will take place unless the pressure of backflow closes the valve. If a check valve is normally closed, the valve will open when there is enough pressure in the direction of flow to meet or exceed the cracking pressure of the valve.

Cracking pressure
Cracking pressure is a key 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 very precise spring pressures or to use elastomer diaphragms to build miniature check valves with very specific and very low cracking pressures.

Basic types of miniature check valves
A miniature check valve is usually installed in-line with its specific positioning dictated by flow control considerations. There are three basic styles of miniature check valves – spring loaded, duckbill and diaphragm.

Rendered color images of spring loaded, duckbill and diaphragm miniature check valves

An important point about check valve installation
Because check valves provide one-way flow, it is important to install them in the correct orientation. An arrow or the symbol for a check valve (a fluidic/pneumatic logic symbol) on the outside 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.

Technical specifications that matter

  • Materials (chemical compatibility)
  • Minimum cracking pressure
  • Sealing pressure or backpressure or back flow
  • Normally closed or normally open
  • How quickly a check valve closes
  • Spring assist, gravity or flow dependent
  • End connection options
  • Zero leakage or some leakage

Other considerations

  • Operating conditions
  • Media viscosity
  • Flow conditions (is there enough)

Whew! Maybe that was a bit much for a mere ‘introduction’ but I like to think that you may come away from this with a more complete understanding of check valve function. An upcoming post will describe the design and function of plastic diaphragm check valves. These are one of the most widely used types of miniature check valves because they are cost effective, adaptable and are particularly suitable in low cracking pressure applications. Their widespread use and broad range of applications requires a detailed discussion of their own. There will also be some ongoing posts about spring loaded and duckbill check valves. They will focus on specific design types and some of the technical issues associated with their use in pneumatic and fluidic flow control applications.

Look for our next blog article where we go beyond the basics of miniature check valves to talk about plastic diaphragm check valves in more detail:

More about Check Valves – Plastic Diaphragm Check Valves


Have you had trouble choosing the right check valve for one of your projects? Did the configuration type or cracking pressure choice cause problems? How about material selection? Help us by telling others about what you learned.

Have questions about check valve features or function? If so, send me an email – You can also ask questions using the comments section below.


Steven C. Williams headshot March, 2018.

About the author
Steven C. Williams my LinkedIn profile link button.
Steven C. Williams, BS, 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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