A check valve is a self-actuating one-way valve — it allows fluid to flow in one direction and automatically prevents reverse flow. There is no handle, no actuator, no operator decision. The valve opens under forward flow pressure and closes when flow stops or reverses. This is the most-installed valve type in Australian industry by unit count: every pump discharge, every parallel piping system, every booster set, every water main, every backflow-prevention application, every self-priming pump suction uses one or more check valves.
Check valve and non-return valve are synonyms — same valve, two names. The plumbing and water industry typically says non-return valve or NRV; the oil, gas, chemical and engineering trades typically say check valve. The terminology is regional and trade-specific, not a product difference. This guide uses both terms interchangeably.
The single biggest failure mode customers experience with check valves is slam — the disc closing violently under reverse flow, generating water hammer that vibrates pipework, cracks welds, breaks pipe supports and, in documented cases, kills people. The water hammer section below is the most important content in this guide. Selecting the right check valve type for the service is the difference between decades of reliable operation and pump-room destruction.
AIMS Industrial stocks check valves across five service tiers: AAP Brass Check Valves (T-type, Y-type, E-type — small bore water, compressed air, oil with WaterMark for potable service), AAP Cast Steel Flanged Check Valves ANSI 150 (oil, petrochem and medium-pressure steam in DN50 through DN150), AAP Resilient Check Valves Flanged Swing Table-D/E (AU waterworks AS 4794 compliance for water mains and reticulation, DN80-DN750), AAP Resilient Check Valves Flanged Ball PN10 Table-D/E (sewage, slurry, and solids-laden service), and the AAP Wafer Check Valve Dual Disc Range (large-bore non-slam DN250-DN500 for HVAC, cooling water, pump discharge). Together this is the deepest check valve supply story in any of the six major valve clusters at AIMS — directly mapped to the five service bands that drive selection.
What this guide is NOT: a backflow prevention device reference (AS 2845.1 RPZD/DCV/PCDA assemblies are a distinct compliance product), a pressure relief valve reference (PSV/PRV — different mechanism), or a control valve reference (modulating control belongs in our Globe Valve Guide). Check valves complete the six-valve cluster alongside butterfly, diaphragm, gate, ball, and globe — and they are the only one that operates without any human intervention.
What is a check valve and how does it work
A check valve has a movable internal element — a disc, ball, plate, or piston — that sits against a stationary seat. When fluid flows in the design direction (forward), the pressure pushes the element off the seat and fluid passes through. When forward flow stops or reverses, the element returns to the seat, blocking reverse flow.
The element returns to seat by one of three mechanisms: gravity (swing check valves rely on the disc weight and reverse flow to close), spring (dual plate, lift check, silent check valves use a coil spring to actively force closure), or flow itself (ball check valves rely on the ball rolling back into the seat under reverse flow). The closure mechanism determines the valve's closure speed — and closure speed is the single most important variable in whether the valve causes water hammer.
Forward flow pressure must exceed a minimum threshold called the cracking pressure for the valve to start opening. Cracking pressure is typically very low on swing check valves (a few hundred millibar) and higher on spring-loaded designs (typically 0.1-0.5 bar). The cracking pressure is what makes spring-loaded check valves close fast — the spring is doing work to keep the element seated until forward pressure exceeds the spring force.
Check valve vs non-return valve — same thing, different name
There is no functional or product difference between a check valve and a non-return valve. They are the same valve. The difference is industry terminology:
- Non-return valve / NRV — dominant in AU plumbing, water reticulation, fire protection, irrigation, and waterworks industries.
- Check valve — dominant in oil and gas, chemical process, refinery, mining, marine, and general engineering trades.
The standards reflect this split: AS 4794 (the AU waterworks standard) uses "non-return valves"; API 594 and ASME B16.34 use "check valves". International suppliers use "check valve" almost universally. Both terms appear interchangeably on product datasheets, catalogues, and spec sheets — if you're searching for one, search for both. AIMS product naming uses both terms across the range depending on the source manufacturer's preferred terminology.
Six check valve types — overview
Six distinct check valve types dominate industrial supply. The choice is driven by service severity, slam risk, head loss tolerance, fluid type, and installation orientation.
| Type | Closure | Slam risk | Head loss | Orientation | Best for |
|---|---|---|---|---|---|
| Swing | Gravity + reverse flow | High | Low | Horizontal only (hinge level) | Water mains, low-velocity service, commodity isolation |
| Dual plate (wafer) | Spring (two half-discs) | Low | Low-medium | Any orientation | Pump discharge, HVAC, cooling water, modern industrial standard |
| Lift | Gravity (vertical lift) or spring | Medium | High | Horizontal or vertical (specify) | High-pressure steam, small bore, severe service |
| Ball | Gravity + flow (ball rolls) | Low | Low | Any orientation | Sewage, slurry, solids-laden fluids, viscous service |
| Silent / nozzle | Spring (axial flow) | Very low | Low | Any orientation | Critical pump discharge, high-cycling service, slam-elimination |
| Foot valve | Gravity + reverse flow | N/A (suction service) | Medium (strainer) | Vertical at pump suction | Self-priming pump suction, fluid transfer, irrigation |
The default modern selection across most industrial pump-discharge applications is the dual plate (wafer) check valve — fast closure (under 0.5 seconds for typical DN150), low head loss, any orientation, and cost-effective at medium bores. Swing checks remain the AU water reticulation standard (AS 4794 compliance), but for everything else the dual plate is now the engineering default.
Swing check valves — the legacy standard
A swing check valve has a single disc hinged from one edge inside the valve body. The disc swings open when forward flow lifts it, swings closed by gravity assistance when flow stops, and seats against a stationary seat ring. It's the oldest check valve design, the simplest, and still dominates AU water reticulation and low-velocity water service.
Advantages: low cost, low head loss when fully open, simple maintenance, well-understood. AAP Resilient Check Valve Flanged Swing Table-E in DN50 through DN750 covers the full AU waterworks range with AS 4794 compliance and WaterMark for potable service. The resilient (rubber-coated) disc seals tight against the metal seat — essentially zero leakage in the closed direction, including against debris that would defeat a metal-to-metal swing seat.
The disadvantage is slam risk. The disc has a long arc to travel before contacting the seat. When upstream pump trips and reverse flow accelerates, the disc closes at maximum velocity — the closure shockwave propagates back up the pipe as water hammer. On large pumps or high-static-head systems, the water hammer can be severe enough to crack pipe welds, break pipe supports, damage pump casings, and (as the documented Canadian incident shows) kill people in the pump room.
Critical install rule: swing check valves must be installed with the hinge axis horizontal — the disc must be able to swing under gravity. If the valve is rotated so the hinge is vertical, the disc may not close fully (hangs open in reverse flow) or may not open fully (hangs against the seat). The body has a directional arrow and an orientation marker — always install per the manufacturer specification.
Dual plate (wafer) check valves — the modern non-slam standard
A dual plate check valve has the body and seat geometry of a wafer butterfly valve, but with two spring-loaded half-disc plates that close against each other on a central hinge pin. Each plate travels only half the bore distance — much shorter arc than a swing disc — and the springs actively close the plates as soon as forward flow drops. Closure happens before reverse flow can develop enough velocity to cause water hammer.
This is the modern engineering default for pump discharge service, HVAC chilled and condenser water, cooling water mains, and any application where slam must be controlled. The combination of short closure arc, active spring closure, and compact wafer body (much shorter face-to-face than a swing check at the same bore) makes the dual plate the answer to most check valve selection questions in industrial service.
API 594 governs dual plate and lug check valves with two face-to-face standardised dimensions:
- Type A (short pattern) — compact face-to-face for tight piping layouts
- Type B (long pattern) — longer face-to-face for higher-pressure-rated bodies and easier removal
The face-to-face choice is pipe-space-driven, not performance-driven — both perform identically. AAP Wafer Check Valve Dual Disc in DN250 through DN500 covers large-bore HVAC and pump discharge service. For smaller-bore applications below DN250, specify equivalent dual plate per the application — call us with the service envelope and we'll specify.
Dual plate check valves work in any orientation — horizontal, vertical-upward flow, vertical-downward flow. The spring is doing the closing work, not gravity. This is a major install advantage on systems with limited space or complex piping geometry.
Lift check valves — straight-line and angle pattern
A lift check valve has a disc or piston that lifts vertically off a horizontal seat as forward flow passes through. Closure is typically gravity-assisted (or spring-assisted for non-slam variants). The geometry restricts the flow path more than swing or dual plate designs — head loss is higher — but the closure is consistent and the valve handles dirty service and pulsating flow better than a swing check.
Lift check valves are typically specified in small bore (DN15 to DN50) high-pressure applications where the head loss penalty is acceptable. Steam plant trap stations, boiler feed water, instrument lines, and small-bore process service all use lift checks. The valve body comes in two geometries:
- Straight-line pattern — inlet and outlet on the same axis, similar to a globe valve body
- Angle pattern — 90° turn in the body, like an angle globe valve
Lift check valves are directional and orientation-specific — most install only with flow upward through the seat (vertical-rise installation). Some lift check variants are designed for horizontal installation with appropriate disc orientation. Always check the manufacturer specification.
Ball check valves — sewage, slurry and solids handling
A ball check valve has a free-moving ball that sits in a seat. Forward flow lifts the ball into a wider section of the body; reverse flow rolls the ball back into the seat. The ball is heavier than fluid (or floats — see below) and seals against the seat by gravity plus reverse flow pressure.
The major application is sewage, slurry, and solids-laden service. A ball check has no narrow flow passages or hinge points where stringy debris, fibres, or solids can hang up. The ball rolls clear of the flow path when open, and the smooth body geometry sweeps debris through. This is the standard for wastewater pump stations, slurry pumps, and any service with suspended solids that would jam a swing or dual plate check valve.
AAP Resilient Check Valve Flanged Ball PN10 Table-D/E in DN150 and above covers the AU waterworks slurry and sewage service range. The ball is rubber-encapsulated for tight sealing — same principle as the resilient swing check valve. Body materials are typically ductile iron or cast iron with internal epoxy coating for waterworks corrosion resistance.
Ball check valves work in any orientation — the ball seats by reverse flow with gravity assist, not by gravity alone. This makes them useful for vertical suction lines and tank-bottom drainage where swing or dual plate would fail.
Silent check / nozzle check valves — when slam must be eliminated
A silent check valve (also called nozzle check, axial flow check, or non-slam check) has a disc or piston with an integrated spring inside a streamlined nozzle-shape body. The spring actively pushes the disc toward the seat continuously; forward flow opens the valve only against spring force. The instant forward flow drops to zero, the spring closes the valve — typically in 0.05 to 0.1 seconds, fast enough to close before reverse flow develops at all.
This is the premium solution for slam-critical applications: large pump discharge where water hammer would damage the system, parallel-pump systems where one trip causes severe reverse-flow conditions, VFD-controlled pumps with variable flow profiles, high-static-head systems, and any application where the noise and vibration of a slamming check valve is unacceptable.
The Eng-Tips engineering forum consensus on slam elimination: "non-slam soft closing nozzle check valves, or disc (not swing) check valves with spring" are the right answer when the service demands it. The cost premium over swing or dual plate is significant (typically 3 to 5 times the price of equivalent dual plate), and the design is overkill for ordinary water service, but on critical pump systems the premium is fully justified by the avoided pump-room damage.
Silent check valves are typically specified by the pump or system engineering team rather than commodity-supplied — call us with the service envelope and we'll source through the supplier network.
Foot valves — self-priming pump suction service
A foot valve is a check valve with an integrated strainer screen, installed at the bottom of a self-priming pump suction line submerged in the supply tank or sump. The check function holds the pump's prime when the pump stops (no fluid drains back into the tank). The strainer keeps suspended debris out of the pump impeller.
Foot valves are critical to self-priming pump operation. Without a foot valve, every pump start requires manual priming — filling the suction line with fluid before starting. With a foot valve, the pump remains primed continuously after the first prime, and starts on demand. This is the standard for fuel transfer pumps, irrigation pumps, water transfer pumps, and any self-priming application.
The foot valve check function is typically a simple flap-on-spring design, lighter duty than a service-pipe check valve because the application is low-pressure suction. The strainer mesh size is selected for the pump impeller clearance — coarse mesh (5-10 mm) for trash pumps, fine mesh (1-2 mm) for clean fluid pumps. Foot valves are sized to match the pump suction pipe — typically same bore as the suction line, occasionally one size larger to reduce strainer pressure drop.
Maintenance note: foot valve strainers must be cleaned periodically. A blocked strainer starves the pump, causes cavitation, and destroys the impeller. On dirty water service (rainwater tanks, dams, bores), expect monthly strainer cleaning; on clean water service, annual inspection is typical.
Water hammer and check valve slam — the #1 failure mode
The mechanism: when an upstream pump trips (power failure, motor protection trip, sudden valve closure downstream), forward flow stops instantly. The fluid downstream of the check valve, decelerating against the system's static head, begins to reverse. A swing check valve disc — hinged from one edge — takes time to travel its full arc to reach the seat. During that travel time, reverse flow accelerates. When the disc finally contacts the seat, it does so at high velocity against the reverse flow column. The impact decelerates the entire reverse-flow column instantly, generating a pressure shockwave that propagates back up the pipe at the speed of sound in water (1,500 m/s). The pressure spike can exceed the pipe's burst pressure rating.
Forum-validated risk factors for check valve slam:
- Sudden pump shutdown — power failure, motor trip, emergency shutdown. The instant flow stop allows reverse flow to develop before a slow-closing check valve responds.
- High static head downstream — multi-storey building water supply, tall process columns, deep wells. The static head drives reverse flow harder.
- Parallel pump systems — when one pump trips while parallel pumps continue, the trip pump's check valve sees reverse flow from the parallel discharge pressure.
- Long discharge piping — more fluid mass to decelerate = bigger water hammer pressure spike.
- Single-plate flap valves without springs — Eng-Tips engineering forum: "single plate flap valves without spring-assisted closing are especially inclined to cause water hammer problems and could chatter against the pipeline wall when opening."
The solutions, in order of effectiveness:
- Specify spring-loaded dual plate (wafer) check valves instead of swing checks for any pump discharge application. The spring closes the plates before reverse flow develops significantly.
- Specify silent / nozzle check valves for high-severity applications — closure under 0.1 seconds eliminates water hammer entirely.
- Install surge anticipators / surge relief valves on the discharge side as a backup if check valve type cannot be changed.
- Soft-start / soft-stop pump control — VFDs that ramp pump speed down on stop reduce the sudden-flow-cessation problem at source.
- Air-vessel / accumulator on the discharge cushions pressure spikes.
Forum practitioner consensus on the choice: "Best choices include non-slam soft closing nozzle check valves, or disc (not swing) check valves with spring. Slowing the time for valves to close can eliminate some potential for water hammer."
Sizing — why oversizing is worse than undersizing
A check valve must be sized for the actual operating flow rate, not just the pipe bore. Oversizing a check valve — fitting a DN150 check valve to handle 100 L/s when the actual flow is 30 L/s — causes serious operational problems that aren't intuitive.
Eng-Tips engineering forum: "Oversizing check valves causes greater pressure drop than a correctly sized valve, and can lead to premature wear at the shaft holding the disk in place." The mechanism: at low flow rate, the check valve disc doesn't lift fully against the stop. The disc floats partway open, oscillating up and down as pressure pulsates with pump pulses. This oscillation wears the shaft, the bushings, and the seat — failure modes that don't appear in a correctly-sized valve.
The rule: select the check valve so the operating flow is in the upper 50-75% of its rated flow range. Better to fit a smaller valve operating at 80% capacity than a larger valve operating at 30%. The correctly-sized valve has its disc fully open against the stop, no oscillation, full sealing on closure.
For systems with widely variable flow (VFD pumps, modulating systems), specify spring-loaded dual plate — the spring forces full plate opening against the stop even at low flow, eliminating the oscillation problem that plagues swing checks at part-flow operation.
Installation orientation — type-specific rules
Each check valve type has specific orientation requirements that determine whether the valve will function correctly.
| Type | Orientation rule | Notes |
|---|---|---|
| Swing | Hinge axis horizontal — body horizontal or vertical-rise flow only | Disc must swing under gravity. Horizontal install with hinge above the disc; vertical install only with flow upward. |
| Dual plate (wafer) | Any orientation | Spring-closed. Horizontal, vertical-up, or vertical-down — all OK. Major install advantage. |
| Lift | Per manufacturer spec — typically flow upward through seat for vertical, hinge-up for horizontal | Disc lifts vertically. Wrong orientation = disc doesn't lift or doesn't close. |
| Ball | Any orientation | Ball seats by reverse flow + gravity assist. |
| Silent / nozzle | Any orientation | Spring-closed axial flow. |
| Foot | Vertical at pump suction with strainer submerged | Strainer must be below minimum tank fluid level. |
Common installation mistakes that cause check valve failure:
- Swing check installed with flow direction reversed (arrow on body ignored)
- Swing check installed in vertical-down flow position (gravity holds disc open)
- Lift check installed horizontally when specified vertical (disc won't lift)
- Check valve installed immediately downstream of a pump volute or elbow — turbulent flow at the inlet causes disc chatter (Eng-Tips practitioner quote: "if a valve is installed on the side of the pipe with elbows and additional fittings, the pressure drop at the inlet can be higher and cause problems like chattering"). Specify minimum 5 pipe diameters of straight pipe upstream.
- Foot valve strainer fitted above minimum tank level (loses prime when tank low)
Materials selection — brass, cast iron, cast steel, stainless
AIMS stocks check valves across material tiers matched to service envelope and pressure class.
| Body material | Service envelope | Temperature | Typical AIMS product |
|---|---|---|---|
| Brass | Small bore water, compressed air, oils, WaterMark for potable | To 200°C | AAP Brass Check Valve T-Type / Y-Type / E-Type |
| Cast iron (resilient) | AU waterworks, water mains, fire service, sewage (ball check variant) | To 60°C (AS 4794 max) | AAP Resilient Swing Table-D/E |
| Cast steel (ANSI 150) | Oil, petrochem, medium-pressure steam, general industrial | To 300°C | AAP Cast Steel Flanged ANSI 150 |
| Ductile iron (wafer dual disc) | HVAC, cooling water, pump discharge, large bore | To 200°C | AAP Wafer Dual Disc DN250-DN500 |
| Stainless steel | Corrosive chemical, pharmaceutical, food and beverage, marine | To 200°C (PTFE seat) or 400°C (metal seat) | AAP SS Check Valve (sourced on request) |
Brass check valves dominate small-bore commodity service — pump suction strainers, instrument lines, compressed air, hot water, condensate, hydraulic isolation. T-type (in-line straight-through body) and Y-type (45° angled body) are the two standard configurations; Y-type has slightly lower head loss and is preferred where pressure drop matters. Brass with DZR (dezincification-resistant) brass is mandatory for potable water service under AS/NZS 4020 and carries WaterMark certification.
Cast iron resilient check valves are the AU waterworks standard. The body is cast iron (typically GG-25 grey iron or ductile iron with internal epoxy coating per AS/NZS 4158). The disc is rubber-encapsulated for tight sealing against debris that would defeat a metal seat. AS 4794:2001 (R2015) certification mandatory for water mains and water reticulation service. Maximum service temperature is 60°C — these are water valves, not steam valves.
Cast steel flanged check valves are the petrochem, oil and gas, and steam plant standard. ANSI Class 150 covers most general industrial service to about 19 bar at 150°C (derating at higher temperatures per ASME B16.34). ANSI 300 doubles the pressure rating for higher-pressure service. Flanged connections per ASME B16.5 or AS 4087.
Ductile iron wafer dual disc valves are the modern industrial standard for medium-to-large bore non-slam service. Compact wafer body, spring-actuated dual plates, low head loss, any orientation install. Standard for HVAC chilled and condenser water systems, cooling water mains, pump discharge in commercial buildings and process plant.
Pressure classes and standards
Check valves are specified by pressure class and the governing standard for the service.
| Standard | Scope | AU usage |
|---|---|---|
| AS 4794:2001 (R2015) | Non-return valves — swing check and tilting disc for waterworks, DN80 to DN750, Class 16-35, max 60°C | Mandatory for AU water mains and reticulation service |
| AS/NZS 4020 | Materials in contact with drinking water — leach testing | Mandatory for potable water service; backs WaterMark certification |
| WaterMark | AU product certification for potable water plumbing products | Required for plumbing trade installation in AU buildings |
| ASME B16.34 | Universal pressure-temperature rating standard for industrial steel valves | Cast steel and forged steel check valves — Class 150, 300, 600, 800+ |
| API 6D | Pipeline valves — large bore high-pressure (DN50+ typical) | Oil and gas pipeline service, severe-duty applications |
| API 594 | Wafer + lug check valves, Type A (short F-to-F) and Type B (long F-to-F) | Dual plate check valves in industrial service — the dominant standard |
| BS 1868 | Steel check valves with flanged or buttweld ends | Legacy plant reference; less common in modern AU supply |
| ISO 5752 | Face-to-face dimensions for industrial valves | Universal cross-reference for valve dimensional standards |
WaterMark certification is a non-negotiable requirement for any check valve installed in AU potable water service — the certification verifies the valve materials are safe for drinking water contact (AS/NZS 4020) and the design meets the relevant performance standard (typically AS 4794 for waterworks-size valves, or AS 1628 for tap-ware-size valves). Without WaterMark, the plumbing inspection won't pass.
AS 4794 is the mandatory standard for non-return valves in AU waterworks service. It covers swing check and tilting disc designs in DN80 to DN750, Class 16 through 35 pressure ratings, with a 60°C maximum service temperature limit. AAP resilient swing check valves are AS 4794 compliant and the standard product for water mains, water reticulation, fire service, and irrigation main lines.
VFD pump systems — special check valve selection
Variable frequency drive (VFD) controlled pumps create operating conditions that destroy standard swing check valves: the pump speed varies continuously, the flow rate varies with it, and at low pump speed the flow may fall well below the check valve's rated capacity. Standard swing check disc oscillation accelerates dramatically under these conditions.
Eng-Tips engineering forum on VFD check valve selection: standard swing checks chatter on VFD service. The recommended pairing is spring-loaded dual plate or silent (nozzle) check valves — the spring forces full plate opening against the stop regardless of flow rate, eliminating the oscillation problem. The added cost is minor compared to the avoided wear damage from chattering.
If a VFD pump is being retrofitted to an existing system with a swing check valve, plan to replace the check valve as part of the VFD installation. Operating a swing check on VFD service typically means valve failure within 6-12 months.
AIMS check valve supply — the five service tiers
AIMS Industrial stocks a comprehensive check valve range across five service tiers matched to AU industrial applications.
| Service band | Pressure / temperature | AAP product | Sizes |
|---|---|---|---|
| Small bore water, air, oils, WaterMark potable | To 200°C, low/medium pressure | Brass Check Valve T-Type / Y-Type / E-Type | DN15-DN65 threaded |
| AU waterworks, water mains, fire service | To 60°C, AS 4794 Class 16-35 | Resilient Swing Table-D/E | DN80-DN750 flanged |
| Sewage, slurry, solids-laden | To 60°C, PN10 Table-D/E | Resilient Ball PN10 | DN150+ flanged |
| Oil, petrochem, medium-pressure steam | To 300°C, ANSI Class 150 | Cast Steel Flanged ANSI 150 | DN50-DN150 flanged |
| HVAC, cooling water, pump discharge, large bore | To 200°C, API 594 | Wafer Dual Disc | DN250-DN500 wafer |
Pairing check valves with companion fittings: pipe flanges for flanged connections, spiral wound gaskets for flange-joint sealing on the steel and high-pressure tiers, gate valves upstream for isolation, globe valves for throttling duty (gate isolates, globe throttles, check enforces direction — the three-function rigging). For the rest of the valve cluster see our Butterfly Valve Guide, Ball Valve Guide, and Diaphragm Valve Guide.
For service outside our standard stock — silent / nozzle check valves for severe slam-elimination, cryogenic-rated bodies, oversized DN500+ valves, severe-service forged Class 800+, or specialty sewage applications — AIMS sources from our supplier network. Contact us or call (02) 9773 0122 with the service envelope (fluid, pressure, temperature, size, end connection, slam risk profile) and we'll specify the right valve for the duty.
Common check valve mistakes — diagnostic table
| Symptom | Likely cause | Fix |
|---|---|---|
| Loud bang when pump trips — water hammer | Swing check slamming closed under reverse flow on high-static-head system | Replace with spring-loaded dual plate (wafer) or silent (nozzle) check valve. Add surge anticipator for high-severity service. |
| Continuous chatter / vibration during pump operation | Check valve oversized for actual flow (disc oscillates); or installed too close to pump volute or elbow | Resize valve to operate in upper 50-75% of rated flow range. Provide minimum 5 pipe diameters of straight pipe upstream. |
| Reverse flow not stopped — fluid drains back when pump stops | Swing check installed in wrong orientation (hinge not horizontal), or debris on seat, or disc detached from hinge pin | Verify body arrow matches flow direction and hinge axis is horizontal. Disassemble + inspect; replace if internal hardware damaged. |
| Swing check disc hangs open during low/intermittent flow | Wrong valve type for variable-flow service | Replace with spring-loaded dual plate or spring-loaded lift check valve. |
| Foot valve loses prime — pump won't self-prime on start | Foot valve seat debris, or strainer blocked, or valve installed above minimum tank fluid level | Pull foot valve, clean strainer, inspect seat. Verify install depth is below minimum tank level. |
| Premature shaft / pivot wear on swing check | Oversized valve with continuous disc oscillation | Replace with correctly-sized valve operating at higher fractional capacity. |
| Check valve passes air or pulses on suction service | Wrong check valve type for suction service (use a foot valve, not an in-line check) | Replace with foot valve at the pump suction with strainer submerged. |
| WaterMark inspector failed plumbing check | Check valve not WaterMark certified, or wrong material spec for potable service | Specify DZR brass or AS 4794-compliant resilient check valve with WaterMark certification. |
Frequently Asked Questions
What is a check valve and what is it used for?
A check valve is a self-actuating one-way valve that allows fluid to flow in one direction and prevents reverse flow. Used on pump discharges (prevent reverse flow when pump stops), water mains (prevent backflow contamination), parallel pump systems (isolate one pump from another), self-priming pump suctions (hold prime via foot valve), and any application where flow must go one way only.
What is the difference between a check valve and a non-return valve?
There is no difference — they are the same valve, called different names in different trades. Non-return valve / NRV is the dominant term in AU plumbing, water reticulation, and waterworks. Check valve dominates oil and gas, chemical, refinery, and general engineering. AS 4794 uses "non-return valve"; ASME B16.34 and API 594 use "check valve". Same product, two names.
Are check valves directional?
Yes — completely directional. The valve only allows flow in one direction (forward) and blocks flow in the reverse direction. Every check valve has a directional arrow cast or stamped on the body indicating the correct flow direction. Installing backwards means the valve is permanently closed against forward flow — the system won't work.
Which way does a check valve install?
Per the directional arrow on the body. Beyond direction, each valve type has orientation rules: swing check valves must install with the hinge axis horizontal (disc swings under gravity); dual plate wafer check valves work in any orientation; lift check valves are typically vertical-flow with the disc rising; foot valves install vertically at the pump suction with the strainer submerged.
Why do check valves slam?
Slam happens when reverse flow develops faster than the valve disc can close. The disc travels through its full arc before contacting the seat, building reverse velocity, then impacts the seat at high velocity — generating a pressure shockwave (water hammer). Swing check valves are most prone to slam because the disc has a long arc to travel. Sudden pump shutdown is the most common trigger.
What causes water hammer in a check valve?
The pressure shockwave from the disc impacting the seat at high velocity against reverse flow. The faster the valve closes, the less reverse flow develops, the less water hammer. Solutions: specify spring-loaded dual plate or silent (nozzle) check valves instead of swing checks for any application with high static head, large pumps, or critical service. Slowing pump shutdown speed (soft-stop VFD) also reduces water hammer.
What is the difference between a swing check valve and a dual plate check valve?
Swing check: single disc hinged from one edge, closes by gravity, long disc travel arc, slam-prone, must install horizontally with hinge axis level. Dual plate (wafer): two half-disc plates with spring closure, short travel, fast closure, no slam, any orientation install, compact wafer body. Dual plate is the modern industrial default; swing remains the AU waterworks standard via AS 4794.
Can a check valve be installed vertically?
Depends on type. Swing checks can install vertically with flow upward only (gravity pulls disc closed onto seat when flow stops). Dual plate wafer checks work in any orientation. Ball checks work in any orientation. Lift checks are typically vertical-only (specify manufacturer requirements). Foot valves are always vertical at pump suction.
What is a silent check valve?
A spring-loaded axial-flow check valve with very fast closure (typically 0.05-0.1 seconds) — fast enough to close before reverse flow develops, eliminating water hammer. Also called nozzle check valve or non-slam check valve. Premium pricing (3-5x dual plate) but justified on critical pump systems, VFD service, large pumps, or high-static-head applications where slam would damage the system.
What is a wafer check valve?
A check valve with a compact body that fits between two pipe flanges like a wafer — much shorter face-to-face than a full-body flanged check. The wafer check is almost always a dual-plate design with spring-loaded half-discs. API 594 governs wafer check valve dimensions in Type A (short pattern) and Type B (long pattern) face-to-face standards.
What is the difference between API 6D and API 594 check valves?
API 6D covers pipeline valves — large-bore high-pressure check valves for oil and gas pipeline service, typically full-bore swing check or piston check designs in the DN50+ range. API 594 covers wafer and lug check valves — the dual-plate spring-actuated compact design used in industrial pump discharge, HVAC, and general service. API 6D is pipeline-spec; API 594 is industrial-spec.
What is a foot valve and how is it different from a check valve?
A foot valve is a check valve with an integrated strainer, installed at the bottom of a self-priming pump suction line submerged in the supply tank. The check function holds the pump's prime when the pump stops; the strainer keeps debris out of the impeller. Standard check valves install in-line in the discharge piping and don't have strainers. Use a foot valve at the suction; use a check valve at the discharge.
What size check valve do I need? Can a check valve be too big?
Size for the operating flow rate so the valve operates in the upper 50-75% of its rated flow range — not just to match the pipe bore. Oversizing causes premature failure: at low flow rate the disc oscillates partway open, wearing the shaft, bushings, and seat. Better to fit a smaller valve at 80% capacity than a larger valve at 30%. For VFD-controlled systems with widely variable flow, specify spring-loaded dual plate to force full plate opening regardless of flow rate.
Why is my check valve chattering?
Three common causes: (1) valve oversized for actual flow rate — disc oscillates partway open; (2) installed too close to a pump volute or elbow — turbulent inlet flow shakes the disc; (3) single-plate flap valve without spring on a pulsating-flow service. Fixes: resize the valve, provide minimum 5 pipe diameters of straight pipe upstream, or replace with a spring-loaded dual plate check valve.
What does AS 4794 cover?
AS 4794:2001 (Reconfirmed 2015) is the Australian Standard for non-return valves used in waterworks service. It covers swing check and tilting disc designs in DN80 through DN750, Class 16 to 35 pressure ratings, maximum service temperature 60°C. Mandatory for water mains, water reticulation, fire service, and irrigation main lines in AU. Resilient swing check valves designed to AS 4794 with WaterMark certification are the standard AU water utility specification.