1. Classification of check valves

Check valves can be classified into various categories based on their structure and purpose. Common classification methods include: lift check valve, swing check valve, spring check valve, etc. Different types of check valves have different characteristics and applicable scenarios, and the selection should be based on specific needs.

2. Selection of check valves

In most cases, the selection of check valves is mainly based on their required closing speed and closing characteristics. However, size is also a crucial factor that cannot be ignored. For particularly important usage scenarios, it is recommended to consult a reputable valve manufacturer for professional advice.

1) Non compressible fluid check valve

For incompressible fluids, the key to choosing a check valve is whether it can effectively cut off backflow when suddenly closed, without generating unacceptable high impact pressure. Usually, this type of valve is considered a low-pressure reducing valve, and two main factors are considered when selecting it.

Firstly, a qualitative assessment of the required closing speed is required. Rukovsky's theory of sudden valve closure provides an evaluation framework, in which the increase in static pressure in the pipeline can be used for this purpose. This theory states that the pressure rise Δ p is related to the flow velocity v before the fluid comes to a standstill, the pressure wave propagation velocity α, the fluid density ρ, and the pipeline diameter B. Through this theory, we can make a reasonable estimate of the required closing speed and select the appropriate check valve.

When evaluating the selection of check valves for incompressible fluids, we need to consider multiple factors. Among them, key parameters include liquid bulk modulus K, liquid density ρ, pipeline inner diameter D, pipeline flow limiting coefficient c, elastic modulus E of pipe wall material, and pipe wall thickness e. Specifically, when using steel pipes with a D/e ratio of 35 and aqueous media, the propagation speed of pressure waves is approximately 1200m/s (or 4000ft/s). This means that when the instantaneous velocity changes by 1m/s, the static pressure will increase by 13.5 bar.

In addition, the closing speed of the valve is also an important consideration factor. If the valve can close within 2L/α of the time required for a pressure wave to travel back and forth (where L is the length of the pipeline and α is the propagation speed of the pressure wave), then the previous pressure wave returning will not be able to cancel out the upcoming pressure wave, resulting in a pressure rise similar to the sudden closure of the valve. This closure method is called quick closing. However, if the closing time of the valve exceeds 2L/α, the returned pressure wave will partially offset the next round of pressure wave, thereby reducing the maximum increase in pressure. This closing method is called slow closing.

It is worth noting that when calculating water hammer caused by pump shutdown, we must fully consider the characteristics of the pump and the rate of change in pump speed after cutting off the power supply. Finally, we need to select the type of check valve that may meet the required closing speed based on these factors.

2) Check valve for compressible fluids

For compressible fluids, the selection principle of check valves is roughly the same as for incompressible fluids. However, it should be noted that when used in gas media, high lift check valves may face flutter issues, which may need to be addressed by adding shock absorbers.

3. Selection guide for check valves:

In situations where automatic prevention of medium backflow is required, check valves should be preferred. Specifically, when the diameter of the pipeline DN is ≤ 40mm, a lift check valve (only for horizontal installation) is a suitable choice; Within the range of DN=50-400mm, rotary check valves (not suitable for vertical pipelines with medium from top to bottom) are more suitable; For large-diameter pipelines with DN ≥ 450mm, it is recommended to use a buffering disc check valve. In addition, double clamp check valves can also be considered for pipelines with DN=100-400mm, and their installation positions are not limited.

4. Main standards for check valves:

When selecting check valves, a series of relevant standards should be followed to ensure the quality and performance of the valve. These standards include GB/T 12233-2006 "General purpose valves - Iron globe valves and lift check valves", GB 12235-2007 "Globe valves and check valves", GB 12236-2008 "General purpose valves - Steel swing check valves", as well as GB/T 13932-92 "General purpose valves - Iron swing check valves", GB 8464-1998 "Internal threaded connection valves for plumbing", etc. Meanwhile, JB/T 53036-1999 "Quality Grading of Check Valve Products" also provides a basis for the quality assessment of check valves.