Often overlooked and misunderstood, check valves play a vital role in water management systems and are a staple in practically every industrial application.
Huade Hydraulic are exported all over the world and different industries with quality first. Our belief is to provide our customers with more and better high value-added products. Let's create a better future together.
A check valve is a component that can be fitted on to the end of a pipe or channel and acts as a one-way/non-return valve that opens under pressure. While water can travel out of the check valve, it cannot travel back through the check valve.
Check valves can come in a range of sizes, designs and materials, all of which can have an effect on the efficiency of the component.
While we will mainly look at check valves from the perspective of their application in various water uses, check valves are also used to process a variety of different fluids, materials and gases.
Check valves perform an essential function in preventing reverse flow – where potentially polluted water is allowed to travel back into a system and contaminate clean water resources.
Check valves are a common sight in industrial systems and are used to remove wastewater and other contaminated fluids. Check valves feature a single inlet and outlet where water can pass through it but not back up it, and it is operated by a pressure differential, according to Proco Products.
A pressure differential, in the context of check valves, is when the pressure of the water inside the valve is greater than the pressure keeping the valve closed. When the pressure on the inlet side is greater, the valve opens automatically, allowing the water to pass through it - this is called cracking pressure.
Cracking pressure is a specified minimum operational pressure that determines when the valve will open and allow water to flow.
In a water system, for example, multiple check valves will be placed in a series to prevent any backflow of contaminated water that could compromise clean water supplies. This also helps to reduce the risk of any reverse flow that could result in water hammer.
The phenomenon known as water hammer, also known as valve slam, is when flow reverses before the valve is completely closed. Once the valve has closed, the sudden change in flow direction and the velocity results in water hammer.
Water hammer proceeds when water in motion is suddenly forced to stop. When the flow of water at the leading edge comes to a stop, but the water behind it continues to move - it starts to compress.
The build-up of the kinetic energy of the water converts to a high amount of pressure energy, creating a hydraulic shock wave that travels at almost the speed of sound, just less than 343 meters per second, through the pipeline.
When repeated extreme pressure surges in connected piping, valves and pumps, it can cause catastrophic damage and ruptures to a water system and its intricate pipework.
Should water hammer occur, it can completely destroy a system that could cost tens of thousands of dollars to repair.
In the event that water hammer occurs, but a water system continues to operate successfully, the repeated impact of water hammer can lead to systems and components becoming fatigued and could seriously compromise the integrity of a system.
Check valves have many purposes and roles in a lot of applications – from your home to large industrial wastewater management models.
This means there is a range of different check valves specially designed to suit the requirements of a specific project.
Swing/flap check valves
Swing/flap check valves or PVC check valves are the simplest design of check valves and are used in small homes to large scale industrial plants.
A metal disc pivots on a hinge or trunnion to prevent reverse flow. Most larger-scale check valves use this design due to their simplicity. With a life span of five to seven years, these valves are often a component that needs to be replaced frequently.
Despite being a popular check valve, due to its cost and simplicity, steel flap and swing check valves can rust, stick and over time become prone to jamming with debris that requires manual cleaning.
When these valves fail, they allow backflow and water hammer to occur. Abrasive sludges and corrosive slurries can create wear and will damage the valves rendering them inoperable.
Ball and spring check valves
A ball and spring check valves are known for their durability and how easy they are to install. They operate when the inlet pressure is greater than the cracking pressure, pushing a ball or disc back and overcoming the strength of the spring.
When the inlet pressure drops below the cracking pressure or backflow occurs, the spring forces the valve to close – stopping the water escaping or travelling back up the pipe.
While ball and spring check valves are known for their reliability, the main drawback to this type of check valve occurs when it needs to be inspected for maintenance. In order to check the valve, it has to been removed completely from the system.
The ball and spring design are used for air check valves. An air check valve has one main function – to allow air flow in one direction whilst preventing air flow from the other side. Air check valves are fitted to air compressors so that it can keep certain part pressurized and others allow other parts to become de-pressurized.
Duck bill check valves
The duck bill check valve came into use in the s and was brought in to replace swing/flap valves. Over the decades it has become increasingly popular in industrial applications.
Working similarly to its namesake, a duck bill check valve open like duck’s bill to allow the water or matter to pass through it.
Duck bill check valves rely on a flexible rubber diaphragm which creates a valve that will remain closed unless positive pressure is applied.
Unlike swing/flap check valves, rubber duck bill check valves are increasingly less vulnerable to rusting, they cannot seize or bind and are considered to be one of the most reliable and long-lasting check valves in the market today – with Proco Products valves lasting up to 35 – 50 years.
Duck bill check valves also do not suffer from mechanical wear, and the rubber design allows for greater elastomer innovations and flexibility.
For projects that require a valve to be submerged on the seafloor or underwater, duck bill check valves offer the advantage of being barnacle and algae resistant – an important feature for saltwater projects such as the Marine Wastewater Outfall Diffusion project in .
Another benefit of a duck bill check valve is its ability to cope with water hammer. Cal Hayes, general manager at Proco Products, said the new series 750 duck bill check valve is an example “because it is made out of rubber, instead of creating a huge valve slam back [water hammer], our rubber is inherent to flex so that when pressure is applied to the outside, it is allowed to flex".
This is another form of a surge relief valve that can protect your piping system from the build-up of pressure surges – manufactures such as Emerson Process Management and Daniel Measurement and Control Inc offer valves of this design.
Other designs
These are the three common check valves types, but there are other designs such as the dual plate types (butterfly check valve) and the ball type and plug type check valve.
Each of these has a unique way in which they operate; the butterfly check valve has two folding disks that move towards a centreline with the forward flow and close with reverse flow.
A ball type and plug type check valve work on the principle of gravity – when there is enough pressure in the flow the ball is lifted, then it drops the ball roll back to close the opening.
An in-line component can refer to any useful object in a circuit that is attached to an adjacent fluid conduit – a check valve placed in a line. They are used to either enhance or upgrade an existing system.
Whereas a check valve’s main focus is to prevent the backflow of water into a system, an in-line check valve is used to prevent the backwards transmission of pressure.
For example, a check valve is generally installed after a pump in a series that makes use of an accumulator, a cold-water tank that facilitates water flow. The in-line check valve prevents stored energy from making its way back into the pump.
Excess energy that is stored in an accumulator can be damaging to a system, and it must be controlled for hydraulic systems to be safe and reliable.
Check valves are typically installed at the discharge point of a piping system to make sure that the fluid or matter that is being pumped out does not flow back into the system. There are a number of different check valves on the market and choosing the right one can be a difficult task.
Here are five things you should consider before choosing the right check valve:
Application
It sounds simple, but properly understanding what application your check valve needs to serve is critical in selecting the correct check valve.
Is it for a water sewage treatment plant? Is it handling corrosive materials? Is the discharge point below water? These are obvious conditions that can easily be overlooked when choosing a check valve.
A usual mistake that is made when selecting a check valve is choosing a valve that fits the system but cannot efficiently handle the flow conditions or the matter passing through the valve.
Water is one of the frequent matters that check valves handle and requires little special treatment. Most, if not all check valves, can handle water efficiently enough - depending on the flow rate.
But what if your check valve is handling corrosive chemicals? While a duck bill check valve is more than suitable to deal with water, can the rubber handle the corrosive chemicals, or will they destroy the check valve?
What if you need a check valve for a water sewage plant, a simple swing/flap check valve could do the task, but what will happen when sludge and debris clog up the metal hinge and stop it from being able to close?
Flow rate
Just as important as knowing what matter your check valve will be handling – it is important to know the rate of flow. Typically, this is recorded in terms of gallons per minute (GPM), gallons per hour (GPH), or litres per second (L/S).
If your application is producing a rate of flow greater than 8 feet per second flowing through the valve, there is an increased chance your valve will deteriorate quicker – regardless of whether you are using a rubber-seated valve or metal-seated valve.
Inlet pressure
Understanding your inlet pressure will help you select which style of check valve best suited for your needs. It can help you decide whether you need an in-line check valve, a slip-on check valve or a flanged style check valve.
Deciding on which one you need to use will depend on where the check valve will be located in your pumping system.
For more hydraulic check valvesinformation, please contact us. We will provide professional answers.
Back pressure
Back pressure is the differential pressure between the inlet and outlet pressures of your system. You need to know if your check valve can withstand the back pressure that it could be exposed to.
For example, say you have a valve installed at the end of a pipe, and it is pumping water in the Thames river. When the Thames water level rises, how deep with the valve be submerged? When the water rises above the valve, that is the point at which back pressure is created. Can your valve cope with that amount of back pressure?
Location
Is your valve discharge point above or below ground? Is it submerged? Is it in a hot sandy country, or on a cold desolate one?
Again, these appear to be obvious conditions that we could never forget – but properly understanding the location of the valve will have a massive impact on the check valve you need to select.
Sand can corrode a metal valve; the sun can deform a rubber duck bill check valve (always ensure you use the proper elastomer to prevent this) and ice can freeze a swing/flap check valve open.
Check valves provide one-way flow, and it is vital to install them in the correct alignment.
Below are two check valve symbols: the top one, flow is allowed from the bottom of the pipe but not the other way, the bottom, the valve will only allow flow when the pressure at the bottom is greater than the pressure at the top (taking into account the rating on the spring).
It could be hard to see the relationship between Formula One and check valves, but there is a connection: elastomers, or rubber. If you want to win Formula one you need to understand tyres, specifically the rubber they use.
Cal Hayes believes he is more in the business of designing new compatible rubbers for different types of chemicals in check valves than designing new check valves.
“There are so many different types of elastomers that can do so many different things. If you look at NASCAR, of if you look at F1, there are people that get paid millions of dollar per year to come up with a better tyre that lasts longer - it’s the same with the check valve.”
The material that is used in the construction of a check valve can determine how long it will last for and what matters it can deal with. In the case of duck bill check valves, the elastomer can have a major impact on the quality, efficiency and life span of the valve.
Check valves are typically installed into the discharge piping of a pumping system to ensure that the material being pumped does not flow backward. With so many check valves available to the market, it can be a challenge to find the right one for your application. Before you start shopping for check valves, make sure you know the answers to these five questions.
What service will the valve be operating in? i.e What is the application? It is critical to know what you want your valve to do so that you can select the correct valve for your specific application. It is quite common to have the wrong check valve installed in a piping system, which typically happens when check valves are selected based on the size of the pipe instead of the flow conditions.
What is the flow rate? It is very important to know the rate of flow in terms of gallons per minute (GPM), gallons per hour (GPH), or liters per second (L/S). If your application has a flow rate of higher than 8 feet per second flowing through the valve, you will experience a higher rate of wear and tear. When flow is higher than 8 feet per second, abrasion will cause deterioration to any valve, regardless of whether it is a rubber-seated valve or a metal-seated valve. The faster the flow, the more wear you will see on your check valve, resulting in decreased life. Knowing your flow rate can help you pick a check valve best suited for your specific application.
What is the inlet pressure? Knowing the inlet pressure will help determine which style of check valve you might need. Do you need a “Slip On” style check valve? Do you need a “flanged” style check valve? Or do you need an “inline” style check valve? A slip on style check valve typically is installed at the end of a discharge pipe. An inline style check valve is typically installed in the middle of a piping system. You can also have a flanged in-line check valve or a flanged end-of-line check valve. Deciding which to use really depends on where the check valve will be located in your pumping system.
What is the back pressure? Back pressure is the differential pressure between the inlet and outlet pressures. For example, imagine a valve is installed at the end of a pipe which is draining fluid into a creek. When that creek rises, how high will the water rise about that valve? When the water rises above that valve, back pressure is created. So with your specific application, you should know how much back pressure the check valve will need to withstand.
Will the valve be installed in a submerged condition? If the valve is installed in a submerged condition, this means that there will always be back pressure on the valve. If that is the case, you want to chose a check valve designed a submerged conditions and manufacture from the correct materials to accommodate the constant back pressures that result from submerged conditions.
It is extremely common to have the wrong check valve put into a piping system. If you are having frequent problems with your check valves, it could be a result of improper selection. Before shopping for a check valve, review your maintenance history, find the answers to the question above, and then consult with the check valve specialists at Proco Products, Inc. Check out the Guides in the Proco Tool Box to learn more.
“Check valves are worthless.”
Every engineer I know has heard that. Many believe it. Quite a few have said it. Yet, check valves are still made and installed. How worthless could they be?
Check valves are designed to allow flow in only one direction. Whether swing check, ball check, or some other design, check valves have one way in, one way out, and an internal mechanism that opens to flow when it is in the right direction and seals against flow when it is in the other direction. Some equipment, such as double diaphragm pumps, use check valves as an essential element of proper operation. Other equipment, such as back flow preventers, use check valves to prevent unsafe operation that normally does not occur under proper operating conditions.
Invariably, it’s the check valves used for safety that get hung with the “worthless” label. Many argue that when it comes to safety, there should be no credit for check valves.
If check valves are of no value, then there is no point in installing them. On the other hand, if they serve a purpose, then there must be some credit to take during a Process Hazard Analysis (PHA) or Layer of Protection Analysis (LOPA).
Check valves are not absolute protections against reverse flow. They prevent bulk reverse flow but will often allow small amounts to flow by. There are many different designs for check valves, but all rely on mechanisms that open to allow flow in one direction and close to prevent flow in the reverse direction. These mechanisms all have the potential to stick open or to be held open by debris. Many of these mechanisms have the potential to corrode or fall out. Given these concerns, it is easy to see how a black-and-white view of safeguards would lead to the conclusions that no credit should be taken for check valves.
Modern risk assessment does not take a black-and-white view of hazards or safeguards but recognizes that everything fails and those failures have a probability that can be calculated. It is this view that allows some credit to be taken for check valves in PHAs and LOPAs, at least when certain conditions are met.
As a matter of practice, a PHA should list all existing safeguards against any particular hazard. Check valves are safeguards against reverse flow, and when they have been installed, they should be listed. How much credit should be taken during the likelihood assessment, however, is a different question.
The HazOp team needs to decide if the check valve is an effective safeguard. Is it used in clean service? Is the hazard one of bulk reverse flow? Does the check valve have a history of successful use? These are all indications that the check valve is an effective safeguard, in which case a level of protection can be achieved. On the other hand, does the check valve have a history of not working? If so, it should receive no credit in the PHA likelihood assessment.
Check valves are almost exclusively used as safeguards, rather than as active design features. The ball checks in double-diaphragm pumps are a notable exception. As a rule then, check valves are potential independent layers of protections (IPLs) in a LOPA scenario. It is not typical that the failure of a check valve is the cause of a LOPA scenario; the cause of the reverse flow comes from some other failure. A check valve failure is typically a failure to protect, not a cause.
To be counted as an IPL, a check valve must satisfy the three criteria of all IPLs: effectiveness, independence, and auditability.
Effectiveness. To be considered effective, an IPL must be able to prevent the LOPA scenario from occurring in the absence of all other IPLs. When it performs as designed, it should be able to prevent the event in question all by itself. This is also true of check valves that are to be considered IPLs. Specific to check valves, the scenario should not depend on preventing small reverse flows or on the check valve being bubble tight when in the closed position. Rather, the scenario should only involve bulk reverse flow. The scenario should not include flow of sticky, dirty, or solids-laden fluids through the check valve, any of which could reasonably be expected to prevent the check mechanism from closing. In any case, any check valve that has a history of being unable to prevent back flow should be considered ineffective.
Independence. Generally, an IPL is independent when the failure of another IPL has no impact on its effectiveness, when its failure has no impact on the effectiveness of another IPL, and when its failure is not the cause of the scenario at hand. Single check valves, being isolated mechanical devices, typically have no difficulty satisfying the criterion of “independence”.
Auditability. An IPL can only be considered auditable when there is a means to verify that the IPL is functioning as intended, and verification is performed as scheduled. If a check valve is installed but cannot be tested or inspected, it is not auditable. If a check valve is installed so it can be tested and inspected, but there is not an active audit program that periodically tests and inspects the check valve, it does not satisfy this criterion. Furthermore, if the check valve is audited and found to not work as designed, it must be repaired. In the case of a check valve failure, effectiveness should also be considered. If a test and inspection reveals the failure of a check valve, serious thought should be given to whether the check valve can be considered effective. A typical audit frequency for check valves is annual; however, a different frequency may be justified once the test history is developed.
If a check valve meets the three criteria for consideration as an IPL then it is creditable as a measure of risk reduction. In a LOPA, a single check valve as an IPL gets credit for one order of magnitude risk reduction: Probability of Failure on Demand (PFD) = 0.1 and Risk Reduction Factor (RRF) = 10.
Often, multiple check valves are installed on the same process pipeline in order to achieve greater risk reduction. There is a limit, however, to the amount of risk reduction that multiple check valves can achieve. At some point, the problem that causes one check valve to fail will cause all check valves in the pipeline to fail. We know this as “common cause failure”, which we must treat with caution. We must take common cause failure into account because it directly affects the credit that we may take when using more than one check valve as a means of protection.
Two Check Valves. If there were no common cause failures, the PFD for two check valves would be:
PFD = 0.1 x 0.1 = 0.01
The RRF for two check valves would be:
RRF = 10 x 10 = 100
As much as 10% of the problems that would cause one check valve to fail, however, are of a nature to cause all check valves in that service to fail—common cause failures. This is especially true when the check valves are of the same design. This means that the calculation for the PFD for two check valves should be:
PFD = (0.1 x 90%) x (0.1 x 90%) + (0.1 x 10%) = 0. → 0.01
Likewise, the RRF should be:
RRF = 1/PFD = 1 / 0. = 55 → 100
Looking at either PFD or RRF, it is reasonable to consider the risk reduction from a pair of check valves in series as approaching two orders of magnitude. In a LOPA, then, it is reasonable to take credit for two check valves, especially when they are of different design, which will make the common cause failure contribution less than 10%.
Three or More Check Valves. Given that two check valves in series is better than a single check valve, does it follow that three check valves in series is better still?
When considering three check valves with no common cause failures, the PFD would be:
0.1 x 0.1 x 0.1 = 0.001.
The RRF for three check valves in series with no common cause failures would be:
RRF = 10 x 10 x 10 = 1,000
Once a 10% common cause failure rate is included, however, the PFD for three check valves gets much worse:
PFD = (0.1 x 90%)3 + (0.1 x 10%) = 0. → 0.01 ≠ 0.001,
Likewise, the RRF would be:
RRF = 1/PFD = 1 / 0. = 93 → 100 ≠ 1,000
Clearly, the risk reduction provided by three check valves is still only two orders of magnitude. Moreover, additional check valves beyond three do not make the risk reduction any better than two orders of magnitude. Therefore, it is appropriate to take credit for one check valve or two check valves that qualify as IPLs, but no more than two.
Check valves are safeguards that prevent hazardous conditions that result from reverse bulk flow. You should acknowledge them in PHAs. As for taking credit for a check valve, don’t be too quick to jump to the “they’re worthless” conclusion. The amount of credit that you take for a check valve should depend on the service and history of the check valve.
In a LOPA, evaluate a check valve as you would any other IPL: for effectiveness, for independence, and for auditability. Typically, the hardest criteria to meet will be auditability. Assuming that a check valve meets the criteria of an IPL, it’s appropriate for you take credit for up to one order of magnitude risk reduction. A second check valve can increase the credit up to two orders of magnitude, though only after you have shown that both check valves meet the criteria of an IPL.
It may be that you decide that the effort to qualify a check valve as an IPL is beyond you. In which case, St. Augustine’s admonition about total abstinence being easier than perfect moderation may come into play. It’s not because it has to be that way; it’s because you choose to make it that way. Otherwise, install them and take credit for them, as your analysis shows is appropriate.
The company is the world’s best hydraulic system relief valve supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.