What a Check Valve Does
A check valve, also called a non return valve, is an automatic valve that permits flow in one direction only. It has no handwheel, no actuator and no external power supply: the disc or flap is moved by the medium itself. When the flow direction reverses, the valve closes and blocks the reverse path.
The purpose of the valve is protection rather than control. It stops reverse flow from draining a line, protects pumps and compressors from reverse rotation, keeps pressure from escaping a vessel through an outlet line, and prevents the mixing of two process streams that must not meet.
How the Disc Opens and Closes
The working principle is a balance between the forces acting on the moving part. In the forward direction the flowing medium produces a differential pressure across the disc that lifts it off the seat. In the reverse direction the same differential acts on the back of the disc and pushes it firmly onto the seat, where line pressure then holds it closed.
Opening: forward flow generates a small pressure difference, and once it exceeds the resistance of the disc weight and any spring, the disc leaves the seat and the valve opens fully.
Full open: at design flow velocity the disc is held out of the flow path, so pressure loss stays low and the disc does not chatter against the seat.
Closing: when flow slows, stops or reverses, the disc returns to the seat. Spring assistance accelerates closure where rapid seating is needed.
Sealing: metal to metal seats give good temperature resistance, while soft seats give tighter shut off at lower temperatures.
Two consequences follow from this mechanism. The valve needs a minimum flow velocity to stay open, so an oversized check valve may chatter and wear out early. And closure is driven by flow reversal, so the closing speed depends on how quickly the reverse flow develops rather than on any actuator setting.
Main Types and Their Working Principles
| Type | Movement | Typical duty |
| Lift or piston check valve | Disc moves axially along the flow path, in horizontal or vertical body arrangements | Water supply and drainage, high pressure pipe networks, pump discharge lines |
| Swing check valve | Disc swings on a hinge pin above the seat | General liquid and gas service, large diameter lines, municipal water mains |
| Dual plate check valve | Two spring loaded half discs folding into the bore | Compact installations, wafer and lug bodies, retrofits where space is limited |
| Axial or nozzle check valve | Streamlined disc moving coaxially with a spring | High velocity and pulsating service, compressor discharge, low pressure drop duties |
| Tilting disc check valve | Disc pivots on an offset axis to reduce slamming | Large diameter lines where water hammer control is important |
Swing check valves are the most widely used family because pressure loss is low and maintenance is simple, but they are not suitable for lines carrying heavy sediment, where the hinge and disc can be buried and fail to close. Lift check valves are better where the medium is clean and the line pressure is steady. Dual plate designs are chosen when installation space is tight, and axial designs are used where pressure loss and dynamic behaviour matter most.
Standards and Materials
Check valves for pipeline and industrial service are normally ordered against internationally recognised specifications. API 594 covers wafer, lug and double flanged check valves and defines face to face dimensions, pressure testing and marking. API 6D covers pipeline valves, including check valves, for pipeline transportation systems. ISO 15761 addresses steel gate, globe and check valves for smaller sizes. Pressure and temperature ratings for flanged steel valves follow ASME B16.34, and pressure testing procedures follow API 598.
Body materials are selected to match the line: carbon steel castings for general oil, gas and water service, stainless steel and duplex stainless steel for corrosive media, and alloy or low temperature steels where the operating temperature demands them. Trim materials, seat type and any hard facing are agreed between purchaser and manufacturer because they determine both the sealing performance and the service life.
Installation and Selection Points
Check flow direction against the arrow cast or marked on the body before installation; a reversed valve will not seal.
Swing check valves should normally be installed in horizontal lines with the hinge pin above the seat, and vertical installation with upward flow is acceptable where the design permits it.
Lift check valves require upward flow for vertical installation and a specific body orientation for horizontal installation.
Leave enough distance between a pump discharge and the check valve so that the flow profile is stable when the disc moves.
Size for the actual flow velocity range, not only for the line size, to avoid chattering at low flow and excessive pressure loss at high flow.
For vertical flow downward or for fast closing duties, choose a spring assisted or axial design and review the surge pressure with the piping engineer.
Inspection and Maintenance Points
Because a check valve is automatic, its failure is often silent until a reverse flow event occurs. Inspection programs therefore focus on evidence of internal damage: noise and vibration in service, rising pressure loss across the valve, and downstream pressure that does not hold after a pump stops. On shutdowns, the seat faces, the hinge pin and the disc guide are the parts to examine, since these are the surfaces that control closure. Where the medium is dirty, upstream strainers or filters should be checked at the same time, because debris trapped on the seat is a common cause of leakage.
FAQ
Q: How does a check valve know when to close?
It does not sense anything. Closure is caused by the fluid itself: when the pressure difference reverses, the medium pushes the disc back onto the seat, and the reverse line pressure then holds it shut.
Q: What is the difference between a swing check valve and a lift check valve?
The swing type rotates its disc on a hinge pin, which gives low pressure loss and suits large lines. The lift type moves its disc axially, which seals well under steady high pressure but is less suitable for dirty media.
Q: Can a check valve be installed vertically?
Yes, with upward flow for most designs. Vertical flow downward requires a spring assisted design and should be confirmed with the manufacturer.
Q: Why does a check valve sometimes make noise?
Noise normally indicates the disc is chattering against the seat because the valve is oversized or the flow is below the velocity needed to hold it fully open.
Q: Does a check valve provide a positive shut off?
A metal seated check valve is not a substitute for an isolation valve. It limits backflow but normally cannot guarantee zero leakage, so isolation must be provided by a dedicated shut off valve.
Q: Which standard should be referenced when ordering check valves?
API 594 for wafer, lug and double flanged check valves, API 6D for pipeline valves, and ASME B16.34 with API 598 for pressure and temperature ratings and pressure testing.
