Check Valve Mechanism Explained: How It Works, Parts, Cracking Pressure Formula and Uses

Pressure lifts the disc from its seat

The disc separates the upstream and downstream chambers. In this upward-opening arrangement, spring preload and disc weight oppose the pressure force. If the forward differential pressure is sufficient, the disc starts to lift and fluid can pass around it.

When forward differential pressure falls, the spring promotes closure. Reverse pressure assists seating. The animation shows these internal parts in section, with blue marks identifying the open flow route.

This is a generic teaching arrangement, not a dimensioned representation of a particular commercial valve. The dashed spring-retainer supports lie behind the illustrated flow plane.

Using a cracking-pressure estimate

The calculation compares the available forward pressure with the closing forces of an upward-opening valve. It can help explain how preload, weight and effective seat area change the onset of lifting.

It does not select a valve’s flow capacity or establish its response during a pump stop. Those require the particular valve’s pressure-loss and dynamic data, together with the pipeline conditions.

Static force balance

Convert effective area from cm² to m² by multiplying by 0.0001. Forward force is differential pressure in pascals multiplied by area. At incipient opening, this equals spring preload plus disc weight.

Thus cracking pressure is (Fspring + Wdisc)/A. The calculator reports this in kPa and psi, and subtracts the closing forces from the applied pressure force to obtain net opening force.

A positive result means the seated force balance favors lifting. It cannot determine the eventual gap because the spring stiffness and fluid forces after opening are not entered. The static condition therefore indicates whether force is available to start lifting the disc, not how far it will open.

Default force balance

With 8 N spring preload, 1.5 N disc weight and 4.91 cm² effective area, the closing force is 9.5 N. The estimated cracking pressure is about 19.35 kPa, or 2.81 psi.

At 25 kPa forward differential pressure, the pressure force is 12.275 N and the net opening force is 2.775 N. The illustration therefore shows a lifted disc. At zero forward differential pressure, the same spring and weight hold it seated.

Doubling the area halves the cracking-pressure estimate for unchanged closing forces. Increasing preload or weight raises it. These effects are calculated directly from the force balance.

Opening force is not lift or flow rate

A real valve’s spring force generally increases as the disc lifts, and flow changes the forces on the disc. A static cracking estimate cannot specify the stable opening, pressure loss or flow rate.

This model also makes no promise about closing time, water hammer, sealing leakage or installation suitability. Use the manufacturer’s documentation and measured operating conditions for those questions.

Calculator questions

Can the opening force determine disc lift?

The inputs determine the seated force balance. Actual opening requires additional information, including spring stiffness and flow forces.

Is disc weight a mass?

No. Enter weight as force in newtons. If starting with mass in kilograms, its standard-gravity weight is approximately 9.80665 times that mass.

Which pressure should I enter?

Use upstream pressure minus downstream pressure in the forward direction. Absolute supply pressure alone is insufficient.

Does this apply unchanged to a horizontal valve?

No. This model assumes gravity acts directly against upward disc motion. A different orientation changes the axial contribution of weight.