An important component of the fire extinguishing system – Check Valve (Part 1)

Jun 2, 2023 | news

What Is A Check Valve?

Check valve refers to a valve whose opening and closing part is a circular valve disc, which acts by its own weight and medium pressure to block the backflow of the medium. It is an automatic valve, also known as a check valve, one-way valve, return valve, or isolation valve. The disc movement mode is divided into lift type and swing type. The lift check valve is similar in structure to the globe valve, except that it lacks the valve stem to drive the disc. The medium flows in from the inlet port (lower side) and flows out from the outlet port (upper side). When the inlet pressure is greater than the sum of disc weight and its flow resistance, the valve is opened. Conversely, the valve is closed when the medium flows backward. The swing check valve has an oblique disc that can rotate around the axis, and its working principle is similar to that of the lift check valve. The check valve is often used as the bottom valve of the pumping device to prevent the backflow of water. The combination of the check valve and globe valve can play the role of safety isolation. The disadvantage is that the resistance is large and the sealing is poor when closed.

Check Valve

Check Valve

Check Valve Working Principle

Check valves are also used on lines that supply auxiliary systems where the pressure may rise above system pressure. Check valves can be mainly divided into swing check valves (rotating according to the center of gravity) and lifting check valves (moving along the axis).

The function of this type of valve is to only allow the medium to flow in one direction and prevent the flow in the opposite direction. Usually, this kind of valve works automatically. Under the action of fluid pressure flowing in one direction, the valve disc opens; when the fluid flows in the opposite direction, the valve seat is acted on by the fluid pressure and the self-weight of the valve disc to cut off the flow.

Among them, the check valve belongs to this type of valve, which includes a swing check valve and a lift check valve. The swing check valve has a hinge mechanism, and a door-like disc freely leans on the inclined seat surface. To ensure that the valve clack can reach the proper position of the seat surface every time, the valve clack is designed in the hinge mechanism, so that the valve clack has enough swing space, and makes the valve clack truly and comprehensively contact with the valve seat. The disc can be made entirely of metal, or it can be inlaid with leather, rubber, or a synthetic cover on the metal, depending on the performance requirements. When the swing check valve is fully open, the fluid pressure is almost unimpeded, so the pressure drop through the valve is relatively small. The disc of the lift check valve is located on the sealing surface of the valve seat on the valve body. Except that the valve disc can rise and fall freely, the rest of the valve is like a globe valve. The fluid pressure makes the valve disc lift from the sealing surface of the valve seat, and the backflow of the medium causes the valve disc to fall back to the valve seat and cut off the flow. According to the conditions of use, the disc can be of all-metal structure, or in the form of a rubber pad or rubber ring inlaid on the disc frame. Like the globe valve, the passage of fluid through the lift check valve is also narrow, so the pressure drop through the lift check valve is larger than that of the swing check valve, and the flow rate of the swing check valve is limited. rare.

Check Valve

Classification Of Check Valves

Structural classification
According to the structure, it can be divided into three types: lift check valve, swing check valve, and butterfly check valve:
1. Lift check valve is divided into two types: vertical and horizontal.
2. Swing check valves are divided into three types: single flap, double flap, and multi flap.
3. The butterfly check valve is a straight-through type.
The connection forms of the above check valves can be divided into four types: threaded connection, flange connection, welding connection, and wafer connection.

Material classification
1. Cast iron check valve
2. Brass check valve
3. Stainless steel check valve
4. Carbon steel check valve
5. Forged steel check valve

Functional Classification
1. DRVZ silent check valve
2. DRVG silent check valve
3. NRVR silent check valve
4. SFCV rubber disc check valve
5. DDCV double disc check valve

Check Valve

Check valve installation

1. Swing check valve: The disc of the swing check valve is disc-shaped and rotates around the shaft of the valve seat channel. Because the channel in the valve is streamlined, the flow resistance is smaller than that of the lift check valve. It is suitable for large-diameter occasions with low flow rates and infrequent flow changes, but it is not suitable for pulsating flow, and its sealing performance is not as good as that of the lifting type. Swing check valves are divided into three types: single-disc type, double-disc type, and multi-disc type. These three types are mainly divided according to the valve caliber. The purpose is to prevent the hydraulic impact when the medium stops flowing or reverses.

2. Lifting check valve: a check valve whose disc slides along the vertical center line of the valve body. Lifting check valves can only be installed on horizontal pipes. On high-pressure small-diameter check valves, the disc can use a ball. The valve body shape of the lift check valve is the same as that of the globe valve (it can be used in common with the globe valve), so its fluid resistance coefficient is relatively large. Its structure is similar to the globe valve, and the valve body and disc are the same as the globe valve. The upper part of the valve disc and the lower part of the bonnet are processed with guide sleeves. The guide sleeve of the valve disc can be lifted and lowered freely in the guide plate of the valve cup. When the medium flows downstream, the valve disc is opened by the thrust of the medium. Drop down on the valve seat to prevent the reverse flow of the medium. The direction of the medium inlet and outlet channels of the straight-through lift check valve is perpendicular to the direction of the valve seat channel; the direction of the medium inlet and outlet channels of the vertical lift check valve is the same as that of the valve seat channel, and its flow resistance is smaller than that of the straight-through type.

3. Disc check valve: a check valve in which the disc rotates around the pin shaft in the valve seat. The disc check valve has a simple structure and can only be installed on horizontal pipelines with poor sealing performance.

4. Pipeline check valve: a valve in which the disc slides along the centerline of the valve body. The pipeline check valve is a new type of valve. It is small in size, light in weight, and good in processing technology. It is one of the development directions of the check valve. However, the fluid resistance coefficient is slightly larger than that of the swing check valve.

5. Compression check valve: This valve is used as a boiler feed water and steam cut-off valve. It has the comprehensive functions of the lift check valve and stops valve or angle valve.

In addition, some check valves are not suitable for pump outlet installation, such as bottom valves, spring type, Y type, etc.