The Difference Between Check Valve and One Way Valve

It is one of the questions that seem as if one is being tricked until one has really gone through the wrong experience. A check valve and a one-way valve are similar in their function of preventing the backflow. This has caused many catalogues to refer to them interchangeably due to their similar capabilities. That should not be the case at all times as sometimes there may be cases when the term should be used differently. Here is the honest explanation of the differences and similarities between check valves and one-way valves along with advice on their usage in the respective situations.

Quick answer
Are they actually the same thing?
How each one works
Types you will see in the field
Materials, sizes and standards
What they cost
How to choose between them
FAQ
References
Conclusion

Quick answer

Short Reply: A check valve is a valve that works passively allowing flow in one direction while preventing it in the opposite one as a result of pressure difference. A one-way valve does the same function but covers more types of devices including spring-actuated and pilot-controlled types that govern flow direction as well as maintaining the operation pressure by means of control. In plumbing and sewage works, both names can be used as synonyms. However, in such areas as hydraulics, pneumatics, and process control, the terms cannot be used interchangeably and replacing them with each other might lead to the situation where the valve would fail to open at the required pressure.

Are they actually the same thing?

Often they are the same thing but for one critical distinction.

They are both classified as non-return valves. They both allow flow in one direction only. They both are directional, which means that direction is very critical in their installation. When buying a 316 stainless steel non-return valve for a sanitary application both terms can be used interchangeably and every retailer will understand what is needed.

One distinction exists with one-way valves that are controlled and not simply passive. A pilot-operated check valve in a hydraulic system is a one-way valve, but it is not a plumbing check valve because it can keep a load stationary up to the pressure signal that releases it rather than merely responding to a flow. It's worth mentioning that this kind of valve is used in cylinders and clamping mechanisms where the whole point is to keep the position fixed while under load.

So here is a simple rule — non-return valves are always one-way valves, but not all one-way valves are non-return valves.

How each one works

There is a common principle at which these two designs operate. It is called "passive working." The operating principle is simple. A disc or some other closure is placed on the seat. When there is the movement of fluid in one direction, the closure is lifted from its seat and the flow proceeds. When the movement turns back, the pressure drops, and the closure drops back and seals off the valve. There is neither a driving mechanism nor electricity, no control signals.

However, the differences lie in the characteristics of the closing mechanism and closure functioning in transient modes.

Closures activated by gravity rely only on the disc weight and pressure from behind. They have to be positioned correctly in order to function properly. Closing can be slow, causing water hammer in pipelines.
Spring-operated closures involve a spring mechanism that immediately acts upon the closure as soon as the forward motion diminishes. Closing is faster, the design can operate at any angle and with lower back pressure. The downside is that the spring tension has to be low enough to allow the entire system to generate the required pressure.
As for pilot-operated closures, they receive hydraulic pressure and a signal that allows for the closure to operate.

Spring selection is where most of the mistakes happen. A spring-loaded valve with 5 psi opening pressure will not function in a gravity-fed pipeline, delivering only 2 psi. This is a design fault, not a valve fault.More detail on the mechanics and where each closure style is used is covered in this guide to how sanitary check valves work, which also explains why hygiene requirements change the internal geometry.

Types you will see in the field

The majority of designs are represented by six models.

Swing check valve is represented by a disc which is rotated. It provides the smallest pressure drop among the available designs being suitable for big-diameter valves. The design must be installed horizontally or at a very small angle to the horizontal line and may hit at high water flow speed.
Ball check valve is a ball mounted on a hole. The model is small, inexpensive, and works excellently with low-viscosity liquid. However, this design will perform badly with solids because of the inability of the ball to clear the seat.
Lift check valve represents a piston or a disc which moves vertically. The design is effective when high pressure is employed which enables tight sealing. The design is required to be used with the flow directed up.
Dual plate or wafer check valve is represented by two half discs combined into a small wafer. This valve has minimum length between the faces, and low opening pressure, which makes this development popular in process piping when saving space is required.
Diaphragm check valve is a flexible diaphragm which opens and closes by means of its flexing. The design has no moving parts so it is the best model for clean and sterile applications.
Tilting disc and axial designs are designed to minimize slam and pressure surge in the large pipes.

There is a fuller breakdown by construction and by application in this catalogue of the types of one-way valves, which is worth reading if you are trying to match a design to a specific fluid rather than to a general duty.

Materials, sizes and standards

Selecting materials depends on the chosen medium, temperature, and hygiene practices.

Brass and bronze — preferred metal for water supply and plumbing works. Machinable, present good corrosion resistance in water, cheap, not resistant to strong acids and high temperatures.
Cast iron is a cheap option for large water supply and sewage pipelines. Heavy, corrodable if not lined properly.
Carbon steel, ASTM A216 WCB — a common material for transporting oil, gas, and high-temperature steam.
304 stainless steel — provides good corrosion resistance in water and food, less sensitive to mild chemicals.
316 and 316L steels — materials used in applications that deal with sanitary and aggressive media. Molybdenum improves the resistance to chlorides and acids significantly; 316L is also a low-carbon type suitable for welding.
Duplex stainless steels and PTFE coated materials are used for seawater, brine, and very aggressive media.

For hygienic applications it is just the part of specifications. It is also very important to ensure the ability to drain valves completely, the absence of dead areas where media can meet contamination, and the usage of seals that meet food contact requirements, such as EPDM, silicone, or PTFE.The practical differences between the two common grades, and when 304 is genuinely good enough, are set out in this 304 versus 316 stainless steel selection guide.

Regarding the connections, the accepted forms are threaded (NPT or BSP), flanged to ANSI, DIN, and JIS, wafer and lug versions, butt weld, and tri-clamp for sanitary application. The pressure ratings range from PN6 for low-pressure water to Class 2500 for heavy-duty application; the sizes range from DN15 to DN1500 and greater. The standard documents to reference are API 594 for check valves in general, API 598 for testing, and 3-A or EHEDG for hygienic construction. If a sanitary valve does not cite one of those two standards, inquire why.

What they cost

In terms of cost, the material and certification used in making valves matter far more than their size, to the surprise of many. Here is some information:

A brass check valve with a size of 1/2 inch costs between $5 to $15.
A stainless check valve with a size of between 1/2 inches and 1 inch costs between $15 to $60.
A tri-clamp sanitary check valve of sizes ranging between 1 to 2 inch and made of 316L steel costs around $60 to $250 depending on whether it carries a 3-A or EHEDG certification.
A flanged check valve of DN100 size or above costs several hundred dollars or more owing to pressure class.

Transitioning from normal stainless valves to hygienic versions is quite significant — the price doubles or triples for the equivalent size in terms of nominal size. This is still an investment worth making for food and pharmaceutical companies since the valves that are not disinfected offshore can lead to significantly more losses in terms of production than the initial costs. Current figures for the hygienic range are set out on our sanitary check valve price guide.

How to choose between them

Answer these five questions one by one, and the solution will often be apparent.

Is it just a matter of stopping something from going back? In this case, you’ll need a check valve. Don’t spend money on a control valve when a check valve is all that is required since it only complicates your task.
Does the application require holding something - a load, pressure, or position? If yes, then a one-way valve with positive seating is essential, possibly with a pilot. A simple swing check cannot be used in a loading application.
What is the pressure available for opening? If this pressure is low, select a low cracking pressure spring or consider either gravity or dual-plate designs. This question is responsible for eliminating the bulk of wrong options.
How clean should the installation be? When the flow is related to food, drink, or pharma industries, hygienic design, 316L material, and 3-A or EHEDG certification will be required.
Will the flow reversal happen suddenly? If so, avoid unsprung swing checks in long pipelines. Instead, choose spring-assisted tilting disc or axial design or do some other things to protect the operation against surges.

Two ways in which a check valve may fail can be designed out. Dirty deposits in the seat is the number one reason for a check valve to stick open, and deploying strainers and filters upstream will pay off. And incorrect position of the valve accounts for almost the rest of the problems. The practical selection criteria for hygienic applications are covered in more depth in this guide to choosing the best sanitary check valve for your application.

FAQ

Is a check valve the same as a one-way valve?

In plumbing, sanitary, and general water uses, these two terms are considered synonymous for a passive non-return product. In hydraulics and process control, they aren’t the same because “one-way valve” has a wider meaning, including spring- and pilot-controlled devices that either maintain or release pressure.

Can a check valve be installed vertically?

It is up to the design. Spring-assisted, dual-plate and diaphragm check valves can be mounted in any orientation. Gravity-dependent designs-swing and lift check valves- require the flow path arranged, so closure falls back onto the seat-way, horizontal for a swing valve and vertical for a lift valve.

Which is better, a ball or a swing check valve?

There is no clear winner in this scenario. In terms of size, cost efficiency and sealing capabilities in clean and low viscosity fluids, the ball check valve will be more effective than the swing check valve. However, if you are dealing with flow rates on the larger end of the scale, the swing variant has substantially lower pressure drop. The downside is that the swing check valve can easily slam shut during quick reversal. So you need to consider the diameter of the line, the nature of the medium and how fast the flow can reverse to make your choice.

What is cracking pressure on a check valve?

It serves as the thrust providing sufficient force to initiate the valve opening process. Gravity-operated or dual-plate valves may require just a small amount to get opened, while a spring-operated valve may require up to 1-5 psi. If your system does not provide sufficient pressure to overpass the cracking pressure, then the valve will remain closed and the pipe will not allow the liquid flow.

Do I need a 316L check valve or is 304 enough?

304 is ideal for handling water, many foods, and not very aggressive chemicals. For halides of any type, marine environments, or heavily washed setups, as well as for acidic materials or when welding is involved, 316 or 316L should be used — 316L is less susceptible to sensitisation at welds. In pharmaceutical and dairy applications, 316L is the material of choice.

References

Conclusion

In most situations you will encounter, a check valve and a one way valve refer to the same operation: the medium passes through only in one direction. The difference is noted only when the valve has to perform some function, like keeping a load, opening against a spring, or unloading on a pilot signal — this is the point when you stop thinking in terms of alternative names and start considering completely different functions.

Pick the simplest device that will do the job and check three things before placing an order: the cracking pressure in the context of your existing pressure; the material in the context of the medium; the design in the context of the orientation and surge conditions in the pipeline. If all three are done, the check valve will sit there doing nothing for many years, which is what you want.