WHAT'S A NON RETURN VALVE

A check valve, clack valve, non-return valve or one-way valve is a mechanical device a valve, which normally allows fluid,liquid or gas to flow through it in only one direction.

Check valves are two-port valves, meaning they have two openings in the body, one for fluid to enter and the other for fluid to leave. There are various types of check valves used in a wide variety of applications. Check valves are often part of common household items. Although they are available in a wide range of sizes and costs, check valves generally are very small, simple, and/or inexpensive. Check valves work automatically and most are not controlled by a person or any external control; accordingly, most do not have any valve handle or stem. The bodies (external shells) of most check valves are made of plastic or metal.

An important concept in check valves is the cracking pressure which is the minimum upstream pressure at which the valve will operate. Typically the check valve is designed for and can therefore be specified for a specific cracking pressure.

Types of check valves

Forward through a hole pushes away a ball that was blocking the far side of the hole, allowing flow to occur. The motion of the ball compresses a spring.
Backward flow pushes a ball against a hole, preventing outward flow through the hole. A spring expands to help hold the ball in place.
A ball check valve in the open position to allow forward flow and closed position to block reverse flow

Backwater valve (for sanitary drainage system) protects lower located rooms against flooding caused by return flow of sewage waters. Such risk occurs most often in sanitary drainage systems connected to combined sewerage systems and in rainwater drainage systems. It may be caused by intense rainfall, thaw or flood. Backwater valve prevents rats and other rodents entering the sanitary and rainwater drainage systems and, consequently, the building interiors. It protects also against unpleasant smells in case of longer breaks in system use.

A ball check valve is a check valve in which the closing member, the movable part to block the flow, is a spherical ball. In some ball check valves, the ball is spring loaded to help keep it shut. For those designs without a spring, reverse flow is required to move the ball toward the seat and create a seal. The interior surface of the main seats of ball check valves are more or less conically-tapered to guide the ball into the seat and form a positive seal when stopping reverse flow.

Ball check valves are often very small, simple, and cheap. They are commonly used in liquid or gel minipump dispenser spigots, spray devices, some rubber bulbs for pumping air, etc., manual air pumps and some other pumps, and refillable dispensing syringes. Although the balls are most often made of metal, they can be made of other materials, or in some specialized cases out of artificial ruby. High pressure hplc pumps and similar applications commonly use small inlet and outlet ball check valves with both balls and seats made of artificial ruby, for both hardness and chemical resistance. After prolonged use, such check valves can eventually wear out or the seat can develop a crack, requiring replacement. Therefore, such valves are made to be replaceable, sometimes placed in a small plastic body tightly-fitted inside a metal fitting which can withstand high pressure and which is screwed into the pump head.

There are similar check valves where the disc is not a ball, but some other shape, such as a poppet energized by a spring. Ball check valves should not be confused with ball valves, which is a different type of valve in which a ball acts as a controllable rotor to stop or direct flow.

A diaphragm check valve uses a flexing rubber diaphragm positioned to create a normally-closed valve. Pressure on the upstream side must be greater than the pressure on the downstream side by a certain amount, known as the pressure differential, for the check valve to open allowing flow. Once positive pressure stops, the diaphragm automatically flexes back to its original closed position.

A swing check valve or tilting disc check valve is check valve in which the disc, the movable part to block the flow, swings on a hinge or trunnion, either onto the seat to block reverse flow or off the seat to allow forward flow. The seat opening cross-section may be perpendicular to the centerline between the two ports or at an angle. Although swing check valves can come in various sizes, large check valves are often swing check valves. The flapper valve in a flush toilet mechanism is an example of this type of valve. Tank pressure holding it closed is overcome by manual lift of the flapper. It then remains open until the tank drains and the flapper falls due to gravity. Another variation of this mechanism is the clapper valve, used in applications such firefighting and fire life safety systems. A hinged gate only remains open in the inflowing direction. The clapper valve often also has a spring that keeps the gate shut when there is no forward pressure.

A stop-check valve is a check valve with override control to stop flow regardless of flow direction or pressure. In addition to closing in response to backflow or insufficient forward pressure (normal check-valve behavior), it can also be deliberately shut by an external mechanism, thereby preventing any flow regardless of forward pressure.

A lift-check valve is a check valve in which the disc, sometimes called a lift, can be lifted up off its seat by higher pressure of inlet or upstream fluid to allow flow to the outlet or downstream side. A guide keeps motion of the disc on a vertical line, so the valve can later reseat properly. When the pressure is no longer higher, gravity or higher downstream pressure will cause the disc to lower onto its seat, shutting the valve to stop reverse flow.

A duckbill valve is a check valve in which flow proceeds through a soft tube that protrudes into the downstream side. Back-pressure collapses this tube, cutting off flow.

Multiple check valves can be connected in series. For example, a double check valve is often used as a backflow prevention device to keep potentially contaminated water from siphoning back into municipal water supply lines. There are also double ball check valves in which there are two ball/seat combinations sequentially in the same body to ensure positive leak-tight shutoff when blocking reverse flow; and piston check valves, wafer check valves, and ball-and-cone check valves.


The principle applications for flap valves are for surface water drainage associated with river, estuary and sea
water outfalls to prevent backflow into the sewer and for final effluent outfalls from wastewater treatment
works to prevent flood damage within the works.


Non return valves or backwater valves are used within sewerage and drainage systems to eliminate the risk of
flood damage by the backflow of sewage or flood water into properties through low level entries such as low
level drain gullies, toilets and washing machine outlets
. The Environment Agency in their publication on
damage limitation for properties at risk from flooding and the Government guidance note “Preparing for
Floods” both recommend the installation of non-return valves (anti-flooding devices) on all such drains and

The Building Regulations 2000, Approved Document H, recommends the installation of non-return valves on
drains from basements in low risk areas and on all drains in areas with a high risk of flooding or sewer

These valves are also used in commercial and industrial application where non-pressure flow control is
required (e.g. fish farming)


DN100 to DN600
A range of PVCu non return valves complying with the requirements of BS EN 13564 - ‘Anti-flooding devices for buildings’. Sizes DN100 to DN200 are certified by the German Institute LGA.

These valves are watertight to a pressure of 0.5 bar (5m Head).

These valves have a spigot and socket with integral seal as standard for direct connection to solid walled PVCu pipe but double spigot versions are available to special order for sizes DN400 and above. They can be connected to other pipe material using Mission Rubber Flexible couplings (see separate technical brochure).

DN100 - 200
DN250 - 300
DN400 - 600

Single Flap with Locking Lever

Couplings Chart

Single Flap without Locking Lever

R = Stainless Steel Flap
D = Outside Diameter Spigot





DN250 to DN500
A range of grp non-return valves designed for connection to any pipe material using Mission Rubber Standard Couplings and Bushes.
Resistant to 1bar (10m head) back pressure
Low weight for ease of installation yet with high mechanical strength
Resistant to all normal sewage effluents operating at normal temperatures (below 80 C) and not containing harmful industrial effluents.
Seals at a low level of back pressure
Easily connected to any type of pipe material
Low maintenance

DN300 - 500



A range of grp flap valves designed for ease of installation due to their low weight but with exceptional mechanical strength and durability.


Resistant to 1bar (10m head) back pressure
(Multi-CD 0.8 bar)

Most sizes can be handled by one person due
to low weight

Low weight ensures ease of installation

High mechanical strength

High resistance to corrosion and ultraviolet
Seals at a low level of back pressure
Low operating head due to low weight
Low maintenance


DN200 to 600
A range of flap valves designed for direct connection to any type of drain or sewer pipe using Mission Rubber Standard Couplings and Bushes


Eliminates need for concrete headwall or pipe flange for fixing e.g. can be connected to the spigot of an outfall pipe passing through gabions

No drilling required

Fast and easy installation

Couplings Chart


DN100 to DN200
A range of small diameter flap valves with a PN 10 flange and an integral foamed EPDM rubber gasket for
direct fixing to a flange or concrete headwall.

Lightweight for fast and easy installation
No need for cement grout when fixing to a headwall

Wall mounting kits available



DN250 to 600
A range of medium diameter flap valves with a PN 10 flange. Integral EPFM rubber sealing rings for sealing to concrete headwall.

Lightweight for fast and easy installation.
Resistant to 0.8bar (8m head) backpressure.
No need for cement grout when fixing to headwall.
Hinged design ensures full face sealing when closed.



DN200 to 1500

A range of medium diameter flap valves with an inclined flap and PN 10 flange. Integral EPFM rubber sealing rings for sealing to concrete headwall.

Lightweight for fast and easy installation.
Inclined flap for additional security against backflow.
No need for cement grout when fixing to headwall.
Hinged design ensures flap closure when no discharge.






Norham Technical Data


Flanges drilled to BS EN 1092 PN10 pitch circle diameter as standard except those marked in red.

Standard bolt holes in the flanges are not drilled to BS EN 1092 diameters (see table) but can be supplied to special order.

Flap valves should be fixed so as to avoid the accumulation of debris around the invert which could prevent the valve operating correctly.

Turbulaence of flow due to severe wave action will shorten the working life of a flap valve. Therefore it is recommended that careful consideration is given to the positioning of valves and they are installed within an outfall bay or otherwise protected from direct wave action by baffles.