14.1: Check Valves
- Page ID
- 116681
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Understanding the Function and Operation of Pneumatic Check Valves
A pneumatic check valve is a basic component that allows fluid flow in only one direction while blocking flow in the opposite direction. These valves are essential in pneumatic systems for controlling the flow of compressed air.
A check valve consists of a valve body with inlet and outlet ports and a moveable member that is typically lightly biased by a spring. This moveable member can vary in design, such as a flapper (called a swing) or plunger but most often it is a ball or poppet.
How a Check Valve Works
Check valves are designed so that fluid can flow through in only one direction. When system pressure at the check valve inlet exceeds the spring force biasing the poppet (usually around 5 psi), the poppet moves off its seat, allowing flow. This is known as the free-flow direction. When fluid tries to flow in the opposite direction, the poppet is pushed against its seat, preventing flow. Basically, it plugs a passage and prevents flow through that passage.
The passage will stay blocked even if there is fluid under pressure on both sides of the poppet or ball. The pressure would be equal on both sides (due to Pascal’s Law), canceling the force created on each side. This case results in the bias spring keeping the ball or poppet seated.
In pneumatic systems, check valves often use resilient seals on the poppet and are commonly used as bypass valves. When used as a bypass, they can allow flow to go around components like flow control valves that would otherwise restrict flow in both directions.
Applications of Pneumatic Check Valves
Check valves have several key applications in pneumatic circuits:
- Internal Compressor Valves: In piston air compressors, check valves serve as intake and discharge valves, controlling the flow of air as it enters and exits the compressor chamber.
- Maintaining Pressure in Storage Receivers: When a compressor reaches the desired pressure in a storage tank or receiver, a check valve between the compressor and the tank prevents air from flowing back into the compressor, ensuring the tank maintains its pressure.
- Quick-Connect Fittings: Check valves in quick-connect fittings enable quick connection and disconnection of pneumatic components without losing air pressure in the supply line.
- Bypass Applications: Check valves are used to bypass flow around certain components in one direction, enabling control over a circuit's operation. For example, a check valve can bypass a flow control valve to control a cylinder’s speed in one direction, while allowing full speed in the opposite direction.
- Specialized Functions: Check valves are also used in advanced pneumatic circuits for applications like logic functions, where they select the higher pressure from two sources (also known as a shuttle valve), and safety features, such as blocking flow in case of a line break to prevent hose whip.
Types of Pneumatic Check Valves
The two most common types of check valves in pneumatic systems are the ball valve and poppet valve:
- Ball Check Valve: This type features a small, durable ball that seals against the valve seat, blocking reverse flow. It opens when inlet pressure pushes the ball away from its seat.
- Poppet Check Valve: The poppet valve design uses a plunger or disk to block flow in one direction. When inlet pressure overcomes the spring force, it lifts the poppet, allowing flow. When flow reverses, the spring pushes the poppet back onto the seat, blocking air passage.
In both types, the free-flow direction is often indicated by the symbol (ball seating into the cup seals the passage in that flow direction) on the valve’s body, guiding users in setting up the circuit properly.