1. What is a check valve?

Check valve, also known as anti-backflow valve, English name is Check Valve (also known as One way valve or non-return valve), is a type of valve whose main function is to only allow the fluid to flow in the direction of flow. in a certain direction, do not allow the refrigerant to circulate in the opposite direction to affect the equipment/system. Therefore, when installing, it should be noted that they must be installed in the correct direction (usually there will be a symbol on the valve body).

2. Structure & working principle of one-way valve

a. Construction of one-way valve

Structure picture of one-way valve

There are many types of check valves such as flap check valve, butterfly check valve, spring check valve, etc., but all of them basically have the same structure, only different in shape, connection, material. materials & mode of operation. The image above is the basic structure of a flanged check valve, we will learn the functions of some main parts.

- Body:  The valve body is usually a metal block, covering & linking the valve parts together. This is the main pressure part, and determines the valve cost. The valve body is usually made from copper, cast iron, steel, stainless steel, etc., depending on the actual requirements of the customer.

- Cover plate:  The part located on the top of the valve, has a circular shape, usually a blind flange, connected to the valve body by bolts, forming a unified, anti-clear block. rust. This flange also works when repairing or maintaining the valve, which can be removed to affect the inside of the valve.

- Disc:  Has a round shape, a diameter larger than the inner diameter of the valve, is a part in direct contact with the medium, so it is usually made from materials that are resistant to corrosion and temperature. & high pressure. The valve disc is attached to the inside of the valve body through a hinge, when receiving direct pressure from the impact flow, the valve disc is raised close to the cover plate - then the valve opens. When the pressure drops, the disc will lower close to the valve seat, preventing backflow - then the valve closes.

- Body seat:  Works in coordination with the valve disc when the valve is closed, preventing reverse flow.

- Hinge & hinge pin:  Has the effect of fixing the valve disc with the inside of the valve body, keeping the valve disc moving in a certain stroke when receiving or stopping the impact force of the medium. .

- Gasket:  The sealing part between the flange of the valve body and the blind flange above the top of the valve.

b. Working principle of check valve

In the idle state, the spring's elastic force will cause the disc to be tight and seal the valve. However, when the fluid flows through the valve, the pressure & flow of the fluid will act on the valve disc. This force will initially be smaller than the spring force and then gradually increase to a level, the force of the fluid flow will be greater than the force of the spring, so the fluid flow will be passed through the valve easily. When the fluid is not circulating or the pressure is lower than the spring force, the valve will close.

Classification of one-way valves:

Check valves are classified into many different types, depending on their mode of operation. Here are some common types of check valves on the market today:

- Swing check valve:   The English name is Swing Check Valve, which is an automatic opening / closing type thanks to the valve disc mechanism - the valve shaft is hinged on the door. This is one of the most commonly used types on the market.

The flap check valve works according to the mechanism when there is flow, the pressure of the flow pushes the valve disc up and allows the flow to pass through the valve, under the pressure of the flow, the valve is continuously opened to let the liquid flow through. When the flow stops, or when it reverses, by its own gravity, the flap will automatically close to prevent the fluid from flowing in the opposite direction.

The check valve is generally flanged for size 2” (DN50) and above, and threaded for size 1.1/2” (DN40) and below.

Advantages of flap check valve:

- Compact design, takes up little installation space.

- Automatic opening and closing, no need for personnel & operating space, just space for maintenance & repair.

- Diverse size, suitable for many pipe sizes.

Disadvantages of flap check valve:

- The structure of the valve has no shaft (using hinges & opening and closing depending on the flow rate) so it is difficult to know the opening position of the valve disc.

- In the event that the medium contains impurities such as garbage, or there is a problem with the hinge pin, the valve will be in the open position continuously, even for reverse flow.

- Butterfly check valve:  The English name is Wafer Check Valve, which is a check valve with a Wafer connection (clamp form); In Vietnamese, it is called a 1-way butterfly valve, or a 1-way butterfly valve. This type of valve has the principle of opening and closing the vane like a butterfly. The valve disc consists of 2 blades, which are attached to the valve shaft. At the same time, use the spring to create a return force to close the valve.

When the valve is in the normally closed state, the two pieces of the valve disc are kept closed by the torsion spring. When there is fluid flowing in the forward direction, the pressure of the fluid causes the two discs to open hinges, allowing the fluid to flow through the valve. When the fluid stops flowing, or the flow reverses, the valve disc is immediately closed by the elastic force of the spring.

This butterfly check valve does not have a flange connection, but only a clamp connection. The valve is linked and tightened by the connecting bolts of the two flanges of the pipeline through the bolt holes located on the valve body.

Advantages of butterfly check valve:

- Compact design, takes up very little installation space.

- Automatic opening and closing, no need for personnel & operating space, just space for maintenance & repair.

Disadvantages of butterfly check valve:

- The structure of the valve has no shaft (using hinges & opening and closing depending on the flow rate) so it is difficult to know the opening position of the valve disc.

- In the event that the medium contains impurities such as garbage, or there is a problem with the hinge pin, the valve will be in the open position continuously, even for reverse flow.

- The valve has a flange connection only, not available with other connection types.

 

- Steam check valve:  Lift Check Valve is a type of valve widely used in steam and compressed air systems thanks to its high tightness and good leak resistance. The valve operates by means of a spring or the weight of the disc itself. When there is air flow in, the pressure of the gas will push the valve disc to rise. The fluid is circulated through the valve. When stopping supply, the valve disc will automatically fall or use spring force to force the valve disc to close.

Advantages of steam check valve:

- Like globe valve, steam check valve has simple structure, high durability & stable operation.

- Automatic opening and closing, no need for personnel & operating space, just space for maintenance & repair.

Disadvantages of steam check valve:

- Steam check valve is only installed for horizontal pipelines.

- This type of valve is large, bulky & very heavy for large sizes.

 

Disc check valve: Also known as a one-way valve, the English name is Disc Check Valve, is a type of valve that works on the mechanism when the medium passes through the valve, the push of the medium will be for the valve disc to slide up. on & allow the flow to flow. When the flow stops, due to the weight of the valve disc, the disc will slide down to the original closed position and press against the sealing rubber gasket, preventing the reverse flow.

 

Non-Slam Check Valve:  The full name of this valve is Non-Slam Check Valve, specially designed to minimize water hammer*. For a non-lam check valve, the closing of the valve will not happen suddenly, meaning no excess pressure peaks will be generated. The disc portion of the check valve has an internal spring that counteracts the open fluid flow pressure. When the flow of the medium is strong enough, the spring compresses and the valve opens; The disc is pushed back by the spring back to the seat surface in the valve when flow decreases and stops, but before the flow direction reverses.

* Water hammer is a phenomenon where the pressure/flow rate drops rapidly, creating a pressure wave in the fluid that bounces throughout the system, can lead to reduced process efficiency, valve damage , leaky coupling with gasket and other problems.

- Spring check valve:  The English name is Spring Check Valve, which is a type of valve that uses the action of a spring, pushing the valve disc to the closed state. For this type of valve, there are many different types and modes of operation, but they are all based on the principle of spring force.

Advantages of spring check valve:

- The valve has a compact design, does not take up much installation space.

- Automatic opening and closing, no need for personnel & operating space, just space for maintenance & repair.

- Simple to disassemble, maintain or replace.

Disadvantages of spring check valve:

- Operates by flow pressure & spring thrust, so springs with appropriate thrust must be selected.

- Springs are in direct contact with the medium, directly affecting the efficiency of the valve, so it is necessary to choose the right spring material, to avoid corrosion, rust & loss of spring's elastic force.

 

In fact, there are a few other types of non-return valves such as Ball Check Valve, One-way Globe Valve (Stop Check Valve), ... but they are not commonly used. In this article, we only ask for permission to share some types of check valves commonly used in the Vietnamese market.