Plastic Check Valves: Design Types and Applications
The correct design type of a plastic check valve determines whether a pipeline is reliably secured against backflow – and whether the system remains protected against pressure surges, back-siphoning or the dry running of a pump. Check valves (also known as non-return valves) allow the medium to pass in one direction only and automatically block the reverse direction, without external energy. In the plastic version they are corrosion-free and media-resistant, making them an integral part of chemical process engineering, water treatment and plant construction. This guide compares the common design types of plastic check valves – ball, cone/spring, swing disc and diaphragm – explains materials, installation position and sizing parameters such as cracking pressure and Kv value, and shows the typical mistakes that in practice lead to backflow, leakage or pressure surge.
Operating principle: how a check valve prevents backflow
A check valve is a self-actuating valve with a moving closure member that is operated exclusively by the medium pressure and – depending on the design type – by a spring or gravity. When the medium flows in the passage direction, the flow pressure lifts the closure member off the seat and opens the cross-section. If the inlet pressure drops or the flow reverses, the back pressure forces the closure member back onto the sealing seat and interrupts the path. There is thus no manual or automatic control – the valve "switches" solely on the basis of the pressure conditions.
The central task is therefore to protect the system: check valves prevent the backflow of the medium, protect pumps against reverse flow and dry running, keep riser pipes filled and protect upstream system components against unwanted contamination. The essential difference between the design types lies in the type of closure member and its guidance – and it is precisely from this that the installation position, cracking pressure, pressure loss and tendency towards pressure surge result.
Plastic check valve: the most important design types at a glance
For a plastic check valve, four design types are particularly widespread in process and water technology. They differ in closure member, return principle and permissible installation position.
Ball check valve
In the ball check valve, a freely moving ball seals against a conical seat. In the passage direction, the ball is lifted by the medium flow and forced out of the cross-section; when the flow reverses or the inlet pressure drops, it falls back onto the seat – through gravity and back pressure. Since the classic version works without a spring and relies on gravity, the vertical installation position with flow from bottom to top is the standard case. The advantages are the simple, robust construction, the low susceptibility to fouling and the good suitability for slightly contaminated or viscous media. The disadvantages are the limited scope for installation position and – with a larger ball mass – the tendency towards noise and pressure surges in rapidly fluctuating flows.
Cone or spring check valve (nozzle check valve)
In the cone or spring check valve, an axially guided cone (or disc) is pressed against the seat by a spring. Only when the inlet pressure exceeds the spring force does the valve open; when the pressure reverses or falls below the cracking pressure, the spring closes the valve immediately and independently of position. The great advantage of this design type is the position-independent installation – horizontal as well as vertical – along with the fast, defined closing, which significantly reduces pressure surges (water hammer). The spring-loaded cone requires a somewhat higher cracking pressure and generates a somewhat higher pressure loss than a freely moving ball. Flow-optimised variants with a guided cone are also referred to as nozzle check valves and are characterised by particularly low-noise, surge-free closing.
Swing check valve (swing disc)
The swing check valve has a disc mounted so that it pivots on one side, which is swung open by the medium flow and, when the flow reverses, is swung back onto the seat by gravity and back pressure. When open, discs offer a large, flow-favourable cross-section and thus a low pressure loss – an advantage with large nominal sizes and high volume flows. A disadvantage is the longer closing travel of the disc: in the event of a sudden loss of flow, the disc can close with a delay and thereby promote pressure surges. The permissible installation position must be observed depending on the design. In plastics engineering, the swing check valve is used above all in pipework systems with larger diameters.
Diaphragm check valve
In the diaphragm check valve, a flexible elastomer diaphragm (e.g. as a lip or umbrella diaphragm) performs both the sealing function and the return. It opens under flow and, when the flow reverses, automatically seals itself shut. This design type has few moving parts, works quietly, closes softly and is well suited to pulsating flows, small nominal sizes as well as dosing and ultra-pure media applications. The diaphragm is a wetted wearing part and must – as with the diaphragm valve – be matched to the medium, temperature and switching frequency.
Design types in direct comparison
The following table summarises the practically relevant differences between the design types. The specific sizing always depends on the medium, operating pressure, temperature, nominal size and installation situation.
| Criterion | Ball | Cone / spring | Swing disc | Diaphragm |
|---|---|---|---|---|
| Closure member | freely moving ball | spring-loaded cone/disc | pivoting disc | flexible diaphragm |
| Return | gravity + back pressure | spring force | gravity + back pressure | elasticity of the diaphragm |
| Installation position | mostly vertical (upward flow) | position-independent | depends on design | depends on design |
| Cracking pressure | low | higher (spring preload) | low | low to medium |
| Pressure loss | low to medium | medium | low (full cross-section) | medium |
| Pressure-surge tendency | medium | low (fast closing) | higher (delayed closing) | low (soft closing) |
| Fouling tolerance | good | medium | good | low to medium |
| Typical application | riser pipe, contaminated media | general, pump protection, any position | large nominal sizes, high volume flows | dosing/ultra-pure media, pulsating flows |
Materials and seals for plastic check valves
As with all plastic valves and fittings, the choice of material determines the resistance to media, temperature and pressure – and thus the service life. The body material and the seal material must be considered separately and always checked against a resistance table for the specific medium in combination with concentration and temperature.
Body materials (thermoplastics)
- PVC-U (rigid PVC): economical standard material for cold water and many acids, alkalis and salt solutions; easy to solvent-cement, operating temperature usually up to around 60 °C.
- PVC-C (post-chlorinated PVC): similar chemical resistance to PVC-U, but higher temperature resistance (often up to around 80 °C).
- PP (polypropylene): good resistance to alkalis and many organic media, weldable, widespread in the chemical and environmental technology sectors.
- PVDF (polyvinylidene fluoride): high resistance to aggressive acids, halogens and solvents, high degree of purity, usable into higher temperature ranges – the first choice for demanding chemical and ultra-pure media applications.
- ABS: tough and impact-resistant, widespread in cooling and cold-water applications.
Seal and spring materials
For seat seals, O-rings and diaphragms, elastomers such as EPDM (many aqueous and alkaline media), FPM/FKM (oils and certain chemicals) or PTFE (broad chemical resistance, aggressive and high-purity media) are used depending on the medium. In spring-loaded versions, the spring material must additionally be made media-resistant – common are springs made of corrosion-resistant stainless steel (e.g. Hastelloy for particularly aggressive media) or, where a metal-free solution is required, of a suitable plastic. In plastic valves and fittings, the seal is the most common cause of failure – the manufacturer’s media resistance list is therefore indispensable.
Sizing: cracking pressure, Kv value, nominal size and pressure rating
For a check valve to open reliably, close securely and not unnecessarily throttle the system, several sizing parameters must be coordinated with one another.
Cracking and closing pressure
The cracking pressure is the differential pressure at which the valve opens. In spring-loaded design types it is set via the spring preload; it must be safely exceeded by the available operating pressure, otherwise the valve does not open fully. A cracking pressure set too high needlessly costs delivery pressure, while one set too low promotes fluttering and backflow. For clean, surge-free closing, a defined, sufficiently high closing pressure is advantageous – here spring-loaded cone valves have the edge.
Kv value, nominal size and pressure loss
The Kv value describes the volume flow in m³/h of water that passes through the valve at 1 bar pressure loss, and serves to match the valve to the operating point. Principle: it is not the pipe nominal size (DN) alone that determines the valve, but the required flow at an acceptable pressure loss. An oversized check valve is operated in partial opening – the closure member flutters, wears and closes imperfectly. An undersized valve generates a high pressure loss. The aim is a nominal size at which the valve reliably opens fully during operation.
Pressure rating and pressure-temperature behaviour
Plastics lose strength as the temperature rises. The nominal pressure rating (e.g. PN 10 or PN 16) applies to a reference temperature, usually 20 °C for water. With warmer media, the permissible operating pressure must be reduced (derating) via the manufacturer’s pressure-temperature diagram. This relationship is often overlooked in practice. The authoritative product standard for non-return valves made of thermoplastic materials is DIN EN ISO 16137 (Industrial valves – Check valves made of thermoplastics), which governs design types, connection dimensions, pressure-temperature assignment and testing. For the connection, the system standards of the respective pipework are also relevant, such as the DIN EN ISO 1452 (PVC-U) and DIN EN ISO 15493 (plastic piping systems for industrial applications) series.
Selection criteria for practice
For the systematic selection of a plastic check valve, it is advisable to check the following points step by step:
- Medium and resistance: the type, concentration, temperature and degree of contamination determine the body, seal and spring material. Always check resistance for the specific operating condition; solids-laden media call for fouling-tolerant design types (ball, swing disc).
- Installation position: this narrows down the design type – pure ball valves usually need the vertical position with upward flow, while spring-loaded cone valves are position-independent. If the position is predetermined, the design type follows from it, not the other way around.
- Cracking pressure and operating point: choose the cracking pressure and Kv value so that the valve reliably opens fully in operation and does not flutter.
- Pressure-surge and noise behaviour: in fast-closing systems (e.g. pump failure), spring-loaded cone and diaphragm valves close with less surge than heavy balls or sluggish swing discs.
- Nominal size, pressure rating, temperature: size the nominal size via the operating point, and choose the pressure rating taking the pressure-temperature derating into account.
- Connection and maintenance: adapt the connection type (solvent-cement, welded, flanged or union connection) to the pipework system, and with wearing parts ensure accessibility for replacement.
Special case: foot valve
A foot valve is a check valve at the lower end of a suction line, usually with an integrated strainer basket (dirt trap). It keeps the suction line and the pump filled when at a standstill, prevents draining and thus makes it easier to restart the pump without re-priming. For aggressive or saline media, foot valves in a plastic version (PVC-U, PP) are widespread. Functionally, the foot valve corresponds to a check valve with an upstream strainer – the design-type, material and sizing criteria mentioned above apply accordingly.
Typical mistakes in selection and operation
- Wrong installation position: a gravity-dependent ball valve installed horizontally does not close reliably. Installation position and design type must match – when in doubt, choose a position-independent spring valve.
- Oversized valve: too large a nominal size leads to partial opening, fluttering and premature wear. Size via the Kv value and operating point, not just via the pipe nominal size.
- Pressure surge underestimated: sluggishly closing design types (heavy ball, large swing disc) can trigger pressure surges in the event of a sudden loss of flow. For pump protection, prefer fast, soft-closing valves.
- Temperature derating ignored: the nominal pressure rating applies to a reference temperature. With warm media, the permissible pressure drops significantly – without derating there is a risk of overloading.
- Incompatible seal or spring: elastomers unsuitable for the medium swell or harden; non-resistant springs corrode. The media resistance list must be observed, and the spring material must be checked as well.
- Contamination not taken into account: solids can damage the seat or prevent closing. With loaded media, provide fouling-tolerant design types and, if necessary, a dirt trap.
Applications in process and water technology
Plastic check valves provide protection wherever backflow must be avoided: as pump protection against reverse flow and dry running, in riser and delivery lines to keep them filled, in dosing and chemical technology to protect upstream system components, as well as in water treatment and in cooling and circulation circuits. Here the design type follows the application – the ball valve in vertical riser pipes, the cone or nozzle valve as position-independent pump protection, the swing disc for large nominal sizes and the diaphragm valve in dosing and ultra-pure media applications.
Conclusion
With plastic check valves, the choice of the correct design type is not a minor matter but determines operational reliability, pressure loss and protection against pressure surges. The ball valve is the robust solution for vertical, contaminated lines, the spring-loaded cone or nozzle valve the position-independent, low-surge all-rounder, the swing disc the low-pressure-loss choice for large nominal sizes and the diaphragm valve the quiet specialist for dosing and ultra-pure media. Decisive for reliability in all cases are the media- and temperature-appropriate choice of material for body, seal and spring, the design type suited to the installation position, and the correct sizing of cracking pressure, Kv value and pressure rating in accordance with DIN EN ISO 16137. Anyone who systematically checks these criteria obtains a low-maintenance, corrosion-free valve that permanently and reliably controls backflow.
Frequently Asked Questions
What design types of plastic check valves are there?
Four design types are widespread in process and water technology: the ball check valve (freely moving ball, mostly vertical installation), the cone or spring check valve (spring-loaded cone, position-independent), the swing check valve (pivoting disc, low pressure loss with large nominal sizes) and the diaphragm check valve (flexible diaphragm, quiet for dosing and ultra-pure media). The design type determines the installation position, cracking pressure, pressure loss and pressure-surge behaviour.
What installation position does a plastic check valve need?
That depends on the design type. Classic ball check valves work in a gravity-dependent manner and are usually installed vertically with flow from bottom to top. Spring-loaded cone or nozzle check valves close via the spring force and are therefore position-independent, i.e. usable both horizontally and vertically. If the installation position is predetermined by the layout, the design type should be selected accordingly.
Which plastic is suitable for a check valve?
The choice depends on the medium and temperature. PVC-U is the standard for cold water and many chemicals, PVC-C and PP offer higher temperature resistance, ABS is tough and impact-resistant for cold water, and PVDF is the first choice for aggressive acids, halogens, solvents and ultra-pure media. Seals are made of EPDM, FPM/FKM or PTFE depending on the medium. In spring-loaded valves, the spring material must additionally be media-resistant.
Which standard applies to plastic check valves?
For non-return valves made of thermoplastic materials, DIN EN ISO 16137 (Industrial valves – Check valves made of thermoplastics) is the authoritative product standard. It governs design types, connection dimensions, pressure-temperature assignment and testing. For the connection to the pipework system, the system standards are additionally relevant, such as DIN EN ISO 1452 for PVC-U and DIN EN ISO 15493 for industrial plastic piping systems.
How do you prevent pressure surges in check valves?
Pressure surges (water hammer) arise when a check valve closes too late during a sudden flow reversal and the returning liquid hits the closed seat hard. Fast, soft closing reduces the load: spring-loaded cone or nozzle check valves and diaphragm valves close in a defined manner and with less surge than heavy balls or sluggish swing discs. In addition, correct nominal-size sizing helps (no fluttering in partial opening) and – on the system side – measures against abrupt switching-off of the pump.
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