What Is A Check Valve: One-way, Air, Sump Pump & What Does It Do?

If you have worked on plumbing, you might have noticed parts of different shapes and sizes. Some are long, while others are short. A check valve has a cylindrical shape and is typically only a few inches long. What does it do, and why is it important to have in your plumbing system?

check valve

The following sections highlight the facts about it. Whether you are someone new to all of these or have some knowledge, you will find this article helpful. Stick around to know more about one-way and their significance. 

What is a Check Valve?

What is a check valve? The simplest explanation is that it restricts fluid flow in one direction. As liquid flows through a cylinder, there may be tendencies for it to flow back. The non-return prevents any backflow from happening. Hence, restricting the flow in one direction. There are two ports, one inlet and one output. They are the holes on each side.

It also has other names. Since fluid flows in one direction only, people also refer to it as a non-return or one-way valve. So why do we typically refer to it? The explanation is simple! It is a device that checks and ensures liquid only flows in a single direction.

It heavily relies on pressure differential, which should be readily available. The inside seals tight in the absence of pressure. The input side requires high pressure for the valve to open. In any case, the input side pressure should be greater than the output side to work correctly. If it is the other way around, the valve will keep close. Fluid will not pass through it.

The closing mechanism varies across these types. There is a section below that provides more details on it. What makes the non-return unique from other types is it does not need a lever or actuator to function correctly. It relies on fluid pressure.

You can find this valve in applications where backflow may cause a problem. It is an effective and cheap solution to solving a possible issue that may arise soon. You might be wondering what the deal is with backflowing fluids? In certain instances, it may cause contamination. An excellent example is a sewer line. A non-return valve prevents waste from slipping back into the system. Blackflows can also damage a system or equipment, especially if it only allows the media to flow in a single direction. For example, an osmosis filter only allows water to flow in a single direction. A one-way valve prevents flowing in the opposite direction from happening. 

Regardless of the case, a check valve drastically reduces the risk of damage to your equipment. It does so by preventing fluid from flowing back or going in the opposite direction. Tideflex and Zoeller are some of the famous brands that sell one-way it. Other manufacturers also sell this piece. It is widely available in the market, so there is no risk of shortage. 

How Does a Check Valve Work?

It needs a minimum upstream pressure to work. Upstream pressure is the difference between outlet and inlet. The valve opens when the pressure reaches the minimum value, allowing fluid to flow through. You do not need any other interactions for it to work. We refer to the minimum upstream pressure as cracking pressure. The valve opens and allows the liquid to flow through it. So at which point does the valve open? There are a few things it is dependent on, such as size and design. To ensure the valve opens up, check your system can generate enough cracking pressure. It can be any value between 3 to 350 psi. It is best to check the specifications. 

Now let’s assume your system falls below the minimum cracking pressure. What is going to happen? If it does, back pressure will happen. It’s where liquid flows from outlet to inlet instead of the other way around. As a result, your check valve should close. It blocks anything from passing through. The blocking mechanism varies per design, assisted by either gravity or spring. If you run into an issue with the spring mechanism, you will have to replace the entire valve.

The installation orientation is vital when setting up for the first time because it only works in one direction. If you install it in the wrong direction, it will not work. It results in no blocking. Most manufacturers mark the valve with an arrow pointing to the correct flow direction. If you do not see any markings, the only option left is to examine them. Installing these in the wrong direction may lead to permanent damage to your system.

A good use case would be in the sewer. You do not want dirty water to flow back to the source, so you use a non-return valve to prevent it from happening. Another good use case is in a dishwasher, which requires an inlet and outlet for clean and used water to flow through. We do not want used water to get back into the dishwasher and mix with the dishes. We add a check valve to prevent it from happening. 

What is a Check Valve Used For?

red check valve

What is the significance of having to install one into your system? Why can’t we skip it and design the system taller on one end, so fluid will always flow in one direction? Manufacturers typically use PVC when producing it.

Its primary use is to prevent backflow from happening, whether it may be gases or liquids. You can use it as a prevention mechanism that comes in different shapes, such as poppet or disc.

The following are its primary uses.

Protects an Equipment from Damage Brought by Backflow

check valve with its benefits

Its primary use is preventing reverse flow. It’s regardless of its type. A spring-loaded check valve uses a spring to seal. It does not need gravity to work correctly. In the event of pressure change, the flow reverses direction. Without the right equipment, it will lead to damage because of the backflow. With the right tool, it protects it from permanent damage.

We use it in different applications, such as priming pumps. It maintains head pressure, line isolation, media injection, etc. The prevention of backflow also stops dirty water from entering the system, in the case of a dishwasher or laundry machine. 

Puts Relief to the Valves with Low Pressure

check valves

A spring-loaded type applies relief to it by using low pressure. The metal seals use thermoplastic or metal, which manufacturers use with stainless steel springs. Other manufacturers also use Inconel or titanium for the spring. It is totally up to the manufacturer.

The material they opt to use does not have a significant performance impact. However, different materials will perform uniquely depending on their properties. The answer to relief of pressure is it pulls away from the valve seat. When does it happen? If the upstream pressure is greater than the spring pressure. When it happens, the media gets to pass through the outlet.

You should know the maximum and set pressure your system can handle during the sizing stage. It saves time and overall cost when you know these things from the start. 

Works as a Vacuum Breaker

Aside from preventing backflow, these non-return valves also act as vacuum breakers. These breakers let air through the system to prevent interruptions in the flow. The most common application is on a tank. We install it at the piping’s top area to relieve pressure when it pumps down. Sudden changes in piping elevation are another common application of these vacuum breakers.

You can use it as a vacuum relief, pulling the required vacuum amount. Whenever there is a sudden change in temperature, it creates a vacuum effect. It can be things such as from very hot to cold temperature change. A vacuum breaker would release the tension arising from the sudden change in temperature. As a result, it prevents damage from happening.

When too much pressure builds up, it may lead to an explosion. It will also cause permanent damage to the system. These will only result in more repair fees and safety compromises. 

When Should Check Valves Be Used?

Now you know what it is and how it works, the next thing you should know is when to use it. We know it prevents anything from flowing in the opposite direction and allows only a singular directional flow.

We use it in three primary roles from an application perspective. They are pilot control, protection, and directional control. 

What is the Difference Between a Check Valve and a Non-Return Valve?

These two are functionally the same. However, there are a few minor details worth noting. The former prevents the backflow of fluid by only allowing it to flow in a single direction. The latter allows fluid to flow in one direction. The former also works specifically to a certain pressure.

Several people use these terms interchangeably, which is not a big problem since they are functionally similar. However, it is best to know the differences between these two. 

Where is a Check Valve Located?

So exactly where can you find the check valve in your system? Is it near the pump or the furthest from it? In most cases, you can find it on the outlet near the pump. It protects it from backflow. 

Most systems have at least two of these, one before the inlet and another after the outlet. It depends on the pressure flowing through the medium to push it close or open. There are two ways to determine its location.

Classification by its Structure

The three structures are butterfly, lift check, and swing check. The butterfly check divides into butterfly single and butterfly double. Secondly, the lift checks divide into straight and vertical through. Lastly, the swing check has further classification into single, double, and multi valves. 

Determining the Installation Location

We typically install it in two locations – one at the end of a pipe in front of the water pump and the other right next to the outlet. 

Types of Check Valves

These come in different sizes, such as but not limited to 3/4, 3/8, 1 1/2, and 2-inch. Aside from sizes, it is also available in several types. It depends on use-case and needs. 

The following types below are some of the most commonly used ones. You may realize the one you are looking for is not listed below. Although it may come in several types, they all serve the same purpose. It restricts the flow to only one direction. 

Sump Pump

The sump pump has a flapper, which we typically install on the sump pump’s discharge pipe. As water pumps through, it forces the flapper to open. It closes when the pump shuts down when gravity pushes it down. Some models will make a clunking noise because of the water hammer.

Pumps are more efficient when coupled with a sump pump. When you run a pump, the water forces through the pipes. When you turn it off, it attempts to fall back to its source. We install the sump pump to prevent short-cycling from happening. We save electricity costs as a result. 


The swing check actuates itself, preventing backflow and directing the flow in only one direction. We also refer to this type as the self-actuated, where it got its name. The seat of the gate has a disc, which swings open to allow flow and close to prevent it. Whenever there is no fluid or motion, the disc shuts close.

The disc’s weight should be heavy enough to prevent any backflow from happening. The swing check has low-pressure drop and experiences less turbulence than other types. We use a lever and weight swing check for applications with high chances of water hammer. On the other hand, we use a lever and spring swing check with high velocity and pressure applications. One thing to note with the swing check is the regular cleaning of the disc. 

Brake Booster

The brake booster is a modern type of vacuum brake booster. It provides additional braking power. How does it do it? It amplifies the force the brake master cylinder delivers. The brake lines receive hydraulic pressure and engage the caliper to reduce the car’s speed.

You can find the brake booster check valve in the vacuum inlet near the hose that connects with the engine intake. Essentially, it expels trapped air from the vacuum brake booster. 

Air Compressor

An air compressor lets air inside from one side to the other. At the same time, it blocks air from the opposite direction passing. Once compressed air reaches this area, it can no longer go back.

There are several reasons why this is in place. Some systems require five of them, while others only need one. It is dependent on the compressor size and model.

Fuel Line

We can find the fuel line type in vehicles, specifically the lines leading to and from the fuel chamber. It closes the return lines, preventing air or fuel from returning to the injection system. 


One-way is a general term to refer to a device that ensures the media passes in one direction only. Water hammer may occur in the absence of this device.

Water & Pool Check Valve

A water and pool check prevents backflow from occurring in your o lines. It prevents water from going back to the pool, which could contain dirt and contaminations. These may cause permanent damage to your system.

Why do you need one? Some states in the United States require a pool check valve if your supply connects to the city water supply. If you have a well at your home, we recommend installing one to prevent contaminants and bacteria from mixing with your pool water.

You can install it close to your water pump. Find the nearest location if you cannot install it right next to the pump. Inline and swing are two of its types. The former utilizes a disc or ball to block water, while the latter uses a flap. 

Vacuum Check Valve

We use it in vacuum applications to prevent air from going in the opposite direction. However, you cannot use it on its own. For maximum effectivity, it must be completely sealed.

Hydraulic Check Valve

The hydraulic check valve is the most basic directional control type we use in a hydraulic system. It permits the free flow of oil in a single direction while blocking it from going in the opposite direction. We use it to control pressure or direction in a hydraulic system. 

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